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 (KTS6Ar-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