Evidence for a Sigmatropic and an Ionic Pathway in the Winstein Rearrangement

Abstract

The spontaneous rearrangement of allylic azides is thought to be a sigmatropic reaction. Presented herein is a detailed investigation into the rearrangement of several allylic azides. A combination of experiments including equilibrium studies, kinetic analysis, density functional theory calculations, and selective ¹⁵N-isotopic labeling are included. We conclude that the Winstein rearrangement occurs by the assumed sigmatropic pathway under most conditions. However, racemization was observed for some cyclic allylic azides. A kinetic analysis of this process is provided, which supports a previously undescribed ionic pathway.

Graphical abstract

INTRODUCTION

Sigmatropic rearrangements are central to organic chemistry and their synthetic applications are numerous. Famous examples include the Cope,¹ Claisen,¹ Wittig,² Carroll,³ Sommelet–Hauser,⁴ Mislow–Evans,⁵ Meisenheimer,⁶ and Overman⁷ rearrangements or modifications thereof.^8–10 In principle, all of these reactions are reversible. However, the most heavily utilized sigmatropic rearrangements have a clear thermodynamic driving force. Most also require heating or a catalyst to achieve efficient reactivity. When viewed in this light, the Winstein rearrangement of allylic azides is an oddity because (i) it occurs at or near room temperature, and (ii) it results in an equilibrium mixture of allylic azides (Scheme 1a).¹¹ This is because the azide functional group is dipolar and symmetric. A C–N, N═N, and C═C bond is present in both the starting material and product of the azide rearrangement, making it near thermoneutral. This parallels the Cope rearrangement, where, in most cases, the starting material and product are similar in energy, resulting in an equilibrium distribution.¹² This also parallels the rearrangement of allylic acetates.^13–15 These unusual properties make the allylic azide rearrangement an interesting candidate for further study.

Scheme 1.

Winstein Rearrangement

The rearrangement of allylic azides was first described by Winstein and co-workers (Scheme 1a).¹¹ In hindsight, this rearrangement was likely observed but not recognized until later by VanderWerf and Heasley.^16,17 Winstein’s initial report describes the rate constant and equilibrium constant for the rearrangement of prenyl azide and crotyl azide. These parameters were minimally sensitive to solvent polarity, indicating a relatively neutral transition state and implying a sigmatropic process. Le Noble found that at high pressure the reaction’s activation volume was consistent with a neutral pathway.¹⁸ Padwa provided the first evidence for stereoselectivity.¹⁹ A single diastereomer was isolated from a tandem insertion rearrangement sequence (Scheme 1b). Spino observed stereoselectivity during a tandem Mitsunobu reaction rearrangement sequence (Scheme 1c).²⁰ A few reports describe an analysis of crotyl and prenyl systems computationally and concluded that the process is cyclic.^21–23 These observations are all consistent with a concerted [3,3] sigmatropic mechanism for the allylic azide rearrangement and the [3,3] mechanism has been generally accepted.

A unique aspect of the allylic azide rearrangement is the abnormally low activation barrier, which allows this process to occur at ambient temperature. The Winstein rearrangement has limited synthetic applications due to difficulties controlling the rearrangement.^24–30 We have an ongoing interest in using the Winstein rearrangement and became interested in more precisely defining the parameters of the rearrangement on a broader set of allylic azides.^31–33 Presented herein is a summary of several experiments that more fully define the energetics and mechanism of this rearrangement. We investigated the effect of competitive conjugation on the equilibrium constant. A rearrangement with ¹⁵N isotopically labeled allylic azide unambiguously confirms the [3,3] pathway. We describe a pathway for the E to Z isomerization of trisubstituted alkenes. These results led to us to probe whether racemization of chiral allylic azides could occur. Under certain conditions, these allylic azides racemize. A kinetic analysis supports an ionic pathway. Each of these pathways is supported by DFT calculations. These combined experiments indicate that more than one pathway is operable for allylic azide isomerization.

RESULTS AND DISCUSSION

Effect of Conjugation on K_eq

Scattered reports provide insight into biasing the azide rearrangement. Hassner originally synthesized cinnamyl azide and reported only a single isomer (Scheme 2a).³⁴ In this example, conjugation to the phenyl ring strongly influences the equilibrium and results in a single observable isomer.³⁵ Panek observed a similar effect with γ-azido-α,β-unsaturated esters (Scheme 2b) and Evans made an analogous observation with 3-azido-vinyl sulfones (Scheme 2c).^36,37 In all cases, the conjugated compound is favored. This is reminiscent of strategies to render the Cope rearrangement effectively irreversible by forming a conjugated product.³⁸ We were interested in re-establishing a detectible equilibrium with conjugated azides. To this end, compounds 15 through 18 were synthesized (Scheme 2d). Each allylic terminus is substituted with a different group for conjugation. After isolation, samples were incubated at 40 °C for 48 h to allow equilibration and then analyzed. The observed equilibrium ratio ranges from 2.3:1 to 1.2:1 for azides 15 through 17, indicating that a detectible equilibrium can be re-established if both termini stabilize the system through conjugation. The effect seemed insensitive to the nature of the aryl (16 vs 17). For nitriles 18a and 18b, the other isomer was not detected. The remainder of this report describes allylic azides that are conjugated to an aryl ring. These systems were intentionally designed to minimize the number of isomers present in the mixture as a means of simplifying analysis.

Scheme 2.

Effects of Conjugation on K_eq

¹⁵N-Labeling Experiment

One means to unambiguously discern between a [1,3], [3,3], and ionic mechanism is to isotopically label one end of the azide group. Sodium azide containing a ¹⁵N-label is commercially available; however, either end of the azide anion could attack an electrophilic carbon center. This would likely generate a 1:1 mixture of proximal and distal ¹⁵N-azides, which would be difficult to analyze. Instead, we turned our attention to an alternate approach (Scheme 3). We synthesized ¹⁵N-amine 19 in two steps from potassium ¹⁵N-phthalimide. The ¹⁵N-label was evident by ¹⁵N NMR and by HRMS. Images of all mass spectra are included in the Supporting Information. The amine was subjected to diazo-transfer conditions, which formed azide 20. This azide spontaneously rearranged to afford ¹⁵N-cinnamyl azide (21), which was isolated as a single isomer due to conjugation. The label was again clearly evident by NMR, IR, and HRMS. Reduction of the azide liberated N₂ and afforded phosphoramidate 22. Analysis of this product by NMR and HRMS indicated the complete loss of the ¹⁵N label. An authentic sample of ¹⁵N-22 was synthesized by a separate route. This experiment provides direct evidence for the [3,3] mechanism of the Winstein rearrangement.

Scheme 3.

Rearrangement of ¹⁵N-Labeled Allylic Azide

E to Z Isomerization

Many allylic azides are isolated as a mixture of alkene isomers.^{11,24,27,39,40} Typically, the observed distribution of isomers is related to the relative thermodynamic stability of the alkenes. Little is known regarding the mechanism of alkene isomerization via the Winstein rearrangement.²² Labadie et al. investigated several isoprenyl azides.²³ That report concluded that allylic azides conjugated to an aryl ring do not rearrange. This is contrary to a prior report that cis-cinnamyl azide isomerizes to trans-cinnamyl azide.⁴¹ We observed that azide 23 forms a mixture of E and Z isomers upon standing (Scheme 4a). The naphthyl containing compound was studied because the isomers were separable by column chromatography.⁴² We conducted a kinetic analysis of the E to Z isomerization by ¹H NMR at 75 °C in C₆D₆. The observed rate constant (k₁ + k₋₁) was 7.2 × 10⁻⁴ s⁻¹ and the observed ratio of 23:24 was 3.2:1 at equilibrium (equilibrium reached after 90 min). The same approximate values were measured by monitoring the reverse Z to E rearrangement (k₁ + k₋₁ = 6.9 × 10⁻⁴ s⁻¹, average reported in Scheme 4). The observed rate is slower than that reported by Winstein for crotyl azide.

Scheme 4.

Equilibration of Azides 23 and 24^a

^a(a) Values determined by ¹H NMR. (b) Free energies (kcal/mol, 1 M standard state, 75 °C) computed at the SMD(CHCl₃)/M06-2X/6-311+G(2df,p)//SMD(CHCl₃)/M06-2X/6-31G(d) level of theory. The vertical scale has been compressed by 15.0 kcal/mol between the ground states and transition states.

