Mechanistic Insight into Ligand-promoted Palladium-Catalyzed C-H
Alkenylation of Simple Arenes with Internal Alkynes : A Computational
Study
Weirong Wu,* a,b Hua Hou, b Yanzhen
Li*a
Abstract The ligand-promoted palladium-catalyzed hydroarylation
of alkynes with arenes without directing group is able to furnish
alkenyl chlorides via a 1,4-chlorine migration or trisubstituted
alkenes. This reaction is challenging due to bidentate N, N ligand and
electron-neutral arenes have rarely been reported to afford good yields.
We carried out density functional theory calculations to better
understand the elementary steps of the reaction and unveil the ligand
effects and origin of substituent-controlled chemoselectivity of
challenging C-H activation. For the n-propyl-substituted substrate, CMD
process is the rate-determining step of the catalytic reaction. And the
chemoselectivity is controlled by oxidative addition with the C-Cl bond
cleavage and protonation process. However, for the reaction with
3,5-dimethylphenyl-substituented substrate, the key step of the whole
catalytic cycle is the protonation process. The stronger electrostatic
attractions, repulsive force and aryl substituent effects result in
reverse chemoselectivity. Bidentate ligand L1 (2-OH-1,10-phenanthroline)
reacts with Pd(OAc)2 to form a most stable square-planer
species, which is different from the one formed by ligand
L2(1,10-phenanthroline). The steric repulsion are found to be mainly
responsible for no product with L2 as the ligand, which is different
from as proviously reported.
Keywords Ligand-promoted, Alkenylation, Vinyl Chlorides,
Chemoselectivity, trisubstituted olefin
Introduction
Alkynes are widely used in the synthesis of various compounds of
practical value, such as non-linear optical materials, liquid crystals,
organic semiconductors and sensors. An important synthetic conversion of
alkynes is alkenylation reaction with arenes and heteroarenes (Scheme
1), which may be also regarded as a hydro (or other group) -arylation of
a acetylene bond1. Alkenyl insertion into aromatic and
heteroaromatic rings by intermolecular or intramolecular methods
provides a reaction route for the synthesis of substituted olefins, for
example, styrene, stilbene.2-9 Substituted olefins are
important intermediates for the preparation of valuable chemicals and
have been extensively studied in the past decades. Currently, there are
four main synthetic strategies for introducing alkenyl groups into
(hetero) aromatic structures using alkynes 1. Among
them, in the first synthesis strategy, various metal-catalyst-catalyzed
carbonmetallation of internal alkynes
is a direct and effective method for the synthesis of multisubstituted
olefins (Scheme 1a). However, these reactions require pre-functionalized
metal-organic reagents 10. In recent years,
significant advances have been achieved by transition metal-catalyzed
C-H activation for construction complex molecules from simple starting
materials.11 Directing group (DG)-assisted C-H
functionalization strategies have been extensively studied, and various
arenes have reacted with alkynes and olefins to form functionalized
olefins12.