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 [(κ2-P,N-Mor-DalPhos)­Pd­(Br)­(Ar*)] (1; 85%),
which was transformed into [(κ3-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 κ3-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 [(κ2-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···alkyne
interactions in these complexes. Treatment of the Pd(0) species [L2Pd­(diphenylacetylene)] (L2 = Mor-DalPhos, 4; L2 = 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 [(κ2-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.