Synthesis of Multi-Substituted Pyridines from Ylidenemalononitriles and their Emission Properties

Abstract

Numerous methodologies to obtain pyridines from ylidenemalononitriles are described in the literature. Nevertheless, they are limited to the use of microwave or conventional heat and few lead to 2, 3, 4 or 2, 3, 4, 5-substituted pyridines as multi-proposal molecular scaffolds or even universal pyridines. Herein, we present a mild and facile solvent-free methodology to obtain a scope of multi-substituted pyridines at room temperature. We also report an example where one of the resulting amino-nicotinonitriles exhibits a preliminary evidence of aggregation-induced emission (AIE).

Introduction

Pyridine moieties play a key role in many bioactive substances, being nicotinonitriles common precursors in the synthesis of pyridine containing active pharmaceutical ingredients (APIs) such as nevirapine.^1,2 In particular, amino-nicotinonitriles (3-cyano-2-aminopyridines) are very common, and two general approaches are used to produce 3-cyano-2-aminopyridines: (1) methods that modify 3-cyano-2-halopyridines³ and (2) methods that build the pyridine from an acyclic precursor.^4–6 Relevant examples are featured in Scheme 1 where acyclic precursors react to produce a variety of cyclic derivatives. Although numerous methodologies are available, few lead to 2, 3, 4 or 2, 3, 4, 5-substituted pyridines, substitution patterns that are prevalent in many pyridine containing species (Scheme 2B).

Scheme 1.

Main methodologies described in the literature to produce 3-cyano-2-aminopyridines

Scheme 2.

A) Developed methodology for efficient functionalization of YMs R₂ position; B) Facile synthesis of highly substituted pyridines from YMs

Our group has explored ylidenemalononitriles (YMs) chemistry for almost a decade.⁷ Our attempts to obtain amino-nicotinonitriles from YM substrates were unsuccessful^6c as combining primary amines and YMs under mild conditions only yielded cyclic amidines (III) (Scheme 2A). When we contemplated this unexpected result, we observed several reaction conditions differences in the methodologies available in the literature compared to the one used by our group; among them, the employment of microwave/heat, long reaction times, and/or substrates containing different substituents.

We hypothesized that the fluorescent amidine which we observed when YMs were reacted with primary amines might be intermediates on the reaction coordinate leading to amino-nicotinonitriles. Our hypothesis was supported by experiments we performed using substrates such as those shown in Scheme 2B.⁸ Herein, we present these results in detail showing examples with varying rates of transition from the cyclic amidine to aminopyridine forms. Moreover, we examine the scope for producing multi-substituted pyridines (i.e. universal pyridines) and provide one example where the product exhibits preliminary evidence of aggregation-induced emission (AIE), a peculiarity that can have further optoelectronic studies and biological applications.⁹

Results and discussion

In our previous study,⁸ we showed that the reaction of sulfenyl chlorides (RSCl) and YMs (I) follows a pattern similar to enamines where an addition followed by elimination provides a regioselective β-substitution (Scheme 2A). We performed the reaction of 17 different YMs (I) with sulfur electrophiles (RSCl) and then cyclized the substituted YMs (II) with primary amines in a click-like reaction to yield III. The resulting products were green or yellow fluorescent cyclic-amidines (Scheme 2A).

We observed a different outcome when 1 reacted with an excess of benzylamine (Scheme 4). In this case, the highly substituted YM produced the expected amidine 2, but as an unstable intermediate that converted to amino-pyridine 3 reasonably fast (< 2 h). Initially, we observed the formation of a product that exhibited yellow fluorescence (2) and the emission spectra transitioned from yellow to blue (visual inspection using 365 nm illumination). Monitoring the reaction by TLC revealed that 2 transitioned to a single less polar product and LC/MS analysis confirmed that the new product had the same molecular mass as the amidine 2.

Scheme 4.

Amino-pyridines obtained from YMs with varying R₁ and R₂ groups

When monitored by ¹H NMR the cyclic amidine chemical shift for the C–H ring hydrogen shifted from 7.5 ppm to 8.3 ppm, a shift consistent with a pyridine (see Supporting Information – Fig. S1). Furthermore, we were able to isolate this new product and grow needle-shaped yellow crystals. X-ray crystallographic analysis of 3 confirmed that the new blue fluorescent species was the amino-nicotinonitrile as suggested by the ¹H NMR (see Supporting Information – Fig. S15).

