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 (<b>5</b>) in a 2:1 ratio under ambient conditions to yield a statistical mixture of the double-1,1-carboboration products (<i>Z</i>,<i>Z</i>)-, (<i>E</i>,<i>Z</i>)-, and <i>(E,E)</i>-<b>6</b>. Photolysis of the product mixture yields a new mixture that is substantially enriched in the (<i>Z</i>,<i>Z</i>)-<b>6</b> isomer. Treatment of <b>5</b> with the P(<i>o</i>-tolyl)<sub>3</sub>/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> frustrated Lewis pair gave the zwitterionic eight-membered heterocycle <b>7a</b>, 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<sup><i>t</i></sup>Bu<sub>3</sub>/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> FLP gave an analogous result, yielding <b>7b</b>. Treatment of 1,2-diethynylbenzene (<b>9</b>) with the P<sup><i>t</i></sup>Bu<sub>3</sub>/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> FLP resulted in deprotonation, forming the phosphonium/alkynylborate salt <b>10</b>. The reaction of triarylphosphine/B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> pairs using the phosphines P(<i>o</i>-tolyl)<sub>3</sub> and P{Ph<sub>2</sub>[2,5-bis(trifluoromethyl)phenyl]} resulted in clean 1,2-P/B FLP additions to a CC triple bond to yield the products <b>11</b> and <b>12</b>, respectively. Eventually, the reaction of <b>9</b> with the poorly nucleophilic phosphine P(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> in combination with B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> gave the zwitterionic product <b>14</b>, 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<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to the remaining alkynyl unit. The products (<i>Z</i>,<i>Z</i>)-<b>6</b>, <b>7a</b>,<b>b</b>, <b>10</b>, <b>12</b>, and <b>14</b> were characterized by X-ray crystal structure analyses.