Acyclic Quaternary Carbon Stereocenters via Enantioselective

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Acyclic Quaternary Carbon Stereocenters via Enantioselective Transition Metal Catalysis Jiajie Feng, Michael Holmes, and Michael J. Krische* Department of Chemistry, Welch Hall (A5300), University of Texas at Austin, 105 East 24th Street, Austin, Texas 78712, United States ABSTRACT: Whereas numerous asymmetric methods for formation of quaternary carbon stereocenters in cyclic systems have been documented, the construction of acyclic quaternary carbon stereocenters with control of absolute stereochemistry remains a formidable challenge. This Review summarizes enantioselective methods for the construction of acyclic quaternary carbon stereocenters from achiral or chiral racemic reactants via transition metal catalysis.

CONTENTS 1. Introduction and Historical Perspective 2. Acyclic Quaternary Carbon Stereocenters 2.1. Electrophilic SN2′ Allylation of Nonstabilized Carbanions 2.2. Conjugate Addition 2.3. Nucleophilic Allylation 2.4. Alkene Functionalization 2.5. Functionalization of Stabilized Carbon Nucleophiles 2.6. Desymmetrization Reactions 3. Conclusion and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References Note Added in Proof

chemists at Schering AG through the desymmetrization of the indicated cyclopentanedione via biocatalytic reduction (Scheme 1, top).14 A related desymmetrization to form a quaternary carbon stereocenter is represented by the Hajos−Parrish− Eder−Sauer−Wiechert reaction, which was reported shortly thereafter (Scheme 1, middle).15,16 Subsequent to these pioneering efforts, numerous methods for the asymmetric synthesis of stereogenic quaternary carbon centers were reported in the literature, including methods beyond desymmetrization. The emergence of enantioselective transition metal catalysis accelerated progress in this area by broadening the palette of C−C bond-forming processes. The first enantioselective transition-metal-catalyzed formation of a quaternary carbon stereocenter was reported in 1978 by Nakamura and Otsuka, who described the cobalt-catalyzed cyclopropanation of 1,1-disubstituted olefins (Scheme 1, bottom).17,18 Enantioselective transition metal catalysis continued to play a key role in this area of research in the following decades.5,9 While substantial progress has been made generating quaternary carbon stereocenters within rings, the creation of acyclic quaternary carbon stereocenters represents a more daunting challenge due to enhanced conformational mobility.19 Because of remarkable advances in the field of transition metal catalysis, organic chemists have begun to address this challenge. Here, we review enantioselective methods for the construction of acyclic quaternary carbon stereocenters from achiral or chiral racemic reactants via transition metal catalysis. Literature is organized on the basis of reaction type. Strategies involving desymmetrization, which encompass diverse reaction types, are treated separately. Diastereoselective formation of acyclic quaternary carbon stereocenters via substrate-directed asymmetric induction is not discussed.6,7,19 Reactions in which transition metals simply serve as chiral Lewis acids are cited, but not explicitly depicted. Catalytic enantioselective formation of cyclic quaternary carbon stereocenters followed by ring opening

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1. INTRODUCTION AND HISTORICAL PERSPECTIVE Quaternary carbon stereocenters occur frequently in natural products, semisynthetic and synthetic pharmaceutical and agrochemical ingredients (Figure 1).1 The vast majority of chemical products that incorporate nonracemic quaternary carbon stereocenters are derived from the chiral pool.2,3 This fact reflects unmet challenges posed by the de novo chemical synthesis of stereocenters bearing four different carbon substituents.4−13 Orbital overlap is more difficult to achieve in the formation of such highly congested stereocenters and, in the case of enantioselective processes, defining the approach of chiral reagents or catalysts is intrinsically more challenging. As illustrated by several historic milestones, significant progress has been made in the formation of quaternary carbon stereocenters in cyclic systems. For example, the very first enantioselective transformation of an achiral starting material to form a quaternary carbon stereocenter was reported in 1966 by © 2017 American Chemical Society

Received: June 28, 2017 Published: September 14, 2017 12564

DOI: 10.1021/acs.chemrev.7b00385 Chem. Rev. 2017, 117, 12564−12580

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Figure 1. Commercial pharmaceutical and agrochemical products bearing cyclic and, less frequently encountered, acyclic quaternary carbon stereocenters.

Scheme 1. Early Examples of the Enantioselective Formation of Quaternary Carbon Stereocenters Involving Cyclic Systems

Scheme 2. Enantioselective Copper-Catalyzed Allylic Substitution To Form Acyclic Quaternary Carbon Stereocenters

construction of acyclic quaternary carbon stereocenters. Copper catalysts figure prominently in these transformations, which operate through a formal SN2′ mechanism. One benefit of utilizing EAS is that the terminal olefin of the product is amenable to diverse derivatization. In 2001, Hoveyda reported the enantioselective coppercatalyzed alkylation of γ,γ-disubstituted allylic phosphates with diethylzinc.23 Using synthetic peptide-based ligand I, good enantioselectivities (78%−90%) were achieved (Scheme 2, condition A). The more selective ligand II was later discovered,

to form acyclic quaternary carbon stereocenters is not covered.6,20

2. ACYCLIC QUATERNARY CARBON STEREOCENTERS 2.1. Electrophilic SN2′ Allylation of Nonstabilized Carbanions

Enantioselective allylic substitution (EAS)21,22 of γ,γ-disubstituted allyl electrophiles with nonstabilized carbanions is one of the earliest methods that was proven effective for 12565


