Uncovering the Mechanism of Homogeneous Methyl Methacrylate Formation with P,N Chelating Ligands and Palladium: Favored Reaction Channels and Selectivities

The catalytic alkoxycarbonylation of alkynes via palladium and
P,N ligands, studied through a prototypical reaction involving propyne
methoxycarbonylation yielding methyl methacrylate, has been explored
at the B3PW91-D3/PCM level of density functional theory. Four different
reaction routes have been probed in detail, spanning those involving
one or two hemilabile P,N ligands and either hydride or carbomethoxy
mechanisms. The cycle that is
both energetically most plausible and congruent with experimental
data involves Pd(0) and two P,N ligands acting cocatalytically in
turn to shuffle protons via both protonation and deprotonation reactions.
Other mechanisms proposed in the literature can be discounted because
they would lead to insurmountable barriers or incorrect selectivities.
For the preferred mechanism, the P,N ligand is found to be crucial
in determining the strong regioselectivity and intrinsically controls
the overall turnover of the catalytic cycle with moderate barriers
(ΔG⧧ of 20.1 to 22.9 kcal/mol)
predicted. Furthermore, the necessary acidic conditions are rationalized
via a potential dicationic channel.