Figure 7 The potential energy surface for Pd(II)-catalyzed
tranformation from 7Ar to productsPR21 and PR22 with
diarylalkyne 2a2 substrate. The free energies are given in
kcal/mol.
TS9Ar-10Ar is stronger than
that in TS9-10 , as indicated by calculated NBO charge: -0.082e
& -0.043e in TS9Ar-10Ar and
-0.063e & -0.049e in TS9-10 for C2 and Cl, respectively. In
addition, the higher unstability ofTS9Ar-10Ar can be attributed to
the stronger electrostatic attractions between Pd & C3 and Pd & Cl,
and the stronger repulsive force of C2-Cl bond in comparison toTS9-10 .
In general, when the substituent in substrate 2a is the
n-propyl (R1 ) group, the major product obtained is alkenyl
chloride PR11 , and the rate-determining step of
the catalytic reaction is CMD process. The origins of chemoselectivity
of the 82:9 ratio of PR11 toPR12 are related to oxidative addition with
C(aryl)-Cl bond cleavage of 8 and protonation process viaTS13-14 . While the substituent is the 3,5-dimethylphenyl
(R2 ) group, the hydroarylation PR22 is
the major product. At this time, the key step of the whole catalytic
cycle is protonation process
(13Ar →TS13Ar-14Ar ).
Three factors, stronger electrostatic attractions between Pd &C3 and Pd
&Cl, repulsive force between C2 and Cl atom and aryl substituent
effects, are responsible for the reverse chemoselectivity .