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

Abstract

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=NMe 2 ( 4); X=NMe 2, Y=Cl ( cis, trans- 5); X=NMe 2, Y=Me [( trans)- 6] and X= t-Bu, Y=Cl ( trans- 8) were synthesized from Si 2(NMe 2) 5Cl, sym-Si 2(NMe 2) 4Cl 2, sym-Si 2(NMe 2) 4Me 2, and sym-Si 2Cl 4( 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 Si 2(NMe 2) 5Cl [X=Y=NMe 2 ( 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(NMe 2) 2 in these reactions was gained from a trapping experiment. Compound 4 upon treatment with SiCl 4, SiBr 4 or PI 3 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 H 2O gave the tetrahydroxy derivative 11. Allowing ( trans)- 6 to react with SiCl 4 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.