Pd-Catalyzed Allylic Alkylation of gem-Alkyl,Aryl-Disubstituted Allyl

Mar 16, 2018 - (E)-gem-Alkyl,aryl-disubstituted allyl carbonates can react with ketones under Pd catalysis using a commercially available NHC ligand b...
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Letter Cite This: ACS Catal. 2018, 8, 3317−3321

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Pd-Catalyzed Allylic Alkylation of gem-Alkyl,Aryl-Disubstituted Allyl Reagents with Ketones: Diastereoselective Construction of Vicinal Tertiary and Quaternary Carbon Centers Fei-Le Yu,¶,† Da-Chang Bai,¶,†,‡ Xiu-Yan Liu,¶,† Yang-Jie Jiang,†,∥ Chang-Hua Ding,*,† and Xue-Long Hou*,†,§ †

State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China ‡ School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan Province 453007, China § Shanghai−Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: (E)-gem-Alkyl,aryl-disubstituted allyl carbonates can react with ketones under Pd catalysis using a commercially available NHC ligand by suppressing the β-H elimination. Ketones with α-tertiary, β-quaternary carbon stereocenters were produced in high yields with high regio- and diastereoselectivities. Kinetic resolution of the reaction product via CBS reduction afforded the optically active ketone as well as the alcohol with high enantioselectivity (S-factor = 35). The utility of the methodology was also demonstrated by transformations of the reaction products. KEYWORDS: palladium catalysis, allylic alkylation, regioselectivity, diastereoselectivity, vicinal tertiary and quaternary stereocenters

P

Scheme 1. Different Modes for the Construction of Carbon Stereocenters in Ketones

alladium-catalyzed allylic alkylation reactions (Pd-catalyzed AA), an important protocol in organic synthesis, have attracted great attention since their discovery approximately 50 years ago.1 Currently, unstabilized carbanions can be used as the nucleophile,2−4 and two vicinal carbon stereocenters can be formed.2c,4c,f,i,5 Ketones with vicinal carbon stereocenters are an important subunit in organic synthesis.6 Several strategies have been developed to use ketones as nucleophiles in Pd-catalyzed AA,2,4b,c,f,i and two carbon stereocenters can be installed in the products.2c,4c,f,i However, only vicinal tertiary/tertiary carbon centers could be established. The formation of vicinal tertiary/ quaternary carbon stereocenters in ketones have not been reported; however, methods of installing quaternary carbon centers at the α-position of ketones are known (Scheme 1).2b,f,g,4b,7,8 Furthermore, 1,1-disubstituted allyl reagents have to be used to construct quaternary carbon stereocenters at the β-position of ketones,7 which should be more challenging in Pd-catalyzed AA reactions. A method for generating ketones bearing vicinal tertiary and quaternary carbon stereocenters via Pd-catalyzed AA reactions remains to be explored. We have achieved regiocontrol in Pd-catalyzed AA reactions of monosubstituted allyl reagents with different nucleophiles,4 which generates the quaternary carbon in the allyl fragment,9b and the method can generate vicinal carbon stereocenters in ketones and carboxylic acid derivatives.4c,e Recently, we found that regioselectivity could also be controlled by simply using a © XXXX American Chemical Society

commercially available N-heterocyclic carbene (NHC) as the ligand in the Pd-catalyzed AA of acyclic ketones with monosubstituted allyl reagents.4i As this is a simple and a facile way to control the regioselectivity in the Pd-catalyzed AA, further investigations have been conducted to determine the Received: December 18, 2017 Revised: March 8, 2018

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DOI: 10.1021/acscatal.7b04313 ACS Catal. 2018, 8, 3317−3321

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ACS Catalysis utility of this strategy. In this Letter, we report our new finding that 1-alkyl-1-aryl disubstituted allyl reagents can be successfully used in Pd-catalyzed AA reactions by suppressing the β-H elimination based upon the understanding of β-H elimination mechanisms and afford ketones with α-tertiary-β-quaternary carbon stereocenters in high yields with excellent regio- and diastereoselectivities (Scheme 1). Optically active allylic alkylated products were obtained by kinetic resolution. Pd/NHC-catalyzed AAs of ketones with monosubstituted allyl reagents proceeds via the oxygen of the ketone enolate attacking the palladium of the Pd-π-allyl complex followed by a [3,3′]-reductive elimination.4i We reasoned that a quaternary carbon stereocenter could be installed using terminal disubstituted allyl compounds through this process because the two substituents should be far from the Pd in the transition state the steric hindrance between the allyl reagent and the nucleophile should be avoided. To test this idea, the reaction of (Z)-tert-butyl (3-phenylbut2-en-al)carbonate (2a) with cyclohexanone (1a) as the nucleophile in the presence of [Pd(η3-C3H5)Cl]2 and an imidazolium salt (IPr·HCl (L1)) was tested. However, only a trace amount of allylated product 3a was observed, and diene 4 was detected as the major component and was generated through the β-H elimination of allyl (Z)-2a (eq 1) in 72% yield.