We turned to density functional theory (DFT) calculations to describe the pathway for the observed alkene isomerization (Scheme 4b). These calculations were conducted at the M06–2X level of theory, both in the gas phase and accounting for solvation effects implicitly with the SMD solvation model (e.g., chloroform, as shown in Scheme 4, see Computational Details).^43,44 The analysis indicates a three step process: (1) endothermic rearrangement to benzylic azide 25, (2) σ bond rotation, and (3) a second Winstein rearrangement to the conjugated isomer 24. The calculated barriers and relative stationary point energetics are in good agreement with the experimental data. In both rearrangements, the transition-state structure is a fairly flat half chair (C–N–N–N–C in plane and C–C–C pucker). The rearrangement is relatively synchronous (C–N bond lengths of 2.036 and 2.069 Å). This study did not address if the transition state is pericyclic or pseudopericyclic.^15,45,46 These results indicate that the Winstein rearrangement is kinetically viable on conjugated systems, even though the isomeric benzylic azide is present at too small a concentration to be readily detected by ¹H NMR.

Chiral Acyclic Allylic Azide

These results led us to consider whether azide 26 could racemize through the Winstein rearrangement (Scheme 5). Stereoselectivity is a key feature to effectively utilizing allylic azides in synthetic contexts.²⁰ Racemization can occur readily if one isomer is achiral (see SI for the calculated profile for crotyl azide) or if the product is the enantiomer of the starting material.^32,47 It was unclear whether a system such as azide 26 would racemize via a sigmatropic process. The examples from Padwa and Spino (Scheme 1b,c) document that racemization was not observed in those instances. However, the example from Padwa is cyclic, cannot form E/Z mixtures, and it is effectively irreversible (kinetically mediated). The example from Spino contains a trans alkene, which may not be able to adopt multiple reactive conformations. Azide 26 rapidly forms a mixture of E/Z isomers, with a rate constant (k₁ + k₋₁) of 5.0 × 10⁻³ s⁻¹ in C₆D₆ at 75 °C and a 2.5:1 equilibrium ratio of 26:27. Equilibrium is reached considerably faster for compounds 26 and 27 (after 20 min) relative to 23 and 24 (90 min). The observation that an additional methyl group speeds the rearrangement is consistent with Winstein’s work where k_rel for prenyl/crotyl azides ranged from 2.6 to 3.6 in a variety of solvents.¹¹ To determine if racemization occurs, we used semipreparative chiral HPLC to isolate enantioenriched azide 26 (>99:1 er). The equilibration of compound 26 was monitored by ¹H NMR at 75 °C. Once equilibrium was reached, the enantiopurity of the sample was measured by chiral HPLC. Only a single enantiomer was observed for both the E isomer (>99:1 er) and Z isomer (>99:1 er). This indicates that racemization via a sigmatropic process is unlikely.

Scheme 5.

Equilibration of Chiral Allylic Azide^a

^a(a) Values determined by ¹H NMR. (b,c) Free energies (kcal/mol, 1 M standard state, 75 °C) computed at the SMD(CHCl₃)/M06-2X/6-311+G(2df,p)//SMD(CHCl₃)/M06-2X/6-31G(d) level of theory. The vertical scale has been compressed by 15.0 kcal/mol between the ground states and transition states.

We investigated the [3,3] mechanism computationally (Scheme 5b,c). The lowest energy pathway is similar to that shown for compounds 23 and 24, where a three step process is responsible for the alkene isomerization (Scheme 5b). The azide rearranges to trans-azide 28, undergoes a bond rotation, and then rearranges a second time to afford (Z)-azide 27. For compounds 26 and 27, a second sigmatropic pathway was identified, which is similar to the first except in the order of events. In the second pathway (Scheme 5c), (E)-azide 26 undergoes a bond rotation and then a rearrangement to afford cis-azide 28. This azide undergoes a second bond rotation and subsequent rearrangement to afford (Z)-azide 27. In both pathways, the initial azide was intentionally assigned as (S)-(E)-azide 26 and the product of both pathways was (R)-(Z)-azide 27. The stereoselectivity in both pathways is consistent with the experimental findings.

A third ionic pathway was identified which was significantly higher in energy. The azide ionizes to form free azide anion and a stabilized allylic cation (Scheme 6). In the gas phase and in chloroform, this pathway was significantly higher in energy relative to the sigmatropic pathway (134.3 and 48.7 kcal/mol, respectively). However, in methanol, the ionic pathway was calculated to be much closer in energy to the sigmatropic pathway, at only 30.0 kcal/mol. This prompted further exploration.

Scheme 6.

Relative Ionization Energies of Azide 26^a

^aFree energies (kcal/mol, 1 M standard state, 75 °C) computed at the M06-2X/6-311+G(2df,p)//M06-2X/6-31G(d) level of theory for gas-phase and SMD-solvated reaction coordinates. The vertical scale has been compressed by 60.0 kcal/mol between chloroform and gas phase.

Identifying a Racemization Pathway

To examine the proposed ionic pathway, a new series of allylic azides was investigated (Scheme 7). These allylic azides are readily prepared from cyclohexenone. The cyclohexyl ring prevents E to Z isomerization and simplifies analysis. These azides could be easily separated by semipreparative chiral HPLC. Once characterized, azide 30e was subjected to prolonged heating in a variety of solvents. After 1 week at 100 °C no or negligible racemization was observed in hexanes, toluene, chloroform, and tetrahydrofuran. In these solvents, even after prolonged heating, these allylic azides can be thought of as static structures that are configurationally and isomerically stable. However, rapid racemization was observed in methanol at 100 °C. As an example, for azide 30b (4-Me), the er decreased considerably after only 1h from >99:1 er to 74:26 er and full racemization occurred within 8 h. Solvent-dependent racemization is consistent with the pathway identified by DFT.

Scheme 7.

Racemization and Solvolysis of Cyclic Azides

Intrigued by the observed racemization, we conducted a more detailed kinetic analysis. The rate of racemization was determined by chiral HPLC. Using the method of initial rates,⁴⁸ the rate was found to be first order in azide. Several analogous substrates were synthesized and subjected to the same analysis. A Hammett plot was generated for racemization (Figure 1). The rate of racemization is highly correlated with the Hammett parameter σ⁺ (R² = 0.99, ρ = −3.9). The relative energies of the ions (31a–f) were calculated at the SMD/M06-2X level. The ion stabilities ranged from 21.4–30.0 kcal/mol (SMD(MeOH)) and the stabilities were also correlated to σ⁺ (see SI). This analysis is consistent with rate determining formation of an ion pair.

Figure 1.

Plot of log(k/k₀) vs σ⁺ for substituent effects on the racemization of azides 30a–f in MeOH. Rates were measured in duplicate, replicates shown.

A more detailed analysis revealed a byproduct of the racemization process, identified as the corresponding methyl ether. At all time points, the methyl ether is racemic. The appearance of this product is consistent with an ionic solvolysis reaction. Adding exogenous tetrabutylammonium azide did not noticeably inhibit the formation of the methyl ether. The rate of methyl ether formation is also correlated with the Hammett parameter σ⁺ (Figure 2, R² = 0.92, ρ = −3.1). When heated in methanol, acyclic azide 26 also racemizes, indicating that other azides can participate in this process.

Figure 2.

Plot of log(k/k₀) vs σ⁺ for substituent effects on the solvolysis of azides 30a–f in MeOH. Rates were measured in duplicate, replicates shown.

CONCLUSION

Presented herein is a description of two competing pathways for the rearrangement of allylic azides. A ¹⁵N-labeling experiment, supported by DFT calculations, indicates that under most circumstances, the rearrangement is sigmatropic. A pathway for the isomerization of a chiral allylic azide, with a trisubstituted alkene, is reported to occur without racemization. A second ionic pathway is available in highly polar media, and we observed it in methanol. This reaction is Hammett constant (σ⁺) correlated and is kinetically consistent with a solvolysis mechanism.

EXPERIMENTAL SECTION

Azide Safety

Azides are known to be high-energy materials, and explosions have been reported when working with azides. In the course of this work, no issues were encountered. All of the azides synthesized in this report have C/N ratios equal to or above the recommended guideline of 3 C/N. Precautionary blast shields were used for all reactions using or producing more than 1 mmol of azide. Blast shields were used both in the fume hood and during rotary evaporation. All waste and aqueous solutions that could be contaminated with azide were kept in individually labeled containers and were kept strictly free of acid to avoid the accidental production of HN₃.

