3.3 Substituent Effects on Chemoselectivity
Interestingly, the reaction affords hydroarylation productPR22 with a yield of 91% when the n-propyl
(R1 ) group in alkyne
(2a1 ) is replaced with the 3,5-dimethylphenyl (R2 )
group (2a2 ), which is contrary to the case discussed above.
This obvious substituent effects on
chemoselectivity urged us to explore the origins. It is found that the
reaction with a diarylalkyne (2a2 ) as the substrate follows a
mechanism similar to that discussed above for the reaction with a
dialkylalkyne (2a1 ) as substrate.
For simplicity’s sake, the following discussion only deals with the
results starting from 5 . Figure 5 summarizes calculated results
for the insertion of diarylalkyne (2a2 ). The T-shape
intermediate 5 first coordinates with the diarylalkyne
(2a2 ) molecule to form the intermediate6Ar ready for subsequent alkyne insertion. This
step is exothermic by -14.0 kcal/mol. Then, the alkyne inserts into the
Pd–C(aryl) bond of 6Ar via
TS6Ar-7Ar with an overall
barrier of 21.6 kcal/mol
(K →TS6Ar-7Ar ), leading
to the formation of alkenyl palladium intermediate7Ar . In comparison with TS6-7 in
Figure 2, it is found that the free energy ofTS6Ar-7Ar is 1.4 kcal/mol
higher than that of TS6-7 , which can be analyzed by their NBO
charge. As shown in Figure 6, the NBO charge at Pd (0.403e) inTS6Ar-7Ar is more positive than
that at Pd (0.392e) in TS6-7 , and the NBO charge at C1
(-0.133e) atom in TS6Ar-7Armore negative than that at C1(-0.128e) in TS6-7 . Evidently, the
electrostatic interaction between Pd
and C1 atoms in TS6Ar-7Ar is
the stronger than that in TS6-7 . The stronger the Pd-C1 bond,
the higher the transition state energy. Moreover, because of the
steric effects, the structureTS6Ar-7Ar is not more stable
than TS6-7 , which can be manifested by the shorter C1-C2 and
Pd-C3 bond distances inTS6Ar-7Ar (2.03 and 2.06 Å,
respectively) compared with the corresponding values in TS6-7(2.07 and 2.08 Å, respectively).
Thus, the reasons that TS6Ar-7Ar in energy is higher than TS6-7 can be attributed to the
stronger electrostatic attraction between Pd and C1 atoms and the
stronger steric effects between C1-C2 and Pd-C3 bonds in