Enantioselective Allylic Alkylation with 4-Alkyl-1,4-dihydro-pyridines

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Enantioselective Allylic Alkylation with 4‑Alkyl-1,4-dihydropyridines Enabled by Photoredox/Palladium Cocatalysis Hong-Hao Zhang, Jia-Jia Zhao, and Shouyun Yu* State Key Laboratory of Analytical Chemistry for Life Science, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China

J. Am. Chem. Soc. 2018.140:16914-16919. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/19/18. For personal use only.

S Supporting Information *

Table 1. Optimization of Reaction Conditionsa

ABSTRACT: Highly regio- and enantioselective allylic alkylation has been achieved enabled by the merger of photoredox and palladium catalysis. In this dual catalytic process, alkyl radicals generated from 4-alkyl-1,4-dihydropyridines act as the coupling partners of the π-allyl palladium complexes. The generality of this method has been illustrated through the reaction of a variety of allyl esters with 4-alkyl-1,4-dihydropyridines. This mechanistically novel strategy expands the scope of the traditional Pd-catalyzed asymmetric allylic alkylation reaction and serves as its alternative and potential complement.

P

alladium-catalyzed asymmetric allylic alkylation (AAA) reactions, pioneered by Trost,1 are among the most powerful protocols with which to construct carbon−carbon bond enantioselectively (Figure 1A).2 To date, various “soft”, or stabilized, nucleophiles (pKa < 25), including malonates, acetoacetates, and enolates, have been used in Pd-catalyzed AAA reactions (Figure 1Aa).2 Conversely, the high reactivity of “hard”, or nonstabilized, alkyl nucleophiles (pKa > 25), such as organic compounds containing main group metals, has

entry

PC

ligand

solvent

yield/%b

ee/%c

B/Lb

1 2 3 4 5 6 7 8 9 10 11e 12 13f

I I I I I I II III I I I − I

L1 L2 L3 L4 L4′ L5 L5 L5 L5 L5 L5 L5 −

THF THF THF THF THF THF THF THF DMF CH3CN CH3CN CH3CN CH3CN

68 66 60 50 48 44 trace 41 40 76 (72d) 0 trace trace

12 28 84 90 −90 90 − 90 88 94 − − −

84:16 87:13 92:8 86:14 86:14 92:8 − 86:14 85:15 >95:5 − − −

a

Reaction conditions: a solution of 1a (0.1 mmol), 2a (0.15 mmol), Pd2(dba)3 (2.5 mol %), ligand (6 mol %), and photocatalyst (2 mol %) in the indicated solvent (2.0 mL) was irradiated by a 45 W blue LEDs for 12 h. bDetermined by GC. cEnantiomeric excess (ee) values were determined by HPLC on a chiral stationary phase. dIsolated yields. eln dark. fWithout Pd2(dba)3. PMP = para-methoxyphenyl.

limited their utility in catalytic processes and their compatibility with functional groups.3 Pd-catalyzed AAA reactions with “hard” nucleophiles have been much less developed (Figure 1Ab).4 As a consequence, the development of efficient protocols for the introduction of novel types of nonstabilized nucleophiles to allylic electrophiles with excellent regio- and Figure 1. Pd-catalyzed asymmetric allylic alkylation. LG = leaving group, PC = photocatalyst. © 2018 American Chemical Society

Received: October 6, 2018 Published: November 26, 2018 16914

DOI: 10.1021/jacs.8b10766 J. Am. Chem. Soc. 2018, 140, 16914−16919

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Figure 2. Scope of monosubstituted allyl acetates and 4-alkyl-1,4-dihydropyridines. Reaction conditions: a solution of 1 (0.1 mmol), 2 (0.15 mmol), Pd2(dba)3 (2.5 mol %), ligand L5 (6 mol %), and photocatalyst I (2 mol %) in CH3CN (2.0 mL) was irradiated by 45 W blue LEDs for 12 h. Isolated yields are presented, and ee values were determined by HPLC on a chiral stationary phase. The B/L ratios were determined by 1H NMR. Unless otherwise noted, B/L > 95:5.

