Stoichiometric Reactivity Relevant to the Mor-DalPhos/Pd-Catalyzed Cross-Coupling of Ammonia and 1-Bromo-2-(phenylethynyl)benzene

While Mor-DalPhos/Pd precatalyst mixtures have in general proven to be highly effective for the monoarylation of ammonia employing a range of (hetero)aryl (pseudo)halide cross-coupling partners, we have observed previously that 1-bromo-2-(phenylethynyl)benzene (Ar*Br) is a challenging substrate for this catalyst system. We report herein on our efforts to examine some possible modes of catalyst inhibition by this substrate. Treatment of [CpPd(allyl)] with Mor-DalPhos in the presence of Ar*Br afforded [(kappa(2)-P,N-Mor-DalPhos)Pd(Br)(Ar*)] (1; 85%), which was transformed into [(kappa(3)-P,N,O-Mor-DalPhos)Pd(Ar*)]+OTf- (3; 83%) upon treatment with AgOTf. The characterization of 3 establishes the ability of the Mor-DalPhos ligand to adopt a kappa(3)-P,N,O structure, which may influence the course of some Pd-catalyzed amination processes. While treatment of 1 with AgOTf in the presence of ammonia, or alternatively treatment of 3 with ammonia, resulted in the clean formation of [(kappa(2)-P,N-Mor-DalPhos)Pd(NH3)(Ar*)]+OTf- (2), our efforts to isolate this compound were thwarted by the facile loss of ammonia from 2 to give 3. Neither NMR spectroscopic nor X-ray crystallographic data obtained for 1 and 3 support the existence of significant Pd center dot center dot center dot alkyne interactions in these complexes. Treatment of the Pd(0) species [L2Pd(diphenylacetylene)] (L-2 = Mor-DalPhos, 4; L-2 = CyPFtBu-JosiPhos, 5) with Ar*Br resulted in divergent behavior: while multiple phosphorus-containing products were observed in the case of 4, under analogous conditions 5 was transformed cleanly into [(kappa(2)-P,P-JosiPhos)Pd(Br)(Ar*)] (6). The identification of 6 was facilitated via independent synthesis from Ar*Br, JosiPhos, and [CpPd(allyl)] (90%). These observations suggest that the inferior performance of Mor-DalPhos relative to JosiPhos in the arylation of ammonia using Ar*Br may be attributable in part to the inefficiency with which putative [(Mor-DalPhos)Pd(alkyne)] species re-enter the catalytic cycle via C-Br oxidative addition.