Synthesis, Reactions, and Stereochemical Studies of New Bicyclic Organosilicons, 3-silabicyclo[3.2.1]octanes and 2-silabicyclo[2.2.1]heptanes

Several members of two new classes of bicyclic organosilicons, 3-silabicyclo[3.2.1]octanes (3-SBO) and 2-silabicyclo[2.2.t]heptanes (2-SBH), were prepared by five- and seven-step synthetic sequences, respectively. Reaction of the bis-organomagnesium reagent of cis-1,3-bis(bromomethyl)cyclopentane with trichloromethylsilane, trichlorophenylsilane, or silicon tetrachloride was used to prepare derivatives of the 3-SBO system. Platinumcatalyzed intramolecular hydrosilation of 4-dichloro­silyl)methyl]cyclopentene or 4-[(chloromethylsilyl)methyl]cyclopentene gave access to the 2-SBH series. New compounds were characterized by elemental analysis, 1H and 13C NMR, IR, and/or mass spectrometry, 29Si NMR spectra were obtained for 3-SBO, endo- and exo-3-methyl­ 3-SBO, 2-SBH, and endo- and exo-2-methyl-2-SBH. A unique mode of fragmentation was observed in the mass spectra of the isomers of 3-methyl-3-SBO involving loss of H2; this fragmentation gave the base peak in the spectrum of exo-3-methyl-J-SBO, whereas a corresponding peak of low intensity was observed for endo-3-methyl-3-SBO. Electron diffraction analyses were obtained for 3-SBO and endo-3-methyl-3-SBO; the latter analysis provided an unambiguous isomer assignment which was extended to six other isomer pairs in the 3-methyl-3-SBO series through correlation of several physical properties. Isomers of 2-methyl-2-SBH derivatives were assigned from the NMR spectral properties. The stereochemistry of several reactions of 3-methyl­ 3-SBO derivatives was studied; the results paralleled those observed for acyclic organosilicons. Studies of the stereochemistry of reactions of 2-methyl-2-SBH derivatives were initiated. LAH reduction of 3-chloro-3-methyl-3-SBO occurred with inversion of configuration, but the stereo­ chemistry of reduction of 2-chloro-2-methy-l2-SBH was more- complex. The isomers of 3-methyl-3-SBO, 3-methoxy-3-methyl­ 3-SBO, 3-methyl-3-phenyl-3-SBO, and 2-methyl-2-SBH were equilibrated by treatment with CsF in polar, aprotic solvents; (+)-α-naphthylphenylmethylsilane (1) was also readily racemized. The order of reactivity toward isomerization was I\u3e2-methyl-2-SBH~3-methoxy-3-methyl­ 3-SBO \u3e 3-methyl-3-SBO \u3e 3-methyl-)-phneyl-3-SBO. Isomerization of 3-methyl-3-SBO was examined with respect to the nature of the fluoride salt, solvent, and added crown ether. The proposed mechanism involves reversible formation of a pentacoordinate intermediate which can undergo pseudorotation. Two new types of reactions were observed for the bicyclic silyl hydrides in the presence of fluoride ion. A facile conversion of Si-H to Si-F was found. Some unique reactions with carboxylic amides were observed in which the silyl hydride was converted to a disiloxane