To assess whether asymmetric induction could be achieved, we chose the racemic allylic acetate (1a) and 4-benzyl-1,4dihydropyridine (2a) as model substrates. Our investigation into the photoredox/Pd-cocatalyzed AAA reaction began with the evaluation of chiral ligands and photocatalysts and yielded the representative results shown in Table 1. Comprehensive reaction condition optimization is provided in the Supporting Information (SI). When a solution of 1a (1 equiv), 2a (1.5 equiv), and K2CO3 (1.5 equiv) in THF was irradiated by 45 W blue LEDs at room temperature for 12 h in the presence of photocatalyst Ir(ppy) 2(dtbbpy)PF6 (I, 2 mol %) and Pd2(dba)3 (2.5 mol %)/(R)-BINAP (L1, 6 mol %), the allylic alkylation reaction proceeded smoothly, affording the desired product (3aa) in a moderate yield (68% determined by GC) and regioselectivity (Branched/Linear (B/L) = 84:16), but with low enantioselectivity (12% ee, Table 1, entry 1). Increasing the steric bulk of the phosphine substituent improved the enantioselectivity (28% ee for L2 and 84% ee for L3, respectively, entries 2 and 3) and regioselectivity. When MeO-BIPHEP (L4) and GARPHOS (L5) were employed, excellent enantioselectivity but lower reactivity were obtained (entries 4 and 6). Several other photocatalysts, such as Ir(dFCF3ppy)2(dtbbpy)PF6 (II) and Ir(dFMeppy)2(dtbbpy)PF6 (III), were examined, but none of them proved to be superior to Ir(ppy)2(dtbbpy)PF6 (I) (entries 7 and 8). The scope of solvent effect was explored, and it was found that CH3CN was the best solvent (entry 10) in terms of efficiency (76% GC yield and 72% isolated yield), regioselectivity (B/L > 95:5), and enantioselectivity (94% ee). No reaction occurred in dark conditions (entry 11), and both the photocatalyst and

enantioselectivities is highly desirable and remains a synthetic challenge. Photoredox catalysis has recently emerged as a powerful tool in synthetic organic chemistry, providing practical strategies for the preparation of many fine chemicals under mild conditions.5 Furthermore, merging photoredox catalysis with transition metal catalysts,6 such as those containing nickel,7 palladium,8 copper,9 or gold,10 has become a popular strategy for expanding the synthetic utility of visible-light photocatalysis and has led to the discovery of novel reaction modes, which are unfeasible or not easily accessible by a single catalytic system. In this regard, Tunge et al. reported photoredox/Pd cocatalyzed decarboxylative allylation of α-amino and phenylacetic allyl esters.11 Xiao and Lu reported α-allylation of N-aryl amines with allylic esters using a similar catalytic system.12 These elegant reports expanded the scope of allylic alkylation chemistry by the assistance of the radical pathway, but in a racemic manner. Highly enantioselective photoredox/Pd cocatalytic reactions have not been reported to date.13 Herein, we would like to report our efforts on the enantioselective photoredox/Pd-cocatalyzed coupling of allyl esters with 4alkyl-1,4-dihydropyridines14 (Figure 1B). Mechanistically, photo-oxidation of 4-alkyl-1,4-dihydropyridine with an excited photocatalyst gives an alkyl radical (I) and low valent photocatalyst.14 Meanwhile, oxidative addition of an allyl ester to Pd(0) forms a Pd-π-allyl species (II). The alkyl radical (I) is then trapped by a π-allylpalladium complex (II) to generate a Pd(III) complex (III), which undergoes reductive elimination to give the allylic alkylation product (vide infra). The presence of an appropriate chiral ligand may encourage enantioselectivity. 16915

DOI: 10.1021/jacs.8b10766 J. Am. Chem. Soc. 2018, 140, 16914−16919

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Figure 3. Scope of disubstituted allyl acetates. Reaction conditions: a solution of 4 (0.1 mmol), 2a or 2b (0.15 mmol), Pd2(dba)3 (2.5 mol %), ligand L5 (6 mol %), and photocatalyst I (2 mol %) in CH3CN (2.0 mL) was irradiated by 45 W blue LEDs for 12 h. Isolated yields are presented, and ee values were determined by HPLC on a chiral stationary phase. Unless otherwise noted, rr > 95:5 and E/Z > 95:5.

