Stoichiometric Reactivity
Relevant to the Mor-DalPhos/Pd-Catalyzed
Cross-Coupling of Ammonia and 1-Bromo-2-(phenylethynyl)benzene
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Abstract
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 [(κ<sup>2</sup>-<i>P</i>,<i>N</i>-Mor-DalPhos)Pd(Br)(Ar*)] (<b>1</b>; 85%),
which was transformed into [(κ<sup>3</sup>-<i>P</i>,<i>N,O</i>-Mor-DalPhos)Pd(Ar*)]<sup>+</sup>OTf<sup>–</sup> (<b>3</b>; 83%) upon treatment with AgOTf. The characterization
of <b>3</b> establishes the ability of the Mor-DalPhos ligand
to adopt a κ<sup>3</sup>-P,N,O structure, which may influence
the course of some Pd-catalyzed amination processes. While treatment
of <b>1</b> with AgOTf in the presence of ammonia, or alternatively
treatment of <b>3</b> with ammonia, resulted in the clean formation
of [(κ<sup>2</sup>-<i>P</i>,<i>N</i>-Mor-DalPhos)Pd(NH<sub>3</sub>)(Ar*)]<sup>+</sup>OTf<sup>–</sup> (<b>2</b>),
our efforts to isolate this compound were thwarted by the facile loss
of ammonia from <b>2</b> to give <b>3</b>. Neither NMR
spectroscopic nor X-ray crystallographic data obtained for <b>1</b> and <b>3</b> support the existence of significant Pd···alkyne
interactions in these complexes. Treatment of the Pd(0) species [L<sub>2</sub>Pd(diphenylacetylene)] (L<sub>2</sub> = Mor-DalPhos, <b>4</b>; L<sub>2</sub> = CyPF<i>t</i>Bu-JosiPhos, <b>5</b>) with Ar*Br resulted in divergent behavior: while multiple
phosphorus-containing products were observed in the case of <b>4</b>, under analogous conditions <b>5</b> was transformed
cleanly into [(κ<sup>2</sup>-<i>P</i>,<i>P</i>-JosiPhos)Pd(Br)(Ar*)] (<b>6</b>). The identification of <b>6</b> 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
