Influence of BH3 and alkaline cation released from the reduction agent on a tandem reduction/acylation reaction - A computational study

摘要

When an alkaline boron hydride is used as reduction reagent two byproducts are released, BH3 and alkaline cation, and both of them could influence the following reaction steps if a multi-step reaction is performed in tandem one-pot fashion. We report a theoretical study on the stability of possible complexes of the Lewis acids, BH3 and alkaline cations, with reaction intermediate, solvent molecules and basic additives in a tandem hydride reduction/acylation reaction of 3-diethylphosphonocoumarin. Both chelate complexes of the intermediate anion with the alkali cations (Li+, Na+, or K+) and complexes of BH3, bound to C or O center of the ambident anionic intermediate have been investigated. Since the formation of the latter complexes blocks the intermediate for further acylation, the reaction could take place only if BH3 is bound in a complex with a strong base as DMAP. The binding energy of BH3 to DMAP was found higher than to the intermediate for Li+ and Na+ as counter cations, while for K+ BH3, is bound to the intermediate stronger than to DMA.P. Formation of the intermediate is facilitated in presence of Li+ cations, but the alteration of the alkali cation does not influence the selectivity of the reaction-C-acylations is preferred in all cases. (c) 2007 Wiley Periodicals, Inc.