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.

2,5-Bis-trimethylsilyl substituted boroles

“…However, likely due to steric pressure of the substituents 1 reversibly eliminates alkyne over time (see ESI †). 91 Our attempts for direct Zr/Sn transmetallation of 1 with Me 2 SnCl 2 to stannole 5 failed (see ESI †). Reactions of 1 with boron halides RBCl 2 neither produced boroles 6 or boryl-borolenes 7 (vide infra) but led to formation of yet unidentified products.…”

Bis(alkynyl)borane: A New Class of Acyclic Boron-Containing π Ligands in η5-Coordination Mode

“…As the Cp 2 Zr source, we chose Rosenthal’s zirconocene synthon Cp 2 Zr­(py)­(Me 3 SiCCSiMe 3 ) ( 5 ), which has been intensively utilized in Tilley’s laboratory for the assembly of macrocycles and helicenes by metallocene-mediated reductive couplings of alkynes . Thus, the bis­(alkynyl)­aminoborane 1 was added to an equimolar amount of 5 in C 6 D 6 at ambient temperature.…”

Rosenthal’s Zirconocene

“…Tilley and co-workers reported the reductive coupling of, amongst others, mesityl-substituted alkynes. 7 The formation of fully regioselective products was observed, and the regioselectivity was attributed largely to steric effects. This led to the polymerization of a mesityl-terminated diyne, which was analyzed after demetalation.…”