630 C12HsBr2 Moes, G. W. H. & Lenstra, A. T. H. (1986). Toxicol. Environ. Chem. 12, 255-266. Molecular Structure Corporation (1994a). MSC/AFC Diffractometer Control Software. MSC, 3200 Research Forest Drive, The Wood- lands, TX 77381, USA. Molecular Structure Corporation (1994b). TEXSAN. Single Crystal Structure Analysis Software. MSC, 3200 Research Forest Drive, The Woodlands, TX 77381, USA. North, A. C. T., Phillips, D. C. & Mathews, E S. (1968). Acta Crvst. A24, 351-359. Pedersen, B. F. (1975). Acta Crvst. B31, 2931-2933. Rajnikant, Watkin, D. & Tranter, G. (1995). Acta Crvst. C5I, 1452- 1454. Rcmming, C., Seip, H. M. & Oymo, I.-M. A. (1974). Acta Chem. Scand. Ser. A, 28, 507-514. Sheldrick, G. M. (1990a). Acta Cryst. A46, 467-473. Sheldrick, G. M. (1990b). SHELXTL. Version 4.21. Siemens Analyti- cal X-ray Instruments Inc., Madison, Wisconsin, USA. Sheldrick, G. M. (1993). SHELXL93. Program for the Refinement of Crystal Structures. Siemens Analytical X-ray Instruments Inc., Madison, Wisconsin, USA. Singh, P. & McKinney, J. D. (1979). Acta Co'st. B35, 259-262. TTMS with alkenes and alkynes (Chatgilialoglu et al., 1992) has been used in particular for intramolecular addition of the corresponding alkyl (Kulicke et al., 1992) or vinyl (Miura et al., 1993) radicals gener- ated on the other additional double bond. As a re- sult of synthetic work in the free-radical cyclization of 1,6- and 1,7-enyne systems through silylation of the triple bond with TTMS, the six-membered cyclic com- pound, (E)-dimethyl 4-isopropyl-3-[tris(trimethylsilyl)- silylmethylene]- 1,1-cyclohexanedicarboxylate, (2), has been isolated in 51% yield as a colourless crystal with a melting point of 356 (1) K. MeOOC COOMe McOOC COOMe + HSi(SiMc3) 3 ~, . . Si(SiMe3)3 Me/~Me M (1) (2) Acta Cryst. (1999). C55, 630-632 (E)-Dimethyl 4-isopropyl-3-[tris(trimethyl- silyl)silylmethylene]-l,l-cyclohexanedi- carboxylate RODOLFOMORENO-FUQUEN, Luz MARINA JARAMILLO- G6MEZ AND JAIME MART[N-FRANCO Departamento de Quimica, Facultad de Ciencias, Universi- dad del Valle, Apartado 25360, Santiago de Cali, Colombia. E-mail: romoreno @ hypatia, univalle, edu. co (Received 16 April 1998; accepted 21 October 1998) Abstract The title isomer carbocyclic compound, C23H48045i4, was prepared by treatment of the 1,7-enyne sys- tem dimethyl (4-methylpent-3-enyl)propanodioate with tris(trimethylsilyl)silicon hydride in the presence of azobisisobutyronitrile. The C~-----C distance is 1.335 (4) A, the C==C--Si bond angle is 132.6 (2) ° and the C-- C==C--Si torsion angle is 6.2 (5) °. The E configuration of the exocyclic double bond has been confirmed by the present X-ray study. Comment Tris(trimethylsilyl)silane (TTMS) has emerged as an al- ternative to the more common tri-n-butyltin hydride in free-radical chain processes (Curran, 1988; Chat- gilialoglu, 1992, 1995). The hydrosilylating ability of © 1999 International Union of Crystallography Printed in Great Britain - all rights reserved No other compounds in which the bulky 'supersilyl' (Me3Si)3Si group is bonded to the methylene exocyclic double bond of the cyclohexane ring such as (2) are available in the literature. The enol system formed by 2,2-dimethyl-l-tris(trimethylsilyl)silylethanol (Frey et al., 1994), compound (3), is however the most similar one to compare with some parameters of the title compound. Parameters of the title compound can also be compared with 1-tris(trimethylsilyl)silyl-3,4,5,6-tetrakis- (trimethylsilyl)cyclohex-l-ene (Puranik & Fink, 1994), compound (4), where the supersilyl group is bonded to an endocyclic double bond of the cyclohexene ring. The crystal-structure determination of compound (2) was undertaken to confirm the E configurations of the exocyclic double bond. The Si--Si and C--Si distances of the tris(trimethylsilyl)silane of the title compound are close to the distances reported for the 'supersilyl' (Me3Si)3Si group (Frey et al., 1994). The C1---C7 distance reveals the existence of a double bond. The C7---Sil distance changes from 1.94,~, in compound (3) and 1.904(3)A in compound (4) to 1.888(3)A in the title compound. This difference suggests that compounds (3) and (4) have more steric strains due to the presence of large pendant groups in their structures. The C1---C7--Sil angle is 133.2 ° in compound (3). This value is close to the value found in the present structure [132.6(2)°]. This relatively large bond angle is likely to be due to the steric effects of the bulky groups in both compounds. A torsion angle of 6.2 (5) ° for C6---C1--C7--Sil shows the E configuration of compound (2). The cyclohexane backbone adopts a chair conformation with 0 = 174 (1)° and ~ = 85 (1)° (Cremer & Pople, 1975). The geminal carboethoxy groups bonded to C5 of the cyclohexane ring are oriented in such a way that the C=O groups maintain Acta Co'stallographica Section C ISSN 0108-2701 © 1999