Chemoselectivity and the regioselectivity of the reaction of dichloropropynylborane with 2-tert- butylbutadiene

VALLEJOS, MARGARITA M.; PELLEGRINET, SILVINA C.

Santiago de Chile

Congreso; 10 th Congress of the World Association of Theoretical and Computational Chemists; 2014

In the present work, the reaction of dichloropropynylborane (1) with 2-tert-butylbutadiene (2) (Scheme 1) has been studied at the B3LYP/6-311++G(d,p) level in order to rationalize the mechanisms associated with the two competing processes (DA reaction and AB), and the origin of the experimental regioselectivity for such reactions.  Three different pathways (A, B and C) for each of the two possible regioisomers, meta and para, were found. The A and B pathways correspond to the DA reaction and the C pathways correspond to the AB process. Pathways A are concerted and their associated TSs have classical [4+2] structures. Pathways B and C correspond to stepwise mechanisms, and are similar in the first part of the reaction path uphill to the intermediate structure from the reactants and then diverge, leading to the cycloadduct and the enyne products, respectively. Reaction pathway C of the meta channel is energetically favored in accordance with the experimental result. We propose that this species rearranges to a seven-membered ring species with [4+3] character, which was also found in the metaB pathway. After that the latter structure turns into the [4+2] structure through a conflict mechanism and as a result the meta cycloaddut of the DA reaction is obtained.