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

Abstract

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 efficient new route to 1,4-disilacyclohexanes with isolated overall yields 10 times higher than previously available syntheses. Molecular and crystal structures of 1 and 2 determined by single-crystal X-ray diffraction show that the saturated compound 1 adopts the ideal chairlike conformation, while the unsaturated analogue 2 has a planar 1,4-disilacyclohexa-2,5-diene ring. Ab initio geometry optimizations and vibrational frequency calculations at the 3-21G*/SCF level of theory confirm that the orientations of the phenyl rings observed in 1 represents the global minimum on the potential-energy surface. Calculations of the transition state for the phenyl ring rotations in 1 reveal a barrier height of ΔG⧧ = 4.09 kcal/mol. Energy profiles of chairlike and boatlike distortions of 1,4-disilacyclohexa-2,5-diene (12) were calculated and compared with the observed deviation from plan...