Mechanistic investigations of the Ni-catalyzed asymmetric reductive alkenylation of N-hydroxyphthalimide (NHP) esters and benzylic chlorides are reported. Investigations of the redox properties of the Ni-bis(oxazoline) catalyst, the reaction kinetics, and mode of electrophile activation show divergent mechanisms for these two related transformations. Notably, the mechanism of C(sp 3 ) activation changes from a Nimediated process when benzyl chlorides and Mn 0 are used to a reductant-mediated process that is gated by a Lewis acid when NHP esters and tetrakis(dimethylamino)ethylene is used. Kinetic experiments show that changing the identity of the Lewis acid can be used to tune the rate of NHP ester reduction. Spectroscopic studies support a Ni II −alkenyl oxidative addition complex as the catalyst resting state. DFT calculations suggest an enantiodetermining radical capture step and elucidate the origin of enantioinduction for this Ni-BOX catalyst.
The
use of a chiral ligand for stereocontrol has assisted the development
of a number of asymmetric functionalization of proximal C–H
bonds. Herein, we report a chiral ligand-controlled, asymmetric remote meta-C–H activation of arenes, leading to asymmetric
C–H olefination and arylation of hydrocinnamic acid derivatives
through desymmetrization with Ac-L-Phe-OH as the chiral ligand using
a Pd(II) catalyst. The origins of the enantioselectivity were explained
with density functional theory calculations. The larger distortion
energy of the substrate part in the C–H bond activation transition
structure
S-TS1 is the major controlling
factor that disfavors the formation of the S-enantiomer
product.
Rh(iii)-catalyzed coupling of phenylhydrazines with 1-alkynylcyclobutanols was realized through a hydrazine-directed C–H functionalization and [4+1] annulation pathway.
We report here a regiospecific [3 + 2] annulation between aminocyclopropanes
and various functionalized alkynes enabled by a P/N-heteroleptic Cu(I) photosensitizer under photoredox
catalysis conditions. Thus, a divergent construction of 3-aminocyclopentene
derivatives including methylsulfonyl-, arylsulfonyl-, chloro-, ester-,
and trifluoromethyl-functionalized aminocyclopentenes could be achieved
with advantages of high regioselectivity, broad substrate compatibility,
and mild and environmentally benign reaction conditions.
The manganese porphyrin-catalyzed C−H bond hydroxylation and amidation of equilenin acetate developed by Breslow and his co-worker have been investigated with density functional theory (DFT) calculations. The hydroxylation of C(sp 2 )−H bond of equilenin acetate leading to the 6-hydroxylated product is more favorable than the hydroxylation of C(sp 3 )−H bond of equilenin acetate, leading to the 11β-hydroxylation product. The computational results suggest that the C(sp 2 )−H bond hydroxylation of equilenin acetate undergoes an oxygenatom-transfer mechanism, which is more favorable than the C(sp 3 )−H bond hydroxylation undergoing the hydrogen-atomabstraction/oxygen-rebound (HAA/OR) mechanism by 1.6 kcal/ mol. That is why, the 6-hydroxylated product is the major product and the 11β-hydroxylated product is the minor product. In contrast, the 11β-amidated product is the only observed product in manganese porphyrin-catalyzed amidation reaction. The benzylic amidation undergoes a hydrogen-atom-abstraction/nitrogen-rebound (HAA/NR) mechanism, in which hydrogen atom abstraction is followed by nitrogen rebound, leading to the 11β-amidated product. The benzylic C(sp 3 )−H bond amidation at the C-11 position is more favorable than aromatic amidation at the C-6 position by 4.9 kcal/mol. Therefore, the DFT computational results are consistent with the experiments that manganese porphyrin-catalyzed C−H bond hydroxylation and amidation of equilenin acetate have different regioselectivities.
The mechanism and origins of stereoselectivity
of chiral iron porphyrin-catalyzed
asymmetric hydroxylation of ethylbenzene were explored with density
functional theory. The hydrogen atom abstraction is the rate- and
stereoselectivity-determining step. In good agreement with experimental
results, the formation of the (R)-1-phenylethanol
product is found to be the most favorable pathway. The transition
state of hydrogen atom abstraction which leads to the (S)-1-phenylethanol product is unfavorable by 1.7 kcal/mol compared
to the corresponding transition state which leads to the (R)-1-phenylethanol product. Enantioselectivity arises from
an attractive π–π stacking interaction between
the phenyl group of ethylbenzene substrate and the naphthyl group
of the porphyrin ligand.
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