Carbon−Carbon Bond-Forming Reductive Elimination from Arylpalladium Complexes Containing Functionalized Alkyl Groups. Influence of Ligand Steric and Electronic Properties on Structure, Stability, and Reactivity

A series of arylpalladium alkyl complexes of the formula [(DPPBz)Pd(Ar)(R)] (DPPBz =
1,2-bis(diphenylphosphino)benzene; R = methyl, benzyl, enolate, cyanoalkyl, trifluoroalkyl,
or malonate) has been prepared to reveal the influence of steric and electronic parameters
on structure, stability, and reactivity. Arylpalladium enolate and cyanoalkyl complexes
ligated by EtPh2P, 1,1‘-bis(diisopropylphosphino)ferrocene (DiPrPF), and BINAP were
prepared to evaluate the effect of the ancillary ligand. The coordination modes of the enolate
and cyanoalkyl complexes were determined by spectroscopic methods, in combination with
X-ray crystallography. In the absence of steric effects, the C-bound isomer was favored
electronically if the enolate or cyanoalkyl group was located trans to a phosphine, and the
O-bound isomer was favored if the enolate was located trans to an aryl group. The
thermodynamic stability of the enolate and cyanoalkyl complexes was controlled by the steric
properties of the enolate or cyanoalkyl group, and complexes with more substitution at the
α-carbon were less stable. Arylpalladium methyl, benzyl, enolate, cyanoalkyl, and trifluoroethyl complexes underwent carbon−carbon bond-forming reductive elimination upon
heating. Reductive elimination was faster from complexes with electron-withdrawing
substituents on the palladium-bound aryl group and with sterically hindered alkyl groups.
The electronic properties of the alkyl group had the largest impact on the rate of reductive
elimination:  electron-withdrawing groups on the α-carbon retarded the rate of reductive
elimination. The rates of reductive elimination correlated with the Taft polar substituent
constants of the groups on the carbon alpha to the metal.

本研究合成了一系列通式为[(DPPBz)Pd(Ar)(R)]的芳基钯烷基配合物（其中DPPBz代表1,2-双(二苯基膦基)苯；R为甲基、苄基、烯醇负离子、氰烷基、三氟烷基或丙二酸酯基），以揭示空间参数与电子参数对配合物结构、稳定性及反应活性的影响。为评估辅助配体对配合物的影响，本研究还合成了由EtPh2P、1,1'-双(二异丙基膦基)二茂铁(DiPrPF)与BINAP配位的芳基钯烯醇负离子配合物及氰烷基配合物。结合光谱学方法与X射线晶体学(X-ray crystallography)，确定了该类烯醇负离子与氰烷基配合物的配位模式。在无空间位阻效应的前提下，若烯醇负离子或氰烷基基团处于膦配体的反位，则电子效应更倾向于生成C-配位异构体；若烯醇负离子处于芳基的反位，则更倾向于生成O-配位异构体。烯醇负离子与氰烷基配合物的热力学稳定性由其自身的空间性质调控，α-碳上取代基越多的配合物稳定性越差。芳基钯甲基、苄基、烯醇负离子、氰烷基及三氟乙基配合物在加热条件下均可发生形成碳-碳键的还原消除反应。当钯结合的芳基上带有吸电子取代基，且烷基基团存在空间位阻时，还原消除反应的速率更快。烷基基团的电子性质对还原消除反应速率的影响最为显著：α-碳上的吸电子基团会延缓还原消除反应的速率。还原消除反应的速率与金属α-位碳上取代基团的塔夫特极性取代基常数(Taft polar substituent constants)呈相关性。