Electronic effects on the beta-alkyl migratory insertion reaction of para-substituted styrene methyl palladium complexes

The beta-methyl migratory insertion chemistry of a series of cis-coordinated styrene methyl complexes of palladium [(phen)Pd(CH3)(p-X-C6H4CH=CH2)Ar-+'B-4(-) (2X) (phen = 1,10-phenanthroline; X = CF3, Cl, H, CH3, OCH3; Ar' = 3,5-(CF32C6H3)] has been investigated. Complexes 2X are prepared in situ from the addition of p-X-styrene to CD2Cl2 solutions of (phen)Pd(CH3)(OEt(2))Ar-+'B-4(-) (1). The X-ray structure of 1 has been determined [P2(1)/c; a = 16.460(4) Angstrom, b = 18.911(3) Angstrom, c = 17.374(3) Angstrom; beta = 117.996(14)degrees; V = 4775.2(15) Angstrom(3); Z = 4] at -78 degrees C. The rearrangement of 2X, via beta-CH3 migratory insertion then arene coordination, to yield (phen)Pd(eta(3)-CH(CH2-CH3)(C6H4-p-X)Ar-+'B-4(-) (3X) has been studied by H-1 NMR spectroscopy. The rearrangement is accelerated by electron-withdrawing groups; a Hammett analysis at -29.2 degrees C reveals that log k values are best fit by sigma(p) parameters: rho(p) = 1.1 +/- 0.1, r = 0.992. The anti-isomer of 3H has been structurally characterized: P2(1)/n; a = 13.950(4) Angstrom, b = 15.582(5) Angstrom, c = 24.004(8) Angstrom; B = 101.961(24)degrees; V = 5105(3) Angstrom(3); Z = 4, Complex 3H reacts further with styrene to form (phen)Pd(eta(3)-CH(CH3)(C6H5)Ar-+'B-4(-) (4) and (E)-beta-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 reversible arrow C6H5CH = CH2 + 2X at -66 degrees C. The electron-rich styrenes bind tightest to Pd; a Hammett plot of these equilibrium constants yields rho(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.