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Scheme 3. Enantioselective Copper-Catalyzed Allylic Substitution of Organomagnesium, Organolithium, and Organoboron Reagents To Form Acyclic Quaternary Carbon Stereocenters

which led to both improved enantioselectivities and broader substrate scope (Scheme 2, condition B).24 In 2004, a secondgeneration catalytic system was developed using a chiral copper−NHC complex generated in situ from a copper precatalyst and the indicated silver−NHC complex (Scheme 2, condition C).25 One year later, an even more effective copper−NHC system was discovered, which enforces higher enantioselectivity at lower loadings of copper (Scheme 2, condition D).26 Notably, upon use of higher dialkylzinc reagents, an inversion in enantioselectivity was observed (not shown).23 The pronounced dependence of enantioselectivity on the structure of the dialkylzinc, along with the pyrophoric nature and poor functional group compatibility of these reagents, intrinsically limits the utility of these methods. Beyond dialkylzinc reagents, organomagnesium,27−29 organolithium,30,31 and organoboron reagents32 have been exploited in copper-catalyzed allylic substitution to form quaternary carbon stereocenters (Scheme 3). Crévisy and Mauduit applied a hydroxyalkyl-substituted NHC-ligand V in copper-catalyzed reactions between Grignard reagents and allylic phosphates, leading to good to excellent regio- and enantioselectivity.29 Feringa found that phosphoramidite ligands VI and VII could be used as ligands for highly regio- and enantioselective substitution of (E)-30 and (Z)-allyl bromides31 with alkyllithium reagents. Previously reported allylic substitutions of (Z)-olefins with organometallic reagents generally displayed low rates and modest levels of enantioselectivity.26,32 In 2014, Sawamura and Ohmiya demonstrated that 9-alkyl-9-BBN reagents participate in copper-catalyzed allylic substitutions of dialkyl-substituted allylic chlorides to form quaternary carbon centers.32 Using the bidentate phosphine ligand VIII, DTBM-MeO-BIPHEP, good to excellent enantioselectivities (71%−90% ee) were observed. The latter method is significant, as it represents a departure from the use of highly basic and moisture-sensitive organometallic nucleophiles. Remarkably, N-heterocyclic carbenes were found to promote enantioselective substitutions of trisubstituted olefins with

Scheme 4. Copper-Free Enantioselective Allylic Substitutions To Form Acyclic Quaternary Carbon Stereocenters

dialkylzinc33 and Grignard reagents34−36 under copper-free conditions (Scheme 4). Hoveyda found that α-alkyl-γ-chloroα,β-unsaturated esters react with Grignard reagents at the αposition in the presence of a bidentate NHC-ligand IX.34 Later, they discovered that previously explored allylic phosphates could be alkylated by organozinc reagents in the absence of a copper salt simply upon switching to the NHC-ligand X.33 Alexakis reported a similar transformation using more readily available Grignard reagents and allyl bromides, which displayed high regio- and enantioselectivity at relatively low loadings of the NHC ligand.35,36 In general, the selectivities observed under copper-free conditions are comparable to those obtained with copper catalysts. Mechanistic studies suggest that the Mg/Zn− NHC complex is the active catalyst in these Cu-free processes.33 Again, the highly basic and pyrophoric nature of these organometallic nucleophiles poses an intrinsic limitation. Vinyl- and arylmetal reagents also participate in SN2′ reactions to form quaternary stereocenters (Scheme 5).37−41 As described by Hoveyda, vinyl-37 and arylaluminum38 reagents prepared in situ through hydroalumination of terminal alkynes 12566


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Scheme 5. Use of Vinyl- and Arylmetal Reagents in Enantioselective Copper-Catalyzed SN2′ Reactions To Form Acyclic Quaternary Carbon Stereocenters

or transmetalation from aryllithium reagents, respectively, engage in enantioselective allylic substitution under the conditions of copper−NHC catalysis (Scheme 5).37,38 Feringa later showed that aryllithium reagents can be deployed directly in copper-catalyzed allylic substitutions of allyl bromides using triazolium-derived NHC ligand XIII, although incomplete regioselectivity was observed.39 More recently, Hoveyda40 and Hayashi41,42 applied more tractable organoboronates to the asymmetric vinylation and arylation of γ,γ-disubstituted allylic phosphates under noncryogenic conditions. Finally, Sawamura and Ohmiya showed that azoles bearing acidic C−H bonds are subject to direct deprotonation-allylic substitution under the conditions of copper catalysis.43 While selectivities were modest, this advance is significant as it avoids the use of premetalated reagents. The range of nucleophilic partners for enantioselective copper-catalyzed SN2′ reactions continues to expand (Scheme 6). Using copper catalysts modified by chiral NHC-ligands in combination with alkynylaluminum reagents44 or allenylboronates,45 Hoveyda reported the formation of enantiomerically enriched alkyne- or allene-bearing quaternary carbon stereocenters, respectively. Using their previously developed chiral naphthol-NHC ligand XVI, Sawamura and Ohmiya recently developed a method for formylation of (Z)-allylic phosphates mediated by dimethylphenylsilane using isocyanides as formyl anion equivalents.46

