title: The B(C6F5)3 boron Lewis acid route to arene-annulated pentalenes.
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.
authors.affiliation: Organisch Chemisches Institut der Universität Münster, Corrensstrasse 40, 48149 Münster (Germany).
authors.forename: Chao
authors.initials: C
authors.lastname: Chen
authors.forename: Marcel
authors.initials: M
authors.lastname: Harhausen
authors.forename: Aiko
authors.initials: A
authors.lastname: Fukazawa
authors.forename: Shigehiro
authors.initials: S
authors.lastname: Yamaguchi
authors.forename: Roland
authors.initials: R
authors.lastname: Fröhlich
authors.forename: Constantin G
authors.initials: CG
authors.lastname: Daniliuc
authors.forename: Jeffrey L
authors.initials: JL
authors.lastname: Petersen
authors.forename: Gerald
authors.initials: G
authors.lastname: Kehr
authors.forename: Gerhard
authors.initials: G
authors.lastname: Erker
keywords: Lewis acids; conjugation; cyclization; dibenzopentalenes; internal alkynyl coupling