Figure 5. Mechanistic investigations and proposed mechanism.

(3ib and 3nb), remain unchanged in this reaction, demonstrating remarkable functional group tolerance. The substrates bearing an electron-donating group in the phenyl ring led to desired products (3aa, 3ja, and 3la) with excellent regio- and enantioselectivities (>95:5 B/L, 92−96% ee). Allylic acetates containing heterocycles, including thiophene and furan, were also suitable for this transformation and gave the products 3ob and 3pb in good yield (71% and 67%) and enantioselectivity (82% ee). We next investigated the scope of 4-alkyl-1,4-dihydropyridines. As indicated in Figure 2b, this protocol was amenable to dihydropyridines bearing benzyl groups with either electrondonating or electron-withdrawing substituents at different positions on the phenyl ring, leading to the desired products (3ha−3hk) in 58−84% yields with good regio- and enantioselectivities (91:9 to >95:5 B/L, 88−94% ee). The dihydropyridine with a thiophenylmethyl substituent gave the heterocyclic product (3hl). The allylic acetate could be alkylated with the bulkier isopropyl and benzhydryl groups to give 3hm and 3hn, and the more sterically demanding tertiary alkyl radicals could also work as the coupling partner to deliver products such as 3ho with all-carbon quaternary centers with excellent enantioselectivity (92% ee). As examples of nonbenzylic alkyl groups, benzyloxymethyl and N-Boc protected aminomethyl groups were also converted to the corresponding products (3hp and 3hq) in acceptable yields with excellent regio- and enantioselectivities (B/L > 95:5, 92− 98% ee). Asymmetric allylation of a racemic 1,3-disubstituted unsymmetric allylic ester, which would introduce E/Z isomers, presents a synthetic challenge in terms of regio- and

Figure 4. A scaled-up experiment and synthetic application.

palladium catalyst were essential to this reaction (entries 12 and 13). Having identified the optimal reaction conditions, we investigated the scope and limitations of this enantioselective allylic alkylation. As shown in Figure 2a, various racemic aromatic allylic acetates reacted with 4-benzyl-1,4-dihydropyridines under the optimized conditions, and the corresponding allylic alkylation products 3aa−3pb were provided in good yields (52−84%) with excellent regio- and enantioselectivities (B/L > 95:5 and 82−96% ee). Common functional groups, such as alkoxyl (3aa, 3ja, and 3la), halides (3db, 3eb, 3kb, and 3mb), trifluoromethyl (3fb), ester (3gb), amide (3hb), aryl 16916

