The Palladium Acetate‐Catalyzed Microwave‐Assisted Hirao Reaction without an Added Phosphorus Ligand as a “Green” Protocol: A Quantum Chemical Study on the Mechanism

Abstract

It was proved by our experiments that on microwave irradiation, the mono‐ or bidentate phosphorus ligands generally applied in the palladium(II)‐catalyzed P–C coupling reaction of aryl bromides and dialkyl phosphites or secondary phosphine oxides may be substituted by the excess of the >P(O)H reagent that exists under a tautomeric equilibrium. Taking into account that the reduction of the palladium(II) salt and the ligation of the palladium(0) so formed requires 3 equivalents of the P‐species for the catalyst applied in a quantity of 5–10%, all together, 15–30% of the P‐reagent is necessary beyond its stoichiometric quantity. In the coupling reaction of diphenylphosphine oxide, it was possible to apply diethyl phosphite as the reducing agent and as the P‐ligand. The reactivities of the diethyl phosphite and diphenylphosphine oxide reagents were compared in a competitive reaction. The mechanism and the energetics of this new variation of the Hirao reaction of bromobenzene with Y2P(O)H reagents (Y=EtO and Ph) was explored by quantum chemical calculations. The first detailed study on simple reaction models justified our assumption that, under the conditions of the reaction, the trivalent form of the >P(O)H reagent may serve as the P‐ligand in the palladium(0) catalyst, and shed light on the fine mechanism of the reaction sequence. The existence of the earlier described bis(palladium complex) {[H(OPh2P)2PdOAc]2} was refuted by high level theoretical calculations. This kind of complex may be formed only with chloride anions instead of the acetate anion. The interaction of palladium acetate and Y2P(O)H may result in only the formation of the [(HO)Y2P]2Pd complex that is the active catalyst in the Hirao reaction. The new variation of the Hirao reaction is of a more general value, and represents the greenest protocol, as there is no need for the usual P‐ligands. Instead, the >P(O)H reagent should be used in an excess of up to 30%. Hence, the costs and environmental burdens may be decreased.

A surprising mechanism lacking the Ni(0) state during the Ni(II)-catalyzed P–C cross-coupling reaction performed in the absence of a reducing agent – An experimental and a theoretical study

“…We aimed at determining the optimum quantity of diphenylphosphine oxide for the reaction performed at 150 °C for 20 min in acetonitrile. On the analogy of our earlier experiences with Pd(OAc) 2 [10] it was expected that one equivalent of the >P(O)H species to the catalyst might ensure the Ni(II)→Ni(0) reduction, while two equivalents would serve the P-ligand via the trivalent tautomeric form >POH. Applying diphenylphosphine oxide in quantities of 1.0, 1.1, 1.2, and 1.3 equivalents, triphenylphosphine oxide (3a) was obtained in yields of 66 %, 76 %, 79 %, and 65 %, respectively (Table 1/Entries 1-4).…”

“…The calculations on the complexation of Ni(II) with the trivalent tautomeric form (Y 2 POH) of diphenylphosphine oxide and diethyl phosphite applying NiCl 2 or NiBr 2 were carried out at the B3LYP level of theory using the 6-31G(d,p) basis set including the explicit-implicit solvent model (Scheme 2) [39][40][41][42]. Earlier, this approach proved to be suitable to describe adequately the reactions studied [10,11]. The limiting factor for the basis set was the large size of a few Ni complexes.…”

“…We found that there is no need to apply mono-or bidentate P-ligands together with the Pd(OAc) 2 catalyst, as the excess of the >P(O)H reagent may serve as the complexing agent in its trivalent (>POH) form. Moreover, the >P(O)H species may also act as the reductant [8][9][10]. The classical mechanism of the Hirao reaction was refined for the case, when no commercial P-ligands were added, and the excess of the diethyl phosphite or diphenylphosphine oxide reagents served as the P-ligand [10].…”

“…Moreover, the >P(O)H species may also act as the reductant [8][9][10]. The classical mechanism of the Hirao reaction was refined for the case, when no commercial P-ligands were added, and the excess of the diethyl phosphite or diphenylphosphine oxide reagents served as the P-ligand [10]. Recently, the formation of the (HO)Y 2 P • • • Pd(0) • • • PY 2 (OH) catalyst was explored in detail, and it was found that the Pd(0) complex displays an enhanced reactivity if the P-ligands contain 2-MeC 6 H 4 or 3-MeC 6 H 4 aryl groups causing sterical hindrance, as compared to the presence of the phenyl group [11].…”

“…The theoretical calculations [18] of Zhao and co-workers justified the involvement of Ni(0) in the usual catalytic cycle starting with oxidative addition of the, in this case, vinyl bromide reactant. This protocol is analogous to the Pd(0)-catalyzed cycle [10].…”