observed when (E)-tert-butyl (3-phenylbut-2-en-al)carbonate (2a) was reacted with ketone 1a under the above reaction conditions (Table 1, entry 1). Table 1. Influence of the Reaction Parameters on the PdCatalyzed Reaction of Cyclohexanone (1a) with Allyl Substrate (E)-2aa

β-H Elimination reactions are a general process in transitionmetal chemistry and might be one of the major reasons that prevent the use of alkyl-substituted allyl reagents in Pdcatalyzed AA reactions. Three mechanisms have been proposed for the β-H elimination reactions (Scheme 2a−c).10 Because

entry

L

solvent

3a+5a (%)b

3a/5ac

3a anti/ sync

2a (%)b

4 (%)b

1d,e 2d 3d,g 4 5 6 7 8 9 10 11 12 13 14h 15i 16i,j

L1 L1 L1 L1 L2 L3 L4 L5 L5 L5 L5 L5 L5 L5 L5 L5

THF THF THF THF THF THF THF THF hexane toluene Et2O DME dioxane dioxane dioxane dioxane

49 69 69 78 12 27 25 81 90 88 78 67 81 95 86 86

96/4 95/5 93/7 95/5 0/100 47/53 60/40 91/9 63/37 60/40 54/46 63/37 92/8 90/10 96/4 96/4

84/16 93/7 89/11 87/13 45/55 70/30 91/9 53/47 71/29 82/18 87/13 91/9 86/14 98/2 98/2

26 15 6 0 61 33 43 0 0 0 0 0 0 0 0 0

-f 7 16 40 28 10 3 0 0

a Reaction conditions: 1a/LDA/(E)-2a/[Pd(η3-C3H5)Cl]2/Ligand = 200/200/100/2.5/5; 0.1 M of (E)-2a. bYield determined by 1H NMR using mesitylene as an internal reference. cDetermined by 1H NMR of the crude product. d1a/LDA/(E)-2a = 150/150/100. eReaction performed at rt. fNot determined. gConducted at reflux. h1a/LDA/ (E)-2a/Pd(OAc)2/S-IPr·HBF4 = 200/200/100/5/5. i1a/LDA/(E)2a/Pd(OAc)2/S-IPr·HBF4 = 150/150/100/5/5; reaction time: 12 h. j Reaction time: 22 h.

Scheme 2. Reported Mechanisms for β-H Elimination Processes

To improve the efficiency of the reaction, the reaction conditions were optimized by varying the reaction parameters (Table 1). Both the yield and the dr increased significantly (to 69% and 93/7, respectively) when the reaction temperature was elevated to 50 °C (entry 2), and similar results were obtained when the reaction was conducted in refluxing THF (entry 3). A further increase in the yield to 78% was realized by changing the ratio of 1a/2a from 1.5/1 to 2.0/1; however, the diastereoselectivity of 3a decreased to 83/17, and β-H elimination product 4 was observed (entry 4 vs entry 2). It was found that the structure of the NHC ligand substantially influenced the efficiency of this reaction. A very low yield of linear product 5a was obtained using the 1,3-diphenyl NHC derived from L2 (entry 5), while a lower diastereoselectivity accompanied by some diene 4 were afforded if N,N’-bis-2,4,6trimethyl phenyl-substituted NHC L3 and bulkier aryl substituted L4 were the ligands (entries 6 and 7). However, a high yield of 3a with high regio- and diastereoselectivities was obtained by using S-IPr·HCl L5, an NHC ligand with an imidazoline ring, and the yield of the desired product was 81%

our reaction proceeded under basic conditions, the β-H elimination reaction might occur from structure (d) via mechanism c, which would give anti-elimination (Scheme 2), and form 4 from (Z)-2a. If this were the case, this β-H elimination process should be suppressed by the use of (E)-tertbutyl (3-phenylbut-2-en-al)carbonate (2a) instead because (e) would be formed instead of (d). Desired branched product 3a with vicinal tertiary and quaternary carbon centers and linear allylation product 5a were obtained in 49% yield (branched/ linear (B/L) ratio being 96/4 and dr being 84/16) along with the recovery of 26% of 2a, and the β-H elimination process was efficiently suppressed. No β-H elimination product 4 was 3318