General Methods

All reactions used magnetic stirring. All reactions conducted at elevated temperatures used aluminum block heating. Temperatures reported are based on an external thermocouple. All commercially available reagents were used without further purification. Dry THF and DCM were obtained from a commercial solvent system utilizing activated alumina columns under an argon pressure. Thin-layer chromatography (TLC) was used to monitor reaction progress. TLC was performed using precoated glass or plastic plates with silica gel impregnated with a fluorescent indicator (254 nm). Visualization was achieved using UV light, PMA, or KMnO₄ stains. Organic solutions were concentrated by rotary evaporation under reduced pressure. Flash chromatography was performed on an automated system utilizing normal phase precolumn load cartridges and gold high performance columns. All proton (¹H) nuclear magnetic resonance spectra (NMR) were recorded on a 400 or 500 MHz spectrometer. All carbon (¹³C) nuclear magnetic resonance spectra were recorded on a 100 or 125 MHz NMR spectrometer. All phosphorus (³¹P) nuclear magnetic resonance spectra were recorded on a 162 MHz NMR spectrometer. All nitrogen (¹⁵N) nuclear magnetic resonance chemical shifts were measured using ¹H–¹⁵N heteronuclear multiple bond correlation (HMBC) on a 93 MHz spectrometer. Data is presented as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet and m = multiplet), integration, and coupling constant in Hertz (Hz). Infrared (IR) spectra were taken with KBr plates. IR spectra were reported in cm⁻¹. Mass spectrometry was conducted at the University of Minnesota Mass Spectometry Laboratory. Details on HPLC analysis and purification are included in the Supporting Information.

Ethyl (E)-4-azido-4-(4-methoxyphenyl)but-2-enoate (15a) and Ethyl (E)-2-azido-4-(4-methoxyphenyl)but-3-enoate (15b)

Azides 15a and 15b were synthesized via a multistep sequence that is outlined here. A procedure was adapted from a known method for the addition of ethyl propiolate to 4-methoxybenzaldehyde and the product was isolated (700 mg, 73%).⁴⁹ Characterization data for this compound has been reported. The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information.

This product was reduced using an adapted procedure.⁵⁰ To an ice-cold suspension of LiAlH₄ (33 mg, 0.9 mmol) in THF (2.5 mL), a solution of ethyl 4-hydroxy-4-(4-methoxyphenyl)but-2-ynoate (200 mg, 0.9 mmol) in THF (2.5 mL) was added dropwise. After 10 min, the ice bath was removed. After 30 min, the reaction was quenched by addition of 1 M HCl. The resulting solution was extracted with DCM (3 × 10 mL). The combined organic phase was washed with brine, dried (MgSO₄), filtered, and concentrated under reduced pressure. Purification by column chromatography (30% EtOAc in hexanes) afforded ethyl (E)-4-hydroxy-4-(4-methoxyphenyl)but-2-enoate (82 mg, 41%) as an oil. Characterization data for this compound has been reported. The material obtained from this method provided an identical ¹H NMR spectrum.⁵¹ An image of the ¹H NMR spectrum is supplied in the Supporting Information.

To a solution of ethyl 4-hydroxy-4-(4-methoxyphenyl)but-2-enoate (67 mg, 0.28 mmol) in CH₂Cl₂ (1 mL) was added TMSN₃ (0.8 mL, 0.6 mmol) and Zn(OTf)₂ (23 mg, 0.063 mmol) at room temperature. The vessel was sealed. After 20 min, the reaction mixture was filtered through silica and washed with CH₂Cl₂. The resulting solution was concentrated under reduced pressure. Purification by column chromatography (gradient elution 0–15% EtOAc in hexanes) afforded a mixture of azide 15a and 15b (15a: 15b = 2.0:1) as an oil (52 mg, 70%): Major isomer (15a): ¹H NMR (500 MHz; CDCl₃) δ 7.24 (d, J = 8.7 Hz, 2H), 6.98–6.89 (m, 3H), 6.14 (d, J = 15.4 Hz, 1H), 5.16 (d, J = 5.5 Hz, 1H), 4.23 (q, J = 7.1 Hz, 2H), 3.84 (s, 3H), 1.32 (t, J = 7.1 Hz, 3H); Minor isomer (15b): ¹H NMR (500 MHz; CDCl₃) δ 7.38 (d, J = 8.8 Hz, 2H), 6.98–6.89 (m, 2H), 6.73 (d, J = 15.8 Hz, 1H), 6.17–6.12 (m, 1H), 4.53 (d, J = 7.7 Hz, 1H), 4.33–4.27 (m, 2H), 3.84 (s, 3H), 1.34 (t, J = 7.1 Hz, 3H); Mixture: ¹³C NMR (125 MHz; CDCl₃) δ 169.1, 165.9, 160.1, 160.0, 144.1, 135.5, 128.9, 128.4, 128.2, 122.5, 118.3, 114.4, 114.1, 65.0, 63.9, 62.1, 60.7, 55.3, 14.2, 14.1; IR (KBr, thin film, cm⁻¹) 2980, 2838, 2103, 1720, 1608, 1513, 1255, 1176; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₃H₁₅N₃O₃Na⁺ 284.1006, found 284.0998.

tert-Butyl (E)-4-azido-4-(4-methoxyphenyl)but-2-enoate (16a) and tert-Butyl (E)-2-azido-4-(4-methoxyphenyl)but-3-enoate (16b)

Azides 16a and 16b were synthesized via a multistep sequence. Following the procedures for compounds 15a and 15b, tert-butyl 4-hydroxy-4-(4-methoxyphenyl)but-2-ynoate was isolated as an oil (840 mg, 87%): ¹H NMR (500 MHz; CDCl₃) δ 7.42 (d, J = 7.8 Hz, 2H), 6.89 (d, J = 8.0 Hz, 2H), 5.47 (s, 1H), 3.80 (s, 3H), 3.49 (s, 1H), 1.50 (s, 9H); ¹³C NMR (126 MHz; CDCl₃) δ 159.8, 152.6, 131.2, 128.2, 114.1, 84.5, 83.9, 78.8, 63.7, 55.3, 28.0; IR (KBr, thin film, cm⁻¹) 3400, 2981, 2936, 2838, 2237,1707, 1612, 1513, 1255, 1156; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₅H₁₈O₄Na⁺ 285.1097, found 285.1097.

tert-Butyl 4-hydroxy-4-(4-methoxyphenyl)but-2-enoate

Title compound was isolated as an oil (220 mg, 56%). ¹H NMR (400 MHz; CDCl₃) δ 7.29–7.26 (m, 2H), 6.94 (dd, J = 14.4, 4.7 Hz, 1H), 6.92–6.88 (m, 2H), 6.05 (dd, J = 15.6, 1.7 Hz, 1H), 5.29 (dd, J = 4.9, 1.3 Hz, 1H), 3.82 (s, 3H), 2.36 (s, 1H), 1.49 (s, 9H); ¹³C NMR (125 MHz; CDCl₃) δ 165.8, 159.6, 147.5, 133.3, 128.0, 122.0, 114.2, 80.6, 73.2, 55.3, 28.1; IR (KBr, thin film, cm⁻¹): 3409, 2977, 2933, 1710, 1512, 1249, 1152; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₅H₂₀O₄Na⁺ 287.1254, found 287.1249.

Allylic azides 16a and 16b were isolated as an oil (1.3:1, 160 mg, 65%): Major isomer (16a): ¹H NMR (500 MHz; CDCl₃) δ 7.25 (d, J = 8.0 Hz, 2H), 6.94 (d, J = 7.8 Hz, 2H), 6.85 (dd, J = 15.4, 5.4 Hz, 1H), 6.06 (d, J = 15.4 Hz, 1H), 5.13 (d, J = 5.2 Hz, 1H), 3.84 (s, 3H), 1.51 (s, 9H); Minor isomer (16b): ¹H NMR (500 MHz; CDCl₃) δ 7.38 (d, J = 8.0 Hz, 2H), 6.90 (d, J = 8.0 Hz, 2H), 6.71 (d, J = 15.8 Hz, 1H), 6.14 (dd, J = 15.8, 7.5 Hz, 1H), 4.38 (d, J = 7.4 Hz, 1H), 3.84 (s, 3H), 1.54 (s, 9H); Mixture: ¹³C NMR (126 MHz; CDCl₃) δ 168.2, 165.1, 160.0, 143.0, 135.0, 128.9, 128.6, 128.3, 128.2, 124.3, 118.8, 114.4, 114.1, 83.2, 80.9, 65.0, 64.3, 55.3, 28.1, 28.0; IR (KBr, thin film, cm⁻¹) 2978, 2933, 2102, 1716, 1512, 1252, 1152; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₅H₁₉N₃O₃Na⁺ 312.1319, found 312.1320.

tert-Butyl (E)-2-azido-4-phenylbut-3-enoate (17a) and tert-Butyl (E)-4-azido-4-phenylbut-2-enoate (17b)

Azides 17a and 17b were synthesized via a multistep sequence that is outlined here. A procedure was adapted from a known method⁵² for the addition of tert-butyl propiolate to benzaldehyde and the product was isolated as an oil (755 mg, 75%): ¹H NMR (500 MHz; CDCl₃) δ 7.52 (d, J = 7.1 Hz, 2H), 7.42–7.36 (m, 3H), 5.54 (s, 1H), 1.52 (s, 9H); ¹³C NMR (125 MHz; CDCl₃) δ 152.6, 138.8, 128.79, 128.78, 126.7, 84.2, 84.0, 79.0, 64.2, 28.0; IR (KBr, thin film, cm⁻¹) 3401, 2981, 2935, 2238, 1707, 1278, 1155; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₄H₁₆O₃Na⁺ 255.0992, found 255.0991.

