SYNTHESIS, STRAIN, CONFORMATIONAL-ANALYSIS, AND MOLECULAR AND CRYSTAL-STRUCTURES OF 1,1,4,4-TETRAPHENYL-1,4-DISILACYCLOHEXANE AND 1,1,4,4-TETRAPHENYL-1,4-DISILACYCLOHEXA-2,5-DIENE

Hydrogenation of 1,1,4,4-tetraphenyl-1,4-disilacyclohexa-2,5-diene (2) to 1,1,4,4-tetraphenyl-1,4-disilacyclohexane (1) provides an efficien t new route to 1,4-disilacyclohexanes with isolated overall yields 10 times higher than previously available syntheses. Molecular and crysta l structures of 1 and 2 determined by single-crystal X-ray diffraction show that the saturated compound 1 adopts the ideal chairlike conform ation, while the unsaturated analogue 2 has a planar 1,4-disilacyclohe xa-2,5-diene ring. Ab initio geometry optimizations and vibrational fr equency calculations at the 3-21G/SCF level of theory confirm that th e orientations of the phenyl rings observed in 1 represents the global minimum on the potential-energy surface. Calculations of the transiti on state for the phenyl ring rotations in 1 reveal a barrier height of Delta G double dagger = 4.09 kcal/mol. Energy profiles of chairlike a nd boatlike distortions of 1,4-disilacyclohexa-2,5-diene (12) were cal culated and compared with the observed deviation from planarity in oth er derivatives of 12. Calculation of ring strain energies revealed tha t diene 12 and its saturated analogue 1,4-disilacyclohexane (4) are al most strain free (1.2 and 0.7 kcal/mol, respectively), resembling thei r carbon analogues.