TS9Ar-10Ar.
Our attention now focuses on the chemoselectivity formation of alkenyl
chloride PR21 and hydroarylation productPR22 . Figure 7 summarizes the calculated
results for the transformation of 7Ar toPR21 and PR22 . As
exhibited by Figure 7, it is not difficult to find that transition stateTS9Ar-10Ar (-11.4 kcal/mol)
associated with reductive elimination is the highest point in the
channel towards alkenyl chloride PR21 , with the
overall barrier of 30.8 kcal/mol
(13Ar →TS9Ar-10Ar ).
For the formation pathway of the hydrogenationPR22 , the highest point isTS13Ar-14Ar (-16.0 kcal/mol)
related to protonation process, and the overall barrier of this channel
is 26.2 kcal/mol
(13Ar →TS13Ar-14Ar ).
It should be noted that, from 7Ar , the reaction
may almost exclusively proceed viaTS13Ar-14A , to give
trisubstituted olefin PR22 due to the large
barrier difference (4.6 kcal/mol) betweenTS9Ar-10Ar andTS13Ar-14A . This result is line
with the experimental phenomenon (PR22 yield is
as high as 91%). The main reason lies in the substituent effects,
namely, replacing alkyl group by aryl group results in a electron
density decrease of alkynyl carbon to faciliates C(alkynyl)-H activation
and makes protonation process easier.
Compare Figures 3 with 7, it can be seen that their obviously
differences are the highest points in the two pathways towards alkenyl
chloride products PR11 andPR21 . The highest point in Figure 3 is
transition state TS8-9 related to oxidation addition, and the
one in Figure 7 is TS9Ar-10Ar associated with reduction elimination. Furthermore, the most obvious
points of energy change in the two pathways are the transition stateTS9-10 (-26.1 kcal/mol) andTS9Ar-10Ar (-11.4 kcal/mol). In
the subsection, to explore the origin of the energy difference of 14.7
kcal/mol, we carried out natural bond orbital (NBO) calculations for the
transition states (see Figure 6). The charges at the Pd, C3 and Cl atoms
in TS9Ar-10Ar are 0.496e,
-0.039e and -0.043e, respectively, and the corresponding values inTS9-10 are 0.484e, -0.038e and -0.049e. It is not difficult to
find that the electrostatic attraction between Pd & C3 and Pd & Cl
atoms in TS9Ar-10Ar is stronger
than those in TS9-10 . The stronger electrostatic attractions
makes it difficult to break the Pd-C3 and Pd-Cl bonds. Moreover, the
repulsive force between C2 and Cl atoms in