Open AccessIsotope Effects and the Mechanism of Photoredox-Promoted [2 + 2] Cycloadditions of Enones
Author(s) -
Kai-Yuan Kuan,
Daniel A. Singleton
Publication year - 2021
Publication title -
journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.2
H-Index - 228
eISSN - 1520-6904
pISSN - 0022-3263
DOI - 10.1021/acs.joc.1c00099
Subject(s) - chemistry , cycloaddition , kinetic isotope effect , selectivity , substrate (aquarium) , enone , photochemistry , reaction mechanism , electron transfer , substituent , molecule , computational chemistry , stereochemistry , organic chemistry , catalysis , deuterium , physics , oceanography , quantum mechanics , geology
13 C kinetic isotope effects (KIEs) for the photoredox-promoted [2 + 2] cycloaddition of enones were determined in homocoupling and heterocoupling examples. The only significant KIEs were observed at the β carbon, indicating that C β -C β bond formation is irreversible. However, these KIEs were much lower than computational predictions, suggesting that product selectivity is determined in part by a step prior to C β -C β bond formation. The results are explained as arising from a competition between C-C bond formation and electron exchange between substrate alkenes. This idea is supported by a relatively small substituent effect on substrate selectivity. The possible rates for electron transfer and bond-forming steps are analyzed, and the competition appears plausible, particularly if the mechanism involves a complex between reduced and neutral enone molecules.