Enantioconvergent cross-couplings of racemic alkylmetal reagents with unactivated secondary alkyl electrophiles: catalytic asymmetric Negishi ?-alkylations of N-Boc-pyrrolidine.
ABSTRACT: Although enantioconvergent alkyl-alkyl couplings of racemic electrophiles have been developed, there have been no reports of the corresponding reactions of racemic nucleophiles. Herein we describe Negishi cross-couplings of racemic ?-zincated N-Boc-pyrrolidine with unactivated secondary halides, thus providing a one-pot, catalytic asymmetric method for the synthesis of a range of 2-alkylpyrrolidines (an important family of target molecules) from N-Boc-pyrrolidine, a commercially available precursor. Preliminary mechanistic studies indicated that two of the most straightforward mechanisms for enantioconvergence (dynamic kinetic resolution of the organometallic coupling partner and a simple ?-hydride elimination/?-migratory insertion pathway) are unlikely to be operative.
Project description:Metal-catalyzed enantioconvergent cross-coupling reactions of alkyl electrophiles are emerging as a powerful tool in asymmetric synthesis. To date, high enantioselectivity has been limited to couplings of electrophiles that bear a directing group or a proximal p/? orbital. Herein, we demonstrate for the first time that enantioconvergent cross-couplings can be achieved with electrophiles that lack such features; specifically, we establish that a chiral nickel catalyst can accomplish Negishi reactions of racemic ?-halosilanes with alkylzinc reagents with good enantioselectivity under simple and mild conditions, thereby providing access to enantioenriched organosilanes, an important class of target molecules.
Project description:Transition-metal catalysis has the potential to address shortcomings in the classic SN2 reaction of an amine with an alkyl electrophile, both with respect to reactivity and to enantioselectivity. In this study, we describe the development of a user-friendly method (reaction at room temperature, with commercially available catalyst components) for the enantioconvergent nucleophilic substitution of racemic secondary alkyl halides (?-iodolactams) by indoles. Mechanistic studies are consistent with the formation of a copper(I)-indolyl complex that reacts at different rates with the two enantiomers of the electrophile, which interconvert under the reaction conditions (dynamic kinetic resolution). This investigation complements earlier work on photoinduced enantioconvergent N-alkylation, supporting the premise that this important challenge can be addressed by a range of strategies.
Project description:In this report, we establish that a readily available nickel/bis(oxazoline) catalyst accomplishes a wide array of enantioconvergent cross-couplings of arylzinc reagents with CF3-substituted racemic secondary alkyl halides, a process that necessitates that the chiral catalyst be able to effectively distinguish between a CF3 and an alkyl group in order to provide good ee. We further demonstrate that this method can be applied without modification to the catalytic asymmetric synthesis of other families of fluorinated organic compounds.
Project description:Carbon-carbon bonds, including those between sp3-hybridized carbon atoms (alkyl-alkyl bonds), typically comprise much of the framework of organic molecules. In the case of sp3-hybridized carbon, the carbon can be stereogenic and the particular stereochemistry can have implications for structure and function1-3. As a consequence, the development of methods that simultaneously construct alkyl-alkyl bonds and control stereochemistry is important, although challenging. Here we describe a strategy for enantioselective alkyl-alkyl bond formation, in which a racemic alkyl electrophile is coupled with an olefin in the presence of a hydrosilane, rather than via a traditional electrophile-nucleophile cross-coupling, through the action of a chiral nickel catalyst. We demonstrate that families of racemic alkyl halides-including secondary and tertiary electrophiles, which have not previously been shown to be suitable for enantioconvergent coupling with alkyl metal nucleophiles-cross-couple with olefins with good enantioselectivity and yield under very mild reaction conditions. Given the ready availability of olefins, our approach opens the door to developing more general methods for enantioconvergent alkyl-alkyl coupling.
Project description:Nickel-catalyzed cross-coupling has emerged as the most versatile approach to date for achieving enantioconvergent carbon-carbon bond formation using racemic alkyl halides as electrophiles. In contrast, there have not yet been reports of the application of chiral nickel catalysts to the corresponding reactions with heteroatom nucleophiles to produce carbon-heteroatom bonds with good enantioselectivity. Herein, we establish that a chiral nickel/pybox catalyst can borylate racemic secondary benzylic chlorides to provide enantioenriched benzylic boronic esters, a highly useful family of compounds in organic synthesis. The method displays good functional group compatibility (e.g., being unimpeded by the presence of an indole, a ketone, a tertiary amine, or an unactivated alkyl bromide), and both of the catalyst components (NiCl2 ?glyme and the pybox ligand) are commercially available.