Scheme 6. Enantioselective Alkynylation, Allenylation, and Formylation of Allylic Phosphates To Form Acyclic Quaternary Carbon Stereocenters

Despite the longstanding use of the Suzuki reaction in chemical synthesis, it was not until recent work by Morken that palladium-catalyzed allylic cross-coupling was exploited for the formation of quaternary carbon stereocenters (Scheme 7).47 The Pd-catalyzed substitution of racemic tertiary allylic 12567


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Shibata and Endo developed a conjugate methylation of β,βdisubstituted α,β-unsaturated enones using trimethylaluminum.68 Using a copper catalyst modified by unusual Spinolbased ligand XIX, high enantioselectivities were achieved at ambient temperatures. Using chiral NHC-modified copper catalysts, Hoveyda expanded the scope of the organoaluminum reagent to encompass vinyl,69 aryl, and more complex alkyl moieties.70 An alternative to pyrophoric alkylaluminum reagents is found in Fletcher’s use of Schwartz’s reagent to generate alkylzirconium reagents in situ from terminal alkenes. This method, which exploits a phosphoramidite-modified copper catalyst, enables delivery of functionalized alkyl nucleophiles.71 Finally, avoiding the all too frequent use of highly basic and moisture-sensitive organometallic nucleophiles, Hoveyda recently reported an enantioselective 1,6-addition of propargyl groups to α,β,γ,δ-unsaturated esters using allenyl boron reagents.72 The use of NHC ligand XXII rather than phosphine ligands was required to promote exclusive 1,6versus 1,4-addition. Considerable progress also has been made on rhodium-based catalysts for asymmetric conjugate addition (Scheme 10). In 2005, Carretero disclosed an asymmetric rhodium-catalyzed addition of vinylboronic acids to β,β-disubstituted α,βunsaturated 2-pyridylsulfones.73 Using Chiraphos as ligand, high levels of enantioselectivity were observed. As described by Shintani and Hayashi, a significant expansion in scope is associated with the use of chiral diene ligands XXIV and XXV, which enable arylation of β,β-disubstituted unsaturated ketones and esters using tetraarylborates74,75 as well as arylboroxines.76 Woodward and Alexakis found that more reactive arylaluminum reagents could be applied to similar Rh-catalyzed transformations using the more common ligand BINAP (not shown).77 A related palladium-catalyzed conjugate addition of arylboronic acids to acyclic enones was reported, but enantioselectivities were quite low (not shown).78

Scheme 7. Enantioselective Formation of Acyclic Quaternary Carbon Stereocenters via Palladium-Catalyzed Allyl−Allyl Cross-Coupling

carbonates with allylboronates takes advantage of 3,3′-reductive elimination of bis-(η1-allyl)palladium species to overcome the intrinsic preference for functionalization of the less-substituted terminus of the allylpalladium electrophile. Notably, isomeric allylic carboxylates display roughly equal regio- and enantioselectivities. 2.2. Conjugate Addition

Catalytic enantioselective conjugate additions48 to electrondeficient olefins are widely utilized for the construction of acyclic quaternary carbon stereocenters.49 Early work in this area focused on the use of transition metal complexes as chiral Lewis acid catalysts for Michael-type additions involving stabilized carbanions. This subject matter will not be covered explicitly, and the reader is directed to references on late transition metal-catalyzed reactions of this type (Rh,50−54 Pd,55−57 Pt,58 La,59 Ni,60 and Ir61). Rather, in this section, we will focus on transition metal-catalyzed additions of nonstabilized carbanions to form acyclic quaternary carbon stereocenters via conjugate addition. In 2005, Hoveyda reported a copper-catalyzed conjugate addition of dialkylzinc to (E)-nitroalkenes using peptide-based ligands (Scheme 8).62 Using the less reactive dimethylzinc, process chemists at Boehringer Ingelheim later adapted this method for conjugate methylations of (Z)-nitroalkenes, which react with generally higher levels of enantioselectivity (not shown).63 Fillion established that alkylidene Meldrum’s acid derivatives are excellent electrophiles in enantioselective conjugate additions of organozinc reagents.64−67 Using a copper catalyst modified by phosphoramidite ligand VII, high yields and enantioselectivities are observed. The generation of quaternary carbon stereocenters via asymmetric conjugate addition to less reactive acyclic α,βunsaturated carbonyl compounds represents a more challenging endeavor, but significant progress has been made (Scheme 9).

2.3. Nucleophilic Allylation

Since the seminal work of Hoffmann in 1978,79 allylmetal reagents have found broad use in asymmetric carbonyl addition.80−82 γ,γ-Disubstituted allylmetal reagents based on boron, silicon, tin, and other metals have been applied successfully to the enantioselective construction of quaternary carbon stereocenters;7 however, the use of stoichiometric metals and chiral auxiliaries is not aligned with the concepts of atom-economy83,84 and green chemistry.85 Catalytic enantioselective methods capable of affecting equivalent transformations, or transformations beyond those accessible using allylmetal reagents, would represent a significant advance.

