Dramatic Ligand Effect in Catalytic Asymmetric Reductive Aldol Reaction of Allenic Esters to Ketones

Regio- and stereoselective copper-catalyzed α,β-protoboration of allenoates: access to Z-β,γ-unsaturated β-boryl esters

A highly efficient regio- and stereoselective method for allenoate borylation has been developed. Using CuCl and bis(pinacolato)diboron in methanol, a variety of allenoates underwent β-boration and α-protonation to afford the corresponding Z-β,γ-unsaturated β-boryl esters under mild conditions with up to 81% yields.

Reductive Coupling of Allenoates with Aldehydes Catalyzed by Halogenotin Hydride

Hydrostannylation of allenoate 1 with halogenotin hydride (Bu2SnClH) was developed to give allylic stannane (I), which then could react with aldehyde 2. The same coupling gave allylic alcohol 3 under...

Copper-Catalyzed Enantioselective Reductive Aldol Reaction of α,β-Unsaturated Carboxylic Acids to Ketones: Silanes as Activator and Transient Protecting Group

We have developed the first enantioselective reductive aldol reaction of unprotected α,β-unsaturated carboxylic acids by employing a copper/bisphosphine catalyst. The reaction features in situ protection and activation of an α,β-unsaturated carboxylic acid by a hydrosilane. The copper enolate formed in situ reacts with a ketone to afford the β-hydroxy carboxylic acid with excellent enantioselectivity (up to 99% ee). The corresponding gram-scale reaction with a low catalyst loading and the derivatization of the β-hydroxy carboxylic acids highlight the practicality of this transformation.

Copper-Catalyzed Reductive Ireland-Claisen Rearrangements of Propargylic Acrylates and Allylic Allenoates

The copper-catalyzed reductive Ireland-Claisen rearrangement of propargylic acrylates led to 3,4-allenoic acids. The use of silanes or pinacolborane as stoichiometric reducing agents and triethylphosphite as a ligand facilitated the divergent and complementary selectivity for the synthesis of diastereomeric anti- and syn-rearranged products, respectively. Copper-catalyzed reductive Ireland-Claisen rearrangement of allylic 2,3-allenoates proceeded effectively only when pinacolborane was used as a reductant to generate various 1,5-dienes in excellent yields and with good diastereoselectivities in some cases. Mechanistic studies showed that the silyl and boron enolates, rather than the copper enolate, underwent a stereospecific rearrangement via a chairlike transition state to afford the corresponding Claisen rearrangement products.

Allenes and Dienes as Chiral Allylmetal Pronucleophiles in Catalytic Enantioselective C=X Addition: Historical Perspective and State-of-The-Art Survey

The use of allenes and 1,3-dienes as chiral allylmetal pronucleophiles in intermolecular catalytic enantioselective reductive additions to aldehydes, ketones, imines, carbon dioxide and other C=X electrophiles is exhaustively catalogued together with redox-neutral hydrogen auto-transfer processes. Coverage is limited to processes that result in both C-H and C-C bond formation. The use of alkynes as latent allylmetal pronucleophiles and multicomponent C=X allylations involving allenes and dienes is not covered. As illustrated in this review, the ability of allenes and 1,3-dienes to serve as tractable non-metallic pronucleophiles has evoked many useful transformations that have no counterpart in traditional allylmetal chemistry.

Cu-catalyzed coupling of vinylidene cyclopropanes with allyl and allenyl boronates

The development of Cu-catalyzed coupling of vinylidene cyclopropanes with allyl or allenyl boronates is reported. The reaction forms a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes with concurrent opening of the cyclopropane ring. In addition, the resulting Cu-enolate intermediate can be intercepted by external electrophiles.

Extended Enolates: Versatile Intermediates for Asymmetric C-H Functionalization via Noncovalent Catalysis

Catalyst-controlled functionalization of unmodified carbonyl compounds is a relevant operation in organic synthesis, especially when high levels of site- and stereoselectivity can be attained. This objective is now within reach for some subsets of enolizable substrates using various types of activation mechanisms. Recent contributions to this area include enantioselective transformations that proceed via transiently generated noncovalent di(tri)enolate-catalyst coordination species. While relatively easier to form than simple enolate congeners, di(tri)enolates are ambifunctional in nature and so control of the reaction regioselectivity becomes an issue. This Minireview discusses in some detail this and other problems, and how noncovalent activation approaches based on metallic and metal free catalysts have been developed to advance the field.