This product was reduced using an adapted procedure.⁵⁰ To an ice-cold suspension of LiAlH₄ (57 mg, 1.7 mmol) in THF (4 mL), a solution of tert-butyl 4-hydroxy-4-phenylbut-2-ynoate (350 mg, 1.7 mmol) in THF (4 mL) was added dropwise. After 15 min, the reaction was quenched by addition of 1 M HCl. The resulting solution was extracted with CH₂Cl₂ (3 × 15 mL). The combined organic phase was washed with brine, dried (MgSO₄), filtered, and concentrated under reduced pressure. Purification by column chromatography (30% EtOAc in hexanes) afforded tert-butyl (E)-4-hydroxy-4-phenylbut-2-enoate (200 mg, 56%) as an oil. Characterization data for this compound has been reported. The material obtained from this method provided an identical ¹H NMR spectrum.⁵³ An image of the ¹H NMR spectrum is supplied in the Supporting Information.

This product was acetylated using an adapted procedure.⁵⁴ To a solution of tert-butyl (E)-4-hydroxy-4-phenylbut-2-enoate (100 mg, 0.4 mmol) in CH₂Cl₂ (0.8 mL) was added acetic anhydride (80 µL, 0.8 mmol), followed by pyridine (30 µL, 0.4 mmol). After 24 h, the reaction was quenched by addition of 1 M HCl. The resulting solution was extracted with CH₂Cl₂ (3 × 5 mL). The combined organic phase was washed with brine, dried (MgSO₄), filtered, and concentrated under reduced pressure. Purification by column chromatography (30% EtOAc in hexanes) afforded tert-butyl (E)-4-acetoxy-4-phenylbut-2-enoate as an oil (75 mg, 64%): ¹H NMR (500 MHz; CDCl₃) δ 7.41–7.36 (m, 5H), 6.92 (dd, J = 15.6, 5.2 Hz, 1H), 6.40 (dd, J = 5.2, 1.6 Hz, 1H), 5.97 (dd, J = 15.6, 1.7 Hz, 1H), 2.15 (s, 3H), 1.50 (s, 9H); ¹³C NMR (125 MHz; CDCl₃) δ 169.7, 165.2, 143.4, 137.4, 128.8, 128.7, 127.4, 123.6, 80.9, 74.3, 28.1, 21.1; IR (KBr, thin film, cm⁻¹) 2979, 2934, 1744, 1716, 1659,1456, 1369, 1230, 1153; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₆H₂₀O₄Na⁺ 299.1254, found 299.1253.

The azide was installed using a known procedure.⁵⁵ Allylic azides 17a and 17b were isolated as an oil (43 mg, 46% yield): Major isomer (17a): ¹H NMR (500 MHz; CDCl₃) δ 7.45–7.32 (m, 5H), 6.86 (dd, J = 15.4, 5.9 Hz, 1H), 6.07 (dd, J = 15.4, 1.6 Hz, 1H), 5.18 (dd, J = 5.9, 1.3 Hz, 1H), 1.51 (s, 9H); Minor isomer (17b): ¹H NMR (500 MHz; CDCl₃) δ 7.45–7.32 (m, 5H), 6.77 (d, J = 15.8 Hz, 1H), 6.27 (dd, J = 15.8, 7.3 Hz, 1H), 4.42 (d, J = 7.3 Hz, 1H), 1.54 (s, 9H); Mixture: ¹³C NMR (125 MHz; CDCl₃) δ 168.2, 165.0, 142.7, 136.7, 135.6, 135.3, 129.1, 128.9, 128.7, 128.6, 127.5, 126.8, 124.7, 121.2, 83.3, 81.0, 65.5, 64.1, 28.1, 28.0; IR (KBr, thin film, cm⁻¹) 2979, 2931, 2104, 1716, 1253, 1152; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₄H₁₇N₃O₄Na⁺ 282.1213, found 282.1216.

(E)-4-Azido-4-(4-methoxyphenyl)but-2-enenitrile (18a) and (Z)-4-Azido-4-(4-methoxyphenyl)but-2-enenitrile (18b)

Azides 18a and 18b were synthesized via a two step sequence. Following a known procedure,⁵⁶ (E)-4-(4-methoxyphenyl)-2-((trimethylsilyl)oxy)but-3-enenitrile was isolated crude as an orange oil (698 mg, 88%). ¹H NMR (400 MHz, CDCl₃) δ 7.37 (d, J = 8.4 Hz, 2H), 6.90 (d, J = 8.4 Hz, 2H), 6.77 (d, J = 15.7 Hz, 1H), 6.08 (dd, J = 15.7, 6.2 Hz, 1H), 5.12 (d, J = 6.2 Hz, 1H), 3.85 (s, 3H), 0.27 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ 160.1, 133.6, 128.3, 127.7, 121.3, 118.6, 114.2, 62.5, 55.3, 0.0; IR (KBr, thin film, cm⁻¹) 2959, 2838, 1607, 1513, 1255, 1176, 845; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₄H₁₉NO₂Na⁺ [M + Na]⁺: 284.1077, found 284.1067.

Starting from (E)-4-(4-methoxyphenyl)-2-((trimethylsilyl)oxy)but-3-enenitrile and following the procedure above for the azide installation for compounds 15 and 16, a mixture of azides 18a and 18b (17 mg, 38%) was isolated as an oil. Major isomer (18a) ¹H NMR (400 MHz, CDCl₃) δ 7.21 (d, J = 8.7 Hz, 2H), 6.97 (d, J = 8.7 Hz, 2H), 6.71 (dd, J = 16.0, 4.6 Hz, 1H), 5.76 (dd, J = 16.0, 2.0 Hz, 1H), 5.18 (dd, J = 4.6, 2.1 Hz, 1H), 3.85 (s, 3H); Minor isomer (18b): ¹H NMR (400 MHz, CDCl₃) δ 7.31 (d, J = 8.8 Hz, 2H), 6.99–6.92 (m, 2H), 6.55 (dd, J = 10.9, 9.2 Hz, 1H), 5.56–5.49 (m, 2H), 3.84 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 160.4, 160.2, 150.7, 150.3, 129.1, 128.2, 127.0, 116.6, 114.7, 114.7, 101.0, 100.8, 64.8, 64.2, 55.4, 55.4; IR (KBr, thin film, cm⁻¹) 2839, 22640, 2103, 1608, 1512, 1252; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₁H₁₀N₂O⁺ [M − N₂]⁺: 186.0788, found 186.0782.

(3-Azidoprop-1-en-1-yl)benzene (9)

A procedure was adapted from a known method⁵⁷ and the product was isolated (32 mg, 60%). The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information. The data provided here is for comparison to the ¹⁵N-labeled compound:¹H NMR (500 MHz; CDCl₃) δ 7.87–7.85 (m, 2H), 7.73 (m, 2H), 7.48–7.46 (d, J = 7.6 Hz, 2H), 7.36 (t, J = 7.4 Hz, 2H), 7.31–7.28 (m, 1H), 6.67 (ddd, J = 17.3, 9.0, 7.6 Hz, 1H), 5.99 (d, J = 7.6 Hz, 1H), 5.40 (d, J = 9.3 Hz, 1H), 5.37 (d, J = 16.8 Hz, 1H); IR (KBr, thin film, cm⁻¹) 3031, 1769, 1715, 1468, 1382, 1352, 989, 718; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₇H₁₃NO₂Na⁺ 286.0838, found 286.0832.