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ACS Catalysis with a B/L ratio of 91/9 and a dr of 91/9 (entry 8 vs entry 4). The studies on the influence of solvent on the reaction showed that nonpolar solvents such as n-hexane and toluene delivered poor regio- and diastereoselectivities (entries 9 and 10), and Et2O and DME gave lower regioselectivity (entries 11 and 12), while 1,4-dioxane provided the same yield and selectivities as THF (entry 13 vs entry 8). Because Cl− ions will accelerate the π−σ−π process of the Pd-π-allyl complex11 even though the interconversion between intermediates d and e should occur via this process,1 we deduced that the efficiency of the reaction may be improved by using (E)-allyl reagents if the reaction proceeds in the absence of Cl− ion; thus, this will reduce the rate of the π−σ−π process and reduce the amount of intermediate (d) formed (Scheme 2). Furthermore, alkyl-substituted allyl reagents, challenging substrates for Pd-catalyzed AA reactions, could be used if the βH elimination process is suppressed under Cl− ion-free conditions. With this idea in mind, further investigations were conducted by changing the Pd species and the counterions of L5. It was found that the combination of Pd(OAc)2 with S-IPr·HBF4 and t-BuOK was the optimal choice. The allylated products were obtained in 95% yield with a 90/10 B/L and an 86/14 dr, and the yield of diene 4 was 3% (entry 14 vs entry 8, Table 1). By changing the mol ratio of 1a/LDA/(E)-2a from 200/200/100 to 150/150/100, the yield decreased from 95% to 86%, but the B/L ratio (96/4) and the dr (98/2) of 3a were improved significantly (entry 15 vs entry 14, Table 1). In addition, extending the reaction time from 12 to 22 h did not affect the yield or regio- and diastereoselectivities of the reaction (entry 16 vs entry 15, Table 1). To our delight, excellent results were obtained when (E)-ethyl,phenyl-substituted allyl 2b was used. The yield of product 3b was 91% with a B/L ratio of 96/4 and a dr of 99/1 (Table 2). Under these reaction conditions, the scope of the Pdcatalyzed reaction of cyclic ketones 1 with (E)-1-alkyl-1-aryldisubstituted allyl substrates 2 was investigated. As shown in Table 2, various alkyl substituent-containing allyl reagents 2 and cyclic ketones with rings of various sizes were suitable for the reaction and provided corresponding products 3a−o with vicinal tertiary and quaternary carbon stereocenters in high yields with excellent regio- and diastereoselectivities (B/L: 90/ 10−96/4 and dr: ≥ 98/2−89/11). The allyl substrates 2 with an aryl ring having either electron-donating or electronwithdrawing substituents (Me, OMe and CF3) at the p- or m-positions reacted smoothly with ketone 1a to efficiently afford allylated products 3g−j. 2-Furyl-substituted allyl substrate 2k was also an excellent electrophile and afforded corresponding product 3k. It was noted that when the alkyl substituent of allyl 2 became bulkier, the reaction yield decreased due to lower conversion (3e vs 3f). The reaction was fully suppressed if iso-propyl or ortho-methyl phenylsubstituted allyls 2 were used as the electrophiles (2l and 2m). Additionally, the reaction of tert-butyl (3-methylbut-2-en-1-yl) carbonate with 1-tetralone afforded isoprene derived from the β-H elimination accompanied by a trace amount of allylated products (for details, see Supporting Information). In addition to cyclohexanone (1a), cycloheptanone, cyclooctanone, 1tetralone, and 1-benzosuberone were competent nucleophiles for this reaction to lead the desired allylated products (3l−o) in high yields with excellent stereoselectivities. No reaction occurred when cyclopentanone was used as the substrate. Acyclic ketones such as 1-phenylbutan-1-one and pentan-3-one

Table 2. Substrate Scope of the Pd-Catalyzed Reaction of Cyclic Ketones 1 with 1,1-Disubstituted Allyl Substrates 2a

a

Reaction conditions: 1/LiHMDS/2/Pd(OAc)2/S-IPr·HBF4 = 150/ 150/100/5.0/5.0; B/L and dr was determined by 1H NMR; isolated yield. bReaction with cyclohexanone (1a). cReaction with (E)-2a.

were also not well-tolerated in this allylic alkylation reaction as they afforded the corresponding allylated products in very low yields with low regio- and diastereoselectivities (3p and 3q). The reaction of allyl 2a with cyclohexanone (1a) was also studied using chiral NHCs. Unfortunately, the enantioselectivity was not satisfactory, and only 16% ee was obtained for product 3a (for details, see SI). However, 44% yield of optically active 3a (61% ee) and 40% yield of reduced product 6 (90% ee) were obtained by kinetic resolution of allylation product 3a via CBS reduction (S-factor = 35, eq 2),12 which provides an alternative approach to preparing optically active 3. The absolute configuration of (+)-3a was determined to be (R,R) (see SI).