We propose that 2 yields 3 via a Dimroth rearrangement as described by Odom and co-workers to similar amidine intermediates.^4l They proposed the amidine rearrangement occurs in the presence of a DBU/phenol system. In our case, we propose that benzylamine serves as both a base and proton donor. Briefly, BnNH₂ reacts with 2 and undergoes a ring-opening followed by a 6-endo-trig cyclization step and benzylamine elimination (Scheme 3).

Scheme 3.

The proposed mechanism to amino-pyridines formation – Dimroth rearrangement mechanism

As discussed before, not all amidines promptly transitioning to the amino-pyridine form and this observation encouraged us to determine what structural features favor the Dimroth rearrangement. Subsequently, a series of YMs with different substitution patterns was exposed to benzylamine (or propylamine) in excess and the rate of conversion to the pyridine was measured (formation of the amidine tends to be from minutes to hours). We varied the substituents at R₁ and R₂ because these two positions show the greatest impact on controlling the rate of the Dimroth rearrangement. YMs possessing R₁ = aryl and R₂ = S-aryl (Scheme 4A), R₁= alkyl and R₂ = S-aryl (Scheme 4B) and finally, R₂= H, Cl, Ph (Scheme 4C) are shown as follows. Compounds 1, 4–9, 16, 18, and 20 react in a matter of 1–2 h to produce the corresponding amino-pyridines (Scheme 4A). Nevertheless, the substrates which are shown in Scheme 4B and 4C are slowly converted (cyclization/aromatization).

We suggest that large groups in R₁ and R₂ produced allylic-strains compared to small groups and that strain raises the ground state of the cyclic amidine and increases the rate of addition of the benzylamine. Steric considerations are not the only features that change the Dimroth rate. Electron withdrawing groups increase the rate as demonstrated by 33, 37, and 43 (2–3 days versus 5–7 days).

Teasing out the mechanism for why each substrate in Scheme 4 is faster or slower is difficult but we are confident that large groups in R₁ and R₂ increase the rate. Additionally, attempts to perform the reaction with secondary amines were done, but with no success in the presented reaction conditions.

We propose that these results are significant for three main reasons: (1) YMs reacting with primary amines are a useful turn-on fluorescent probe^7c and understanding the design principles that control the stability of the cyclic amidine vs. formation of the pyridine will aid those who wish to exploit that application; (2) compounds in Scheme 4A and B are a relatively rare class of amino-pyridine where the 5-position contains a sulfur substituent being an effective scope demonstration; (3) members of this class of pyridines may have relevant photophysical properties such as aggregation-induced emission.

For further characterization of the new pyridine derivatives, we measured the photophysical properties for a cross-section of molecules in Scheme 4 and the data are presented in Table 1. In DCM, these compounds exhibit similar blue emission (460–487 nm), excited-state lifetimes (2.6–3.1 ns), and fluorescence quantum yields (0.07–0.16) indicating that the photophysical properties are primarily dominated by the pyridine core with minimal influence from the aryl substituents (i.e. Cl, OMe, H).

Table 1.

Photophysical properties of 3, 10, 11, and 17 in DCM in ambient conditions (~10 μM)

Compound	Absorbance λ (nm)	Emission at rta			k r (×107 s−1)b	k nr (×108 s−1)c
		λmax (nm)	τ (ns)	ɸPL
3	358	474	3	0.09	3	3
10	354	460	3.1	0.16	5.2	2.7
11	358	486	2.6	0.07	2.8	3.6
17	360	487	3	0.08	2.5	3.1

^(a)Emission data acquired using dilute solutions and lifetimes calculated from monoexponential fits.

^(b)k_r =ɸ/τ.

^(c)k_nr = (1−ɸ)/τ.

Additionally, compounds also exhibit a large Stokes shift (_~ 6,500 cm⁻¹) that is similar to the cyclic amidine analogs.⁸ Despite having a similar structure to those in Table 1, compound 13 (highlighted in grey in Scheme 4) was non-emissive in DCM. It is not usual for nitroaromatics to quench emission via a photoinduced electron transfer (PET).¹⁰

Given that nitroaromatic containing compound 29 was also non-emissive, a similar mechanism may be active here. Remarkably, in contrast to in solution, crystals of 13 exhibit yellow-green emission (see Supporting Information – Fig. S2). The lack of emission in dilute solution and strong fluorescence in the solid-state is typically described as aggregation-induced emission (AIE).¹¹

From the X-ray crystal structure of 13 (see Supporting Information – Fig. S16), the nitrobenzene group is nearly perpendicular (79°) to the pyridyl core. This orthogonality reduces electronic communication between the nitroaryl group and the fluorophore core, which could slow PET, resulting in emission in the solid-state. In solution, there is greater rotational freedom and more favorable electronic coupling resulting in PET quenching. In contrast, for 29 where the 4-nitrothiophenol substituent cannot be orthogonal to the pyridyl core, PET quenching could still be active in the solid-state. Alternatively, the crystal of 13 may inhibit non-radiative decay through the restriction of intramolecular rotation (RIR).⁹ However, if that is the case, it is not clear why RIR would not enhance the emission of 29 in the solid-state, nor why 13 is emissive in solution when a similar rotational deactivation mechanism would be active. It is worth noting that in the crystal structure of 13 there is a hydrogen bond interaction between the amino group (NH) and nitrile (CN) forming a dimer and a π-stacking interaction between aryl groups R₁ and R₂, which can support the inhibited rotation (Figure 1).

