Studies Toward Improved Enantiocontrol in the Asymmetric Cu-Catalyzed Reductive Coupling of Ketones and Allenamides: 1,2-Aminoalcohol Synthesis

Enantioselective Synthesis of α-Hydroxy Allyl Ketones via BINAP-CuH-Catalyzed Hydroacylation

Catalytic hydrocupration of unsaturated carbon-carbon bonds to generate organometallic nucleophiles has recently become an attractive alternative to conventional stoichiometric reagents in the stereoselective synthesis. Herein, we have developed an efficient and economical method to synthesize enantiopure α-hydroxy allyl ketones via a copper hydride (CuH)-catalyzed hydroacylation of alkoxyallenes, a significant advancement given the scarcity of reports on such scaffolds in the literature. DFT calculations reveal that this reaction proceeds through the nucleophilic attack of a kinetically favourable Z-selective allyl-copper intermediate on acid anhydrides via a six-membered chair-like transition state, stabilized by strongly attractive non-covalent interactions that ultimately leads to high level of enantioselectivities using the simple BINAP ligand. This method successfully overcomes the challenges of over-reduction of carbonyl functionality in the presence of CuH-complex, olefin isomerization and the presence of a highly enolizable α-stereocenter, which can lead to erosion in enantioselectivities, making our strategy highly desirable. The reaction exhibits a wide range of substrate scope including symmetrical as well as carbonic anhydrides with both aromatic, and aliphatic substitutions. In addition, α-substituted acid anhydrides provide exclusive syn-selective α,α'-disubstituted allyl ketones in excellent enantiomeric ratios, where the nucleophilic allylation occurs on one of the carbonyls containing the matched α-stereocenter, confirmed with mechanistic studies.

Copper-Catalyzed Diastereo-, Enantio-, and (Z)-Selective Aminoallylation of Ketones through Reductive Couplings of Azatrienes for the Synthesis of Allylic 1,2-Amino Tertiary Alcohols

We introduce a method for the (Z)-selective aminoallylation of a range of ketones to prepare allylic 1,2-amino tertiary alcohols with excellent diastereo- and enantioselectivity. Copper-catalyzed reductive couplings of 2-azatrienes with aryl/alkyl and dialkyl ketones proceed with Ph-BPE as the supporting ligand, generating anti-amino alcohols with >98% (Z)-selectivity under mild conditions. The utility of the products is highlighted through several transformations, including those that leverage the (Z)-allylic amine moiety for diastereoselective reactions of the alkene. Calculations illustrate Curtin-Hammett control in the product formation over other possible isomers and the origin of (Z)-selectivity.

Asymmetric Synthesis of Trisubstituted Vicinal Diols through Copper(I)-Catalyzed Diastereoselective and Enantioselective Allylation of Ketones with Siloxypropadienes

Chiral trisubstituted vicinal diols are a type of important organic compounds, serving as both common structure units in bioactive natural products and chiral auxiliaries in asymmetric synthesis. Herein, by using siloxypropadienes as the precursors of allyl copper(I) species, a copper(I)-catalyzed diastereoselective and enantioselective reductive allylation of ketones was achieved, providing both syn-diols and anti-diols in good to excellent enantioselectivity. DFT calculations show that cis-γ-siloxy-allyl copper species are generated favorably with either 1-TBSO-propadiene or 1-TIPSO-propadiene. Moreover, the steric difference of TBS group and TIPS group distinguishes the face selectivity of acetophenone, leading to syn-selectivity for 1-TBSO-propadiene and anti-selectivity for 1-TIPSO-propadiene. Easy transformations of the products were performed, demonstrating the synthetic utility of the present method. Moreover, one chiral diol prepared in the above transformations was used as a suitable organocatalyst for the catalytic asymmetric reductive self-coupling of aldimines generated in situ with B2(neo)2.

Development of regiodivergent asymmetric reductive coupling reactions of allenamides to access heteroatom-rich organic compounds

Organic compounds of biological importance often contain multiple stereogenic C-heteroatom functional groups (e.g. amines, alcohols, and ethers). As a result, synthetic methods to access such compounds in a reliable and stereoselective fashion are important. In this feature article, we present a strategy to enable the introduction of multiple C-heteroatom functional groups in a regiodivergent cross-coupling approach through the use of reductive coupling chemistry employing allenamides. Such processes allow for opportunities to access different heteroatom substitution patterns from the same starting materials.