Isomeric Metamorphosis: Si₃E (E = S, Se, and Te) Bicyclo[1.1.0]butane and Cyclobutene

Group 14 and 16 hybrid heavy bicyclo[1.1.0]butanes (tBu2MeSi)4Si3E (E = S, Se, and Te) 2a−c have been prepared by the [1 + 2] cycloaddition reaction of trisilirene 1 and the corresponding chalcogen. Bicyclo[1.1.0]butanes 2 have exceedingly short bridging Si−Si bonds (2.2616(19) Å for 2b and 2.2771(13) Å for 2c), a phenomenon explained by the important contribution of the trisilirene−chalcogen π-complex character to the overall bonding of 2. Photolysis of 2a and 2b produced their valence isomers, the heavy cyclobutenes 3a and 3b, featuring flat four-membered Si3E rings and a planar geometry of the SiSi double bond. The mechanism of such isomerization was studied using deuterium-labeled 2a−d6 to ascertain the preference of the pathway, involving the direct concerted symmetry-allowed transformation of bicyclo[1.1.0]butane 2 to cyclobutene 3.