The cis-trans isomerization of 1,2,5,6-tetrasilacycloocta-3,7-dienes: Analysis by mechanistic probes and density functional theory

A series of alkyl- and aryl-substituted derivatives of cis, cis-1,2,5,6-tet rasilacyclooeta-3,7-diene were prepared. Isomerization of these compounds t o the corresponding trans,trans-1,2,5,6-tetrasilacyclooeta-3,7-dienes by ex posure to Ru and Zr hydride complexes was explored. Experimental probes of the isomerization were consistent with a stepwise mechanism involving metal hydride addition/elimination rather than one involving radical intermediat es. Analysis of the low energy conformers of the various cis and trans isom ers of 1,1,2,2,5,5,6,6-octamethyl-1,2,5,6-tetrasilacyclooeta-3,7-diene usin g density functional theory suggested the following trend in stability: tra ns,trans > cis,trans > cis,cis. The calculated trend in stability was consi stent with the experimentally observed unidirectional isomerization of the carbon-carbon double bonds from all cis to all trans and supports a cis,tra ns isomer as a tenable intermediate.