Carbon-silicon rings

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

The structures of compounds 55a,c and 56a,c were established by means of NMR spectroscopy and mass spectrometry. Due to the different polarity of the C=N and C=P triple bonds, the silicon ring atom in 55a,c is bound to the nitrogen atom, and in 56a,c to the carbon atom of the C=P moiety. The molecular structure of 55a was further determined by single-crystal X-ray diffraction analysis (Fig. 16).14 The four-membered SiNAsC framework is slightly puckered (folding angle N—Si—C/Si—C—As 7°), and... 

CARBON-SILICON/GERMANIUM-NITROGEN-METAL RINGS... 

Several other factors play a role in the differential stabilization of three- and four-membered carbon and silicon rings. 

The use of a silyl ether temporary linkage introduced by Nishiyama [82J and Stork [77,83] allows the facile cleavage of the five-membered ring formed in the cyclization by oxidation of the carbon-silicon bond. This procedure has been successfully used for the hydroxymethylation of sugars at position 3, 4, and 6 [84-86] (Scheme 24). 

A short time ago numerous triorganyl-cyclotriphosphanes have become accessible as pure substances on various synthetic routes (2). Moreover, three-membered phosphorus heterocycles with carbon, silicon, germanium, arsenic, boron, and sulfur as hetero ring atoms could be synthesized (2). Recently, we have found that such compounds with elements of the fifth periodtare als stable enough fop existence. Through the reaction of K(Bu )P-P(Bu )K with Bu S C1 or Bu -SnC at -78°C, the diphosphastib rane ... 

CARBON-SILICON-SULFUR (or OXYGEN or NITROGEN) RINGS... 

Examples of carbene insertions into the carbon-silicon bond of SCBs have been known since 1967, when Seyferth studied the behavior of SCBs exposed to dichlorocarbene, which was generated by thermolytic activation of phenyl(bromodichloromethyl)mercury <1967JA1538>. The reaction produces a mixture of products arising from Si-C and C-H bond insertions, with the major products being the ring-expanded silacyclopentanes that result from Si-C bond insertions (Scheme 30). 

Ring expansion in conjunction with Tamao-type oxidation of carbon-silicon bonds provides access to 1,4-diols. The l-(l-iodoalkyl)-l-silacyclobutanes are available from 1-chlorosilacyclobutanes (addition of vinyl, Scheme 34) <1991TL6383>. The utility of silacyclopentanes formed by the ring expansion of SCB for the synthesis of diols has been reported <1992TL7031, 1995BCJ625>. 

The Si=As double bond of 104 reacted with the strongly polarized C-N triple bond in benzonitrile to furnish the [2+2] cycloaddition product 37 (Scheme 27), while similar addition across a C-P triple bond afforded the novel heterocycle 55. According to the reverse polarity of the C-P triple bond compared with the C-N triple bond, the silicon ring atom in 55 is bound to the carbon atom <19960M1845>. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

The following examples further illustrate the selectivity that can emerge from this analysis (Scheme 6). Divinylcyclopropane (30) underwent exclusively the cleavage of the siloxy-substituted ring with activation enhancement of ca. 5 kcal mol", whereas the silyl-substituted counterpart (31) preferred the cleavage of the carbon-substituted ring due to the decelerating effect of silicon. ... 

The NMR spectrum of metallacycle 22 revealed two resonances at 7.8 and 8.2 ppm, three resonances at 128.3, 132.4, and 148.1 ppm, and a single resonance at 191.2 ppm, due to Et2Si carbons, phenylene ring carbons, and alkene carbons, respectively <1996OM1101>. In addition, the Si NMR spectrum showed a single resonance at —2.7 ppm at much lower field that that of the silicon atoms in 5,6-benzo-l,4-disilacyclohexa-2,5-dienes, indicating that compound 22 must have a 4,5-benzo-l,3-disilacyclopent-4-ene structure. 

The cleavage of the carbon-silicon bond of 1-silyl-l-vinylcyclopropane has been achieved by the fluoride ion. The reaction has been performed138) in only one case (Scheme 65) but has severd to generate a pentadienyl anion which was regioselectively hydroxy-alkylated on the cyclopropane ring 138) (Scheme 65). This reaction has been successfully used for the synthesis of ( ) a-vetispirene138) (Scheme 65). 

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