Figure 1.

X-ray crystal structure of 13. A) Hydrogen bond interaction between amino group (NH) and nitrile (CN) groups are depicted with a dashed red line. B) π-stacking interaction between aryl groups R₁ and R₂

Experimental section

It is unclear what role, if any, excited-state proton transfer may play in the unusual phase-dependent photophysics of 13. To gain further insights into the photophysics of 13, we performed solvent dependent emission measurements (see Supporting Information – Fig. S3). Compound 13 is non-emissive in polar solvents (i.e. EtOH, EtOAc, MeCN, DCM), but exhibited yellow-green emission in non-polar solvents (i.e. mesitylene, toluene, pentane, and hexanes). Interestingly, while the concentrations were similar, the solubility of 13 decreases from polar to non-polar solvents with the obvious particulate formation in pentane and hexanes. The addition of EtOAc to the non-polar solutions resulted in the disappearance of insoluble particles and a complete loss of fluorescence (see Supporting Information – Fig. S3D). The decreased solubility and increased emission in non-polar solvents further support an AIE mechanism. NMR experiments were also carried out using a 25 mM solution of 13 in a deuterated solvent. Different samples were prepared where the ratio of CDCl₃:toluene-d₈ was varied from 100:0 to 0:100. An increase in the fraction of the non-polar solvent resulted in an upfield chemical shift (Δδ = 0.4 ppm) for the hydrogen in the amino group (NH) (see Supporting Information – Fig. S4). This observation reinforces the idea of aggregates formation and the chemical shift is due to the prevalence of the hydrogen bonds in toluene-d₈.

For more quantitative insights into the solvent dependent photophysics, we measured emission spectra for 13 in ratios of toluene:DCM ranging from 100:0 (100%) to 70:30 (70%) (see Supporting Information – Fig. S12). With a solution of < 70% toluene, minimal emission was observed. In contrast, from 70% to 90% toluene, there was a subtle blue shift in emission from 610 to 605 nm and a linear increase in emission intensity (see Supporting Information – Fig. S13). At > 90% toluene, there was an increase in intensity and a further blueshift to 570 nm. These results are consistent with the above observations (i.e. increased aggregation and enhanced emission in non-polar solvents) further supporting an AIE mechanism. In this case, the AIE is likely accompanied by polarity dependent solvatochromism.

While these observations are intriguing, we are yet to definitively establish the mechanism of AIE in 13. Possible mechanisms include RIR,¹² restricted access to the dark state (RADS),¹³ inhibited excited-state proton transfer,¹⁴ or as suggested above, solid-state inhibition of PET. The latter is particularly intriguing in that, as far as we know, this would be the first example of AIE via inhibited PET. Ultrafast transient absorption studies would be necessary to definitively establish an AIE mechanism.

It is important to acknowledge that the emission quantum yield of these dyes (~10%) is well below many commercially available probes used in fluorescent labeling and microscopy. However, it is worth mentioning that no efforts have yet been made to optimize their photophysical properties which could no doubt be improved with known fluorophore design strategies (i.e. increased rigidity, inhibited deactivation modes, etc.). Regardless, this class of dyes exhibits a unique combination of 1) readily tunable emission, 2) turn-on fluorescence in the presents of primary amines, 3) high solvent sensitivity, and in some cases 4) aggregate induced emission. This amalgamation of properties and stimuli responsiveness could open door to a plethora of multiparameter sensing opportunities and applications including chiral recognition, ionic sensing, biomolecule detection, and cell imaging.¹⁵

Conclusions

In conclusion, we presented a solvent-free methodology developed under mild conditions to obtain a scope of multi-substituted pyridines from YMs. We demonstrated that the cyclization time can be considerably reduced from days to hours by varying the YMs substituents. Such feature is especially observed in substrates possessing large steric groups in the R₁ and R₂ positions. Not only the stability/reactivity of the YMs can be tuned by varying the YM substituents, but also the fluorescence λ_max of the cyclic amidine intermediate, as discussed in our previous work.⁸ Photophysics studies showed that most of the amino-pyridines obtained in this present study exhibit blue emission. Even so, amino-nicotinonitrile 13 presented yellow-green fluorescence at the solid-state and when solubilized in non-polar solvents. Analysis of 13 in different solvents and their emissions in different non-polar solvent concentrations, preliminarily support a molecule with AIE properties, being 13 a promising compound for future studies and applications in optics, electronics, energy, and biosciences.

