Heck-Type Coupling of Fused Bicyclic Vinylcyclopropanes: Synthesis of 1,2-Dihydropyridines, 2,3-Dihydro-1H-Azepines, 1,4-Cyclohexadienes, 2H-Pyrans, and 1,3-Butadienes
2021
Herein, we report a versatile approach for the
endocyclic ring-opening of bicyclic vinylcyclopropanes triggered by Heck
arylations. Key step for this transformation is a [1,3]-migratory shift of Pd
allowing the ring expansion of cyclopropanated pyrroles, piperidines, furans as
well as cyclopentadienes to grant access to the corresponding
1,2-dihydropyridines, 2H-pyrans,
2,3-dihydro-1H-azepines and
1,4-cyclohexadienes, respectively. Additionally, gem-disubstituted
cyclopropanated furans showed unexpected behavior by giving
diastereoselectively asymmetrically substituted dienes. Mechanistic studies and
theoretical calculations point towards a facile [1,3]-migratory shift
of Pd along the cyclopropane moiety, which can successfully compete with the usual
termination step of a Heck reaction via a syn-b-hydride elimination.
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