Chemoselective Oxidation of Alcohols in the Presence of Amines Using an Oxoammonium Salt

Abstract

The oxidation of alcohols in the presence of reactive amines employing the commercially available oxoammonium cation, “Bobbitt’s salt” is described. The oxidation is accomplished under acidic conditions and subsequent treatment with a suitable base affords a convenient one-pot method to access imines in good to excellent isolated yields (74–99%).

Graphical Abstract

1. Introduction

Oxoammonium cations are versatile organic oxidants that can accomplish a variety of oxidative transformations in a selective manner (Figure 1).¹ The tetrafluoroborate counterion is frequently used for the preparation of bench-stable, crystalline oxoammonium salts such as the commercially available Bobbitt’s salt² (1); which can be prepared in molar quantities and easily handled at the undergraduate level.³ The reactivity of these oxidants is often varied by the choice of solvent, the counterion, the use of external bases, or through modification of the electron-withdrawing or steric nature of the parent scaffold.^4–9 While previous reports mainly focus on the alteration of the electronic and steric aspects of oxoammonium cations, the stability of oxoammonium salts under a wide range of conditions (different pH values, temperatures, solvents, etc.) is often overlooked and can be leveraged to elicit different chemical outcomes. Bobbitt’s salt (1) and its nitroxide precursor ACT (2) are particularly attractive oxidants due to not only their stability at neutral pH ranges, but also their stability at both high pH (10–12)¹⁰ and low pH (2–3) ranges.¹¹

Figure 1.

Commonly used oxoammonium salts and nitroxides.

As demonstrated by Bobbitt, Bailey and coworkers¹² the sequential chemoselective oxidation of diol 4 to aldehydes 5 and 6 can be accomplished in both high yield and in a regioselective manner using 1 under slightly acidic conditions in methylene chloride solvent. Replacement of the methylene chloride solvent with wet acetonitrile alters the outcome, and aliphatic aldehydes are preferentially oxidized by 1 over benzaldehydes to carboxylic acids (e.g., 6 → 7).

Banwell and coworkers have shown that the oxidation of 1,2-diols into 1,2-diones without oxidative cleavage of the C-C bond can be accomplished using ACT (2) in the presence of excess of p-toluenesulfonic acid (p-TsOH), which catalyzes the rapid disproportionation of 2 into the N-oxoammonium salt oxidant (e.g. I; Figure 1) and a hydroxylamine salt (e.g., II; Figure 1) both possessing the tosylate counterion (Scheme 2).¹² Given the compatibility of 4-acetamido oxoammonium salts to the low pH environment employed in this method, we gathered that 1 had the potential to be able to accomplish the selective oxidation of alcohols in the presence of basic amines under low pH conditions. Primary amines are rapidly oxidized by 1 under basic conditions via hydride removal from the amine substrate to the corresponding imine, which undergoes further oxidation to the nitrile product (Scheme 2).¹³ In the absence of an external base, the amine substrate becomes protonated and resistant to oxidation by 1. Amines themselves are inherently susceptible to oxidative processes and an extensive variety of protecting groups (e.g., carbamate, amide, sulfonamide, N-alkyl, aminoacetals, among others) have been developed to limit their participation during the synthetic manipulation of other functional groups during complex molecule synthesis.¹⁴

Scheme 2.

Previously reported oxidation of alcohols under acidic conditions and amines under basic conditions using N-oxoammonium cations and the selective oxidation of alcohols in the presence of amines.

We reasoned that by conducting oxidations using 1 at low pH, amine substrates would be rendered inert to oxidative processes (e.g., 15), thus enabling the selective oxidation of alcohols by 1 in the presence of amines (Scheme 2). Our developed approach eliminates the need for prior protecting group placement on oxidation sensitive amines providing a more direct route for oxidative transformations of amine-containing substrates and is complementary to transition metal-based approaches.¹⁵

2. Results and Discussion

Initial investigations were focused on whether the desired oxidation of an alcohol by 1 could be accomplished in the presence of an amine at a pH ~3 (Table 1). Benzylamine (13a) was chosen as a model amine system and the oxidation of benzyl alcohol (12a) by 1 was optimized in acetonitrile solvent by varying amounts of p-TsOH (Entries 1–3; Table 1). These exploratory oxidations revealed that a minimum of 1 molar equivalent of 1 was required and improved isolated yields (94%) could be obtained with 2.2 equivalents of p-TsOH. The oxidation of 12a in the presence of 13a by 1 was also evaluated in methylene chloride solvent, and similarly, the results showed that with increasing equivalents of p-TsOH the isolated yields of 16a improved (Entries 4–8; Table 1). Isolated yields of 16a could also be improved by increasing the molar equivalents of 1 used (Entries 4–8; Table 1).

