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Theoretical Probe to the Mechanism of Pt-Catalyzed C–H Acylation Reaction: Possible Pathways for the Acylation Reaction of a Platinacycle
journal contribution
posted on 2019-12-13, 20:11 authored by Elizabeth Warden, Libero Bartolotti, Shouquan Huo, Yumin LiDensity functional theory (DFT) and nudged elastic band
(NEB) theory
have been used to study the possible pathways for the acylation of
cycloplatinated complex A derived from 2-phenoxypyridine,
which is conceived as the key step in the platinum-catalyzed acylation
of 2-aryloxypyridines. Geometry optimization indicates that the previously
proposed intermediate, an arenium ion species as a result of analogous
aromatic substitution, is not an energy minimum, but rather cationic
Pt-arene η2-complex E is obtained as
a stable intermediate. NEB simulations suggest that the minimum energy
pathway for the acylation reaction has energy barrier of 33.6 kcal/mol
and consists of the following steps: (1) Nucleophilic substitution
at acetyl chloride by the platinum of the reactant A forms
five-coordinate Pt(IV) acylplatinum complex B with an
energy barrier of 21.7 kcal/mol. (2) B undergoes 1,2-acyl
migration from the platinum to the cyclometalated carbon through a
three-membered platinacycle transition state to give Pt-arene η2-complex E with an energy barrier of 14.0 kcal/mol.
(3) E undergoes ligand exchange with chloride to form
neutral Pt-arene η2-complex F. (4) F undergoes ligand substitution with acetonitrile to give
the product and the energy barrier is small (10.6 kcal/mol). The rate-determining
step is the 1,2-acyl migration step. It is interesting to note that
intermediate F was not included in the proposed mechanism
but was identified by the NEB simulations. Five-coordinate Pt(IV)
acylplatinum complex B undergoes barrierless ligand coordination
with chloride to form neutral formal oxidative addition acylplatinum
complex D; however, D is less stable than
reactant A by 2.9 kcal/mol, which also implies that the
isolation of an oxidative addition product Pt(IV) complex may be very
challenging. The direct reductive elimination of D to
form product P has a higher energy barrier (36.6 kcal/mol).