Frustrated Lewis Pair Reactions With Bis-Acetylenic Substrates: Exploring the Narrow Gap Separating Very Different Competing Reaction Pathways

Tris(pentafluorophenyl)borane reacts with 9,9-dipropargylfluorene (5) in a 2:1 ratio under ambient conditions to yield a statistical mixture of the double-1,1-carboboration products (Z,Z)-, (E,Z)-, and (E,E)-6. Photolysis of the product mixture yields a new mixture that is substantially enriched in the (Z,Z)-6 isomer. Treatment of 5 with the P(o-tolyl)₃/B(C₆F₅)₃ frustrated Lewis pair gave the zwitterionic eight-membered heterocycle 7a, which is the product of a 1,1-carboboration of one alkynyl unit followed by an intramolecular 1,2-addition of the in situ generated new FLP to the remaining alkynyl moiety. The reaction of the P^tBu₃/B(C₆F₅)₃ FLP gave an analogous result, yielding 7b. Treatment of 1,2-diethynylbenzene (9) with the P^tBu₃/B(C₆F₅)₃ FLP resulted in deprotonation, forming the phosphonium/alkynylborate salt 10. The reaction of triarylphosphine/B(C₆F₅)₃ pairs using the phosphines P(o-tolyl)₃ and P{Ph₂[2,5-bis(trifluoromethyl)phenyl]} resulted in clean 1,2-P/B FLP additions to a CC triple bond to yield the products 11 and 12, respectively. Eventually, the reaction of 9 with the poorly nucleophilic phosphine P(C₆F₅)₃ in combination with B(C₆F₅)₃ gave the zwitterionic product 14, which is thought to result from a two-step reaction involving 1,1-carboboration followed by an exo-dig cyclization by means of a 1,2-P/B FLP reaction of the in situ generated alkenylborane Lewis acid and P(C₆F₅)₃ to the remaining alkynyl unit. The products (Z,Z)-6, 7a,b, 10, 12, and 14 were characterized by X-ray crystal structure analyses.