beta-Silylcarbenes from isolable diazosilanes.
Manganese dioxide oxidation of the hydrazone derivative of tert-butyldimethylsilyl acetophenone gave 2-tert-butyldimethylsilyl-1-phenyldiazoethane (17) an isolable diazocompound. Thermal and Rh(II)-catalyzed decomposition of diazosilane 17 in cyclohexane led to 1-tert-butyldimethylsilyl-1-phenylethylene (19) as the major product. The formation of alkene 19 presumably involves (tert-butyldimethylsilyl)methylphenylcarbene (21), which undergoes preferential 1,2-silyl migration as opposed to 1,2-hydrogen migration. Thermal decomposition of 17 in cyclohexane under oxygen gave substantial amounts of tert-butyldimethylsilyl acetophenone, presumably by reaction of the intermediate carbene with oxygen. Thermal decomposition of 17 in methanol led to alkene 19 and 2-tert-butyldimethylsilyl-1-methoxy-1-phenylethane (22) as major products, along with a significant amount of trans-1-tert-butyldimethylsilyl-2-phenylethylene (20). Kinetic studies indicate that these products are not derived from acid-catalyzed decomposition of the diazocompound 17. Formation of the methyl ether product 22 suggests the involvement of a beta-silyl carbocation intermediate, and solvent isotope effect studies indicate that this cation is at least partially derived from protonation of diazocompound 17 by neutral methanol. Photochemical decomposition of 17 in methanol produced the alkene 19 (97%) along with a small amount (2.4%) of the methyl ether 22. Capture of a photochemically generated carbene 21 by methanol is the proposed origin of this minor product. Geometry optimization of trimethylsilylmethylphenylcarbene (8) and carbene 21 at the HF/6-31G computational level led to a conformation consistent with a hyperconjugative interaction between the vacant p-orbital of these carbenes and the adjacent C-Si bond. Carbenes 8 and 21 are not energy minima at the B3LYP/6-31G level, where they rearrange to alkenes without barrier via silyl migration. These theoretical findings contrast with the proposed trapping of carbene 21 by methanol and oxygen.
Creary
Xavier
X
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA. creary1@nd.edu
Butchko
Mark A
MA