Table 1.

Optimization of oxidative formation of imines.

entry	alcohol	oxidant (equiv)	equiv of p-TsOH	solvent	methoda	result
1	12a	1 (1.05)	1.0	CH3CN	A	16a; 46%b
2	12a	1 (1.05)	1.5	CH3CN	A	16a; 52%b
3	12a	1 (1.05)	2.2	CH3CN	A	16a; 94%b
4	12a	1 (1.05)	1.0	CH2Cl2	B	16a; 37%b
5	12a	1 (1.05)	1.5	CH2Cl2	B	16a; 66%b
6	12a	1 (1.5)	1.5	CH2Cl2	B	16a; 87%b
7	12a	1 (2.0)	2.2	CH2Cl2	B	16a; 73%b
8	12a	1 (2.0)	3	CH2Cl2	B	16a; 99%b
9	12b	1 (1.05)	0	CH3CN	A	16ac; n.d.d
10	12b	1 (1.05)	0	CH2Cl2	B	16ac; n.d.d
11	12a	PIDA (1.05)	2.2	CH2 Cl2	B	-c,d,e
12	12a	PIFA (1.05)	2.2	CH2 Cl2	B	-c,d,e

^aAll reactions were conducted on a 2 mmol scale relative to the starting alcohol 12a or 12b, using the indicated equivalents of oxidant and p-TsOH with the given general procedure (A or B) as detailed in the experimental section below.

^bRefers to the isolated yield of product.

^cDetermined by GC-MS analysis of an aliquot of the crude reaction mixture; see electronic supplementary information for details.

^dIsolated yield was not determined.

^eAfforded a mixture of products and no desired imine 16a was observed. PIDA = phenyliodine(III) diacetate, PIFA = phenyliodine(III) bis(trifluoroacetate).

By using between 1.5 and 3.0 molar equivalents of p-TsOH a sufficient equilibrium is established ensuring the amine substrate 13a is adequately protonated as its corresponding tosylate salt 15 and resistant to oxidation by 1. This was confirmed via attempted oxidation of 4-methoxybenzyl alcohol (12b) by 1 in the absence of p-TsOH (Entries 9–10, Table 1). In the event, benzylamine (13a) is preferentially oxidized over 12b to an aldimine intermediate¹³ which is intercepted by either another molecule of 13a affording 16a by loss of ammonia, or by adventitious water to afford benzaldehyde 14a which condenses with 13a to afford 16a by loss of water. Analysis of an aliquot of the reaction mixture by GC-MS reveals that in the absence of p-TsOH the oxidation is non-selective as substantial amounts of 12b remain, 14a is observed, and 16a is the major product instead of desired imine 16b.

The unique stability and effectiveness of 1 under low pH conditions (pH ~ 3) was compared to two common hypervalent iodine reagents phenyliodine(III) diacetate (PIDA) and phenyliodine(III) bis(trifluoroacetate) (PIFA) that can be used for alcohol oxidations (Entries 11–12, Table 1).¹⁶ Both PIDA and PIFA were ineffective in affording the desired imine product 16a at the pH necessary to protect the amine during alcohol oxidation.

To demonstrate the effectiveness of the p-TsOH to prevent amine oxidation by 1, we subjected dibenzylamine (17) to the oxidation conditions (Figure 2). ¹H NMR analysis of an aliquot of the reaction demonstrates that tosylate salt 18 is unchanged after 1 h of reaction time in the presence of 1 (Figure 2). Furthermore, when both alcohol 12b and amine 17 are subjected to the reaction conditions after 1 h of reaction time, no 12b remains, the formed tosylate salt 18 is unchanged, and the formation of aldehyde 14b is clearly observed (Figure 2). These studies unambiguously show that 1 selectively oxidizes benzylic alcohols to their corresponding aldehydes in the presence of amine tosylate salts and under low pH conditions.

