Interplay of Steric and Electronic Effects on the Bonding Components in Aromatic Ring Agostic Interactions

Density
functional theory (DFT) calculations on the effect of steric
size adjacent to an agostic interaction in ligand assisted Pd–C
bond formation involving aromatic rings gives insight into why the
synthetic reaction can fail as the size of an alkyl group is increased.
In [PdCl2(1-tetralone oxime)] agostic complexes, changing
the C(7)-substituent on the ligand through the series H, Me, CHMe2, and CMe3 progressively reduces agostic and syndetic
donations. For (N)–CMe3 imine complexes, CMe3 steric pressure at C(7) switches off agostic donation and
increases syndetic donation significantly, especially where the aromatic
ring can rotate. Electron withdrawal from the aromatic ring in this
type of system has little effect, but electron donation into the ring
invokes η1-covalency, especially with strong π-donation.
This covalency can be switched off by further π-donation and
the syndetic donation restored. These steric effects can be expected
to impact the success of C–C bond formation chemistry derived
from Pd–C cyclometalation reactions involving agostic and syndetic
donations.

本研究通过密度泛函理论（DFT）计算，探究了配体辅助芳香环参与的钯-碳键形成过程中，与agostic相互作用（agostic interaction）相邻的位阻尺寸对反应的影响，揭示了烷基尺寸增大时合成反应失败的内在机制。在[PdCl₂(1-四氢萘酮肟)]型agostic配合物中，将配体C(7)位取代基依次替换为氢、甲基、异丙基与叔丁基，可逐步削弱agostic相互作用与协同供体作用（syndetic donation）。对于(N)-叔丁基亚胺配合物而言，C(7)位的叔丁基位阻会彻底阻断agostic相互作用，并显著增强协同供体作用，该效应在芳香环可自由旋转的体系中尤为显著。此类体系中，从芳香环吸电子对反应的影响甚微，但向芳香环供电子则会引发η¹共价性（η¹-covalency），强π供体作用下该效应尤为突出。进一步增强π供体作用可关闭该η¹共价性，使协同供体作用得以恢复。上述位阻效应预计会对基于agostic相互作用与协同供体作用的钯-碳环金属化反应衍生的C-C键形成化学的反应成功率产生影响。