RESUMO
The Pt-catalyzed enantioselective diboration of terminal alkenes can be accomplished in an enantioselective fashion in the presence of chiral phosphonite ligands. Optimal procedures and the substrate scope of this transformation are fully investigated. Reaction progress kinetic analysis and kinetic isotope effects suggest that the stereodefining step in the catalytic cycle is olefin migratory insertion into a Pt-B bond. Density functional theory analysis, combined with other experimental data, suggests that the insertion reaction positions platinum at the internal carbon of the substrate. A stereochemical model for this reaction is advanced that is in line both with these features and with the crystal structure of a Pt-ligand complex.
Assuntos
Alcenos/química , Boratos/química , Platina/química , Catálise , Cinética , Ligantes , Modelos Moleculares , Conformação Molecular , Oxirredução , Estereoisomerismo , Especificidade por SubstratoRESUMO
More with boron: The development of catalytic enantioselective 1,2-diboration of 1,3-dienes enables a new strategy for enantioselective carbonyl allylation reactions (see scheme). These reactions occur with outstanding levels of stereoselection and can be applied to both monosubstituted and 1,1-disubstituted dienes. The carbonyl allylation reactions provide enantiomerically enriched functionalized homoallylic alcohol products.
Assuntos
Alcadienos/química , Compostos Alílicos/química , Boro/química , Álcoois/síntese química , Álcoois/química , Alcadienos/síntese química , Compostos Alílicos/síntese química , Catálise , EstereoisomerismoRESUMO
Addition of nitrosobenzene to pinacol allylboronates leads to oxidation of the organoboron with concomitant rearrangement of the substrate alkene. This reaction appears to proceed by allylboration of the nitroso group in analogy to carbonyl and imine allylation reactions. Remarkably, the N-O bond is cleaved during the reaction such that simple alcohols are the final reaction product.
Assuntos
Álcoois/síntese química , Alcadienos/química , Compostos Alílicos/química , Compostos de Boro/química , Compostos Nitrosos/química , Peróxidos/química , Álcoois/química , Catálise , Técnicas de Química Combinatória , Oxirredução , EstereoisomerismoRESUMO
The Pt-catalyzed enantioselective addition of bis(pinacolato)diboron to simple monosubstituted alkenes is described. This reaction occurs in the presence of a readily available chiral phosphonite ligand and is effective with a variety of terminal alkene substrates. Importantly, the reaction can operate with catalyst loadings of only 1 mol % Pt. While oxidation of the intermediate 1,2-bis(boronate) ester provides the chiral 1,2-diol as the reaction product, the intermediate may also be subjected to homologation/oxidation to furnish a chiral 1,4-diol as the reaction product.
Assuntos
Alcenos/química , Boro/química , Platina/química , Catálise , Estereoisomerismo , Especificidade por SubstratoRESUMO
Asymmetric 1,4-dihydroxylations of 1,3-dienes, and other transformations, are initiated by the Pt-catalyzed enantioselective addition of bis(pinacolato)diboron (B(2)(pin)(2)) to conjugated dienes. The studies reported in this communication suggest that both cyclic and acyclic substrates will participate in this reaction; however, dienes which are unable to adopt the S-cis conformation are unreactive. For most substrates, 1,4-addition is the predominant pathway. In addition to oxidation to the derived 2-buten-1,4-diol, stereoselective carbonyl allylation with the intermediate bis(boronate) ester is also described.