Electronic Effects on the β-Alkyl Migratory Insertion Reaction of para-Substituted Styrene Methyl Palladium Complexes

The β-methyl migratory insertion chemistry of a series of cis -coordinated styrene methyl complexes of palladium [(phen)Pd(CH 3 )( p -X-C 6 H 4 CH CH 2 ) + Ar‘ 4 B - ( 2X ) (phen = 1,10-phenanthroline; X = CF 3, Cl, H, CH 3, OCH 3; Ar‘ = 3,5-(CF 3 ) 2 C 6 H 3 )] has been investigated. Complexes 2X are prepared in situ from the addition of p -X-styrene to CD 2 Cl 2 solutions of (phen)Pd(CH 3 )(OEt 2 ) + Ar‘ 4 B - ( 1 ). The X-ray structure of 1 has been determined [ P 2 1 / c; a = 16.460(4) Å, b = 18.911(3) Å, c = 17.374(3) Å; β = 117.996(14)°; V = 4775.2(15) Å 3; Z = 4] at −78 °C. The rearrangement of 2X, via β-CH 3 migratory insertion then arene coordination, to yield (phen)Pd(η 3 -CH(CH 2 CH 3 )(C 6 H 4 - p -X) + Ar‘ 4 B - ( 3X ) has been studied by 1 H NMR spectroscopy. The rearrangement is accelerated by electron-withdrawing groups; a Hammett analysis at −29.2 °C reveals that log k values are best fit by σ p parameters: ρ p = 1.1 ± 0.1, r = 0.992. The anti -isomer of 3H has been structurally characterized: P 2 1 /n; a = 13.950(4) Å, b = 15.582(5) Å, c = 24.004(8) Å; β = 101.961(24)°; V = 5105(3) Å 3; Z = 4. Complex 3H reacts further with styrene to form (phen)Pd(η 3 -CH(CH 3 )(C 6 H 5 ) + Ar‘ 4 B - ( 4 ) and ( E )-β-methylstyrene; kinetic and isotopic labeling experiments have been employed to determine the mechanism of this benzyl exchange reaction. The relative binding affinities of the p -X-styrenes of 2X have been determined for the equilibria p -X-C 6 H 4 CH CH 2 + 2H ⇌ C 6 H 5 CH CH 2 + 2X at −66 °C. The electron-rich styrenes bind tightest to Pd; a Hammett plot of these equilibrium constants yields ρ p = −2.2 ± 0.1 and r = 0.999. The kinetic and thermodynamic data indicate that both the ground and transition-states are stabilized by electron-rich styrenes and, of the two, the substituent effects are greatest in the ground-states. Since the relative rates are determined by the relative differences in energy of the ground and transition states, the migratory insertion reaction of 2X is accelerated by styrenes bearing electron-withdrawing substituents.