Figure 2.

¹H NMR analysis of the oxidation of 12b by 1 in the presence of dibenzylamine (17).

Two optimized procedures were selected allowing the use of 1 in acetonitrile solvent with heating (General Procedure A) or methylene chloride solvent at room temperature (General Procedure B). The developed procedures are both operationally simple and provide comparable yields.

Several aryl alcohols (12a-f) underwent oxidation by 1 in the presence of benzylamine (13a) in acetonitrile solvent allowing for the successful production of imines (16a-f; Scheme 3). The oxidation is conducted as follows: to a 0.2 M solution of the alcohol in acetonitrile solvent is added to 1.2 molar equivalents of benzylamine (13a) along with 2.2 molar equivalents of p-TsOH and the mixture allowed to stir for 15 minutes. After this time, the amine substrate is adequately protonated as a tosylate salt (e.g., 15) and 1 is added to the mixture to afford a yellow slurry. The oxidation is colorimetric and the initially yellow slurry fades to an almost colorless solution, and the resulting hydroxylamine salt (3) begins to precipitate out of solution as an off-white solid as 1 is consumed.

Scheme 3.

Oxidation of alcohols (12a-f) in the presence of benzylamine (13a) by 1 to afford imines (16a-f). ^a Yields refer to isolated yields of the reaction run on 1.0 mmol scale relative to the alcohol starting material using 2.2 molar equiv of p-TsOH, 1.2 equiv of 13a, 1.05 equiv of 1, and 3.5 equiv of K₂CO₃ after purification through a plug of SiO₂. ^b Time = 1 h, step 1; 1 h, step 2. ^c Time = 6 h, step 1; 12 h, step 2.

The time required for oxidation of the alcohols to their corresponding aldehydes varied since the hydride removal by 1 under acidic conditions is largely dictated by the electronic character of the substrate;¹⁷ the overall progress is easily monitored by thin-layer chromatography (TLC). Benzyl alcohols with electron-donating or weakly electron-withdrawing substituents (e.g., 12a-b; 12d-f) were oxidized by 1 in less than 1 h, whereas electron-withdrawn benzyl alcohols such as 4-nitrobenzyl alcohol (12c) required up to 6 h. Upon completion of alcohol oxidation as determined by TLC analysis (20% EtOAc/hexanes), 3.5 molar equivalents of potassium carbonate (K₂CO₃) are added along with activated 3Å molecular sieves. The mixture was then heated to 50 °C for 1 h (electron-rich systems; Scheme 3) or 12 h (electron-withdrawn systems; Scheme 3) to complete imine formation as monitored by TLC analysis. Upon completion, the acetonitrile solvent was removed under reduced pressure and the residue was taken up in diethyl ether. An aqueous extraction with K₂CO₃ afforded the crude imine product that was then purified by passage through a plug of silica gel using a 1% triethylamine/diethyl ether solution. Collection of the filtrate and removal of the solvent under reduced pressure afforded the desired imine products (16a-f) in good to excellent isolated yields (74–95%). Under the developed conditions, some of the more electron-rich substrates (e.g., 16b, 16e, and 16f) afforded a mixture of imine isomers (~ 20:1 E:Z ratio).

Aliphatic alcohols such as 1-butanol and 1-octanol were evaluated under these conditions, but due to the slow nature of the oxidation of aliphatic alcohols with 1 under acidic or neutral conditions,² the desired imine products were contaminated with equal amounts of dimeric ester (e.g., octyl octanoate from 1-octanol) that were not able to be separated by column chromatography. The addition of silica gel to the reaction mixture to speed up the alcohol oxidation² did not improve the outcome.

To further evaluate the scope of the transformation, several benzylamines (e.g., 13a; 13g-j), aliphatic amines of varying steric encumbrance (e.g., 13i-p), and aniline (13k) were evaluated against benzyl alcohol in methylene chloride solvent (Scheme 4). Under these conditions, the oxidation of 12a to benzaldehyde (14a) required 45 minutes when an excess of 1 was employed and imine formation in the presence of magnesium sulfate as a dehydrating agent could be accomplished in 24 h upon addition of K₂CO₃ at 23 °C (Scheme 4). The progress of the imine formation was monitored by TLC analysis using 10:89:1 EtOAc:hexanes:NEt3 solution as the eluent and visualization was achieved with ninhydrin stain. The work-up procedure was able to be further simplified to eliminate the use of a silica gel plug. The methylene chloride solvent was removed under reduced pressure and the residue was taken-up in diethyl ether. The resulting suspension was then filtered prior to an extractive work-up of the ethereal solution with K₂CO₃, which afforded the desired imine products in >93% isolated yield after removal of the solvent (Scheme 4).

