The B(C 6 F 5 ) 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(C 6 F 5 ) 3 to yield the multiply annulated, planar conjugated π‐system 13 (50 % yield). In the course of this reaction, a C 6 F 5 group was transferred from boron to carbon. Treatment of 12 with CH 3 B(C 6 F 5 ) 2 proceeded similarly, giving a mixture of 13 (C 6 F 5 ‐transfer) and the product 15 , which was formed by CH 3 ‐group transfer. 1,2‐Bis(phenylethynyl)benzene ( 8 a ) reacts similarly with CH 3 B(C 6 F 5 ) 2 to yield a mixture of the respective C 6 F 5 ‐ and CH 3 ‐substituted dibenzopentalenes 10 a and 16 . The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C 6 F 5 ) 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(C 6 F 5 ) 3 /P( o ‐tolyl) 3 . The overall formation of the C 6 F 5 ‐substituted products formally require HB(C 6 F 5 ) 2 cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl‐containing starting material 21 with B(C 6 F 5 ) 3 , which gave the respective annulated pentalene product 23 that had the HB(C 6 F 5 ) 2 moiety 1,4‐added to its thiophene ring. Compounds  12 – 14 , 23 , and 26 were characterized by X‐ray diffraction.