Scheme 8. Enantioselective Copper-Catalyzed Conjugate Addition of Organozinc Reagents To Form Acyclic Quaternary Carbon Stereocenters

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Scheme 9. Enantioselective Copper-Catalyzed Conjugate Addition of Organoaluminum, Organozirconium, and Organoboron Reagents To Form Acyclic Quaternary Carbon Stereocenters

Scheme 10. Enantioselective Rhodium-Catalyzed Conjugate Addition of Organoboron Reagents To Form Acyclic Quaternary Carbon Stereocenters

In 2008, the Krische group found that primary alcohols react with allyl acetate under the conditions of iridium-catalyzed hydrogen transfer to form allyliridium−aldehyde pairs, enabling catalytic enantioselective carbonyl allylation in the absence of stoichiometric metals.86−89 A related byproduct-free process, the coupling of primary alcohols with isoprene oxide, provides products of tert-(hydroxy)-prenylation (Scheme 11).89 Because Curtin−Hammett-type effects are associated with isomerization of the transient (E)- and (Z)-σ-allyliridium intermediates, the quaternary carbon stereocenter forms with high levels of antidiastereo- and enantioselectivity. This method was used to construct oxetanes90 and cyclopropanes91 bearing quaternary carbon stereocenters, as well as the terpenoid natural products oridamycin A, triptoquinones B and C, and isoiresin (not shown),92 which incorporate vicinal quaternary carbon stereocenters.93,94

In a remarkable advance, iridium catalysts modified by PhanePhos were found to promote the coupling of methanol with dienes to form products of hydrohydroxymethylation bearing quaternary carbon center (Scheme 11).95 Mechanistic studies corroborate a scenario wherein methanol dehydrogenation triggers rapid, reversible diene hydrometalation en route to regioisomeric allyliridium−formaldehyde pairs. However, due to Curtin−Hammett effects, complete levels of regioselectivity are observed. In a similar manner, iridium-PhanePhos catalysts also promote the coupling of methanol with CF3allenes,96 representing a rare example of the catalytic enantioselective formation of acyclic CF3-bearing quaternary carbon stereocenters. In these methanol-mediated couplings of dienes and allenes, asymmetric induction relies on the intervention of a single geometrical isomer of the σ-allyliridium species accompanied by high levels of enantiotopic π-facial 12569


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Scheme 11. Enantioselective Iridium-Catalyzed Carbonyl Allylation To Form Acyclic Quaternary Carbon Stereocenters

Scheme 12. Enantioselective NHK-Coupling of Allyl Chlorides and Aldehydes To Form Acyclic Quaternary Carbon Stereocenters

discrimination in the carbonyl addition event. Whereas the vast majority of catalytic methods for the formation of acyclic quaternary carbon stereocenters exploit organometallic nucleophiles and require low reaction temperatures, the indicated transfer hydrogenative couplings bypass the use of premetalated reagents and proceed under noncryogenic conditions with complete levels of atom-efficiency. There is only one additional study on catalytic enantioselective carbonyl allylation to form acyclic quaternary carbon stereocenters, which is the reductive coupling of γ,γdisubstituted allyl chloride and aldehydes described by Zhang (Scheme 12).97 This work takes advantage of Fürstner’s 1996 report on the chromium-catalyzed Nozaki−Hiyama−Kishi reaction mediated by stoichiometric manganese metal,98 along with chiral ligand XXVIII previously developed by Kishi.99 One equivalent of ZrCp2Cl2, a highly mass-intensive reagent, and one equivalent of cesium iodide were required to improve catalyst turnover and reaction rate. Although good levels of anti-diastereo- and enantioselectivity were observed, the use of multiple stoichiometric metallic reagents and manganese metal diminishes the appeal of this method, especially given the earlier disclosure of related alcohol−allene couplings that provide identical products with complete atom-efficiency.100

Scheme 13. Enantioselective Nickel-Catalyzed Hydrovinylation of α-Substituted Styrenes To Form Acyclic Quaternary Carbon Stereocenters

resulting in the enantioselective formation of quaternary carbon stereocenters (Scheme 13).103 The combination of a nickel catalyst and chiral spirophosphoramidite ligand XXIX led to excellent levels of asymmetric induction for reactants bearing secondary alkyl substituents. In a contemporaneous report, RajanBabu applied a nickel catalyst modified by the binaphthyl phosphoramidite ligand XXX to affect enantioselective hydrovinylation of α-ethylstyrenes at significantly lower catalyst loadings.104−106 An intermolecular oxidative Heck reaction of arylboronic acids with trisubstituted olefinic alcohols to construct quaternary carbon stereocenters was reported by Sigman in 2014 (Scheme 14).107 Under halide-free conditions using a

2.4. Alkene Functionalization

Unactivated olefins and styrenes are vastly abundant. Alkene hydrovinylation101,102 represents an ideal C−C bond-forming method for the byproduct-free conversion of such feedstocks into chiral building blocks. In 2006, Zhou reported the first hydrovinylation of α-substituted styrenes with ethylene gas 12570


