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(C 6 F 5 ) 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 t Bu 3 /B(C 6 F 5 ) 3 FLP gave an analogous result, yielding 7b . Treatment of 1,2-diethynylbenzene ( 9 ) with the P t Bu 3 /B(C 6 F 5 ) 3 FLP resulted in deprotonation, forming the phosphonium/alkynylborate salt 10 . The reaction of triarylphosphine/B(C 6 F 5 ) 3 pairs using the phosphines P( o -tolyl) 3 and P{Ph 2 [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(C 6 F 5 ) 3 in combination with B(C 6 F 5 ) 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(C 6 F 5 ) 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.