Synthesis of 1,4-disilacyclohexa-2,5-dienes

Title compounds of the type 2,3,5,6-tetraphenyl-1,4-di-X-1,4-di-Y-1,4-disilacyclohexa-2,5-diene wherein X=Y=NMe2 (4); X=NMe2, Y=Cl (cis, trans-5); X=NMe2, Y=Me [(trans)-6] and X=t-Bu, Y=Cl (trans-8) were synthesized from Si2(NMe2)5Cl, sym-Si2(NMe2)4Cl2, sym-Si2(NMe2)4Me2, and sym-Si2Cl4(t-Bu)2, respectively, in the presence of diphenylacetylene at 200 °C. Similarly the analogous title compound from the combination of 1-phenyl-1-propyne and Si2(NMe2)5Cl [X=Y=NMe2 (cis and trans-7) was synthesized. In all cases where cis/trans diastereomers could arise from two different silicon substituents (5, 6, 8) the trans isomer was the sole or dominant product. Evidence for the intermediacy of the silylene Si(NMe2)2 in these reactions was gained from a trapping experiment. Compound 4 upon treatment with SiCl4, SiBr4 or PI3 provided the corresponding 1,1,4,4-tetrahalo derivatives 9a–c, respectively. Treatment of 4 with MeOH or PhOH gave the 1,1,4,4-tetramethoxy and tetraphenoxy analogues 9d and 9e, respectively. The tetrachloro derivative 9a upon LAH reduction led to the corresponding tetrahydro compound 10, while the reaction of 9a with H2O gave the tetrahydroxy derivative 11. Allowing (trans)-6 to react with SiCl4 provided a ca. 1:1 cis/trans ratio of the derivative 12 in which X=Cl, Y=Me, and possible pathways that rationalize this loss of stereochemistry are proposed. Synthesis of trans-13 in which X=t-Bu, Y=H was achieved by LAH reduction of 8. All of the title compounds except 8 experience free phenyl rotation at room temperature. At −30 °C this rotation in 8 is essentially halted. The molecular structures of 4, 8, 9c, 9e, 10 and 13 were determined by X-ray crystallography.