The procedure to deprotect the phthalimide is based off of a known method.⁵⁸ To a solution of 2-(1-phenylallyl)isoindoline-1,3-dione (0.10 g, 0.40 mmol) in EtOH (5 mL) was added ethanolamine (0.24 mL, 3.8 mmol). The vessel was sealed and heated to 40 °C. After 18 h, the solution was poured over 6 M HCl (10 mL) and extracted with CH₂Cl₂ (1 × 10 mL). The aqueous layer was then basified with NaOH (pH > 10) and the resulting solution was extracted with CH₂Cl₂ (3 × 15 mL). The combined organic phases were washed with brine, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The resulting compound was isolated as a yellow oil (23 mg, 44%) and was used without further purification. The material obtained from this method provided an identical ¹H NMR spectrum to previous report.⁵⁹ An image of the ¹H NMR spectrum is supplied in the Supporting Information. The data provided here is for comparison to the ¹⁵N-labeled compound: ¹H NMR (400 MHz; CDCl₃) δ 7.39–7.34 (m, 4H), 7.30–7.26 (m, 1H), 6.05 (ddd, J = 16.9, 10.4, 6.3 Hz, 1H), 5.27 (d, J = 16.9 Hz, 1H), 5.14 (d, J = 10.2 Hz, 1H), 4.55 (d, J = 6.1 Hz, 1H), 1.63 (s, 2H); IR (KBr, thin film, cm⁻¹): 3368, 3271, 3062, 3027, 2856, 1639, 1601, 1492, 1452, 995, 919, 701; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₁₁N⁺ 133.0886, found 133.0848, [M − H]⁺ calcd for C₉H₁₀N⁺ 132.0803, found 132.0803.

Using a known diazotransfer procedure,⁶⁰ the product was isolated (24 mg) in 44% yield. The material obtained from this method provided an identical ¹H NMR spectrum to a previous report.³⁴ An image of the ¹H NMR spectrum is supplied in the Supporting Information. The data provided here is for comparison to the ¹⁵N-labeled compound; ¹H NMR (500 MHz; CDCl₃) δ 7.44–7.43 (m, 2H), 7.38–7.35 (m, 2H), 7.32–7.28 (m, 1H), 6.68 (d, J = 15.8 Hz, 1H), 6.27 (dt, J = 15.8, 6.6 Hz, 1H), 3.98 (d, J = 6.6 Hz, 2H); IR (KBr, thin film, cm⁻¹) 3029, 2923, 2100, 1493, 1448, 1236, 968; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₉N₃⁺ 159.0791, found 159.0747, [M − N₃]⁺ calcd for C₉H₉⁺ 117.0699, found 117.0693.

1-Phenylprop-2-en-1-amine¹⁵ (19)

Using the procedure above for compound 9 with phthalimide-¹⁵ potassium salt (98 atom % ¹⁵N), 2-(1-phenylallyl)isoindoline-1,3-dione-¹⁵ was isolated (340 mg) in 74% yield as an oil: ¹H NMR (500 MHz; CDCl₃) δ 7.86 (dd, J = 5.4, 3.1 Hz, 2H), 7.73 (dd, J = 5.5, 3.0 Hz, 2H), 7.47 (d, J = 7.6 Hz, 2H), 7.36 (t, J = 7.5 Hz, 2H), 7.31–7.28 (m, 1H), 6.68 (dddd, J = 17.8, 10.2, 7.6 Hz, J_N–H = 1.1 Hz, 1H), 5.99 (d, J = 7.6 Hz, 1H), 5.40 (d, J = 10.2 Hz, 1H), 5.37 (d, J = 17.6 Hz, 1H); ¹³C NMR (125 MHz; CDCl₃) δ 167.6 (d, J_C–N = 11.9 Hz), 138.6, 134.3, 134.0, 131.9 (d, J_C–N = 7.9 Hz), 128.6, 127.83, 127.77, 123.3, 119.1, 56.8 (d, J_C–N = 8.6 Hz); ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 171; IR (KBr, thin film, cm⁻¹) 3467, 3062, 3031, 1767, 1716, 1612, 1468, 1382, 1086; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₇H₁₃¹⁵NO₂Na⁺ 287.0809, found 287.0816.

Using the phthalimide deprotection procedure used for the synthesis of 9, ¹⁵N-1-phenylprop-2-en-1-amine (19) was isolated (110 mg, 61%) as an oil and used without further purification: ¹H NMR (500 MHz; CDCl₃) δ 7.37–7.34 (m, 4H), 7.28–7.27 (m, 1H), 6.05 (ddd, J = 17.0, 9.1, 7.3 Hz, 1H), 5.27 (d, J = 17.1 Hz, 1H), 5.14 (d, J = 9.2 Hz, 1H), 4.55 (d, J = 7.1 Hz, 1H), 1.57 (s, 2H); ¹³C NMR (125 MHz; CDCl₃) δ 144.5, 142.3 (d, J_C–N = 4.2 Hz), 128.6, 127.1, 126.6, 113.7 (d, J_C–N = 6.6 Hz), 58.4 (d, J_C–N = 16.3 Hz); ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 38; IR (KBr, thin film, cm⁻¹) 3362, 3284, 3081, 3062, 3027, 2852, 1639, 1601, 1492, 1452, 918, 701; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₁₁¹⁵N⁺ 134.0856, found 134.0829, calcd for C₉H₁₀¹⁵N⁺ [M −H]⁺ 133.0778, found 133.0774.

¹⁵N-(3-Azidoprop-1-en-1-yl)benzene (21)

Using the diazotransfer procedure for the synthesis of 9, the product was isolated as an oil (45 mg, 80%): ¹H NMR (400 MHz; CDCl₃) δ 7.44 (d, J = 7.5 Hz, 2H), 7.37 (t, J = 7.5 Hz, 2H), 7.30 (m, 1H), 6.68 (d, J = 15.8 Hz, 1H), 6.28 (dt, J = 15.7, 6.6 Hz, 1H), 3.98 (d, J = 6.5 Hz, 2H); ¹³C NMR (101 MHz; CDCl₃) δ 136.0, 134.6, 128.7, 128.2, 126.6, 122.4, 53.0; ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 214; IR (KBr, thin film, cm⁻¹) 3028, 2923, 2077, 1493, 1448, 968; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₉N₃⁺ 160.0761, found 160.0767, [M − N₃]⁺ calcd for C₉H₉⁺ 117.0699, found 117.0694.

Diethyl cinnamylphosphoramidate (22)

To a solution of ¹⁵N-(3-azidoprop-1-en-1-yl)benzene (21) (21 mg, 0.12 mmol) in THF (0.5 mL) was added triethyl phosphite (24 µL, 0.14 mmol). After 3 h, KOH (97 mg, 1.7 mmol) in water (0.25 mL) was added, and the solution was heated to 40 °C. After 2 h, the solution was heated to 80 °C. After an additional 2 h, the solution was poured over water (5 mL) and extracted with CH₂Cl₂ (3 × 8 mL). The combined organic phases were washed with brine, dried (Na₂SO₄), filtered, and concentrated under reduced pressure. The resulting yellow oil (29 mg, 86%) was used without further purification: ¹H NMR (400 MHz; CDCl₃) δ 7.38–7.31 (m, 4H), 7.28–7.23 (m, 1H), 6.56 (d, J = 15.8 Hz, 1H), 6.24 (dt, J = 15.8, 6.0 Hz, 1H), 4.16–4.06 (m, 4H), 3.74–3.68 (m, 2H), 2.88 (td, J_H–P = 17.0, 7.1 Hz, 1H), 1.35 (t, J = 7.1 Hz, 6H); ¹³C NMR (100 MHz; CDCl₃) δ 136.6, 131.0, 128.6, 127.64, 127.5 (d, J_C–P = 7.3 Hz), 126.3, 62.4 (d, J_C–P = 5.3 Hz), 43.5, 16.3 (d, J_C–P = 7.1 Hz);³¹P NMR (162 MHz; CDCl₃) δ 8.7; IR (KBr, thin film, cm⁻¹) 3246, 2983, 2932, 2906, 1702, 1450, 1238, 963; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₃H₂₀NO₃PNa⁺ 292.1073, found 292.1067.

3-Phenylprop-2-en-1-amine

Using a known procedure,⁶¹ 2-cinnamylisoindoline-1,3-dione was isolated (600 mg, 76%). The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information. The data provided here is for comparison to the ¹⁵N-labeled compound: ¹H NMR (400 MHz; CDCl₃) δ 7.89 (dd, J = 5.5, 3.0 Hz, 2H), 7.75 (dd, J = 5.4, 3.1 Hz, 2H), 7.39–7.37 (m, 2H), 7.33–7.24 (m, 3H), 6.69 (d, J = 15.8 Hz, 1H), 6.28 (dt, J = 15.8, 6.5 Hz, 1H), 4.47 (d, J = 6.5 Hz, 2H); IR (KBr, thin film, cm⁻¹) 3043, 3024, 1770, 1704, 1427, 1396, 1108, 727; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₇H₁₃NO₂Na⁺ 286.0838, found 286.0828.

