In:
Pure and Applied Chemistry, Walter de Gruyter GmbH, Vol. 91, No. 1 ( 2019-01-28), p. 121-134
Abstract:
The microwave-assisted Hirao reaction of bromobenzene and diarylphosphine oxides was performed at 120 °C using triethylamine as the base, and 5% 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-MeC 6 H 4 ) 2 P(O)H ( A ) and (3,5-diMeC 6 H 3 ) 2 P(O)H ( B ), than with Ph 2 P(O)H ( C ) and (4-MeC 6 H 4 ) 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 “PdP 2 ” 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 “PdP 2 ” 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 6-31G(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 “PdP 2 ” 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.
Type of Medium:
Online Resource
ISSN:
1365-3075
,
0033-4545
DOI:
10.1515/pac-2018-1004
Language:
English
Publisher:
Walter de Gruyter GmbH
Publication Date:
2019
detail.hit.zdb_id:
2022101-0
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