Catalytic enantioselective intermolecular reductive aldol reaction 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.

Synthesis of β-Silyl-α-amino Acid Derivatives by Cu-Catalyzed Regio- and Enantioselective Silylamination of α,β-Unsaturated Esters

A copper-catalyzed silylamination of α,β-unsaturated esters with silylboranes and hydroxylamines has been developed to afford the corresponding β-silyl-α-amino acid derivatives, which are of great interest in medicinal and pharmaceutical chemistry. Additionally, by using a suitable chiral bisphosphine ligand, the asymmetric induction is possible, delivering the optically active β-silyl-α-amino acids with synthetically acceptable diastereomeric ratios (55:45-82:18 dr) and high enantiomeric ratios (81:19-99:1 er).

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.

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.

Intertwined Analytical, Experimental and Theoretical Studies on the Formation and Structure of a Copper Dienolate

The reaction of a silyl dienolate, a Cu(II) salt and TBAT yielding the corresponding copper dienolate is addressed. A combined NMR and cyclic voltammetry analysis first highlight the role of TBAT in the Cu(II) to Cu(I) reduction and the structure of the precatalytic species. From these first results a second set of NMR and theoretical studies enable the determination of the structure and the mechanism of formation of the copper dienolate catalytic species. Finally, we showed that that the copper catalyst promote the E/Z s-cis/s-trans equilibration of the silyl dienolate precursor through a copper dienolate intermediate. All of these results unveil some peculiarities of the catalytic and asymmetric vinylogous Mukaiyama reaction.

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.

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 aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

A Ru(iii) – catalyzed α-cross-coupling aldol type addition reaction of activated olefins with isatins

A α-cross-coupling aldol type addition reaction activated olefins with isatins has been described in the presence of ruthenium(iii) chloride and tributyltin hydride (TBTH) at room temperature.

Copper-catalyzed cascade reactions of α,β-unsaturated esters with keto esters

A copper-catalyzed cascade reaction of α,β-unsaturated esters with keto esters is reported. It features a copper-catalyzed reductive aldolization followed by a lactonization. This method provides a facile approach to prepare γ-carboxymethyl-γ-lactones and δ-carboxymethyl-δ-lactones under mild reaction conditions.

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.