Using the phthalimide deprotection procedure for the synthesis of 9, the product was isolated (130 mg, 47%). The material obtained from this method provided an identical ¹H NMR spectrum to a previous report.⁶² An image of the ¹H NMR spectrum is supplied in the Supporting Information. The data provided here is for comparison to the ¹⁵N-labeled compound: ¹H NMR (500 MHz; CDCl₃) δ 7.39 (d, J = 7.4 Hz, 2H), 7.33 (t, J = 7.4 Hz, 2H), 7.24 (t, J = 7.3 Hz, 1H), 6.52 (d, J = 15.9 Hz, 1H), 6.35 (dt, J = 15.7, 6.0 Hz, 1H), 3.50 (d, J = 5.8 Hz, 2H), 1.48 (s, 2H); IR (KBr, thin film, cm⁻¹) 3263, 3078, 2848, 1636, 1565, 1453, 965, 684; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₁₁N⁺ 133.0886, found 133.0879, [M − H]⁺ calcd for C₉H₁₀N⁺ 132.0808, found 132.0804.

(3-Azidoprop-1-en-1-yl)benzene.¹⁵

Using the procedure for the synthesis of 2-cinnamylisoindoline-1,3-dione, with potassium ¹⁵N-phthalimide, the product was isolated (510 mg, 89%): ¹H NMR (500 MHz; CDCl₃) δ 7.89 (t, J = 2.7 Hz, 2H), 7.74 (t, J = 2.7 Hz, 2H), 7.37 (d, J = 7.5 Hz, 2H), 7.32–7.29 (m, 2H), 7.24 (t, J = 7.3 Hz, 1H), 6.68 (d, J = 15.8 Hz, 1H), 6.28 (dt, J = 15.1, 6.5 Hz, 1H), 4.47 (d, J = 6.5 Hz, 2H); ¹³C NMR (125 MHz; CDCl₃) δ 168.0 (d, J_C–N = 13.1 Hz), 136.2, 134.0, 133.8, 132.2 (d, J_C–N = 7.5 Hz), 128.5, 127.9, 126.5, 123.3, 122.7, 39.7 (d, J_C–N = 9.8 Hz); ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 163; IR (KBr, thin film, cm⁻¹) 3024, 1769, 1702, 1469, 1381, 1095, 726; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₇H₁₃¹⁵NO₂Na⁺ 287.0809, found 287.0801.

Using the phthalimide deprotection procedure for the synthesis of 9, the product was isolated (42 mg, 56%) and used without further purification: ¹H NMR (500 MHz; CDCl₃) δ 7.39 (d, J = 7.5 Hz, 2H), 7.33 (t, J = 7.3 Hz, 2H), 7.24 (t, J = 7.2 Hz, 1H), 6.53 (d, J = 15.9 Hz, 1H), 6.35 (dt, J = 15.4, 6.3 Hz, 1H), 3.51 (d, J = 5.8 Hz, 2H), 1.84 (s, 2H); ¹³C NMR (125 MHz; CDCl₃) δ 137.2, 131.1, 129.5, 128.5, 127.3, 126.2, 44.3 (d, J_C–N = 3.8 Hz); ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 23; IR (KBr, thin film, cm⁻¹) 3258, 3021, 2838, 1632, 1565, 1435, 1395,957, 738; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₁₁¹⁵N⁺ 134.0856, found 134.0845, [M − H]⁺ calcd for C₉H₁₀¹⁵N⁺ 133.0778, found 133.0779.

Using the diazotransfer procedure for the synthesis of 9, the product was isolated in (31 mg, 54%): ¹H NMR (500 MHz; CDCl₃) δ 7.44 (d, J = 7.4 Hz, 2H), 7.38 (t, J = 7.4 Hz, 2H), 7.31 (m, 1H), 6.69 (d, J = 15.8 Hz, 1H), 6.28 (dt, J = 15.1, 6.5 Hz, 1H), 3.98 (d, J = 6.5 Hz, 2H); ¹³C NMR (126 MHz; CDCl₃) δ 136.0, 134.6, 128.7, 128.2, 126.7, 122.4, 53.0 (d, J_C–N = 4.1 Hz); ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 73; IR (KBr, thin film, cm⁻¹) 3028, 2927, 2102, 1492, 1449, 1224, 968, 747, 693; HRMS (EI-TOF) m/z [M]⁺ calcd for C₉H₉N₃⁺ 160.0761, found 160.0742, [M − N₃]⁺ calcd for C₉H₉⁺ 117.0699, found 117.0694.

¹⁵N-Diethyl cinnamylphosphoramidate.¹⁵

Using the procedure for the synthesis of 22, the product was isolated (25 mg, 95%) and used without further purification; ¹H NMR (500 MHz; CDCl₃) δ 7.38–7.31 (m, 4H), 7.28–7.24 (m, 1H), 6.57 (d, J = 15.8 Hz, 1H), 6.24 (dt, J = 15.8 6.8 Hz, 1H), 4.11 (apparent q, J = 7.3 Hz, 4H), 3.71 (td, J_H–P = 12.2, 7.0 Hz, 2H), 2.85 (apparent d, J_N–H = 84 Hz, 1H), 1.35 (t, J = 7.1 Hz, 6H); ¹³C NMR (125 MHz; CDCl₃) δ 136.6, 131.0 (d, J_C–N = 1.5 Hz), 128.6, 127.7, 127.6 (d, J_C–P = 6.1 Hz), 126.3, 62.4 (d, J_C–P = 5.4 Hz), 43.4 (d, J_C–N = 8.3 Hz), 16.2 (d, J_C–P = 7.1 Hz). (Note: J_C–P vs J_C–N were assigned by comparison to 22); ¹⁵N NMR (93 MHz; CDCl₃, using ¹H–¹⁵N HMBC) δ 43; ³¹P NMR (162 MHz, CDCl₃) δ 8.68 (d, J_P–N = 40.9 Hz); IR (KBr, thin film, cm⁻¹): 3211, 2981, 2904, 1444, 1237, 1031, 966; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₃H₂₀¹⁵NO₃PNa⁺ 293.1043; found 293.1047, [2M + Na]⁺ calcd for C₂₆H₄₀¹⁵N₂O₆P₂Na⁺ 563.2195, found 563.2207.

2-(4-Azidobut-2-en-2-yl)naphthalene

Using a known procedure⁴² the product was isolated (520 mg, 65%). Characterization data for this compound has been reported. The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information.

5-Azido-2,3,4,5-tetrahydro-1,1′-biphenyl (rac-30e)

Azides 30 were prepaired by a multistep sequence that is outlined here. A procedure was adapted from a known method for the formation of tertiary alcohols and 3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol was isolated as an oil (1.38 g, 75%).⁶³ Characterization data for this compound has been reported. The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information.

To a solution of 3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (0.28 g, 1.6 mmol) in CH₂Cl₂ (7.0 mL) was added TMS-N₃ (0.41 mL, 3.2 mmol) and Zn(OTf)₂ (57 mg, 0.16 mmol) at room temperature. The vessel was sealed. After 20 min, the solution was quenched with triethylamine (0.5 mL) and MeOH (0.5 mL). The reaction mixture was filtered through basic alumina and washed with excess CH₂Cl₂. The resulting solution was concentrated under reduced pressure. Purification by column chromatography (gradient elution 0–15% EtOAc in hexanes) afforded rac-30e as an oil (0.25 g, 1.3 mmol, 79%). Characterization data for this compound has been reported.⁴² The material obtained from this method provided an identical ¹H NMR. An image of the ¹H NMR spectrum is supplied in the Supporting Information.

(4-Azidopent-2-en-2-yl)benzene (26)

Following the azide formation procedure above for compound 30e, the product (0.24 g, 57%) was isolated as an oil: ¹H NMR (500 MHz, CDCl₃) (E-isomer) δ 7.47–7.42 (m, 2H), 7.40–7.35 (m, 2H), 7.33–7.30 (m, 1H), 5.73 (dq J = 9.0, 1.5 Hz, 1H), 4.49 (dq, J = 9.0, 6.6 Hz, 1H), 2.17 (d, J = 1.4 Hz, 3H), 1.37 (d, J = 6.6 Hz, 3H); ¹H NMR (500 MHz, CDCl₃) (Z-isomer) δ 7.50–7.15 (m, 5H), 5.47 (dq, J = 9.9, 1.6 Hz, 1H), 4.04 (dq, J = 9.9, 6.6 Hz, 1H), 2.12 (d, J = 1.4 Hz, 4H), 1.24 (d, J = 6.6 Hz, 3H); ¹³C NMR (125 MHz, CDCl₃) (E-isomer) δ 142.5, 139.3, 128.3, 127.6, 126.7, 126.0, 55.5, 20.7, 16.6; ¹³C NMR (125 MHz, CDCl₃) (Z-isomer) δ 141.5, 140.7, 128.4, 127.7, 127.3, 126.2, 56.0, 25.7, 20.8; IR (KBr, thin film, cm⁻¹) 2979, 2096, 1444, 1231; HRMS (CI-TOF) m/z [M − N₃]⁺ calcd for C₁₁H₁₃⁺ 145.1012, found 145.1005.

