Version 2 2024-01-23, 14:37Version 2 2024-01-23, 14:37
Version 1 2024-01-20, 14:11Version 1 2024-01-20, 14:11
journal contribution
posted on 2024-01-23, 14:37authored byLu Qian, Cui Yu, Lan Gan, Xixia Tang, Yulei Wang, Guixia Liu, Xuebing Leng, Zhao Sun, Yinlong Guo, Xiao-Song Xue, Zheng Huang
Despite half a century’s advance in the field
of transition-metal-catalyzed
asymmetric alkene hydrogenation, the enantioselective hydrogenation
of purely alkyl-substituted 1,1-dialkylethenes has remained an unmet
challenge. Herein, we describe a chiral PCNOx-pincer iridium
complex for asymmetric transfer hydrogenation of this alkene class
with ethanol, furnishing all-alkyl-substituted tertiary stereocenters.
High levels of enantioselectivity can be achieved in the reactions
of substrates with secondary/primary and primary/primary alkyl combinations.
The catalyst is further applied to the redox isomerization of disubstituted
alkenols, producing a tertiary stereocenter remote to the resulting
carbonyl group. Mechanistic studies reveal a dihydride species, (PCNOx)Ir(H)2, as the catalytically active intermediate,
which can decay to a dimeric species (κ3-PCNOx)IrH(μ-H)2IrH(κ2-PCNOx) via a ligand-remetalation pathway. The catalyst deactivation
under the hydrogenation conditions with H2 is much faster
than that under the transfer hydrogenation conditions with EtOH, which
explains why the (PCNOx)Ir catalyst is effective for the
transfer hydrogenation but ineffective for the hydrogenation. The
suppression of di-to-trisubstituted alkene isomerization by regioselective
1,2-insertion is partly responsible for the success of this system,
underscoring the critical role played by the pincer ligand in enantioselective
transfer hydrogenation of 1,1-dialkylethenes. Moreover, computational
studies elucidate the significant influence of the London dispersion
interaction between the ligand and the substrate on enantioselectivity
control, as illustrated by the complete reversal of stereochemistry
through cyclohexyl-to-cyclopropyl group substitution in the alkene
substrates.