Cationic Rhodium-Catalyzed Hydroboration: Investigations into Electronic and Ligand Effects; Lewis-Acid Reaction Acceleration and Regioselectivity Change
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Authors
Lata, Christopher
Date
2009-10-20T20:54:49Z
Type
thesis
Language
eng
Keyword
Chemistry , Hydroboration
Alternative Title
Abstract
Mechanistic aspects of the asymmetric rhodium catalyzed hydroboration of vinyl
arenes with catecholborane (HBCat) and pinacolborane (HBPin) have been probed.
Competition studies conducted with catalysts bearing asymmetric ligands Josiphos or Quinap
reveal substituent effects of varying sensitivity correlating to Hammett parameter sigma. In
reactions involving the Quinap ligand, rho-values of +1.97 and +1.02 are observed for
HBCat and HBPin, respectively. In Josiphos-ligated systems, rho-values of +1.38 and -3.17
are observed, indicating that the reaction proceeds through differing mechanisms depending
on the borane employed. Labeling experiments reveal differences in the reversibility of the
hydride insertion step of reaction depending on the ligand and borane implemented in
hydroboration.
A new methodology has been developed for regioselective rhodium-catalyzed
hydroboration of internal olefins with pinacolborane. The addition of co-catalytic Lewis
acids results in the activation of cationic rhodium catalysts in chlorinated solvents. Increased
catalytic activity and regioselectivity are observed under these conditions. For internal alkyl
olefins, selectivity for non-isomerized secondary boronate esters was found originate from
the non-coordinating solvents employed in the reaction. For aromatic olefins, increased
regioselectivity and reaction acceleration are both effects of the Lewis acid additive. Study
of the reaction revealed that these properties are linked to Lewis acid-base mediated
heterolytic B-H bond cleavage of pinacolborane, and suggests that the reagent is transferred
to the rhodium catalyst as borenium ion and hydride moieties. Inhibition studies confirmed
that Lewis acid is required throughout the reaction and does not solely act as an initiator.
Description
Thesis (Master, Chemistry) -- Queen's University, 2009-10-12 20:56:41.263
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