Reaction of diphenylphosphanylacetylene with RB(C6F5)2 reagents: evidence for a remarkable sequence of synergistic frustrated Lewis pair addition reactions.

Diphenylphosphanylethyne (3a) reacts with tris(pentafluorophenyl)borane at room temperature by a typical frustrated Lewis 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 5a. Product 5 features a pentafulvene-reminiscent structure with a pair of phosphonium units in the ring and B(C6F5)3 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 6a becomes the major product, featuring an intramolecular P···B interaction. The reaction of 3a with H3CB(C6F5)2 also gives a heterocyclic dimer (11), except that here a substituent H/CH3 exchange by an addition/elimination pathway has taken place. In the B(C6F5)3-derived system, we were able to trap an alleged intermediate of this rearrangement reaction by adding n-butyl isocyanide. Five products in the Ph2P- and (p-tolyl)2P-derived systems were characterized by X-ray diffraction.