Reaction
of Diphenylphosphanylacetylene with RB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> Reagents: Evidence for a Remarkable
Sequence of Synergistic Frustrated <i>Lewis</i> Pair Addition
Reactions
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Abstract
Diphenylphosphanylethyne
(<b>3a</b>) reacts with tris(pentafluorophenyl)borane
at room temperature by a typical frustrated <i>Lewis</i> pair (FLP) reaction. It undergoes a sequential series of 1,2-phosphane/borane
additions to the alkyne unit in an overall 2:2 molar ratio to selectively
form the dimeric product <b>5a</b>. Product <b>5</b> features
a pentafulvene-reminiscent structure with a pair of phosphonium units
in the ring and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> substituents
at the periphery. At elevated temperature, the reaction becomes less
selective, now favoring the formation of cis- and trans-1,1-carboboration
products from a 1:1 stoichiometry. After photolytic trans/cis isomerization,
the vicinal FLP <b>6a</b> becomes the major product, featuring
an intramolecular P···B interaction. The reaction of <b>3a</b> with H<sub>3</sub>CB(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> also gives a heterocyclic dimer (<b>11</b>), except that here
a substituent H/CH<sub>3</sub> exchange by an addition/elimination
pathway has taken place. In the B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>-derived system, we were able to trap an alleged intermediate
of this rearrangement reaction by adding <i>n</i>-butyl
isocyanide. Five products in the Ph<sub>2</sub>P- and (<i>p</i>-tolyl)<sub>2</sub>P-derived systems were characterized by X-ray
diffractio
