Unsymmetrical Zirconacyclopentadienes from Isolated Zirconacyclopropenes with 1-Alkynylphosphine Ligands

Abstract

The reaction of one equiv of 1-alkynylphosphines, R 2 PC≡CR′ (R = Et, i Pr, or Ph and R′ = Ph or Mes), with Cp 2 Zr(pyr)(η 2 -Me 3 SiC≡CSiMe 3 ) resulted in formation of monoalkyne complexes. In the case where R = Et, i Pr, or Ph and R′ = Ph, a “ligand free” zirconacyclopropene complex is produced. These complexes are stabilized by intermolecular donation of the phosphorus lone-pair in the dimeric complexes [Cp 2 Zr(η 2 -R 2 PC≡CPh)] 2 (R = Et, i Pr, or Ph). However, with R = Ph and R′ = Mes, the zirconocyclopropene-pyridine complex Cp 2 Zr(pyr)(η 2 -Ph 2 PC≡CMes) is formed. Homocoupling of the 1-alkynylphosphines was demonstrated by reaction of a second equiv of Ph 2 PC≡CPh with [Cp 2 Zr(η 2 -Ph 2 PC≡CPh)] 2 to give the diphosphinozirconacyclopentadiene Cp 2 Zr[2,5-(Ph 2 P) 2 -3,4-Ph 2 C 4 ] with high regioselectivity (77%). The zirconacyclopropene complexes also react with one equiv of PhC≡CPh or EtC≡CEt to give zirconacyclopentadienes in which the phosphino substituent preferentially adopts the 2-position (α) of the zirconacyclopentadiene ring. These unsymmetrical zirconacyclopentadienes undergo substitution of the R 2 PCCR′ moiety with the less bulky alkynes PhC≡CPh or EtC≡CEt. The substituents on the 1-alkynylphosphines significantly influence the rates of alkyne substitution such that sterically more demanding substituents in either the α- (-P i Pr 2 ) or β- (-Mes) position of the zirconacycle lead to faster exchange. The α-phosphinozirconacyclopentadienes were readily converted to 1-phosphinobutadienes via reaction with benzoic acid. The zirconacyclopentadiene Cp 2 Zr[2-Ph 2 P-3,4,5-Ph 3 C 4 ] was converted to the corresponding thiophene oxide by the oxo-transfer reaction with sulfur dioxide. In the case of ((1 Z,3 E )-3-ethyl-2-phenylhexa-1,3-dienyl)diphenylphosphine and (3,4,5-triphenylthiophen-2-yl oxide)diphenylphosphine, the molecules were isolated as their phosphine oxides. Reactions of the zirconacyclopropene complexes [Cp 2 Zr(η 2 -Ph 2 PC≡CPh)] 2 and Cp 2 Zr(pyr)(η 2 -Ph 2 PC≡CMes) with the diyne (F 5 C 6 )C≡C−1,4-C 6 H 4 −C≡C(C 6 F 5 ) gave bis(zirconacycle)s terminated with phosphino groups. These bis(zirconacycle)s were converted to the corresponding phosphino-terminated oligomers by protonolysis with hydrochloric acid. In addition, the Ph 2 PCCMes moiety of Cp 2 Zr[2-Ph 2 P-3-Mes-4-(C 6 F 5 )C 4 ]−1,4-C 6 H 4 −Cp 2 Zr[2-Ph 2 P-3-Mes-4-(C 6 F 5 )C 4 ] was exchanged with PhC≡CPh to give the phenylene(zirconacyclopentadiene) Cp 2 Zr[2,3-Ph 2 -4-(C 6 F 5 )C 4 ]−1,4-C 6 H 4 −Cp 2 Zr[2,3-Ph 2 -4-(C 6 F 5 )C 4 ].