Scheme 4.

Oxidation of amines (13a-1) in the presence of benzyl alcohol (12a) by 1 to afford imines (16h-q). ^a Yields are isolated yields of the reaction run on 2.0 mmol scale relative to the benzyl alcohol starting material using 2.0 molar equiv of 1, 1.2 equiv of amine, 3 equiv of p-TsOH, 1 mmol of MgSO₄, and 3 equiv of K₂CO₃ after an extractive work-up.

3. Conclusions

In summary, we report an operationally simple method for the oxidation of alcohols in the presence of amines using commercially available N-oxoammonium salt 1. The stability of 1 under acidic conditions allows masking the reactivity of the free amine by employing p-TsOH as an additive and upon aldehyde formation, treatment of the reaction mixture with K₂CO₃ provides access to imines in a single-pot operation. The oxidation can be accomplished in acetonitrile solvent with heating (50 °C) with the imine product purified through a short plug of deactivated silica gel or in methylene chloride solvent at room temperature (23 °C) with product purification accomplished through an extractive work-up, allowing for synthetic flexibility. Aryl alcohols along with benzylamines, aliphatic amines of varying steric encumbrance, and aniline are all well tolerated.

4. Experimental Section

4.1. General Procedures

Unless otherwise stated, reagents were used as received from the manufacturers. Reactions that required heating were heated to the required temperature using an appropriate-sized IKA aluminum reaction block equipped with an IKA temperature probe. The reactions were monitored by normal phase thin-layer chromatography (TLC) using Millipore Sigma glass-backed 60 Å plates (indicator F-254, 250 μM) using hexanes/ethyl acetate/triethylamine as the eluent system and were visualized using UV light at 254 nm followed by use of ninhydrin stain for visualization. Acetonitrile, diethyl ether, toluene, and methylene chloride were of HPLC grade. Manual flash chromatography was performed using the indicated solvent systems (ACS grade) with Silicycle SiliaFlash^® P60 (230–400 mesh) silica gel as the stationary phase. Room temperature (r.t.) refers to 23 °C, unless otherwise stated.

NMR spectra (¹H, ¹³C, ¹⁹F) were recorded at 298 K on a Brüker Avance^™ 300, 400, or 600 MHz NMR fitted with an autosampler. Chemical shifts (δ) are reported in parts per million (ppm) and coupling constants (J) are reported in hertz (Hz). Proton spectra recorded in deuterated chloroform are referenced to the residual ¹H signal of CHCl₃ at δ = 7.26 and are reported relative to TMS at δ = 0.00; carbon spectra are referenced to the central ¹³C signal for CDCl₃ at δ = 77.23 and are reported relative to TMS at δ = 0.00. NMR spectra acquired at Indiana University of Pennsylvania were taken in deuterated chloroform spiked with TMS as an internal standard, and NMR spectra acquired at Old Dominion University were taken in deuterated chloroform without TMS. The ¹H and ¹³C NMR spectra recorded for all products were fully in accord with those reported in the literature. in

High resolution mass spectra (HRMS) were obtained by the Old Dominion University COSMIC Mass Spectrometry Center on a Bruker 10 T APEX -Qe FTICR-MS spectrometer using +ESI or −ESI and reported for the molecular ion ([M]⁺, [M+H]⁺, [M+Na]⁺, respectively).