Experimental

General Information

All commercially available reagents were purchased from Acros Organics, Sigma-Aldrich, TCI, or Alfa Aesar and used as received. Thin layered chromatography (TLC) was performed using silica gel 60 F254 plates. Hydrogen nuclear magnetic resonance (¹H NMR) spectra and carbon nuclear magnetic resonance (¹³C NMR) spectra were recorded on a Bruker-600 MHz. Chemical shifts for protons are reported in parts per million downfield from tetramethylsilane. Chemical shifts for carbon are reported in parts per million (ppm) downfield from tetramethylsilane or referenced to residual solvent. Data are represented as follows: chemical shift, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sextet = sext, heptet = hept, octet = oct, dd = doublet of doublets, dt = doublet of triplets, m = multiplet), coupling constants in Hertz (Hz), integration. HPLC chromatographs were acquired either on an Agilent 1260 Infinity system using an Agilent Poroshell 120 EC-C18 column (2.7 μm, 4.6 mm × 50 mm) applying a gradient of 5% ACN in H₂O to 95% ACN in H₂O from 0.5 min to 6.5 min at a flow rate of 1.5 mL/min. High-resolution mass spectra were obtained through the Virginia Commonwealth University Chemical and Proteomic Mass Spectrometry Core Facility using a Orbitrap Velos mass spectrometer from Thermo Electron Corporation. For the crystal structure determination, single crystals were mounted on a MiTeGen Microloop and used for data collection on a XtaLAB MM007-HF system (Cu Ka radiation, λ = 1.54184 Å) coupled with a Hybrid Photon Counting detector (Dectris Eiger 4M). CrysAlis PRO software (Rigaku OD, 2015)¹⁶ was used for data collection, data processing, cell refinement, and data reduction. The structures were solved with ShelXTL¹⁷ using Intrinsic Phasing and refined with the olex2-refine¹⁸ refinement package using Gauss-Newton minimization. H atom positions were calculated geometrically and refined using a rigid model. The final difference Fourier maps showed no peaks of chemical significance. The names of all compounds were generated using the PerkinElmer ChemDraw Ultra v.12.0.2 software package.

General Procedure – Synthesis of Multi-Substituted Pyridines.

1 mmol of the enamine (1, 4–9, 16, 18, 20, 22–26, 32, 34, 36, 38–40) was dissolved in 2 mL of primary amine (benzylamine or propylamine) at room temperature. The reaction was left stirring vigorously until all the enamine was entirely consumed and a blue fluorescent spot was observed by TLC when irradiated with a 254 nm UV-lamp. The reaction mixture was concentrated under vacuum in the rotatory evaporator and the product (3, 10–15, 17, 19, 21, 27–31, 33, 35, 37, 41–43) was purified by column chromatography in EtOAc/Hexanes. All enamines (1, 4–9, 16, 18, 20, 22–26, 32, 34, 36, 38–40) used in this work to obtain the final aminopyridines were already described in our previous publication. Their detailed experimental procedures and full characterization are available.⁸

2-(benzylamino)-5-((4-methoxyphenyl)thio)-4-phenylnicotinoni- trile (3).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 91% yield (385 mg, 0.91 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.27 (s, 1H), 7.38 – 7.34 (m, 3H), 7.32 – 7.28 (m, 4H), 7.27 – 7.21 (m, 1H), 7.20 – 7.17 (m, 2H), 6.93 (d, J = 8.6 Hz, 2H), 6.65 (d, J = 8.6 Hz, 2H), 5.56 (t, J = 5.5 Hz, 1H), 4.65 (d, J = 5.5 Hz, 2H), 3.68 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.2, 158.0, 156.7, 156.6, 138.1, 135.4, 132.9, 129.3, 128.8, 128.6, 128.3, 127.8, 127.7, 126.1, 119.9, 115.9, 114.7, 92.9, 55.3, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₆H₂₂N₃OS 424.1483; Found 424.1481. X-ray available (Fig. S15).

