SCHEDULED MAINTENANCE
Pyridine coordination enabled stepwise PT/ET N–H transfer and metal-independent C–C cleavage mechanism for Cu-mediated dehydroacylation of unstrained ketones†
*a
Xinzheng
Yang
*d
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 β-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.
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