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Scheme 14. Enantioselective (Oxidative) Heck Reaction of Arylboronic Acids, Indoles, and Vinyl Triflates with Trisubstituted Olefinic Alcohols To Form Remote Acyclic Quaternary Carbon Stereocenters

aldehydes was developed by Yoshida, which involves capture of the π-allylpalladium intermediate by an enamine derived from a chiral α-stereogenic amino acid.124,125 Most recently, Stoltz and Marek reported decarboxylative Tsuji−Trost allylations of fully substituted amide enolates to form acyclic quaternary carbon stereocenters.126 Remote electronic effects of the phosphine ligand XXXVII were required to enforce optimal levels of enantioselectivity. In 2013, Carreira developed an iridium-catalyzed Tsuji− Trost allylation of α,α-disubstituted aldehydes in which enantiomeric phosphoramidite ligands XXXVIII are used in combination with pseudo-enantiomeric primary amines (Scheme 16).127 Depending on the chiral ligand and chiral amine, all four stereoisomers of the product can be formed in a stereoselective fashion. This process requires an aryl substituent on each coupling partner to enforce good levels of enantioselectivity. On the basis of this work along with Breit’s prior observation that alkynes may serve as precursors to electrophilic π-allylrhodium species,128 Dong recently developed an analogous rhodium-catalyzed stereodivergent coupling of 4-aryl-2-butynes with α,α-disubstituted aldehydes.129 The authors only report reactions of the (R)-BINAP ligand XXXIX. Depending on the choice of chiral amine, two of the four stereoisomers of the product can be obtained selectively. In 2013, Stoltz demonstrated that iridium complexes modified by phosphoramidite ligand XXXX catalyze Tsuji− Trost allylations of α-substituted β-keto-esters to form quaternary carbon stereocenters with excellent control of diastereo- and enantioselectivity (Scheme 17).130 Recently in 2017, these conditions were adapted to the use of masked acyl cyanide pronucleophiles.131 The iridium catalyst modified by Carreira’s phosphoramidite ligand (S)-XXXVIII was used in combination with triethyl borane, which was required to promote ionization of the trisubstituted allylic carbonate. In 2015, Evans reported that rhodium complexes modified by chiral phosphite ligand XXXXI catalyze enantioselective Tsuji− Trost allylations of lithiated α-substituted benzyl nitriles to form acyclic quaternary carbon stereocenters (Scheme 18).132 These conditions were applicable to α-branched aldehydes.133 Remarkably, mechanistic studies reveal a mixture of prochiral (E)- and (Z)-enolates is generated under these conditions; yet high levels of enantioselectivity are nevertheless achieved.

palladium catalyst modified by pyridine oxazoline ligand XXXI, olefin carbopalladation is followed by β-hydride elimination and migration of the resulting double bond along the alkyl chain, ultimately terminating in aldehyde or ketone formation. This method was later extended to the use of indoles108 and vinyl triflates109 as coupling partners using pyridine oxazoline ligands XXXII and XXXIII, respectively. This method enables enantioselective formation of remote quaternary carbon stereocenters from highly tractable and accessible coupling partners. 2.5. Functionalization of Stabilized Carbon Nucleophiles

Formation of acyclic quaternary carbon stereocenters through the functionalization of stabilized carbon nucleophiles largely encompasses reactions of enols and enolates with carbon electrophiles. Such transformations pose many challenges, including the regio- and stereoselective generation of fully substituted enol(ate) derivatives.110 The use of transition metal complexes simply as chiral Lewis acid catalysts111,112 or photoredox catalysts113 will not be covered explicitly, and the reader is directed to leading references on reactions of this type. The Tsuji−Trost reaction of prochiral enolates is the most common method for formation of acyclic quaternary carbon stereocenters via enolate alkylation.114 As the use of 1,1disubstituted π-allyl precursors suffers from incomplete regioselectivity,115 work in this area exclusively involves the alkylation of fully substituted enolates. Palladium catalysts are widely used in transformations of this type (Scheme 15). In 1996, Sawamura and Ito reported a Pd/Rh dual-catalytic system for the allylation of α-substituted cyanoesters using the bis(ferrocene) ligand XXXIV.116 In these processes, palladium mediates formation of the π-allyl electrophile, while rhodium coordinates the nitrile moiety of the cyanoester to direct enantioselectivity. Hou performed the asymmetric allylation of α,α-disubstituted amides with ferrocene derived ligand XXXV.117 A similar enolate allylation of α-aryl-β-hydroxyacrylates was reported by Hossain.118 Building on precedent for the merger of transition metal and organocatalysis,119,120 List reported a palladium-catalyzed allylation of α,α-disubstituted aldehydes wherein chiral phosphate anions direct enantioselectivity in the addition of enamines to π-allylpalladium cations.121,122 An oxidative variant of this process was later disclosed by Gong.123 A related allylation of α,α-disubstituted 12571