Asymmetric Synthesis of 1,2-Dihydronaphthalene-1-ols via Copper-Catalyzed Intramolecular Reductive Cyclization

We describe a copper-catalyzed intramolecular reductive cyclization of easily accessible benz-tethered 1,3-dienes containing a ketone moiety. This process provided biologically active 1,2-dihydronaphthalene-1-ol derivatives in good yields with excellent enantio- and diastereoselectivity. Mechanistic investigations using density functional theory revealed that (Z)- and (E)-allylcopper intermediates formed in situ from the diene and copper catalyst undergo isomerization and selective intramolecular allylation of the (E)-allylcopper form of the major product through a six-membered boatlike transition state. The resulting products were further transformed to fully saturated naphthalene-1-ols by reactions of the olefin moiety.

Catalytic Reductive Aldol and Mannich Reactions of Enone, Acrylate, and Vinyl Heteroaromatic Pronucleophiles

Catalytic reductive coupling of enone, acrylate, or vinyl heteroaromatic pronucleophiles with carbonyl or imine partners offers an alternative to base-mediated enolization in aldol- and Mannich-type reactions. In this review, direct catalytic reductive aldol and Mannich reactions are exhaustively catalogued on the basis of metal or organocatalyst. Stepwise processes involving enone conjugate reduction to form discrete enol or (metallo)enolate derivatives followed by introduction of carbonyl or imine electrophiles and aldol reactions initiated via enone conjugate addition are not covered.

Cu-catalyzed C-C bond formation of vinylidene cyclopropanes with carbon nucleophiles

The development of Cu-catalyzed addition of carbon nucleophiles to vinylidene cyclopropanes was reported. The reactions with 1,1-bisborylmethane provided homopropargylic boronate products by forming a C-C bond at the terminal carbon atom of the allene moiety of vinylidene cyclopropanes. Alkynyl boronates are also suitable nucleophile precursors in reactions with vinylidene cyclopropanes, and skipped diynes were obtained in high yields. In addition, the Cu-enolate generated from the initial addition of nucleophilic copper species to vinylidene cyclopropanes can be intercepted by an external electrophile. As such, vinylidene cyclopropane serves as a linchpin to connect a nucleophile and an electrophile by forming two carbon-carbon bonds sequentially.

Metal- and Hydride-Free Pentannulative Reductive Aldol Reaction

Traditionally, the reductive aldol reaction is a metal-catalyzed and hydride-promoted coupling between enones and aldehydes. We present a phosphine-mediated diastereoselective intramolecular reductive aldol reaction of α-substituted dienones and aldehydes, which is metal-free and hydride-free. The synthetic utility of the reductive aldol adducts is demonstrated by elaborating them in one step to indeno[1,2- b]furanones, indeno[1,2- b]pyrans, and dibenzo[ a, h]azulen-8-ones.

Organo-Catalyzed Regio- and Geometry-Specific Construction of β-Hydroxyl-α-vinyl Carboxylic Esters: Substrate Scope, Mechanistic Insights, and Applications

A green protocol has been developed for the synthesis of β-hydroxyl-α-vinyl carboxylic esters using aldehydes and α,β-unsaturated esters bearing an activated γ proton as starting materials under Morita-Baylis-Hillman (MBH) reaction conditions. Diverse β-hydroxyl-α-vinyl carboxylic esters have been synthesized regiospecifically in moderate to good yields with only E geometric selectivity. Other remarkable features include atom efficiency, environmental benignancy, and mild reaction conditions. Furthermore, the reaction products could be readily converted into tetrahydrofuran, dihydrofuran, and furan derivatives.

Lewis-Base-Catalyzed Reductive Aldol Reaction To Access Quaternary Carbons

A synthetic method for the efficient construction of β-hydroxylactones and lactams bearing α-quaternary carbon centers is described. This transformation relies on an electronically differentiated Lewis base catalyst, which is uniquely capable of promoting a reductive aldol reaction of α,α-disubstituted and α,α,β-trisubstituted enones. This approach provides a valuable synthetic alternative for carbon-carbon bond formation in complex molecular settings due to its orthogonal reactivity compared to that of traditional aldol reactions. Based on this method described herein, lactones, lactams, and morpholine amides bearing α-quaternary carbon centers are accessible in yields up to 85% and 50:1 dr.

