title: Selective mono- to perarylations of tetrabromothiophene by a cyclobutene-1,2-diylbisimidazolium preligand.
abstract: (Cyclobut-1-ene-1,2-diyl)bis(1-methylimidazolium)tetrafluoroborate is applied as preligand in palladium-catalyzed cross-coupling reactions starting from tetrabromothiophene for the synthesis of mono-, bi-, tri-, and tetraaryl-substituted thiophenes bearing up to four different aryl rings. A synthetic kit for preparations of nine different substitution patterns of arylated thiophenes is presented by application of only one single catalyst system. In agreement with DFT calculations, which predict energetically low rotational barriers in triaryl-3-bromothiophenes and tetraarylthiophenes, no NOE effects between adjacent aryl rings are detectable. The regioselectivity of their syntheses has therefore been elucidated by reduction of triaryl-3-bromothiophene to 2,3,5-triarylthiophene followed by HMBC, HSQC, and NOESY NMR measurements. Additionally, results of an X-ray single structure analysis are presented.
authors.affiliation: Institute of Organic Chemistry, Clausthal University of Technology, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany.
authors.forename: Alireza
authors.initials: A
authors.lastname: Rahimi
authors.forename: Jan C
authors.initials: JC
authors.lastname: Namyslo
authors.forename: Martin H H
authors.initials: MH
authors.lastname: Drafz
authors.forename: Julian
authors.initials: J
authors.lastname: Halm
authors.forename: Eike
authors.initials: E
authors.lastname: Hübner
authors.forename: Martin
authors.initials: M
authors.lastname: Nieger
authors.forename: Nicola
authors.initials: N
authors.lastname: Rautzenberg
authors.forename: Andreas
authors.initials: A
authors.lastname: Schmidt