5-Azido-4′-methoxy-2,3,4,5-tetrahydro-1,1′-biphenyl (rac-30a)

Following the procedure above for compound 30e, 4′-methoxy-3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (3.0 g, 47%) was isolated as a waxy solid. Characterization data for this compound has been reported.⁶³ The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information. With slight modifications, Cu(hfac)₂ instead of Zn(OTf)₂ with ice bath cooling, the product rac-30a (0.58 g, 86%) was isolated as a waxy solid. ¹H NMR (500 MHz, CDCl₃) δ 7.39 (d, J = 8.4 Hz, 2H), 6.91 (d, J = 8.5 Hz, 2H), 6.07–6.01 (m, 1H), 4.12–4.10 (m, 1H), 3.84 (s, 3H), 2.53–2.48 (m, 1H), 2.44–2.39 (m, 1H), 2.00–1.91 (m, 2H), 1.84–1.77 (m, 2H); ¹³C NMR (125 MHz, CDCl₃) δ 159.4, 141.8, 133.5, 126.6, 119.8, 113.7, 57.0, 55.3, 28.4, 27.4, 19.8; IR (KBr, thin film, cm⁻¹) 2935, 2836, 2089, 1607, 1513, 1251, 1037; HRMS (CI-TOF) m/z [M − N₃]⁺ calcd for C₁₃H₁₅O⁺ 187.1117, found 187.1110.

5-Azido-4′-methyl-2,3,4,5-tetrahydro-1,1′-biphenyl (rac-30b)

Following the above procedure for compound 30e, 4′-methyl-3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (3.9 g, 68%) was isolated as an oil. Characterization data for this compound has been reported.⁶³ The material obtained from this method provided and identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information. The product rac-30b (0.53 g, 71%) was isolated as an oil: ¹H NMR (400 MHz, CD₃CN) δ 7.41–7.34 (d, J = 8.3 Hz, 2H), 7.19 (d, J = 8.0 Hz, 2H), 6.10 (dt, J = 3.8, 1.8 Hz, 1H), 4.18–4.14 (m, 1H), 2.56–2.36 (m, 2H), 2.35 (s, 3H), 1.95–1.85 (m, 2H), 1.83–1.71 (m, 2H); ¹³C NMR (125 MHz, CD₃CN) δ 142.1, 138.1, 137.7, 129.0, 125.2, 120.5, 56.9, 28.0, 26.9, 20.1, 19.5; IR (KBr, thin film, cm⁻¹) 2939, 2864, 2093, 1235; HRMS (CI-TOF) m/z [M − N³]⁺ calcd for C₁₃H₁₅⁺ 171.1168, found 171.1166.

5-Azido-4′-(tert-butyl)-2,3,4,5-tetrahydro-1,1′-biphenyl (rac-30c)

Following the procedure above for compound 30e, 4′-(tert-butyl)-3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (1.3 g, 72%) was isolated as an oil: ¹H NMR (400 MHz, CDCl₃) δ 7.45 (apparent d, J = 8.6 Hz, 2H), 7.39 (apparent d, J = 8.6 Hz, 2H), 6.05 (dt, J = 10.0, 3.7 Hz, 1H), 5.82 (d, J = 10.1 Hz, 1H), 2.22–2.07 (m, 2H), 2.06–1.99 (m, 1H), 1.91 (td, J = 10.0, 3.1 Hz, 1H), 1.87–1.77 (m, 1H), 1.69–1.61 (m, 1H), 1.36 (s, 9H); ¹³C NMR (100 MHz, CDCl₃) δ 149.7, 144.8, 132.4, 130.4, 125.2, 125.0, 72.0, 39.4, 34.4, 31.4, 25.1, 19.3; IR (KBr, thin film, cm⁻¹) 3050, 2817, 1571, 1401, 1161; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₆H₂₂ONa⁺ 253.1563, found 253.1574. Using ice bath cooling, rac-30c (0.26 g, 79%) was isolated as an oil: ¹H NMR (400 MHz, CD₃CN) δ 7.42 (s, 4H), 6.12 (dt, J = 3.8, 1.8 Hz, 1H), 4.19–4.13 (m, 1H), 2.58–2.46 (m, 1H), 2.46–2.36 (m, 1H), 1.95–1.85 (m, 2H), 1.84–1.67 (m, 2H), 1.33 (s, 9H); ¹³C NMR (126 MHz, CD₃CN) δ 150.8, 142.1, 138.1, 125.3, 125.1, 120.6, 56.9, 34.1, 30.5, 28.0, 26.9, 19.5; IR (KBr, thin film, cm⁻¹) 2960, 2866, 2093, 1162; HRMS (CI-TOF) m/z [M − N₃]⁺ calcd for C₁₆H₂₁⁺ 213.1638, found 213.1631.

5-Azido-4′-fluoro-2,3,4,5-tetrahydro-1,1′-biphenyl (rac-30d)

Following the procedure above for compound 30e, 4′-fluoro-3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (0.83 g, 68%) was isolated as an oil. ¹H NMR (400 MHz, CDCl₃) δ 7.50–7.42 (m, 2H), 7.02 (apparent triplet, J = 7.02, 2H), 6.04 (dt, J = 10.1, 3.7 Hz, 1H), 5.76 (d, J = 10.0, 1H), 2.23 (s, 1H), 2.19–2.03 (m, 2H), 2.02–1.92 (m, 1H), 1.89–1.72 (m, 2H), 1.66–1.56 (m, 1H); ¹³C NMR (125 MHz, CDCl₃) δ 161.8 (d, J_C–F = 243.3 Hz), 143.6 (d, J_C–F = 3.0 Hz), 132.1, 130.9, 127.2 (d, J_C–F = 7.9 Hz), 114.7 (d, J_C–F = 21.1 Hz), 71.9, 39.7, 25.0, 19.2; ¹⁹F NMR (376 MHz, CDCl₃) δ –116.5; IR (KBr, thin film, cm⁻¹) 3378, 2938, 1601, 1508, 1221, 1175, 833; HRMS (EI-TOF) m/z [M]⁺ calcd for C₁₂H₁₃FO⁺ 192.0945, found 192.0951. The product rac-30d (0.17 g, 79%) was isolated as an oil: ¹H NMR (400 MHz, CDCl₃) δ 7.46–7.36 (m, 2H), 7.11–7.00 (m, 2H), 6.05 (dt, J = 3.6, 1.8 Hz, 1H), 4.14–4.08 (m, 1H), 2.55–2.35 (m, 2H), 2.03–1.90 (m, 2H), 1.88–1.74 (m, 2H); ¹³C NMR (125 MHz, CDCl₃) δ 162.5 (d, J_C–F = 245.3 Hz), 141.5, 137.1 (d, J_C–F = 3.4 Hz), 127.1 (d, J_C–F = 7.8 Hz), 121.4 (d, J_C–F = 1.5 Hz), 115.2 (d, J_C–F = 21.2 Hz), 56.8, 28.2, 27.5, 19.7; ¹⁹F NMR (471 MHz, CDCl₃) δ –114.6; IR (KBr, thin film, cm⁻¹) 2940, 2096, 1601, 1510, 1231, 1161, 826; HRMS (CI-TOF) m/z [M − N₃]⁺ calcd for C₁₂H₁₂F⁺ 175.0918, found 175.0924.

5-Azido-4′-(trifluoromethyl)-2,3,4,5-tetrahydro-1,1′-biphenyl (rac-30f)

Following the procedure above for compound 30e, 4′-(trifluoromethyl)-3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (1.4 g, 78%) was isolated as an oil. Characterization data for this compound has been reported.⁶³ The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information. The product rac-30f (0.42 g, 83%) was isolated as an oil: ¹H NMR (500 MHz, CD₃CN) δ 7.69 (d, J = 8.3 Hz, 2H), 7.64 (d, J = 8.5 Hz, 2H), 6.24 (dt, J = 3.8, 1.8 Hz, 1H), 4.23–4.17 (m, 1H), 2.56–2.50 (m, 1H), 2.47–2.41 (m, 1H), 2.05–1.95 (m, 1H), 1.95–1.86 (m, 1H), 1.84–1.72 (m, 2H); ¹³C NMR (125 MHz, CD₃CN) δ 144.9, 141.0, 128.8 (q, J_C–F = 32.0 Hz), 126.0, 125.8, 125.3 (q, J_C–F = 3.7 Hz), 124.5 (q, J_C–F = 269.4 Hz), 56.7, 27.8, 26.8, 19.4; ¹⁹F NMR (471 MHz, CD₃CN) δ –63.0; IR (KBr, thin film, cm⁻¹) 2934, 2836, 2091, 1606, 1513, 1250, 1181; HRMS (CI-TOF) m/z [M − N₃]⁺ calcd for C₁₃H₁₂F₃⁺ 225.0886, found 225.0875.

