Cationic Ir(III) Alkylidenes
Are Key Intermediates
in C–H Bond Activation and C–C Bond-Forming Reactions
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Abstract
This work describes the chemical reactivity of a cationic
(η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)Ir(III) complex that
contains
a bis(aryl) phosphine ligand, whose metalation determines its unusual
coordination in a κ<sup>4</sup>-<i>P</i>,<i>C</i>,<i>C′</i>,<i>C″</i> fashion. The
complex (<b>1</b><sup><b>+</b></sup> in this paper) undergoes
very facile intramolecular C–H bond activation of all benzylic
sites, in all likelihood through an Ir(V) hydride intermediate. But
most importantly, it transforms into
a hydride phosphepine species <b>4</b><sup><b>+</b></sup> by means of an also facile, base-catalyzed, intramolecular dehydrogenative
C–C coupling reaction. Mechanistic studies demonstrate the
participation as a key intermediate of an electrophilic cationic Ir(III)
alkylidene, which has been characterized by low-temperature NMR spectroscopy
and by isolation of its trimethylphosphonium ylide. DFT calculations
provide theoretical support for these results