2-(benzylamino)-4-phenyl-5-(phenylthio)nicotinonitrile (10).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 83% yield (326 mg, 0.83 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.49 (s, 1H), 7.44 – 7.38 (m, 7H), 7.35 – 7.31 (m, 1H), 7.26 – 7.23 (m, 2H), 7.20 – 7.16 (m, 2H), 7.16 – 7.11 (m, 1H), 7.00 (d, J = 7.52 Hz, 2H), 5.73 (t, J = 5.5 Hz, 1H), 4.77 (d, J = 5.5 Hz, 2H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.6, 158.5, 158.3, 137.9, 137.1, 135.4, 129.3, 128.9, 128.8, 128.8, 128.7, 128.5, 128.2, 127.8, 127.7, 126.3, 117.2, 115.8, 93.1, 45.6. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₅H₂₀N₃S 394.1378; Found 394.1379.

2-(benzylamino)-4-(4-chlorophenyl)-5-((4-methoxyphenyl)thio) nicotinonitrile (11).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 89% yield (406 mg, 0.89 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.39 (s, 1H), 7.43 – 7.39 (m, 2H), 7.39 – 7.35 (m, 4H), 7.35 – 7.30 (m, 1H), 7.22 – 7.19 (m, 2H), 7.02 – 6.99 (m, 2H), 6.77 – 6.73 (m, 2H), 5.68 (t, J = 5.5 Hz, 1H), 4.75 (d, J = 5.5 Hz, 2H), 3.78 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.2, 158.0, 156.9, 155.6, 138.0, 135.5, 133.8, 132.7, 130.1, 129.7, 128.9, 128.8, 128.7, 128.6, 128.3, 127.8, 127.7, 125.9, 119.6, 115.7, 114.8, 92.7, 55.4, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₆H₂₁ClN₃OS 458.1094; Found 458.1090.

2-(benzylamino)-4-(4-methoxyphenyl)-5-((4-methoxyphenyl)thio) nicotinonitrile (12).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 85% yield (385 mg, 0.85 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.23 (s, 1H), 7.31 – 7.15 (m, 7H), 6.97 (d, J = 8.8 Hz, 2H), 6.89 (d, J = 8.6 Hz, 2H), 6.68 (d, J = 8.8 Hz, 2H), 5.54 (t, J = 5.5 Hz, 1H), 4.65 (d, J = 5.5 Hz, 2H), 3.78 (s, 3H), 3.69 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 162.0, 160.4, 159.2, 158.1, 156.5, 156.3, 138.1, 132.8, 130.3, 128.8, 127.8, 127.6, 127.6, 126.2, 120.0, 116.3, 114.7, 113.7, 92.9, 55.3, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₇H₂₄N₃O₂S 454.1589; Found 454.1591.

5-((4-methoxyphenyl)thio)-4-(4-nitrophenyl)-2-(propylamino) nicotinonitrile (13).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 84% yield (352 mg, 0.84 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.45 (s, 1H), 8.27 (d, J = 7.7 Hz, 2H), 7.39 (d, J = 7.7 Hz, 2H), 6.92 (d, J = 7.9 Hz, 2H), 6.72 (d, J = 7.9 Hz, 2H), 5.43 (t, J = 4.8 Hz, 1H), 3.76 (s, 3H), 3.53 (q, J = 6.6 Hz, 2H), 1.70 (sext, J = 7.3 Hz, 2H), 1.02 (t, J = 7.3 Hz, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.2, 158.4, 157.8, 154.8, 148.1, 141.9, 132.1, 129.8, 126.0, 123.5, 118.0, 115.4, 114.8, 92.0, 55.4, 43.5, 22.6, 11.3. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₂H₂₁N₄O₃S 421.1334; Found 421.1336. X-ray available (Fig. S16).

2-(benzylamino)-4-(4-chlorophenyl)-5-((2,2,2-trifluoroethyl)thio) nicotinonitrile (14).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 88% yield (381 mg, 0.88 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.58 (s, 1H), 7.51 (d, J = 8.4 Hz, 2H), 7.41 – 7.36 (m, 4H), 7.36 – 7.30 (m, 3H), 5.77 (t, J = 5.5 Hz, 1H), 4.78 (d, J = 5.5 Hz, 2H), 2.86 (q, J = 9.5 Hz, 2H). ¹⁹F{¹H} NMR (565 MHz, CDCl₃): δ_F −65.71. ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.1, 158.7, 157.2, 139.3, 137.7, 136.0, 133.4, 130.2, 128.9, 128.8, 127.8, 127.8, 127.8, 125.9, 124.1, 115.4, 115.1, 114.1, 92.8, 45.6, 38.2 (q, J = 32.1 Hz), 33.8, 31.9, 31.6, 29.7, 29.7, 29.6, 29.6, 29.5, 29.3, 29.1, 28.9, 22.7. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₁H₁₆ClF₃N₃S 434.0706; Found 434.0681.

