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

Abstract

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. ■ INTRODUCTIONAlkyne alkoxycarbonylation is a transformation with 100% atom efficiency that forms acrylate esters. 1−8 Methoxycarbonylation of propyne yields methyl methacrylate (MMA), a small-molecule feedstock crucial in industry due to its polymer poly(methyl methacrylate), more commonly known as Perspex. This material has a wide range of uses, which include important surgical roles, 9 cosmetics and coatings, and as a rigid transparent plastic for windows, especially in transport. 10 There is also a growing demand from use in LCD screens. 11 Functionalization of propyne yielding the branched product of methoxycarbonylation has received considerable attention, 2,3,5−7 and such chemistry has been extended to higher alkynes such as ethynyl benzene 12 and alkynols. 13 Transition metals are key to many industrial processes, 14−16 and in the example of MMA formation from propyne, palladium complexes with P,N chelating ligands play a key role in this transformation (Scheme 1).Drent has reported that the presence of a P,N ligand, 2-pyridyldiphenylphosphine (2-PyPPh 2 ), is necessary for both high selectivity for the branched product and a high turnover frequency (TOF) for carbonylation. 2,3 Ligands based upon 3-PyPPh 2 , 4-PyPPh 2 , and PPh 3 exhibit a reduced efficiency, suggesting that both the presence and location of the nitrogen atom are important. 2-PyPPh 2 allows the methoxycarbonylation of propyne to proceed under mild conditions of 45°C, attaining a turnover frequency of 40 000 mol (mol Pd h) −1 with a selectivity of ∼99% toward MMA. P,N ligands are known to coordinate in a number of binding modes. While monocoordination and multiple unidentate coordination are typically through the softer phosphorus atom, 17 many structures have been isolated that show chelation. 18−24 2-PyPPh 2 may also coordinate metal (hetero)dimers, 4,25−27 and structures involving iridium suggest that two 2-PyPPh 2 ligands ...

Palladium-catalysed alkyne alkoxycarbonylation with P,N-chelating ligands revisited: a density functional theory study

“…For the original pathway D, introduction of an NMe 2 group in 4position of the 2-Py moiety was predicted to lower the overall barrier notably (by 3.6 kcal mol -1 ). 29,30 In contrast, in our new mechanism E, the same substitution raises the overall barrier from 16.8 kcal mol -1 to 18.2 kcal mol -1 (see energy span values in upper half of Table 3). This increase in the overall barrier should decrease the TOF relative to that of the original system, in good agreement with the experimental results discussed in Section 1.…”

“…Bühl's pathway D, including computed driving forces for elementary steps (G 298K in kcal mol -1 ). 29,30 Based on this mechanism, an increase in basicity of the 2-pyridyl moiety should facilitate the critical proton transfer steps, and indeed it was predicted computationally that the 4dimethylamino-2-pyridyl ligand (4-Me 2 N-Py)PPh 2 should lower the overall barrier of the whole cycle significantly, thus increasing the overall catalytic activity. 29 In the Pringle group we have put this prediction to the test for the methoxycarbonylation of phenylacetylene.…”

“…A number of possible pathways (previously labelled A -D) were considered, of which only one appeared to be consistent with observed activities and selectivities (pathway D, Scheme 2). 29,30 This mechanism involves proton shuttling by the pyridyl groups in the initiation and termination steps. The pendant pyridyl moiety (when protonated) can act as an in situ acid, protonating coordinated propyne followed by thermodynamically favoured CO insertion and then deprotonating coordinated methanol to promote rapid ester formation.…”

Structural and theoretical studies of the methoxycarbonylation of higher olefins catalysed by (Pyrazolyl‐ethyl)pyridine palladium (II) complexes

“…First to be studied was the effect of acid promoters using inorganic, organic and Lewis acids (Table , entries 1–6). The role of the acid promoter in the methoxycarbonylaion reaction is believed to be the generation of the Pd‐hydride, considered as the active species . The most reactive acid was HCl (91%), while the organic acids, methyl sulfonic (MSA) and para‐tolylsulfonic ( p ‐TsOH) were inactive.…”

Palladium complexes of (benzoimidazol-2-ylmethyl)amine ligands as catalysts for methoxycarbonylation of olefins

“…57 For complexes 1-5, both of these mechanisms are highly possible due to coordinative unsaturation of the palladium metal center resulting from possible loss of PPh 3 group. At this stage, we are cannot unambiguously establish the preferred mechanistic pathway.…”