Some transformations of product 3a were carried out (eq 3). Allylated product 3a was diastereoselectively reduced by L3319

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ACS Catalysis

allyl metal complexes. In Comprehensive Organic Synthesis, 2nd ed., Vol. 4; Molander, G. A., Knochel, P., Eds; Elsevier: Oxford, 2014; pp 648− 698. (2) Some examples using ketones in Pd-catalyzed AA reaction: (a) Trost, B. M.; Self, C. R. On the palladium-catalyzed alkylation of silyl-substituted allyl acetates with enolates. J. Org. Chem. 1984, 49, 468−473. (b) Trost, B. M.; Schroeder, G. M. Palladium-catalyzed asymmetric alkylation of ketone enolates. J. Am. Chem. Soc. 1999, 121, 6759−6760. (c) Braun, M.; Laicher, F.; Meier, T. Diastereoselective and enantioselective palladium-catalyzed allylic substitution with nonstabilized ketone enolates. Angew. Chem., Int. Ed. 2000, 39, 3494−3497. (d) Burger, E. C.; Tunge, J. A. Asymmetric allylic alkylation of ketone enolates: an asymmetric Claisen surrogate. Org. Lett. 2004, 6, 4113−4115. (e) Behenna, C.; Stoltz, B. M. The enantioselective Tsuji allylation. J. Am. Chem. Soc. 2004, 126, 15044− 15045. (f) Trost, B. M.; Xu, J. Palladium-catalyzed asymmetric allylic α-alkylation of acyclic ketones. J. Am. Chem. Soc. 2005, 127, 17180− 17181. (g) Bélanger, É.; Cantin, K.; Messe, O.; Tremblay, M.; Paquin, J.-F. Enantioselective Pd-catalyzed allylation reaction of fluorinated silyl enol ethers. J. Am. Chem. Soc. 2007, 129, 1034−1035. (h) Zhao, X.; Liu, D.; Guo, H.; Liu, Y.; Zhang, W. C−N bond cleavage of allylic amines via hydrogen bond activation with alcohol solvents in Pdcatalyzed allylic alkylation of carbonyl compounds. J. Am. Chem. Soc. 2011, 133, 19354−19357. (3) Some examples using other unstabilized carbanions: (a) Weiß, T. D.; Helmchen, G.; Kazmaier, U. Synthesis of amino acid derivatives via enantio- and diastereoselective Pd-catalyzed allylic substitutions with a non-stabilized enolate as nucleophile. Chem. Commun. 2002, 1270− 1271. (b) Ibrahem, I.; Córdova, A. Direct catalytic intermolecular αallylic alkylation of aldehydes by combination of transition-metal and organocatalysis. Angew. Chem., Int. Ed. 2006, 45, 1952−1956. (c) Trost, B. M.; Thaisrivongs, D. A. Palladium-catalyzed regio-, diastereo-, and enantioselective benzylic allylation of 2-substituted pyridines. J. Am. Chem. Soc. 2009, 131, 12056−12057. (d) Sha, S. C.; Zhang, J.; Carroll, P. J.; Walsh, P. J. Raising the pKa limit of “soft” nucleophiles in palladium-catalyzed allylic substitutions: application of diarylmethane pronucleophiles. J. Am. Chem. Soc. 2013, 135, 17602− 17609. (e) Tao, Z. L.; Zhang, W. Q.; Chen, D. F.; Adele, A.; Gong, L. Z. Pd-catalyzed asymmetric allylic alkylation of pyrazol-5-ones with allylic alcohols: the role of the chiral phosphoric acid in C−O bond cleavage and stereocontrol. J. Am. Chem. Soc. 2013, 135, 9255−9258. (f) Mao, J.; Zhang, J.; Jiang, H.; Bellomo, A.; Zhang, M.; Gao, Z.; Dreher, S. D.; Walsh, P. J. Palladium-catalyzed asymmetric allylic alkylations with toluene derivatives as pronucleophiles. Angew. Chem., Int. Ed. 2016, 55, 2526−2530. (4) (a) You, S.-L.; Hou, X.-L.; Dai, L.-X.; Zhu, X.-Z. Highly efficient ligands for palladium-catalyzed asymmetric alkylation of ketone enolates. Org. Lett. 2001, 3, 149−151. (b) Yan, X. X.; Liang, C. G.; Zhang, Y.; Hong, W.; Cao, B. X.; Dai, L. X.; Hou, X. L. Highly enantioselective Pd-catalyzed allylic alkylations of acyclic ketones. Angew. Chem., Int. Ed. 2005, 44, 6544−6546. (c) Zheng, W.-H.; Zheng, B.-H.; Zhang, Y.; Hou, X.-L. Highly regio-, diastereo-, and enantioselective Pd-catalyzed allylic alkylation of acyclic ketone enolates with monosubstituted allyl substrates. J. Am. Chem. Soc. 2007, 129, 7718−7719. (d) Zhang, K.; Peng, Q.; Hou, X.-L.; Wu, Y. D. Highly enantioselective palladium-catalyzed alkylation of acyclic amides. Angew. Chem., Int. Ed. 2008, 47, 1741−1744. (e) Chen, J.-P.; Ding, C.-H.; Liu, W.; Hou, X.-L.; Dai, L.-X. Palladium-catalyzed regio-, diastereo-, and enantioselective allylic alkylation of acylsilanes with monosubstituted allyl substrates. J. Am. Chem. Soc. 2010, 132, 15493− 15495. (f) Chen, J.-P.; Peng, Q.; Lei, B.-L.; Hou, X.-L.; Wu, Y.-D. Chemo- and regioselectivity-tunable Pd-catalyzed allylic alkylation of imines. J. Am. Chem. Soc. 2011, 133, 14180−14183. (g) Chen, T. G.; Fang, P.; Dai, L. X.; Hou, L. H. Palladium-catalyzed asymmetric allylic alkylation reaction of 2-mono-substituted indolin-3-ones. Synthesis 2015, 47, 134−140. (h) Li, X. H.; Wan, S. L.; Chen, D.; Liu, Q. R.; Ding, C. H.; Fang, P.; Hou, X. L. Enantioselective construction of quaternary carbon stereocenter via palladium-catalyzed asymmetric allylic alkylation of lactones. Synthesis 2016, 48, 1568−1572. (i) Bai,

