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.
A series of tosylhydrazone derivatives of exo-6-substituted bicylo[2.2.2]octan-2-ones have been prepared. Thermal decomposition of the sodium salts of these tosylhydrazones gives carbene-derived products from 1,3-migration of either the C6 hydrogen (perturbed) or the C7 hydrogen (unperturbed), along with smaller amounts of alkenes derived from 1,2-hydrogen migration. The exo-6-substituent strongly activates 1,3-hydrogen migration in the case of SiMe(3) and weakly activates it in the case of CH(3) substitution. Thiomethoxy and carbomethoxy are weakly deactivating, while cyano and methoxy groups are strongly deactivating. B3LYP/6-31G* calculations on these substituted carbenes and transition states are in qualitative agreement with the ease of 1,3-hydrogen migration of perturbed vs unperturbed hydrogen. These experimental results and computational studies suggest carbene stabilization due to the exo-6-silyl group. They also suggest a reactant-like transition state for 1,3-hydrogen migration in which the inductive effect influences ease of migration. In the case of the exo-6-methoxy group, the inductive effect overwhelms any potential resonance-stabilizing effects.
Thermal decomposition of the in situ generated lithium salt of the tosylhydrazone derivative of cyclopropyl trimethylsilylmethyl ketone gave 1-cyclopropyl-1-trimethylsilylethylene, a product of exclusive silyl migration. Thermal decomposition of the sodium salts of tosylhydrazone derivatives of 1-trimethylsilylcyclopropyl alkyl ketones also gave methylenecyclopropane products derived from trimethylsilyl migration. These reactions were interpreted in terms of rapid trimethylsilyl migration to carbene-like centers that compete effectively with ring expansion processes of cyclopropylcarbenes. Computational studies (B3LYP/6-31G) suggest that cyclopropyl stabilization of carbenes is more effective than beta-trimethylsilyl stabilization. However, beta-trimethylsilyl stabilized conformations are easily attained, and these conformations can lead to silyl migrations. There are two minimum energy conformations of methyl-1-trimethylsilylcyclopropylcarbene, 27, and the rotational barrier to interconversion of these conformations (5.4 kcal/mol) is substantially lower than in the parent cyclopropylcarbene (15 kcal/mol). The onset of a stabilizing interaction in the transition state between the carbene vacant orbital with the adjacent Si-C sigma-orbital is proposed. Computational studies also show a very small (2.0 kcal/mol) barrier for trimethylsilyl migration in trimethylsilylmethyl cyclopropylcarbene, 11.
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