title: Mesityl alkyne substituents for control of regiochemistry and reversibility in zirconocene couplings: new synthetic strategies for unsymmetrical zirconacyclopentadienes and conjugated polymers.
abstract: Reaction of 2 equivs of MesC[triple bond]CPh with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) afforded the zirconacyclopentadiene Cp(2)Zr[2,5-Ph(2)-3,4-Mes(2)C(4)]. The regiochemistry of this isomer (betabeta with respect to the mesityl substituents) was determined through single-crystal X-ray analysis and 2D (NOESY, HSQC, HMBC) NMR experiments. This selectivity is attributed largely to a steric-based directing effect of the o-methyl ring substituents since coupling of 1,3-dimethyl-2-(phenylethynyl)benzene with zirconocene gave a single regioisomer (o-xylyl groups in both beta-positions) while coupling of 1,3-dimethyl-5-(phenylethynl)benzene gave a statistical distribution of zirconacyclopentadiene regioisomers. The coupling reaction of 2 equivs of MeC[triple bond]CMes or PrC[triple bond]CMes with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) at ambient temperature gave the betabeta regioisomers, Cp(2)Zr[2,5-Me(2)-3,4-Mes(2)C(4)] and Cp(2)Zr[2,5-Pr(2)-3,4-Mes(2)C(4)], respectively, as the major products. Heating solutions of these zirconacycles at 80 degrees C for several hours resulted in an increase in the amount of the unsymmetrical product. For reaction mixtures of PrC[triple bond]CMes and Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) the major (and apparently thermodynamic) product under these reaction conditions was Cp(2)Zr[2,4-Mes(2)-3,5-Pr(2)C(4)]. The steric strain in the mesityl-substituted zirconacycles allowed for facile substitution reactions of MesC[triple bond]CPh or PrC[triple bond]CMes by less bulky alkynes (i.e., tolan and 3-hexyne) to give the unsymmetrical ziconacyclopentadienes Cp(2)Zr[2,4,5-Ph(3)-3-MesC(4)], Cp(2)Zr[2-Ph-3-Mes-4,5-Et(2)C(4)], and Cp(2)Zr[2-Pr-3-Mes-4,5-Ph(2)C(4)]. Reaction of a mesityl-terminated diyne containing a rigid dihexylfluorenylene spacer with zirconocene afforded poly(p-fluorenylenedienylene) after demetalation with benzoic acid.
authors.affiliation: Department of Chemistry, University of California, Berkeley, Berkeley, California 94720-1460, USA.
authors.forename: Adam D
authors.initials: AD
authors.lastname: Miller
authors.forename: John F
authors.initials: JF
authors.lastname: Tannaci
authors.forename: Samuel A
authors.initials: SA
authors.lastname: Johnson
authors.forename: Hyuk
authors.initials: H
authors.lastname: Lee
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: McBee
authors.forename: T Don
authors.initials: TD
authors.lastname: Tilley