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 PtBu3/B(C6F5)3 FLP gave an analogous result, yielding 7b. Treatment of 1,2-diethynylbenzene (9) with the PtBu3/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.

三(五氟苯基)硼烷与9,9-二炔丙基芴(5)以2:1的摩尔比在常温常压条件下反应，得到双1,1-碳硼化产物的统计混合物：(Z,Z)-、(E,Z)-及(E,E)-构型的6。对该产物混合物进行光解，可得到显著富集(Z,Z)-6异构体的新混合物。以三(邻甲苯基)膦/三(五氟苯基)硼受阻路易斯对（frustrated Lewis pair, FLP）处理5，可得到两性离子型八元杂环7a，其生成路径为：先对一个炔基单元发生1,1-碳硼化，随后原位生成的新型FLP对剩余炔基片段进行分子内1,2-加成。采用三叔丁基膦/三(五氟苯基)硼FLP进行反应可得到类似结果，生成7b。以三叔丁基膦/三(五氟苯基)硼FLP处理1,2-二乙炔基苯(9)会发生去质子化反应，生成鏻/炔基硼酸盐盐10。使用三芳基膦/三(五氟苯基)硼对（其中膦分别为三(邻甲苯基)膦和P{Ph2[2,5-双(三氟甲基)苯基]}）可与C≡C三键发生高选择性的1,2-P/B FLP加成反应，分别得到产物11和12。最终，以弱亲核性膦三(五氟苯基)膦与三(五氟苯基)硼组合处理9，可得到两性离子产物14，该产物被认为通过两步反应生成：先发生1,1-碳硼化，随后原位生成的烯基硼烷路易斯酸与三(五氟苯基)膦对剩余炔基发生1,2-P/B FLP反应，经外切型炔烃环化完成环化。产物(Z,Z)-6、7a、7b、10、12及14均通过X射线晶体结构分析完成了表征。