2-(benzylamino)-4-(4-chlorophenyl)-5-(decylthio)nicotinonitrile (15).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 97% yield (475 mg, 0.97 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.47 (s, 1H), 7.50 – 7.47 (m, 2H), 7.40 – 7.36 (m, 4H), 7.36 – 7.30 (m, 3H), 5.65 (t, J = 5.5 Hz, 1H), 4.76 (d, J = 5.5 Hz, 2H), 2.43 (t, J = 7.1 Hz, 2H), 1.39 – 1.12 (m, 20H), 0.90 (t, J = 7.1 Hz, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.0, 157.2, 156.1, 138.0, 135.6, 134.0, 130.3, 128.8, 128.7, 127.8, 127.7, 118.4, 115.9, 92.5, 45.5, 36.1, 31.9, 29.6, 29.5, 29.4, 29.3, 29.0, 28.9, 28.4, 22.7, 14.1. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₃₁H₃₉ClN₃S 520.2555; Found 520.2556.

2’-(benzylamino)-5’-((4-methoxyphenyl)thio)-[3,4’-bipyridine]-3’-carbonitrile (17).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 70% yield (296 mg, 0.70 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.68 (dd, J = 4.9, 1.7 Hz, 1H), 8.53 (d, J = 2.0 Hz, 1H), 8.47 (s, 1H), 7.58 (dt, J = 7.9, 2.0 Hz, 1H), 7.40 – 7.29 (m, 6H), 6.95 (d, J = 8.8 Hz, 2H), 6.72 (d, J = 8.8 Hz, 2H), 5.80 (t, J = 5.5 Hz, 1H), 4.76 (d, J = 5.5 Hz, 2H), 3.75 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.1, 158.1, 157.4, 153.4, 150.2, 149.0, 137.8, 136.1, 132.3, 131.5, 128.7, 127.8, 127.6, 125.8, 122.8, 119.5, 115.4, 114.7, 92.8, 55.3, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₅H₂₁N₄OS 425.1436; Found 425.1437.

2-(benzylamino)-5-((4-methoxyphenyl)thio)-4-(quinolin-3-yl) nicotinonitrile (19).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 67% yield (317 mg, 0.67 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.73 (s, 1H), 8.37 (s, 1H), 8.04 (d, J = 8.6 Hz, 1H), 7.85 (d, J = 1.8 Hz, 1H), 7.65 – 7.60 (m, 2H), 7.45 – 7.40 (m, 1H), 7.24 – 7.17 (m, 4H), 7.16 – 7.11 (m, 1H), 6.76 (d, J = 8.6 Hz, 2H), 6.46 (d, J = 8.6 Hz, 2H), 5.84 – 5.75 (m, 1H), 4.61 (d, J = 5.5 Hz, 2H), 3.51 (d, J = 2.6 Hz, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.9, 158.1, 157.3, 153.4, 149.1, 147.6, 137.8, 136.1, 132.2, 130.3, 129.1, 128.6, 128.5, 128.5, 128.0, 127.6, 127.4, 127.4, 126.9, 126.7, 125.7, 119.5, 119.7, 115.4, 114.5, 92.8, 55.0, 45.2. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₉H₂₃N₄OS 475.1592; Found 475.1594.

2-(benzylamino)-5-((4-methoxyphenyl)thio)-4-(naphthalen-2-yl) nicotinonitrile (21).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 94% yield (444 mg, 0.94 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.25 (s, 1H), 7.77 – 7.72 (m, 2H), 7.67 (d, J = 7.9 Hz, 1H), 7.55 (s, 1H), 7.42 – 7.35 (m, 2H), 7.25 – 7.05 (m, 6H), 6.83 (d, J = 8.6 Hz, 2H), 6.52 (d, J = 8.6 Hz, 2H), 5.59 (t, J = 5.5 Hz, 1H), 4.61 (d, J = 5.5 Hz, 2H), 3.57 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.2, 158.1, 156.6, 143.3, 138.1, 133.4, 132.9, 132.8, 128.8, 128.7, 128.5, 128.4, 128.2, 128.0, 127.8, 127.7, 127.6, 127.0, 126.9, 126.7, 126.4, 126.1, 125.9, 120.1, 116.0, 114.7, 93.1, 55.3, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₃₀H₂₄N₃OS 474.1640; Found 474.1603.

