The B(C6F5)3 Boron Lewis Acid Route to Arene‐Annulated Pentalenes |
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| Authors: | Dr Chao Chen Dr Marcel Harhausen Prof Aiko Fukazawa Prof Shigehiro Yamaguchi Dr Gerald Kehr Prof Gerhard Erker |
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| Affiliation: | 1. Organisch Chemisches Institut der Universit?t Münster, Corrensstrasse 40, 48149 Münster (Germany);2. Department of Chemistry Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya, 464‐8602 (Japan) |
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| Abstract: | 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 π‐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 ( 8 a ) reacts similarly with CH3B(C6F5)2 to yield a mixture of the respective C6F5‐ and CH3‐substituted dibenzopentalenes 10 a 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. Compounds 12 – 14 , 23 , and 26 were characterized by X‐ray diffraction. |
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| Keywords: | conjugation cyclization dibenzopentalenes internal alkynyl coupling Lewis acids |
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