Synthesis, photochemical decomposition and DFT studies of 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclopropane

• The title silacyclopropane has two substructures for silylene generation: a silacyclopropane ring and an aryltrisilane chain.
• The title silacyclopropane underwent photochemical and thermal 2 + 1 fragmentations to give bis(dimethylphenylsilyl)silylene.
• TD-DFT calculations show the excitation of electron from the silacyclopropane ring to the phenyl ring.
• Bis(dimethylphenylsilyl)silylene was trapped with triethylsilane, trimethylmethoxysilane and bis(trimethylsilyl)acetylene.

The compound, 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclopropane (1), is a potential photochemical source of silylenes either through a 2 + 1 fragmentation of the silacyclopropane ring core or through α-elimination from the aryltrisilane chain. Upon 254 nm photolysis, the silacyclopropane (1) exclusively undergoes 2 + 1 ring fragmentation with the formation of bis(dimethylphenylsilyl)silylene and tetramethylethylene. This clearly shows the heightened photoreactivity of the silacyclopropane ring compared with the aryltrisilane chain. The extruded bis(dimethyphenyl)silylene is intercepted with trapping agents such as triethylsilane, methoxytrimethylsilane and bis(trimethylsilyl)acetylene to form thermally stable adducts. Time Dependent-Density Functional Theory (TD-DFT) calculations at the B3LYP/6-31G* level reveal that an electron is excited from a Si–C σ orbital of the silacyclopropane ring (HOMO) to empty π* orbitals of phenyl group (LUMO). This weakens the Si–C bonds of the silacyclopropane ring which is believed to result in the observed 2 + 1 photofragmentation.

The 2,2,3,3-tetramethyl-1,1-bis(dimethylphenylsilyl)silacyclpropane (1) has two potential photochemical pathways for generating silylenes: through photofragmentation of the silacyclopropane ring or through extrusion via an aryltrisilane chain. Photochemical and thermal reactions of 1 demonstrated the dominance of silacycloprpoane ring fragmentation by the quantitative extrusion of bis(dimethylphenylsilyl)silylene and the tetramethylethylene.Figure optionsDownload as PowerPoint slide