Addition of Silylenes

Motoyoshiya, H. Aoyama, T. Waka-hara, T. Akasaka, M. Fujitsuka, O. Ito, Angew. Chem. 2002, 114, 845 Angew. Chem. Int. Ed. Engl. 2002, 41, 817. 

Sugita, Y. Nakamura, E. Kasa-SHiMA, E. Yashima, Y. Okamoto, J. Nishi-MUEA,/. Am. Chem. Soc. 1997, 119, 926. 

Herrmann, H. Ringsdorf, C. Thil-gen, Helv. Chim. Acta 1993, 76, 2445. 

Eernandez-Paniagua, B. Illescas, N. Martin, C. Seoane, P. de ia Cruz, 

Gorgues, P. Hudhomme, J. Orduna, J. Garin, Tetrahedron Lett. 1997, 38, 81. 

---

In the past decade there has been considerable mechanistic study of the addition of silylenes to 1,3-dienes. From the first report of the formation of l-silacyclopent-3-enes as major products thirty years ago, a stepwise mechanism including vinylsilirane intermediates was proposed, but concerted 1,4-addition was also considered1-5. Vinylsilirane intermediates were clearly identified in trapping experiments by 19752, but questions have remained about the concertedness of both the primary addition and rearrangement steps... 

The current view of the mechanism of addition of silylenes to 1,3-dienes is based on the following experimental observations ... 

Complex product mixtures have been obtained from addition of silylenes to 3,6-bis(trifluoromethyl)-l,2,4,5-tetrazine and 2,5-bis(trifluoromethyl)-l,3,4-oxadiazole172. 

Formal 1,4-addition of silylenes to 2,2 -bipyridyls leads to the formation of deep violet, highly air-sensitive adducts (equation 73)173 174. 

D. Lei, A Mechanistic Study of the Addition of Silylenes to 1,3-Dienes in Both the Forward and Reverse Directions, Doctoral Dissertation, Washington University, St. Louis, August 1988. 

The kinetically stabilized silylene 132 easily reacted with carbon disulfide to afford the four-membered Si-S-Si-C framework. The formation of product 133 most probably proceeded via double addition of silylene 132 to the C=S double bond followed by rearrangement of the spirocyclic intermediate (Scheme 48) <1996PAC895>. 

Transition metal complexes were known to facilitate the addition of silylene to acetylenes from a variety of different sources.60,61,90,91 These conditions, however, often required heating, and the initially formed silacyclopropene often incorporated a second molecule of the acetylene to afford a silole.92,93 With their discovery of low-temperature silver-mediated di-ferf-butylsilylene transfer conditions from cyclohexene silacyclopropane 58 to olefins, Woerpel and coworkers set out to investigate the... 

The recoil-atom reaction with phosphine-ethylene mixture (38) produced a product identified as 31SiH3CH2CH2PH2. This finding was rationalized by a mechanism involving addition of silylene to the 71-bond of ethylene ... 

Addition of silylene units to unsaturated systems is a rich area of reaction chemistry The mechanism by which such species inserts into alkenes has been explored. ... 

With naphthalene two silicon units are also added, but in 81% yield [reaction (b)]. These results differ from the previously reported [1+4] addition of silylenes to anthracene. A [1 + 2] cycloaddition product is also obtained (in 58% yield) when Dip2Si (Dip = diisopropylphenyl) is combined with C o in toluene. ... 

Unsaturated 1,4-disilacyclohexane derivatives are also of interest. A facile approach is the addition of silylene (316) to acetylene at low temperatures which, in the first stage, leads to 317 and then dimerizes to 1,1,4,4-tetramethyl-l,4-disilacyclohexa-2,5-diene (368) at higher temperature (equation 166)168. 

Increased interest in the photochemistry of organosilicon compounds appears to be developing. Silacyclopropanes and silahexatrienes are now being proposed as photochemical intermediates on the basis of trapping experiments the former compounds appear to be produced by addition of silylenes R2Si to alkenes... 

The formation of 1,4-disilacyclohexadiene from the addition of silylene to alkynes could be explained by two separate mechanisms as indicated in the scheme in equation (117). Mechanism a involves the addition of SiXY to the acetylene to give a substituted silacyclopropene, which subsequently dimerizes to give the disilacyclohexadiene compound. Mechanism b first calls for the... 

Overall, for the addition of silylenes to alkynes to give 1,4-disilacyclo-hexadienes, the operation of mechanism b seems to be firmly established. The complete operation of mechanism a is probably in doubt, but the conversion of silacyclopropene to disilacyclobutene (patch c) is still possible. However, since the primary steps of both mechanisms, the addition of silylene to n bonds and the dimerization of silylenes, are both well established, the relative importance of the two mechanisms should be kinetically controlled by the concentration of the silylenes. At some lower concentration of the reacting silylenes, the silacyclopropene route may actually prevail. 

Direct calculations of the reaction pathways, by which carbenes are added to alkenes, clarify some important details of the intrinsic mechanism of such reactions. At the level of ab initio calculations, the reaction of singlet methylene with ethylene has been studied in the most detailed fashion [48]. Like the addition of silylene [49], this reaction proceeds without a barrier with the attacking particle moving under the plane of alkene ( internal approach). External approach of methylene and other carbenes is energetically less... 

Silicon and germanium also exhibit carbene-like chemistry. A review comparing carbene and silylene chemistry was published. All the examples of germylene chemistry are from theoretical studies of their structure and their interaction with alkenes. A practical demonstration of silylene chemistry was reported. Ab initio calculations have been performed on the addition of silylenes to alkenes and they shed new light on the presence of intermediates, the structure of the transition state and the effect of substituents. ... 

---