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 potentialenergy surface. Calculations of the transition state for the phenyl ring rotations in 1 reveal a barrier height of ∆G q ) 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 planarity in other derivatives of 12. Calculation of ring strain energies revealed that diene 12 and its saturated analogue 1,4-disilacyclohexane (4) are almost strain free (1.2 and 0.7 kcal/mol, respectively), resembling their carbon analogues.

Synthesis and Unusual Reactivity of Acyl-Substituted 1,4-Disilacyclohexa-2,5-dienes

“…Siloles (silacyclopenta-2,4-dienes) are the most thoroughly studied representatives of this class of compounds because they are readily prepared on a preparative scale with a wide range of substitution patterns. − High-efficiency light-emitting diodes, organic solar cells, and high-mobility field-effect transistors have all been reported based on silole derivatives, indicating the potential importance of siloles to materials chemistry. Related studies involving six-membered 1,4-disilacyclohexadienes were more or less restricted to synthetic issues − until Ottosson and co-workers discovered that this class of compounds may complement or even outperform siloles in the design of electronically active materials. − In particular for 1,1,4,4-tetrakistrimethylsilyl-1,4-disilacyclohexa-2,5-diene 1 , strong cyclic cross-hyperconjugation was detected as shown by its long wavelength UV absorption and its unusually low oxidation potential and valence photoionization energies.…”