Control of the regioselectivity in catalytic transformations involving amphiphilic bis-allylpalladium intermediates: mechanism and synthetic applications.
Various dialkyl-substituted allyl chloride derivatives (2d-i) undergo regioselective palladium-catalyzed coupling reactions with allylstannanes (1a,b) and benzylidenemalonitrile (4), providing functionalized 1,7-octadienes in good yield. The catalytic reaction proceeds through an unsymmetrical amphiphilic bis-allylpalladium intermediate. An introductory electrophilic attack on the terminal position of the unsubstituted allyl moiety is followed by a nucleophilic attack on the alkyl-substituted allyl ligand. A theoretical analysis was performed by applying density functional theory at the B3PW91/DZ+P level to study the substituent effects on the electrophilic attack. According to the theoretical results, the high regioselectivity can be ascribed to the electronic effects of the alkyl substituents: The terminal alkyl groups destabilize the eta(1),eta(3)-bis-allylpalladium intermediate of the reaction; in addition, the alkyl substitution increases the activation barrier for the electrophilic attack.
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden.
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Solin
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Narayan
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KJ
Szabó