Journal
PURE AND APPLIED CHEMISTRY
Volume 91, Issue 1, Pages 121-134Publisher
WALTER DE GRUYTER GMBH
DOI: 10.1515/pac-2018-1004
Keywords
ab initio calculations; cross-coupling; ICPC-22; microwave heating; P-ligands; palladium; reaction mechanisms
Categories
Funding
- National Research Development and Innovation Fund [K119202]
- New National Excellence Program of the Ministry of Human Capacities [UNKP-18-3-I-BME-118]
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The microwave-assisted Hirao reaction of bromobenzene and diarylphosphine oxides was performed at 120 degrees C using triethylamine as the base, and 50% of palladium acetate as the catalyst in ethanol. 5 % Excess of the >P(O)H reagent served as the reducing agent, while another 10 % as the preligand (in the >POH tautomeric form). It was found that the P-C coupling reaction was significantly faster with (2-MeC6H4)(2)P(O)H (A) and (3,5-diMeC(6)H(3))(2)P(O)H (B), than with Ph2P(O)H (C) and (4-MeC6H4)(2) P(O)H (D). Moreover, species A and B could be applied as selective P-ligands in the reaction of bromobenzene with C or D. Dependence of the effectiveness of PdP2 catalysts with diarylphosphine oxide preligands on the methyl substituents followed a reversed order as the reactivity of the diarylphosphine oxide species in the P-C coupling itself. Formation of the PdP2 catalyst from palladium acetate and diarylphosphine oxide has never been studied, but now it was evaluated by us at the B3LYP level of theory applying 6-316(d,p) for C,H,P,O and SDD/MW28 for Pd including the explicit-implicit solvent model. The novel mechanism requiring three equivalents of the >P(O)H species for each of the palladium acetate molecule was in agreement with the preparative experiments. The ligation of palladium(0) with different P(III) species comprising the >POH form of the >P(O)H reagent was also studied, and the critical role of the steric hindrance on the ligation, and hence on the activity of the PdP2 catalyst was substantiated. Last but not least, the influence of the Me substituents in the aromatic ring of the P-reagents on the energetics of the elemental steps of the Hirao reaction itself was also evaluated.
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