posted on 2022-12-16, 17:34authored byBa L. Tran, Jeremy D. Erickson, Amy L. Speelman, R. Morris Bullock
The ability of Cu–H complexes
to undergo selective insertion
of unsaturated hydrocarbons under mild conditions has rendered them
valuable, versatile catalysts. The direct formation of Cu allyl intermediates
from unfunctionalized 1,3-dienes and transient Cu hydrides is an appealing
strategy for upgrading conjugated diene feedstocks. However, empirical
mechanistic studies of the underlying elementary steps and characterization
of key intermediates in Cu–H catalysis are sparse. Using [(NHC)CuH]2 (NHC = N-heterocyclic carbene), we examined
the steric effects of NHC ligands on two key elementary steps of CuH-catalyzed
carbonyl allylation: the insertion of a diene into the Cu–H
bond to produce a Cu–allyl complex, and the formation of C–C
bonds from stoichiometric allylations of ketones and aldehydes. The
resulting allyl and homoallylic alkoxide complexes have been characterized
by NMR spectroscopy and single-crystal X-ray diffraction. Employing
isolable (NHC)Cu–allyl complexes, we further evaluated the
roles of the ligand size, electronic properties of carbonyl substrates,
coordinating groups within the substrate, and solvent on the regioselectivity,
diastereoselectivity, and relative rate of the C–C bond formation
step. In contrast to the clean allylation of ketones, allylation of
aldehydes provided a rare example of a formal β-hydride elimination
reaction from a secondary homoallylic alkoxide species. Mechanistic
studies of key elementary steps provide insights for a range of catalytic
reactions of dienes mediated by hydride complexes.