Facile Group IV Based C-H Bond Activation: Confirmation of Classic
Benzyne Mechanism by DFT. Extension to Adamantyl Activation.
Abstract
Group IV organometallic complexes are promising systems for C-H bond
activation. We are interested in the C-H bond activation of the CH2
positions of the adamantyl group, since these positions are particularly
hard to activate and to functionalize. As a potential platform for
activation of that important alkyl group, we consider the alkyl bonded
to the cyclopentadienyl in a substituted bis-cyclopentadienyl group IV
metal diphenyl complex. The mechanism proposed in the classic paper
reporting such activation using Zr(IV) (Erker and Mühlenbernd, 1987)
involves an η2-benzyne complex intermediate. This current work reports a
computational analysis of the problem through Density Functional Theory
(DFT). We found that the two-step mechanism proposed for activation of
C(Me)2-Ph or tert-Bu groups using Zr(IV) is indeed confirmed by DFT and
that it can be extended to Ti and Hf. We further found that the system
can be successfully extended to the adamantyl group. The first step
involves formation of the benzyne complex, which can also be described
as a metallacyclopropene. In the second step, the cyclopentadienyl-bound
alkyl is activated in the coordination sphere of the metal via proton
transfer to the bound benzyne, which, if the metallacyclopropene
description is chosen, resembles a σ-bond metathesis. The C-H bond
activation of adamantyl through this approach is thermodynamically and
kinetically feasible. Selective α-CH bond activation should be
achievable with Ti (under thermodynamic control), and selective γ-CH
bond activation with Zr (under kinetic or thermodynamic control).