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Scheme 15. Enantioselective Palladium-Catalyzed Tsuji−Trost Reactions To Form Acyclic Quaternary Carbon Stereocenters

carbenoids undergo indole addition to generate transient enolates that engage in further asymmetric Mannich-type reaction. A BINOL-derived chiral phosphoric acid protonates the imine, activating it toward addition and enforcing high levels of diastereo- and enantioselectivity. This reactivity pattern was successfully deployed using related π-nucleophiles, pyrroles, and anilines (not shown).138,139

Beyond Tsuji−Trost-type allylations of prochiral enolates, in 2006 Ma disclosed a copper-catalyzed enantioselective arylation of α-methyl-β-ketoesters using hydroxy proline XXXXII as chiral ligand (Scheme 19).134 The requisite aryl iodide requires an ortho-trifluoroacetamide to enhance conversion and enantioselectivity. Shibasaki and Kanai reported a coppercatalyzed asymmetric decarboxylative Mannich reaction of αsubstituted β-cyanoacetic acids.135 Good diastereo- and enantioselectivity was observed for aryl aldimines; however, reactions of alkyl-substituted imines were less selective. Finally, Nishibayashi developed an enantioselective copper-catalyzed propargylation of indoles using pybox ligand XXXXIV, which enables formation of CF3-bearing acyclic quaternary carbon centers.136 In 2012, Hu reported an asymmetric three-component coupling of indoles, α-diazo esters, and imines to generate acyclic quaternary stereocenters (Scheme 20).137 Rhodium

2.6. Desymmetrization Reactions

Catalytic enantioselective reactions that transform substrates with prochiral centers into chiral compounds by removing elements of symmetry are a powerful class of transformations that enable concomitant formation of multiple stereogenic centers.140−144 Such desymmetrization reactions have the advantage that creation of the quaternary center and the induction of chirality occur in separate events. While numerous desymmetrizing cyclizations have been developed,144 the formation of acyclic quaternary carbon stereocenters through 12572


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Scheme 16. Enantioselective Iridium- and Rhodium-Catalyzed Tsuji−Trost Reactions for Stereodivergent Formation of Acyclic Quaternary Carbon Stereocenters

Scheme 17. Enantioselective Iridium-Catalyzed Tsuji−Trost Reactions To Form Acyclic Quaternary Carbon Stereocenters

Scheme 18. Enantioselective Rhodium-Catalyzed Tsuji−Trost Reactions To Form Acyclic Quaternary Carbon Stereocenters

transition-metal catalyzed desymmetrization remains uncommon. Because of the rigid nature of the reactant in the stereodetermining transition state, enantioselective ring-opening of strained cyclic systems represents the most common strategy for the formation of acyclic quaternary carbon stereocenters via desymmetrization (Scheme 21). In 2003,

Uemura reported a palladium-catalyzed arylative ring-opening of prochiral 3,3-disubstituted cyclobutanols.145 These processes occur through enantioselective β-carbon elimination from the Pd(II) alkoxide to generate a neopentyl alkylpalladium intermediate, which upon reductive elimination releases product. A related rhodium-catalyzed desymmetrizing ringopening isomerization of cyclobutanols was subsequently 12573


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Scheme 19. Miscellaneous Enantioselective Transformations of Stabilized π-Nucleophiles To Form Acyclic Quaternary Carbon Stereocenters

Scheme 20. Enantioselective Three-Component Coupling of Indoles, α-Diazo Esters, and Imines To Form Acyclic Quaternary Carbon Stereocenters

Scheme 21. Desymmetrization via Enantioselective Cleavage of Strained Rings To Form Acyclic Quaternary Carbon Stereocenters

developed by Cramer.146 Through deuterium labeling experiments, the authors corroborate 1,3-migration of the neopentyl alkylrhodium intermediate to form a rhodium enolate. In 2007, Hoveyda reported the enantioselective ring-opening metathesis of 3,3-disubstituted cyclopropenes through use of a Hoveyda−Grubbs-type catalyst modified by chiral NHC-ligand XXXXVI (Scheme 22).147 The formation of acyclic quaternary carbon centers occurred with moderate to excellent control of alkene stereochemistry and, for the major (E)-isomer, excellent levels of enantioselectivity. A few years later, a highly (Z)selective molybdenum-based metathesis catalyst was developed by the same research group, allowing access to enantiomerically enriched cis-olefins bearing quaternary carbon stereocenters.148

Very little work beyond ring-opening reactions of strained cyclic systems appears in the literature. A C−H activation initiated Heck-type reaction of diphenylacetic acids was disclosed in 2010 by Yu (Scheme 23).149 Here, a palladium catalyst modified by Boc-protected isoleucine, ligand XXXXVIII, enabled discrimination between enantiotopic ortho-C−H bonds by way of a carboxylate directing group. In 2014, Yu reported the palladium-catalyzed C−H functionalization of enantiotopic gem-dimethyl groups using chiral hydroxamic acid ligand XXXXIX.150 While enantioselectivities are modest, the ability to access quaternary carbon centers from gem-dimethyl compounds is truly remarkable. To our knowledge, the only other example of the desymmetrization of an 12574


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the value of stereochemical complexity in small-molecule pharmaceutical research,152 advancement in this area will be certain to accelerate progress in the drug discovery enterprise. It is our hope that the present monograph will spur further growth in this challenging area of research.

