Pyridine coordination enabled stepwise PT/ET N-H transfer and metal-independent C-C cleavage mechanism for Cu-mediated dehydroacylation of unstrained ketones.

ABSTRACT

A density functional theory study of copper-mediated dehydroacylation of 4-phenyl-2-butanone to the corresponding olefin reveals a flexible N-H transfer process and a metal-independent C-C cleavage mechanism. When N'-methylpicolinohydrazonamide (MPHA) acts as the activating reagent, N-H cleavage can easily take place via stepwise proton transfer/electron transfer (PT/ET) and the rate-determining step is C-C homolysis with a total free energy barrier of 22.6 kcal mol-1, which is consistent with experimental observation of no kinetic isotope effects (KIE) at ß-H. Besides, copper is found to have little influence on C-C cleavage, but is responsible for triggering single electron oxidation of the pre-aromatic intermediate (PAI). When replacing MPHA with picolinohydrazonamide (PHA), the second N-H transfer is 2.7 kcal mol-1 more favorable than C-C cleavage and dominates the pathway to aromatization, which explains there being no C-C cleavage product well. When N'-methylbenzohydrazonamide (MBHA) is adopted, the lack of pyridine coordination significantly reduces the stability of CuII and N-H transfer proceeds via a much more difficult proton coupled electron transfer (PCET) pathway, thus making N-H cleavage a rate-determining step with a total free energy barrier of up to 28.1 kcal mol-1.