selectride to give alcohol 6 in 92% yield, which was then cyclized in the presence of NBS to furnish product 7 with a fully substituted tetrahydrofuran ring in 79% yield. Dihydroxylation/periodate cleavage of 3a followed by reduction and reductive amination gave bicyclic pyrrolidine 8 in 63% overall yield. The reaction can also be conducted on a gram scale. The reaction of allyl 2a (1.25 g) with cyclohexanone (0.75 g) proceeded smoothly with 2.5 mol % Pd(OAc)2 and S-IPr· HBF4 as the catalyst to produce 0.98 g of product 3a in 86% yield with 97/3 B/L and 96/4 dr. The present work demonstrates that β-H elimination could be suppressed by changing the configuration of the allyl substrates, and alkyl-substituted allyl reagents could then be successfully applied in Pd-catalyzed AA reactions. Vicinal tertiary and quaternary stereocenters in ketones were established in excellent regio- and diastereoselectivities by a simple and efficient method using a commercially available NHC ligand. The utility of this protocol has been demonstrated by the conversion as well as the kinetic resolution of the allylic alkylation products. This study offers a new platform for Pdcatalyzed AA reactions. Further investigations on an asymmetric variant along with other aspects of the aforementioned methodology are in progress.

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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/acscatal.7b04313. X-ray data of (±)-3a (CIF) X-ray data of 9 (CIF) Optimization data, experimental procedures, NMR and HPLC spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Da-Chang Bai: 0000-0001-7342-2966 Xue-Long Hou: 0000-0003-4396-3184 Author Contributions ¶

(F.-L.Y., D.-C.B., X.-Y.L.) These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was received from the National Natural Science Foundation of China (NSFC) (21532010, 21472214, 21772215, and 21421091), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20030100), the Chinese Academy of Sciences, the Technology Commission of Shanghai Municipality, and the Croucher Foundation of Hong Kong.

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REFERENCES

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Letter

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DOI: 10.1021/acscatal.7b04313 ACS Catal. 2018, 8, 3317−3321