The B(C6F5)3 Boron Lewis Acid Route to Arene-Annulated Pentalenes

4,5-Dimethyl-1,2-bis(1-naphthylethynyl)benzene (12) undergoes a rapid multiple ring-closure reaction upon treatment with the strong boron Lewis acid B(C6F5)(3) to yield the multiply annulated, planar conjugated -system 13 (50% yield). In the course of this reaction, a C6F5 group was transferred from boron to carbon. Treatment of 12 with CH3B(C6F5)(2) proceeded similarly, giving a mixture of 13 (C6F5-transfer) and the product 15, which was formed by CH3-group transfer. 1,2-Bis(phenylethynyl)benzene (8a) reacts similarly with CH3B(C6F5)(2) to yield a mixture of the respective C6F5- and CH3-substituted dibenzopentalenes 10a and 16. The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C6F5)(3)-substituted species (26, 27) consequently formed by in situ deprotonation upon treatment of the respective 1,2-bis(alkynyl)benzene starting materials (24, 8) with the frustrated Lewis pair B(C6F5)(3)/P(o-tolyl)(3). The overall formation of the C6F5-substituted products formally require HB(C6F5)(2) cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl-containing starting material 21 with B(C6F5)(3), which gave the respective annulated pentalene product 23 that had the HB(C6F5)(2) moiety 1,4-added to its thiophene ring. Compounds12-14, 23, and 26 were characterized by X-ray diffraction.