2-(benzylamino)-4-methyl-5-(phenylthio)nicotinonitrile (27).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 93% yield (308.2 mg, 0.93 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.33 (s, 1H), 7.30 – 7.26 (m, 4H), 7.25 – 7.20 (m, 1H), 7.19 – 7.14 (m, 2H), 7.09 – 7.05 (m, 1H), 7.00 – 6.96 (m, 2H), 5.56 (t, J = 5.5 Hz, 1H), 4.66 (d, J = 5.5 Hz, 2H), 2.40 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.7, 158.6, 157.0, 138.0, 136.9, 129.1, 128.7, 127.6, 127.7, 127.6, 126.9, 125.9, 116.5, 115.8, 93.5, 45.4, 19.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₀H₁₈N₃S 332.1221; Found 332.1218.

2-(benzylamino)-5-((4-methoxyphenyl)thio)-4-methyl nicotinonitrile (28).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 85% yield (307.2 mg, 0.85 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.35 (s, 1H), 7.39 – 7.29 (m, 5H), 7.17 – 7.13 (m, 2H), 6.85 – 6.81 (m, 2H), 5.56 (t, J = 5.5 Hz, 1H), 4.73 (d, J = 5.5 Hz, 2H), 3.79 (s, 3H), 2.50 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.8, 158.4, 157.3, 155.8, 138.1, 130.8, 128.8, 127.7, 127.6, 126.5, 118.9, 115.9, 114.9, 93.4, 55.4, 45.4, 19.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₁H₂₀N₃OS 362.1327; Found 362.1326.

2-(benzylamino)-4-methyl-5-((4-nitrophenyl)thio)nicotinonitrile (29).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 99% yield (372.7 mg, 0.99 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.42 (s, 1H), 8.11 (d, J = 8.8 Hz, 2H), 7.42 – 7.32 (m, 5H), 7.09 (d, J = 8.8 Hz, 2H), 5.78 (t, J = 5.5 Hz, 1H), 4.78 (d, J = 5.5 Hz, 2H), 2.49 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.4, 159.2, 157.3, 147.3, 145.5, 137.7, 128.9, 128.8, 127.7, 125.3, 124.3, 115.3, 113.3, 93.9, 45.5, 19.4. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₀H₁₇N₄O₂S 377.1072; Found 377.1069.

2-(benzylamino)-4-methyl-5-((2,2,2-trifluoroethyl)thio) nicotinonitrile (30).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 72% yield (242.9 mg, 0.72 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.45 (s, 1H), 7.39 – 7.29 (m, 5H), 5.67 (t, J = 5.5 Hz, 1H), 4.73 (d, J = 5.5 Hz, 2H), 3.19 (q, J = 9.7 Hz, 2H), 2.63 (s, 3H). ¹⁹F{¹H} NMR (565 MHz, CDCl₃): δ_F −66.1. ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.0, 158.7, 156.7, 137.9, 128.7, 127.7, 127.6, 126.1, 124.3, 116.5, 115.6, 93.0, 45.3, 39.0 (q, J = 32.1 Hz), 19.4. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₁₆H₁₅F₃N₃S 338.0939; Found 338.0939. X-ray available (Fig. S17).

2-(benzylamino)-5-(decylthio)-4-methylnicotinonitrile (31).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 53% yield (224.5 mg, 0.53 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.35 (s, 1H), 7.38 – 7.28 (m, 5H), 5.52 (t, J = 5.5 Hz, 1H), 4.72 (d, J = 5.5 Hz, 2H), 2.66 (t, J = 7.3 Hz, 2H), 2.60 (s, 3H), 1.53 (quint, J = 7.7 Hz, 2H), 1.42 – 1.24 (m, 18H), 0.90 (t, J = 7.1 Hz, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.1, 157.3, 155.6, 138.2, 128.7, 127.6, 127.5, 119.3, 116.0, 93.0, 45.2, 36.3, 31.9, 29.6, 29.5, 29.4, 29.3, 29.2, 29.1, 28.6, 22.6, 19.5, 14.1. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₆H₃₈N₃S 424.2788; Found 424.2788.

2-(benzylamino)-5-chloro-4-methylnicotinonitrile (33).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 88% yield (226.8 mg, 0.88 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.19 (s, 1H), 7.38 – 7.28 (m, 5H), 5.49 (br s, 1H), 4.69 (d, J = 5.7 Hz, 2H), 2.49 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 157.2, 150.8, 149.8, 138.2, 128.7, 127.7, 127.6, 120.1, 115.5, 93.3, 45.4, 18.7. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₁₄H₁₃ClN₃ 258.0800; Found 258.0801. X-ray available (Fig. S18).