Project description:In recent years, a wide array of methods for achieving nickel-catalyzed substitution reactions of alkyl electrophiles by organometallic nucleophiles, including enantioconvergent processes, have been described; however, experiment-focused mechanistic studies of such couplings have been comparatively scarce. The most detailed mechanistic investigations to date have examined catalysts that bear tridentate ligands and, with one exception, processes that are not enantioselective; studies of catalysts based on bidentate ligands could be anticipated to be more challenging, due to difficulty in isolating proposed intermediates as a result of instability arising from coordinative unsaturation. In this investigation, we explore the mechanism of enantioconvergent Kumada reactions of racemic ?-bromoketones catalyzed by a nickel complex that bears a bidentate chiral bis(oxazoline) ligand. Utilizing an array of mechanistic tools (including isolation and reactivity studies of three of the four proposed nickel-containing intermediates, as well as interrogation via EPR spectroscopy, UV-vis spectroscopy, radical probes, and DFT calculations), we provide support for a pathway in which carbon-carbon bond formation proceeds via a radical-chain process wherein a nickel(I) complex serves as the chain-carrying radical and an organonickel(II) complex is the predominant resting state of the catalyst. Computations indicate that the coupling of this organonickel(II) complex with an organic radical is the stereochemistry-determining step of the reaction.
Project description:Classic nucleophilic substitution reactions (SN1 and SN2) are not generally amenable to the enantioselective variants that use simple and racemic alkyl halide electrophiles. The merging of transition metal catalysis and radical chemistry with organometallic nucleophiles is a versatile method for addressing this limitation. Here, we report that visible light-driven catalytic asymmetric photoredox radical coupling can act as a complementary and generic strategy for the enantioconvergent formal substitution of alkyl haldies with readily available and bench-stable organic molecules. Single-electron reductive debrominations of racemic ?-bromoketones generate achiral alkyl radicals that can participate in asymmetric Csp3-Csp3 bonds forming cross-coupling reactions with ?-amino radicals derived from N-aryl amino acids. A wide range of valuable enantiomerically pure ?2- and ?2,2-amino ketones were obtained in satisfactory yields with good-to-excellent enantioselectivities by using chiral phosphoric acid catalysts to control the stereochemistry and chemoselectivity. Fluoro-hetero-quaternary and full-carbon quaternary stereocenters that are challenging to prepare were successfully constructed.
Project description:The addition of terminal alkynes to racemic ?-stereogenic ?-keto esters was achieved in high levels of stereoselectivity, affording versatile tertiary propargylic alcohols containing two stereocenters. This environmentally benign enantioconvergent reaction proceeds with perfect atom economy, requires no solvent, and is catalyzed by a non-toxic zinc salt. The alkyne moiety can be leveraged in downstream transformations including hydrogenation to the corresponding saturated tertiary alcohol, which represents the product of a formal enantioconvergent aliphatic nucleophile addition.
Project description:Although nickel-catalyzed stereoconvergent couplings of racemic alkyl electrophiles are emerging as a powerful tool in organic chemistry, to date there have been no systematic mechanistic studies of such processes. Herein, we examine the pathway for enantioselective Negishi arylations of secondary propargylic bromides, and we provide evidence for an unanticipated radical chain pathway wherein oxidative addition of the C-Br bond occurs through a bimetallic mechanism. In particular, we have crystallographically characterized a diamagnetic arylnickel(II) complex, [(i-Pr-pybox)Ni(II)Ph]BAr(F)4, and furnished support for [(i-Pr-pybox)Ni(II)Ph](+) being the predominant nickel-containing species formed under the catalyzed conditions as well as a key player in the cross-coupling mechanism. On the other hand, our observations do not require a role for an organonickel(I) intermediate (e.g., (i-Pr-pybox)Ni(I)Ph), which has previously been suggested to be an intermediate in nickel-catalyzed cross-couplings, oxidatively adding alkyl electrophiles through a monometallic pathway.
Project description:The first method for the stereoconvergent cross-coupling of racemic ?-halonitriles is described, specifically, nickel-catalyzed Negishi arylations and alkenylations that furnish an array of enantioenriched ?-arylnitriles and allylic nitriles, respectively. Noteworthy features of this investigation include: the highly enantioselective synthesis of ?-alkyl-?-aryl nitriles that bear secondary ?-alkyl substituents; the first examples of the use of alkenylzinc reagents in stereoconvergent Negishi reactions of alkyl electrophiles; demonstration of the utility of a new family of ligands for asymmetric Negishi cross-couplings (a bidentate bis(oxazoline), rather than a tridentate pybox); in the case of arylzinc reagents, carbon-carbon bond formation at a remarkably low temperature (-78 °C), the lowest reported to date for an enantioselective cross-coupling of an alkyl electrophile; a mechanistic dichotomy between Negishi reactions of an unactivated versus an activated secondary alkyl bromide.