5-Methoxy-2,3,4,5-tetrahydro-1,1′-biphenyl (32e)

This procedure was adapted from a known method.⁶⁴ To a solution of 3,4-dihydro-[1,1′-biphenyl]-1(2H)-ol (189 mg, 1.08 mmol) in MeCN (5 mL) and MeOH (1 mL) was added salicylic acid (15 mg, 0.11 mmol). The resulting solution was heated at 40 °C. After 18 h, the reaction was diluted with ethyl acetate and quenched by the addition of saturated aqueous NaHCO₃. The resulting solution was extracted with EtOAc (3 × 10 mL). The combined organic phase was washed with brine, dried (MgSO₄), filtered, and concentrated under reduced pressure. Purification by column chromatography (gradient elution 0–20% EtOAc in hexanes) afforded the product (150 mg, 0.797 mmol, 73%) as an oil. Characterization data for this compound has been reported.⁶⁴ The material obtained from this method provided an identical ¹H NMR spectrum. An image of the ¹H NMR spectrum is supplied in the Supporting Information.

4′,5-Dimethoxy-2,3,4,5-tetrahydro-1,1′-biphenyl (32a)

Following the above procedure for compound 32e at room temperature, the product 32a (0.16 g, 81%) was isolated as a waxy solid: ¹H NMR (400 MHz, CDCl₃) δ 7.39 (d, J = 8.8 Hz, 2H), 6.88 (d, J = 8.9 Hz, 2H), 6.13 (dt, J = 3.6, 1.7 Hz, 1H), 3.99–3.92 (m, 1H), 3.81 (s, 3H), 3.45 (s, 3H), 2.52–2.42 (m, 1H), 2.42–2.32 (m, 1H), 2.00–1.82 (m, 2H), 1.81–1.67 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 159.0, 139.8, 134.1, 126.5, 122.7, 113.6, 75.0, 55.8, 55.2, 27.7, 27.6, 19.7; IR (KBr, thin film, cm⁻¹) 2935, 2834, 1607, 1513, 1462, 1250, 1181, 1097, 1037, 822; HRMS (ESI-TOF) m/z [M + Na]⁺calcd for C₁₄H₁₈O₂Na⁺ 241.1199, found 241.1206.

5-Methoxy-4′-methyl-2,3,4,5-tetrahydro-1,1′-biphenyl (32b)

Following the above procedure for compound 32e, the product 32b (0.16 g, 76%) was isolated as an oil: ¹H NMR (400 MHz, CDCl₃) δ 7.34 (apparent d, J = 8.2 Hz, 2H), 7.15 (apparent d, J = 7.9 Hz, 2H), 6.16 (dt, J = 3.5, 1.8 Hz, 1H), 3.98–3.94 (m, 1H), 3.45 (s, 3H), 2.53–2.32 (m, 2H), 2.37 (s, 3H), 2.00–1.85 (m, 2H), 1.82–1.67 (m, 2H); ¹³C NMR (125 MHz, CDCl₃) δ 140.3, 138.7, 137.0, 128.9, 125.3, 123.5, 75.0, 55.8, 27.7, 27.5, 21.1, 19.6; IR (KBr, thin film, cm⁻¹) 2935, 2861, 1513, 1348, 1098, 808; HRMS (ESI-TOF) m/z [M + Na]⁺ calcd for C₁₄H₁₈ONa⁺ 225.1250, found 225.1253.

4′-(tert-Butyl)-5-methoxy-2,3,4,5-tetrahydro-1,1′-biphenyl (32c)

Following the above procedure for compound 32e, the product 32c (0.18 g, 87%) was isolated as an oil: ¹H NMR (400 MHz, CD₃CN) δ 7.40 (s, 4H), 6.19–6.13 (m, 1H), 3.96–3.89 (m, 1H), 3.38 (s, 3H), 2.51–2.41 (m, 1H), 2.41–2.32 (m, 1H), 1.98–1.86 (m, 2H), 1.73–1.60 (m, 2H), 1.33 (s, 9H); ¹³C NMR (125 MHz, CD₃CN) δ 150.3, 139.4, 138.7, 125.2, 125.0, 124.0, 74.8, 55.0, 34.1, 30.6, 27.5, 27.3, 19.5; IR (KBr, thin film, cm⁻¹): 2947, 2866, 1159, 1104; HRMS (ESITOF) m/z [M + Na]⁺ calcd for C₁₇H₂₄ONa⁺ 267.1719, found 267.1721.

4′-Fluoro-5-methoxy-2,3,4,5-tetrahydro-1,1′-biphenyl (32d)

Following the above procedure for compound 32e, the product 32d (0.15 mg, 82%) was isolated as a waxy solid: ¹H NMR (400 MHz, CDCl₃) δ 7.40–7.37 (m, 2H), 7.02 (apparent t, J = 8.7 Hz, 2H), 6.13 (dt, J = 3.6, 1.8 Hz, 1H), 4.01–3.92 (m, 1H), 3.45 (s, 3H), 2.52–2.41 (m, 1H), 2.39–2.31 (m, 1H) 2.01–1.87 (m, 2H), 1.81–1.66 (m, 2H); ¹³C NMR (100 MHz, CDCl₃) δ 162.2 (d, J_C–F = 244.6 Hz), 139.5, 137.7 (d, J_C–F = 3.3 Hz), 127.0 (d, J_C–F = 7.9 Hz), 124.3, 115.0 (d, J_C–F = 21.1 Hz), 74.9, 55.9, 27.8, 27.4, 19.6; ¹⁹F NMR (376 MHz, CDCl₃) δ –115.4; IR (KBr, thin film, cm⁻¹) 2938, 2818, 1601, 1509, 1099, 825; HRMS (EI-TOF) m/z [M]⁺ calcd for C₁₃H₁₅FO⁺ 206.1101, found 206.1093.

5-Methoxy-4′-(trifluoromethyl)-2,3,4,5-tetrahydro-1,1′-biphenyl (32f)

Following the above procedure for compound 32e using 0.5 equiv salicylic acid, the product 32f (0.10 g, 52%) was isolated as a waxy solid. ¹H NMR (400 MHz, CDCl₃) δ 7.59 (d, J = 8.0 Hz, 2H), 7.53 (d, J = 8.2 Hz, 2H), 6.25 (dt, J = 3.5, 1.8 Hz, 1H), 4.01–3.94 (m, 1H), 3.46 (s, 3H), 2.54–2.33 (m, 2H), 2.03–1.88 (m, 2H), 1.81–1.67 (m, 2H); ¹³C NMR (125 MHz, CDCl₃) δ 145.1, 139.3, 129.2 (q, 32.2 Hz), 126.5, 125.7, 125.2 (q, 3.7 Hz), 124.3 (q, 270.1 Hz), 74.8, 56.0, 27.6, 27.3, 19.5; ¹⁹F NMR (471 MHz, CDCl₃) δ –62.5; IR (KBr, thin film, cm⁻¹) 2936, 2820, 1615, 1326, 1122, 1070; HRMS (EI-TOF) m/z [M]⁺ calcd for C₁₄H₁₅F₃O⁺ 256.1070, found 256.1078.

Computational Details

All calculations were performed at the DFT level using the M06-2X density functional⁴⁴ as implemented in Gaussian09.⁶⁵ The 6-31G(d) basis set was used for all atoms.^66,67 The structures of reactants, intermediates, products, and transition states were fully optimized with the grid = ultrafine option in gas phase and in CHCl₃ (ε = 4.71) and MeOH (ε = 32.61) using the SMD approach.⁴³ Transition states were identified by having one imaginary frequency in the Hessian matrix. It was confirmed that transition states connect with the corresponding intermediates. All frequencies below 50 cm⁻¹ were replaced by 50 cm⁻¹ when accounting for thermal contributions to vibrational partition functions.⁶⁸ Single-point calculations were performed using the 6-311+G(2df,p) basis set.^69,70 Final values are reported as free energies computed at the specified temperature and 1 M standard state.