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Oxygen-directed intramolecular hydroboration.

ABSTRACT: Metal-free homoallylic oxygen-directed intramolecular hydroboration is reported. Regioselectivities from 20:1 to 82:1 favoring the 1,3-dioxy-substituted products have been achieved using Me2S.BH3/TfOH followed by standard oxidative workup. Branching at the C5 position improves regioselectivity.

SUBMITTER: Rarig RA

PROVIDER: S-EPMC2646882 | biostudies-other | 2008 Jul

REPOSITORIES: biostudies-other

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Oxygen-directed intramolecular hydroboration.

Rarig Robert-André F RA Scheideman Matthew M Vedejs Edwin E

Journal of the American Chemical Society 20080624 29

Metal-free homoallylic oxygen-directed intramolecular hydroboration is reported. Regioselectivities from 20:1 to 82:1 favoring the 1,3-dioxy-substituted products have been achieved using Me2S.BH3/TfOH followed by standard oxidative workup. Branching at the C5 position improves regioselectivity. ...[more]

PMID: 18572941

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Project description:A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic ?,?-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic ?,?-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)2S2](+), (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh-H (preferred pathway) or Rh-B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B-C or H-C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major ?-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh-H bond (pathways I, I-1, and I-2), (2) a potential pathway proceeding via migratory insertion into the Rh-B bond (pathway II), and two potential competing routes to a ?-borylated byproduct (pathway III). The results find that the Rh-H migratory insertion pathway I-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing ?-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.

Dataset Information

Oxygen-directed intramolecular hydroboration.

Publications

Oxygen-directed intramolecular hydroboration.

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