Synthesis and Reactivity of Phosphametallacycles:  Sterically Induced Epimerizations and Retrocycloadditions

Elimination of methane from Cp 2 Zr(PR*H)(Me) (R* = C 6 H 2 -2,4,6- t- Bu 3 ) ( 4 ) in the presence of diphenylacetylene or phenylpropyne afforded the [2 + 2] cycloaddition products Cp 2 Zr(P(R*)C(Ph) CPh) ( 3 ) and Cp 2 Zr(P(R*)C(Me) CPh) ( 5 ), respectively. Alternatively, the [2 + 2] cycloaddition reaction between (Cp 2 Zr PR*)(PMe 3 ) ( 1 ) and phenylacetylene yielded Cp 2 Zr(P(R*)C(H) CPh) ( 6 ). Metallacycles 3 and 5 undergo facile [2 + 2] retrocycloaddition reactions; addition of 1 equiv of phenylpropyne to 3 resulted in an equilibrium mixture of 3 and 5 . In contrast, addition of phenylacetylene to 3 yielded Cp 2 Zr(C⋮CPh)(C(Ph) C(Ph)PHR*) ( 7 ), the product of C H activation of the terminal alkyne. Attempts to synthesize a phosphametallacycle with less sterically hindered substituents on phosphorus by reaction of Cp 2 ZrMeCl, diphenylacetylene, and LiHPMes (Mes = C 6 H 2 -2,4,6-Me 3 ) instead led to the formation of Cp 2 Zr(P(Mes)P(Mes)C(Ph) CPh) ( 8 ). Probing the mechanism of formation of 8 by reaction of (Cp 2 ZrCl) 2 (μ-PMes) with Li 2 PMes in the presence of diphenylacetylene afforded Cp 2 Zr(P(Mes)C(Ph) CPh) 9 . However, reaction of 9 with H 2 PMes instead resulted in the formation of the unstable compound Cp 2 Zr(C(Ph) C(Ph)PMesH)(PMesH) ( 10 ). Phosphametallacyclobutene 3 reacts with tert -butyl isocyanide, acetone, cyclohexanone, benzonitrile, benzaldehyde and styrene oxide to give the insertion products Cp 2 Zr(C( N- t -Bu)P(R*)C(Ph) CPh) ( 12 ), Cp 2 Zr(OCMe 2 P(R*)C(Ph) CPh) ( 13 ), Cp 2 Zr(O( c -CC 5 H 10 )P(R*)C(Ph) CPh) ( 14 ), Cp 2 Zr(N C(Ph)P(R*)C(Ph) CPh) ( 15 ), Cp 2 Zr(OCHPhP(R*)C(Ph) CPh) ( 16 ), and Cp 2 Zr(OCH 2 CHPhP(R*)C(Ph) CPh) ( 17 ), respectively. Compounds 16 and 17 were also obtained by reaction of either benzaldehyde or styrene oxide with 13, 14, or 15, via [4 + 2] retrocycloadditions. Epimerization at phosphorus has been identified in complexes 3, 5, 6, 8, 9, and 12 − 15, and has been attributed to steric congestion in these species. Spectroscopic methods, X-ray crystallography, and molecular orbital calculations have been employed to address this issue.