作者:Angela R. Woodward、Heather E. Burks、Louis M. Chan、James P. Morken
DOI:10.1021/ol052312i
日期:2005.11.1
[reaction: see text] Palladium-catalyzed enantioselective diboration of prochiral allenes generates a reactive chiral allylboron intermediate which is a versatile reagent for the allylation of carbonyls. Experiments that improve the enantioselectivity of this process, examine the substrate scope, and are directed toward functionalization of the allylation intermediate are described.
Palladium-Catalyzed Enantioselective Diboration of Prochiral Allenes
作者:Nicholas F. Pelz、Angela R. Woodward、Heather E. Burks、Joshua D. Sieber、James P. Morken
DOI:10.1021/ja044167u
日期:2004.12.1
Pd-catalyzed diboration of prochiral allenes occurs exclusively at the internal position and is remarkably accelerated in the presence of Lewis basic ligand structures. On the basis of preliminary observations, a chiral ligand was employed, and the enantiomeric excess of a variety of diboration products was found to be in the range of 86-92% ee. The chiral diboron reaction products should be useful in organic synthesis, and preliminary experiments suggest that they may participate in allylation reactions with a high level of chirality transfer.
Pd-catalyzed enantioselective diboration of prochiral allenes provides adducts which participate in highly selective allylation reactions with primary imines. The allylation product is a vinyl boronate which may be oxidized to give nonracemic Mannich products (87-97% ee). Alternatively, enantiomerically enriched homoallylic amine derivatives may be obtained by protonation and Suzuki cross-coupling of the vinyl boronate.
Development, Mechanism, and Scope of the Palladium-Catalyzed Enantioselective Allene Diboration
作者:Heather E. Burks、Shubin Liu、James P. Morken
DOI:10.1021/ja070572k
日期:2007.7.1
In the presence of a chiral phosphoramidite ligand, the palladium-catalyzed diboration of allenes can be executed with high enantioselectivity. This reaction provides high levels of selectivity with a range of aromatic and aliphatic allene substrates. Isotopic-labeling experiments, stereodifferentiating reactions, kinetic analysis, and computational experiments suggest that the catalytic cycle proceeds by a mechanism involving rate-determining oxidative addition of the diboron to Pd followed by transfer of both boron groups to the unsaturated substrate. This transfer reaction most likely occurs by coordination and insertion of the more accessible terminal alkene of the allene substrate, by a mechanism that directly provides the eta(3) pi-allyl complex in a stereospecific, concerted fashion.
Modular Asymmetric Synthesis of 1,2-Diols by Single-Pot Allene Diboration/Hydroboration/Cross-Coupling
作者:Nicholas F. Pelz、James P. Morken
DOI:10.1021/ol0616891
日期:2006.9.1
Chiral allyl vinyl boronates are generated by catalytic enantioselective diboration of prochiral allenes. They may then be reacted, in situ, with a hydroborating reagent to form a novel triboron intermediate. The least hindered and most reactive C-B bond then participates in cross-coupling wherein the coupling is brought about by the same catalyst as that which catalyzed the diboration reaction. The