THE RELUCTANT TITANIUM-CATALYZED CYCLOTRIMERIZATION OF 1-PHENYL-2-(TRIMETHYLSILYL) ACETYLENE - CRYSTAL-STRUCTURE OF 1,3,5-TRIPHENYL-2,4,6-TRIS(TRIMETHYLSILYL)BENZENE

The cyclotrimerization of 1-phenyl-2-(trimethylsilyl)acetylene (PTMSA) catalyzed by the TiCl4/Et(2)AlCl (Ti:Al = 10-20) system affords 1,3,5 -triphenul-2,4,6-tris(trimethylsilyl)benzene (I) (minor product) and e thyl-triphenyl-bis(trimethylsilyl)benzene (II) (major product) in low yields. An X-ray crystal structural study of I revealed that there are two different conformers in the unit cell. Compound I has a planar ce ntral ring with alternating values of angles at carbon atoms bearing t he trimethylsilyl group (av. 116.8(6)degrees) and at carbon atoms bear ing the Ph group (av. 123.1(9)degrees). The steric hindrance between t he substituents is relieved by the almost perpendicular orientation of the Ph rings with respect to the central ring (81 +/- 5 degrees). The replacement of one trimethylsilyl group by an Et group from Et(2)AlCl probably precedes the ring closure yielding II. The presence of trime thylsilyl groups on the carbon atoms involved in this step hinders the ring closure and gives rise to what are probably linear oligomers of PTMSA.