4.2. Experimental Procedures and Data of Synthetic Products

4.2.1. General Procedure A-Acetonitrile Solvent with Heating

A 25-mL round-bottomed flask was charged with 1.00 mmol of benzyl alcohol (1 equiv), 0.129 g of benzylamine (1.2 mmol, 1.2 equiv), 0.419 g of p-TsOH (2.2 mmol; 3 equiv) and 10 mL of acetonitrile. The colorless solution was stirred for 15 minutes at room temperature at which time 0.315 g of Bobbitt’s reagent (1.05 mmol; 1.05 equiv) was added, forming a yellow, colored suspension. The reaction was stirred at room temperature for 1 h and monitored by TLC (ethyl acetate:hexanes (20:80 v/v) and visualized by UV (254 nm). After the oxidation was complete, 0.483 g of potassium carbonate (3.50 mmol; 3.5 equiv) and 10–15 beads of 3Å molecular sieves was added to the suspension. The flask was then fitted with a Vigreux column and placed under a nitrogen atmosphere and heated to 50 °C for 1 h and reaction progress was monitored by TLC (ethyl acetate:hexanes: triethylamine (20:79:1 v/v)) and visualized by Ninhydrin stain.¹⁸ When the reaction was complete, acetonitrile was then removed by means of rotary evaporation (note: acetonitrile must be completely removed prior to diethyl ether addition to precipitate 2 and 3), 25mL of diethyl ether was then added, and the reaction mixture stirred for an additional 20 minutes. The suspension was then vacuum filtered using a Buchner funnel fitted with filter paper (70mm, Whatman #1), washed once with 25 mL of diethyl ether and the collected precipitate discarded. The ethereal solution was then washed with three 25 mL aliquots of saturated potassium carbonate and 25 mL of brine. The organic layer was dried over sodium sulfate, passed through a plug of deactivated silica gel (2g SiO₂ deactivated with 1% NEt₃ in ether), and the filtrate was concentrated by rotary evaporation to provide the imine product (74–95% yield; see Scheme 3).

(E)-N-benzyl-1-phenylmethanimine (16a)¹⁹

¹H (300 MHz, CDCl₃) δ 8.40 (s, 1H), 7.79 (m, 2H), 7.42 (m, 3H), 7.34 (m, 4H), 7.29 (m, 1H), 4.83 (s, 2H); ¹³C (75 MHz, CDCl₃) δ 162.2, 139.5, 136.3, 131.0, 128.8, 128.7, 128.5, 128.2, 127.2, 65.3.

(E/Z)-N-benzyl-1-(4-methoxyphenyl)methanimine (16b)²⁰

Isolated as a 20:1 mixture of E:Z isomers^15d that were not separated. NMR data reported is for the major E isomer: ¹H (400 MHz, CDCl₃) δ 8.35 (s, 1H), 7.78 (d, J = 8.8 Hz, 2H), 7.39 (m, 4H), 7.31 (m, 1H), 6.97 (d, J = 9.0 Hz, 2H), 4.83 (s, 2H), 3.84 (s, 3H); ¹³CNMR (100 MHz, CDCl₃) δ 161.8, 161.3, 139.8, 129.9, 129.3, 128.5, 128.0, 127.0, 114.1, 65.0, 55.4.

(E)-N-benzyl-1-(4-nitrophenyl)methanimine (16c)²⁰

¹H (400 MHz, CDCl₃) δ 8.46 (s, 1H), 8.27 (d, J = 8.5 Hz, 2H), 7.94 (d, J = 9.0 Hz, 2H), 7.39–7.33 (m, 4H), 4.89 (d, J=1.0 Hz, 2H); ¹³CNMR (100 MHz, CDCl₃) δ 159.5, 141.7, 138.5, 129.0, 128.7, 128.1, 127.4, 123.9, 65.3.

(E)-N-benzyl-1-(4-chlorophenyl)methanimine (16d)²⁰

¹H (400 MHz, CDCl₃) δ 8.36 (s, 1H), 7.75 (d, J = 8.4 Hz, 2H), 7.39–7.44 (m, 6H), 7.33 (m, 1H), 4.85 (s, 2H); ¹³CNMR (100 MHz, CDCl₃) δ 139.1, 136.7, 134.7, 129.5, 128.9, 128.5, 128.0, 127.1, 65.0.

(E/Z)-N-benzyl-1-(4-methyl)methanimine (16e)¹⁹

Isolated as a 20:1 mixture of E:Z isomers^15d that were not separated. NMR data reported is for the major E isomer: ¹H (600 MHz, CDCl₃) δ 8.26 (s, 1H) 7.59 (d, J = 8.1 Hz, 2H), 7.25 (m, 4H), 7.18 (m, 1H), 7.14 (d, J = 8.4 Hz, 2H); ¹³CNMR (150 MHz, CDCl₃) δ 162.1, 141.2, 139.6, 133.8, 129.5, 128.7, 128.4, 128.2, 127.1, 65.2, 21.7.