Scheme 22. Desymmetrization via Enantioselective RingOpening Metathesis of 3,3-Disubstituted Cyclopropenes To Form Acyclic Quaternary Carbon Stereocenters

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Michael J. Krische: 0000-0001-8418-9709 Notes

The authors declare no competing financial interest. Biographies Jiajie Feng obtained a B.S. degree in chemistry from Peking University in 2012, where he conducted undergraduate research with Professor Jianbo Wang. He moved overseas the same year to pursue a doctoral degree at the University of Texas at Austin under the supervision of Professor Michael J. Krische as a Marye Anne Fox Endowed Presidential Fellow. During his graduate studies, he developed a byproduct-free iridium-catalyzed coupling of primary alcohols with vinyl epoxides to form all-carbon quaternary stereocenters and applied this method to construct several terpenoid natural products. After receiving his Ph.D. degree in 2017, Jiajie accepted a position in the process research and development team at AbbVie.

acyclic reactant to form an acyclic quaternary carbon center is the copper-pybox-catalyzed enantioselective mono-benzoylation of prochiral 2,2-disubstituted 1,3-propanediols (not shown).151 However, in this transformation, the transition metal simply serves as a chiral Lewis acid catalyst.

3. CONCLUSION AND OUTLOOK While significant progress toward enantioselective methods for the construction of acyclic quaternary carbon stereocenters via transition metal catalysis has been made in recent years, many important challenges remain. Beyond reactivity and selectivity, the development of safer and more atom-efficient methods is required. In particular, catalytic processes that bypass stoichiometric organometallic reagents, and the issues of safety and waste generation that attend their use, would be especially desirable. The direct use of abundant chemical feedstock as reactants or reagents, rather than materials derived therefrom, would enhance efficiency by minimizing cost, as would the identification of high-turnover/recyclable catalysts. Gratifyingly, chemists have begun to address these problems, and a small number of reports that may be viewed as “proof-of-concept” can be found in this Review. Given the growing recognition of

Michael Holmes obtained a B.S. degree in Chemistry and Mathematics from the University of Canterbury in 2010 where he performed research in the laboratory of Professor Owen Curnow. He initiated doctoral studies under the supervision of Professor Robert Britton at Simon Fraser University in 2011, where he developed methods for the construction of tetrahydrofuranol containing natural products. Following receipt of his Ph.D. degree in 2016, he joined the laboratory of Professor Michael J. Krische at the University of Texas at Austin as an Eli Lilly postdoctoral fellow, where he developed a byproduct-free iridium-catalyzed coupling of methanol with allenes to form CF3-bearing all-carbon quaternary stereocenters. Professor Michael J. Krische obtained a B.S. degree in Chemistry from the University of California at Berkeley (1989), where he performed research with Professor Henry Rapoport. After a year abroad as a Fulbright Fellow, he initiated doctoral studies at Stanford University with Professor Barry Trost as a Veatch Graduate Fellow. Following

Scheme 23. Desymmetrization via Enantioselective Palladium-Catalyzed C−H Functionalization To Form Acyclic Quaternary Carbon Stereocenters

