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

The β-methyl migratory insertion chemistry of a series of cis-coordinated styrene methyl complexes of palladium [(phen)Pd(CH3)(p-X-C6H4CHCH2)+Ar‘4B- (2X) (phen = 1,10-phenanthroline; X = CF3, Cl, H, CH3, OCH3; Ar‘ = 3,5-(CF3)2C6H3)] has been investigated. Complexes 2X are prepared in situ from the addition of p-X-styrene to CD2Cl2 solutions of (phen)Pd(CH3)(OEt2)+Ar‘4B- (1). The X-ray structure of 1 has been determined [P21/c; a = 16.460(4) Å, b = 18.911(3) Å, c = 17.374(3) Å; β = 117.996(14)°; V = 4775.2(15) Å3; Z = 4] at −78 °C. The rearrangement of 2X, via β-CH3 migratory insertion then arene coordination, to yield (phen)Pd(η3-CH(CH2CH3)(C6H4-p-X)+Ar‘4B- (3X) has been studied by 1H 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 σp parameters: ρp = 1.1 ± 0.1, r = 0.992. The anti-isomer of 3H has been structurally characterized: P21/n; a = 13.950(4) Å, b = 15.582(5) Å, c = 24.004(8) Å; β = 101.961(24)°; V = 5105(3) Å3; Z = 4. Complex 3H reacts further with styrene to form (phen)Pd(η3-CH(CH3)(C6H5)+Ar‘4B- (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-C6H4CHCH2 + 2H ⇌ C6H5CHCH2 + 2X at −66 °C. The electron-rich styrenes bind tightest to Pd; a Hammett plot of these equilibrium constants yields ρp = −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.