(1E/Z,2E)-N-benzyl-3-phenylprop-2-en-1-imine (16f)²¹

Isolated as a 20:1 mixture of 1E,2E:1Z,2E isomers^15d that were not separated. NMR data reported is for the major 1E,2E isomer: ¹H (600 MHz, CDCl₃) δ 7.97 (d, J = 7.4 Hz, 1H), 7.33 (d, J = 7.4 Hz, 2H), 7.21–7.14 (m, 7H), 7.12 (t, J = 7.0 Hz, 1H), 6.83 (m, 2H), 4.56 (s, 2H); ¹³CNMR (150 MHz, CDCl₃) δ 163.5, 142.1, 139.3, 135.8,129.3, 128.9, 128.6, 128.2, 128.1, 127.3, 127.1, 65.3.

4.2.2. General Procedure B – Methylene Chloride Solvent without Heating

A 100-mL round-bottomed flask was charged with 2.40 mmol of amine (1.2 equiv), 0.217 g (2.00 mmol; 1 equiv) of benzyl alcohol, 1.14 g of p-TsOH (6.00 mmol; 3 equiv) and 50 mL of methylene chloride. The colorless solution was stirred for 20 minutes at room temperature at which time 0.720 g of Bobbitt’s reagent (4.00 mmol; 2 equiv) was added, forming a yellow, colored suspension. The reaction mixture was stirred at room temperature for 45 minutes and monitored by TLC (ethyl acetate:hexanes: triethylamine (10:89:1 v/v) and visualized by UV (254 nm). After the oxidation was complete, 0.827 g of potassium carbonate (6.00 mmol; 3 equiv) and 0.120 g of magnesium sulfate (1.00 mmol) was added to the suspension. Stirring was continued at room temperature for an additional 24 hours and the reaction progress was monitored by TLC and visualized with a ninhydrin stain.¹⁸ When the reaction was complete, methylene chloride was then removed by means of rotary evaporation (note: methylene chloride must be completely removed prior to diethyl ether addition to precipitate 2 and 3), 50 mL of diethyl ether was then added, and the reaction mixture stirred for an additional 20 minutes. The suspension was then vacuum filtered using a Buchner funnel fitted with filter paper (70 mm, Whatman #1), washed once with 15 mL of diethyl ether and the collected precipitate discarded. The ethereal solution was then washed with three 15 mL aliquots of saturated potassium carbonate and 3 mL of brine. The organic layer was dried over sodium sulfate and concentrated by rotary evaporation to provide the imine product (93–95% yield; see Scheme 4).

(E)-N-(4-chlorobenzyl)-1-phenylmethanimine (16g)²²

¹H (300 MHz, CDCl₃) δ 8.40 (s, 1H), 7.82 – 7.79 (m, 2H), 7.45 (m, 3H), 7.35 – 7.28 (m, 4H), 4.79 (s, 2H); ¹³C (75 MHz, CDCl₃) δ 162.4, 138.0, 136.1, 132.8, 131.1, 129.4, 128.8, 128.7, 128.4, 64.4.

(E)-N-(2-bromobenzyl)-1-phenylmethanimine (16h)

¹H (300 MHz, CDCl₃) δ 8.43 (s, 1H), 7.82 (m, 2H), 7.57 (d, J = 7.9 Hz, 1H), 7.45 (m, 4H), 7.31 (m, 1H), 7.15 (m, 1H), 4.90 (s, 2H); ¹³C (75 MHz, CDCl₃) δ 163.1, 138.9, 136.3, 132.7, 131.1, 130.0, 128.8, 128.7, 128.5, 127.7, 123.8, 64.5; HRMS (ESI-ICR) m/z calcd for C₁₄H₁₂BrN [M+H]⁺ calc. 274.0226, found 274.0225.