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(15) Hajos, Z. G.; Parrish, D. R. Werkwijze Voor de Bereiding van 1,3-Dioxycycloalkanen. German Patent DE 2102623, Jul 29, 1971. (16) Eder, U.; Sauer, G.; Wiechert, R. New Type of Asymmetric Cyclization to Optically Active Steroid CD Partial Structures. Angew. Chem., Int. Ed. Engl. 1971, 10, 496−497. (17) Nakamura, A.; Konishi, A.; Tatsuno, Y.; Otsuka, S. A Highly Enantioselective Synthesis of Cyclopropane Derivatives through Chiral Cobalt(II) Complex Catalyzed Carbenoid Reaction. General Scope and Factors Determining the Enantioselectivity. J. Am. Chem. Soc. 1978, 100, 3443−3448. (18) Nakamura, A.; Konishi, A.; Tsujitani, R.; Kudo, M.; Otsuka, S. Enantioselective Carbenoid Cyclopropanation Catalyzed by Chiral vicDioximatocobalt(II) Complexes Prepared from Natural Camphor and β-Pinene. Mechanism and Stereochemistry. J. Am. Chem. Soc. 1978, 100, 3449−3461. (19) Marek, I.; Minko, Y.; Pasco, M.; Mejuch, T.; Gilboa, N.; Chechik, H.; Das, J. P. All-Carbon Quaternary Stereogenic Centers in Acyclic Systems through the Creation of Several C−C Bonds per Chemical Step. J. Am. Chem. Soc. 2014, 136, 2682−2694. (20) Marek, I.; Masarwa, A.; Delaye, P.-O.; Leibeling, M. Selective Carbon−Carbon Bond Cleavage for the Stereoselective Synthesis of Acyclic Systems. Angew. Chem., Int. Ed. 2015, 54, 414−429. (21) Helmchen, G.; Kazmaier, U.; Förster, S. Enantioselective Allylic Substitutions with Carbon Nucleophiles. In Catalytic Asymmetric Synthesis, 3rd ed.; Ojima, I., Ed.; John Wiley & Sons: Hoboken, NJ, 2010; pp 497−641. (22) Ding, C.-H.; Hou, X.-L. Nucleophiles with Allyl Metal Complexes. In Comprehensive Organic Synthesis II; Knochel, P., Molander, G. A., Eds.; Elsevier: Amsterdam, Netherlands, 2014; Vol. 4, pp 648−698. (23) Luchaco-Cullis, C. A.; Mizutani, H.; Murphy, K. E.; Hoveyda, A. H. Modular Pyridinyl Peptide Ligands in Asymmetric Catalysis: Enantioselective Synthesis of Quaternary Carbon Atoms Through Copper-Catalyzed Allylic Substitutions. Angew. Chem., Int. Ed. 2001, 40, 1456−1460. (24) Kacprzynski, M. A.; Hoveyda, A. H. Cu-Catalyzed Asymmetric Allylic Alkylations of Aromatic and Aliphatic Phosphates with Alkylzinc Reagents. An Effective Method for Enantioselective Synthesis of Tertiary and Quaternary Carbons. J. Am. Chem. Soc. 2004, 126, 10676−10681. (25) Larsen, A. O.; Leu, W.; Oberhuber, C. N.; Campbell, J. E.; Hoveyda, A. H. Bidentate NHC-Based Chiral Ligands for Efficient CuCatalyzed Enantioselective Allylic Alkylations: Structure and Activity of an Air-Stable Chiral Cu Complex. J. Am. Chem. Soc. 2004, 126, 11130−11131. (26) Van Veldhuizen, J. J.; Campbell, J. E.; Giudici, R. E.; Hoveyda, A. H. A Readily Available Chiral Ag-Based N-Heterocyclic Carbene Complex for Use in Efficient and Highly Enantioselective RuCatalyzed Olefin Metathesis and Cu-Catalyzed Allylic Alkylation Reactions. J. Am. Chem. Soc. 2005, 127, 6877−6882. (27) Falciola, C. A.; Tissot-Croset, K.; Reyneri, H.; Alexakis, A. βand γ-Disubstituted Olefins: Substrates for Copper-Catalyzed Asymmetric Allylic Substitution. Adv. Synth. Catal. 2008, 350, 1090− 1100. (28) Falciola, C. A.; Alexakis, A. High Diversity on Simple Substrates: 1,4-Dihalo-2-Butenes and Other Difunctionalized Allylic Halides for Copper-Catalyzed SN2′ Reactions. Chem. - Eur. J. 2008, 14, 10615− 10627. (29) Magrez, M.; Le Guen, Y.; Baslé, O.; Crévisy, C.; Mauduit, M. Bidentate Hydroxyalkyl NHC Ligands for the Copper-Catalyzed Asymmetric Allylic Substitution of Allyl Phosphates with Grignard Reagents. Chem. - Eur. J. 2013, 19, 1199−1203. (30) Fañanás-Mastral, M.; Pérez, M.; Bos, P. H.; Rudolph, A.; Harutyunyan, S. R.; Feringa, B. L. Enantioselective Synthesis of Tertiary and Quaternary Stereogenic Centers: Copper/Phosphoramidite-Catalyzed Allylic Alkylation with Organolithium Reagents. Angew. Chem., Int. Ed. 2012, 51, 1922−1925. (31) Fañanás-Mastral, M.; Vitale, R.; Pérez, M.; Feringa, B. L. Enantioselective Synthesis of All-Carbon Quaternary Stereogenic

receipt of his Ph.D. degree (1996), he joined the laboratory of Professor Jean-Marie Lehn at the Université Louis Pasteur as an NIH Post-Doctoral Fellow. In 1999, he joined the faculty at the University of Texas at Austin. He was promoted directly to the rank of full professor (2004) and shortly thereafter appointed the Robert A. Welch Chair in Science (2007). Professor Krische has pioneered a new class of C−C bond formations that merge the characteristics of carbonyl addition and catalytic hydrogenation. Professor Krische’s research has garnered numerous awards, including the NSF-CAREER Award (2000), Cottrell Scholar Award (2002), Eli Lilly Granteeship for Untenured Faculty (2002), Frasch Award in Chemistry (2002), Dreyfus Teacher-Scholar Award (2003), Sloan Fellowship (2003), Johnson & Johnson Focused Giving Award (2005), Solvias Ligand Prize (2006), Presidential Green Chemistry Award (2007), ACS Corey Award (2007), Dowpharma Prize (2007), Novartis Lectureship (2008), Tetrahedron Young Investigator Award (2009), Humboldt Senior Research Award (2009−2011), Mukaiyama Award (2010), Glaxo-Smith-Kline Scholar Award (2011), ACS Cope Scholar Award (2012), JSPS Fellow (2013), Eun Lee Lectureship, Korea (2015), and Royal Society of Chemistry, Pedlar Award (2015).

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NOTE ADDED IN PROOF After completion of the present review, Breit reported an enantioselective rhodium-catalyzed addition of 1,3-diketones to 1,1-disusbtituted allenes using phosphoramidite ligand XXXVIII.153

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