DOI: 10.1021/jacs.8b10766 J. Am. Chem. Soc. 2018, 140, 16914−16919

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Journal of the American Chemical Society stereoselectivities.15 To address this challenge and expand the scope of this reaction further, disubstituted allyl acetates were explored (Figure 3). Excellent regio-, enantio-, and E/Zselectivities (88−98% ee, 88:12 to >95:5 rr, E/Z > 95:5) were observed for the reactions of 4-benzyl-1,4-dihydropyridines (2a or 2b) with unsymmetrical 1-alkyl-3-phenylallyl acetates. The allylation products 5ab−5ib were produced in this way with yields of 57−80% (Figure 3a). gem-Alkyl, aryldisubstituted allyl acetates also underwent this transformation smoothly, affording products 5jb and 5ka with all-carbon quaternary stereogenic centers with high enantioselectivity (80% ee) (Figure 3b).16 Interestingly, when 1,3-diphenylallyl acetate (4l) was alkylated with 2a under our established conditions with L6 as the ligand, the thermodynamically less stable Z-isomer of 5la was obtained as the major product (86% yield, 9:91 E/Z, and 96% ee). This result has not been observed in Pd-catalyzed AAA reactions. Photochemical isomerization of (E)-5la led to (Z)-5la.17 When an organic dye, Eosin Y, was used as the photocatalyst instead of I, the photochemical isomerization process was not observed and the E-isomer of 5la was isolated as the sole product (61% yield, >95:5 E/Z, 88% ee) (Figure 3c). With the aim of showing the practicality and utility of this photoredox/Pd-cocatalyzed AAA reaction, a gram scale synthesis of 3hb was performed. As shown in Figure 4a, when 5 mmol of 1h was alkylated with 2b under our established conditions, 3hb was obtained in comparative isolated yield with excellent regio- and enantioselectivities (78%, 92% ee, >95:5 B/L). The (R)-configuration of 3hb was established unambiguously by single crystal X-ray diffraction analysis.18 The remaining products of this reaction could be assigned the same stereochemistry based on the assumption that all the reactions proceed through a similar pathway. The key intermediate (5br) for the asymmetric synthesis of a natural estrogenic metabolite (S)-Equol19 could be synthesized with this method in good yield (60%) and enantioselectivity (92% ee) (Figure 4b). Control experiments were conducted to gain some insight into the mechanism of this reaction. The isomeric allylic acetates 1a and 1q reacting with 2a gave similar results (Figure 5a). This phenomenon suggests that π-allylpalladium complex might be the key intermediate of this reaction.2 When the reaction of the chiral allylic acetate (S)-4m was performed with (R)-L4 or (S)-L4′ as the ligand, the product 5ab with the opposite configurations was obtained (Figure 5b). The stereochemistry of the product is controlled by the chiral ligand. Based on these observations and previous published work on photoredox/Pd cocatalysis,8,11,12 a plausible mechanism is proposed, as shown in Figure 5c. The reductive quenching process of the visible-light excited Ir(III)* by dihydropyridine 2a leads to an alkyl radical 7 together with a low-valent Ir(II) complex.14 The dimer of 7 could be detected by GC-MS, and this is evidence for the existence of the radical 7. Meanwhile, oxidative addition of an allyl ester (1a) to Pd(0) gives a Pd-π-allyl species (9). The alkyl radical 7 is then trapped by a π-allylpalladium complex (9) to generate the Pd(III) complex (12).8b,c,g,n This Pd(III) complex can undergo reductive elimination to give an allylic alkylation product (3aa) and a Pd(I) species (13).8f,20 Finally reduction of 13 by the Ir(II) complex regenerates Ir(III) and Pd(0) closing both catalytic cycles. Competitively, the Pd(II) species (9) can also be reduced to a Pd(I) complex by Ir(II) and can then dissociate into Pd(0) and an allylic radical (10). The

mixture of dimers of 10 could also be detected by GC-MS. Alternatively, the Pd(III) intermediate (12) can also be reduced by Ir(II) to give a Pd(II) species. Reductive elimination of the Pd(II) species delivers the final product (3a).8h,o In summary, we have described the first photoredox/Pdcocatalyzed enantioselective coupling of allyl esters with 4alkyl-1,4-dihydropyridines. Alkyl radicals generated from 4alkyl-1,4-dihydropyridines act as coupling partners with the πallyl palladium complexes. The generality of this method is illustrated through the reaction of a variety of allyl esters with 4-alkyl-1,4-dihydropyridines. This mechanistically novel strategy expands the scope of the traditional Pd-catalyzed AAA reactions and serves as their alternative and potential complement. This novel synergistic photoredox/Pd catalytic system will be transferable to other transformations, providing new opportunities for asymmetric metallaphotoredox catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b10766. Experimental details, NMR spectra, and details of experiments (PDF) Crystallographic data for 3hb (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Shouyun Yu: 0000-0003-4292-4714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21672098, 21732003), the National Key Research and Development Program of China (2018YFC0310900), and the Fundamental Research Funds for the Central Universities (020814380092) is acknowledged.

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REFERENCES

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DOI: 10.1021/jacs.8b10766 J. Am. Chem. Soc. 2018, 140, 16914−16919

Communication

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DOI: 10.1021/jacs.8b10766 J. Am. Chem. Soc. 2018, 140, 16914−16919