(E)-N-(4-methoxybenzyl)-1-phenylmethanimine (16i)²²

¹H (300 MHz, CDCl₃) δ 8.37 (s, 1H), 7.78 (m, 2H), 7.42 (m, 3H), 7.25 (m, 2H), 6.89 (d, J = 8.3 Hz, 2H), 4.76 (s, 2H), 3.80 (s, 3H); ¹³CNMR (75 MHz, CDCl₃) δ 161.8, 158.9, 136.4, 131.6, 130.9, 129.4, 128.8, 128.5, 114.1, 64.7, 55.5.

(E)-N-(4-methylbenzyl)-1-phenylmethanimine (16j)²²

¹H (300 MHz, CDCl₃) δ 8.27 (s, 1H), 7.73 (s, 2H), 7.34 (s, 3H), 7.14 (m, 4H), 4.71 (s, 2H), 2.28 (s, 3H); ¹³C (75 MHz, CDCl₃) δ 159.1, 133.8, 133.43, 133.42, 128.1, 126.7, 126.5, 125.9, 125.6, 125.3, 62.1, 18.5.

(E)-N,1-diphenylmethanimine (16k)²²

¹H (300 MHz, CDCl₃) δ 8.47 (s, 1H), 7.92 (m, 2H), 7.49 (m, 3H), 7.41 (m, 2H), 7.24 (m, 3H); ¹³C (75 MHz, CDCl₃) δ 160.6, 152.3, 136.4, 131.6, 129.4, 129.0, 129.0, 126.1, 121.1.

(E)-N-cyclohexyl-1-phenylmethanimine (16l)¹⁹

¹H (300 MHz, CDCl₃) δ 8.32 (s, 1H), 7.73 (m, 2H), 7.40 (m, 3H), 3.24 – 3.15 (m, 1H), 1.86 – 1.58 (m, 7H), 1.35 (q, 3H); ¹³C (75 MHz, CDCl₃) δ 158.8, 136.8, 130.5, 128.7, 128.2, 70.2, 34.6, 25.8, 25.0.

(E)-N-tert-butyl-1-phenylmethanimine (16m)²³

¹H (300 MHz, CDCl₃) δ 8.29 (s, 1H), 7.78–7.75 (m, 2H), 7.42–7.37 (m, 3H), 1.32 (s, 9H); ¹³CNMR (75 MHz, CDCl₃) δ 155.3, 137.3, 130.3, 128.7, 128.1, 57.4, 29.0.

(E)-N-butyl-1-phenylmethanimine (16n)²²

¹H (300 MHz, CDCl₃) δ 8.27 (s, 1H), 7.73 (m, 2H), 7.41 (m, 3H), 3.62 (t, J= 7.1 Hz, 2H), 1.71 – 1.65 (m, 2H), 1.44 – 1.34 (m, 2H), 0.96 (t, J = 7.3 Hz, 3H); ¹³C (75 MHz, CDCl₃) δ 160.9, 136.5, 130.6, 128.7, 128.2, 61.7, 33.2, 20.6, 14.1.

(E)-N-pentyl-1-phenylmethanimine (16o)²⁴

¹H (300 MHz, CDCl₃) δ 8.27 (s, 1H), 7.74 (m, 2H), 7.41 (m, 3H), 3.62 (t, J = 6.9 Hz, 2H), 1.72 (m, 2H), 1.36 (m, 4H), 0.92 (t, J = 7.0 Hz, 3H); ¹³CNMR (75 MHz, CDCl₃) δ 160.8, 136.5, 130.6, 128.7, 128.1, 62.0, 30.8, 29.7, 22.7,14.2.

(E)-N-(2-ethylhexyl)-1-phenylmethanimine (16p)

¹H (300 MHz, CDCl₃) δ 8.26 (s, 1H), 7.76–7.73 (m, 2H), 7.41 (m, 3H), 3.55 (dt, J = 6.1, 1.5 Hz, 2H), 1.7 (m, 1H), 1.45 – 1.29 (m, 8H), 0.92 (m, 6H); ¹³C (75 MHz, CDCl₃) δ 160.9, 136.7, 130.5, 128.7, 128.2, 65.3, 40.7, 31.6, 29.2, 24.7, 23.3, 14.3, 11.2. HRMS (ESI-ICR) m/z calcd for C₁₅H₂₃N [M+H]⁺ calc. 218.1903, found 218.1902.

Scheme 1.

Chemoselective alcohol oxidations by 1.

Data Availability

Data can be found in Appendix A and will be made available on request.