Catalytic enantioselective OFF ↔ ON activation processes initiated by hydrogen transfer: concepts and challenges

Hydrogen transfer initiated processes are eco-compatible transformations allowing the reversible OFF ↔ ON activation of otherwise unreactive substrates. The minimization of stoichiometric waste as well as the unique activation modes provided by these transformations make them key players for a greener future for organic synthesis. Long limited to catalytic reactions that form racemic products, considerable progress on the development of strategies for controlling diastereo- and enantioselectivity has been made in the last decade. The aim of this review is to present the different strategies that enable enantioselective transformations of this type and to highlight how they can be used to construct key synthetic building blocks in fewer operations with less waste generation.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Copper-catalyzed asymmetric addition of olefin-derived nucleophiles to ketones

Enantioenriched alcohols found in an array of bioactive natural products and pharmaceutical agents are often synthesized by asymmetric nucleophilic addition to carbonyls. However, this approach generally shows limited functional-group compatibility, requiring the use of preformed organometallic reagents in conjunction with a stoichiometric or substoichiometric amount of chiral controller to deliver optically active alcohols. Herein we report a copper-catalyzed strategy for the stereoselective nucleophilic addition of propargylic and other alkyl groups to ketones, using easily accessible (poly)unsaturated hydrocarbons as latent carbanion equivalents. Our method features the catalytic generation of highly enantioenriched organocopper intermediates and their subsequent diastereoselective addition to ketones, allowing for the effective construction of highly substituted stereochemical dyads with excellent stereocontrol. Moreover, this process is general, scalable, and occurs at ambient temperature.

Enantioselective CuH-Catalyzed Reductive Coupling of Aryl Alkenes and Activated Carboxylic Acids

A new method for the enantioselective reductive coupling of aryl alkenes with activated carboxylic acid derivatives via copper hydride catalysis is described. Dual catalytic cycles are proposed, with a relatively fast enantioselective hydroacylation cycle followed by a slower diastereoselective ketone reduction cycle. Symmetrical aryl carboxyclic anhydrides provide access to enantioenriched α-substituted ketones or alcohols with excellent stereoselectivity and functional group tolerance.

Copper-Catalyzed trans-Hydroboration of Terminal Aryl Alkynes: Stereodivergent Synthesis of Alkenylboron Compounds

A Cu-catalyzed highly Z-stereoselective hydroboration of alkynes with 1,8-naphthalenediaminatoborane (HB(dan)) is developed. DPEphos (bis[(2-diphenylphosphino)phenyl]ether)-ligated Cu catalysts produced alkenylboron compounds from terminal alkynes with excellent Z-stereoselectivity. In contrast, using a SIPr-CuCl complex as the precatalyst exclusively produced E-hydroboration products at mild conditions. Both catalytic procedures form alkenylboron products stereocomplementary to each other, constituting stereodivergent hydroboration of alkynes through Cu catalysis. Deuterium labeling and isomerization studies support the Z-selective hydroboration via trans-addition of the boron reagent to terminal alkynes as opposed to precedent noble-metal-catalyzed trans-hydroborations.

Advances in tandem reactions with organozinc reagents

A review of recent advances in tandem reactions with organozinc reagents that highlight their utility in organic synthesis.

Redox-triggered C-C coupling of diols and alkynes: synthesis of β,γ-unsaturated α-hydroxyketones and furans by ruthenium-catalyzed hydrohydroxyalkylation

Direct ruthenium-catalyzed CC coupling of alkynes and vicinal diols to form β,γ-unsaturated ketones occurs with complete levels of regioselectivity and good to complete control over the alkene geometry. Exposure of the reaction products to substoichiometric quantities of p-toluenesulfonic acid induces cyclodehydration to form tetrasubstituted furans. These alkyne-diol hydrohydroxyalkylations contribute to a growing body of merged redox-construction events that bypass the use of premetalated reagents and, hence, stoichiometric quantities of metallic by-products.

Synthesis, structure and catalytic properties of [Ru(dppp)2(CH3CN)Cl][BPh4] and isolation of catalytically active [Ru(dppp)2Cl][BPh4]: ruthenium catalysed alkyne homocoupling and tandem alkyne–azide cycloaddition

The complex, [Ru(dppp)₂(CH₃CN)Cl][BPh₄] catalyses coupling of alkynes and subsequent alkyne–azide cycloaddition reaction.

Enantio- and diastereoselective synthesis of syn-β-hydroxy-α-vinyl carboxylic esters via reductive aldol reactions of ethyl allenecarboxylate with 10-TMS-9-Borabicyclo[3.3.2]decane and DFT analysis of the hydroboration pathway

An enantio- and diastereoselective synthesis of syn-β-hydroxy-α-vinyl carboxylate esters 3 via the reductive aldol reaction of ethyl allenecarboxylate (2) with 10-trimethylsilyl-9-borabicyclo[3.3.2]decane (1R) has been developed. Density functional theory calculations suggest that the allene hydroboration involves the 1,4-reduction of 2 with the 1R, leading directly to dienolborinate Z-(O)-8a.