2-(benzylamino)-4,5-diphenylnicotinonitrile (35).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 86% yield (310 mg, 0.86 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.41 (s, 1H), 7.49 – 7.45 (m, 2H), 7.44 – 7.40 (m, 2H), 7.39 – 7.32 (m, 4H), 7.26 – 7.21 (m, 5H), 7.07 – 7.02 (m, 2H), 5.79 (t, J = 5.5 Hz, 1H), 4.85 (d, J = 5.5 Hz, 2H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 158.0, 153.0, 152.8, 138.4, 136.6, 135.7, 129.5, 129.3, 128.8, 128.7, 128.4, 128.3, 128.1, 127.7, 127.5, 126.8, 126.1, 116.6, 91.9, 45.4. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₅H₂₀N₃ 362.1657; Found 362.1656. X-ray available (Fig. S19).

2’-(benzylamino)-[3,4’-bipyridine]-3’-carbonitrile (37).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 96% yield (274 mg, 0.96 mmol).¹H NMR (600 MHz, CDCl₃): δ_H 8.80 (d, J = 2.2 Hz, 1H), 8.73 (d, J = 5.0 Hz, 1H), 8.36 (d, J = 5.1 Hz, 1H), 7.94 (dt, J = 8.0, 2.1 Hz, 1H), 7.44 (dd, J = 7.9, 4.8 Hz, 1H), 7.40 – 7.34 (m, 4H), 7.32 – 7.28 (m, 1H), 6.69 (d, J = 5.1 Hz, 1H), 5.81 (t, J = 5.1 Hz, 1H), 4.78 (d, J = 5.7 Hz, 2H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.2, 152.4, 150.8, 150.7, 149.7, 138.2, 135.4, 132.6, 128.7, 127.6, 127.5, 123.4, 116.3, 112.7, 90.0, 45.4. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₁₈H₁₅N₄ 287.1296; Found 287.1292. X-ray available (Fig. S20).

2-(benzylamino)-4-phenylnicotinonitrile (41).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 91% yield (259.7 mg, 0.91 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.33 (d, J = 5.1 Hz, 1H), 7.62 – 7.58 (m, 2H), 7.55 – 7.48 (m, 3H), 7.44 – 7.36 (m, 4H), 7.35 – 7.30 (m, 1H), 6.72 (d, J = 5.1 Hz, 1H), 5.71 (t, J = 4.8 Hz, 1H), 4.79 (d, J = 5.5 Hz, 2H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.2, 154.4, 152.0, 138.4, 136.7, 129.7, 128.8, 128.7, 128.1, 127.7, 127.5, 116.8, 113.1, 90.1, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₁₉H₁₆N₃ 286.1344; Found 286.1343.

2-(benzylamino)-4-(4-methoxyphenyl)nicotinonitrile (42).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 95% yield (299.6 mg, 0.95 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.29 (d, J = 5.3 Hz, 1H), 7.59 – 7.55 (m, 2H), 7.42 – 7.35 (m, 4H), 7.34 – 7.30 (m, 1H), 7.05 – 7.01 (m, 2H), 6.69 (d, J = 5.3 Hz, 1H), 5.67 (t, J = 5.3 Hz, 1H), 4.78 (d, J = 5.5 Hz, 2H), 3.88 (s, 3H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 160.9, 159.3, 154.0, 151.8, 138.5, 129.6, 128.9, 128.7, 127.7, 127.5, 117.2, 114.3, 112.8, 89.7, 55.3, 45.5. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₂₀H₁₈N₃O 316.1450; Found 316.1446.

2-(benzylamino)-4-(4-nitrophenyl)nicotinonitrile (43).

It was prepared following the General Procedure. Purification by flash chromatography EtOAc/Hexanes from 0:100 to 20:80 afforded the desired product in 99% yield (327.0 mg, 0.99 mmol). ¹H NMR (600 MHz, CDCl₃): δ_H 8.42 – 8.35 (m, 3H), 7.75 (d, J = 8.6 Hz, 2H), 7.42 – 7.30 (m, 5H), 6.71 (d, J = 5.1 Hz, 1H), 5.73 (t, J = 5.0 Hz, 1H), 4.79 (d, J = 5.5 Hz, 2H). ¹³C{¹H} NMR (150 MHz, CDCl₃): δ_C 159.1, 152.7, 151.9, 148.5, 142.9, 138.1, 129.3, 128.8, 127.8, 127.7, 124.1, 116.1, 112.6, 89.8, 45.6. HRMS (ESI-FTMS) m/z: [M+H]⁺ Calcd for C₁₉H₁₅N₄O₂ 331.1197; Found 331.1198.