Silenes Peterson reaction

TABLE 4. Silenes R1R2Si=CR3R4 synthesized by the sila-Peterson reaction and related reactions... 

The Synthesis of Transient Silenes using the Principle of the Peterson Reaction... 

Krempner, C., Reinke, H., Oehme, H. The synthesis of transient silenes using the principle of the Peterson reaction. Organosilicon Chemistry II From Molecules to Materials, [Muenchner Silicontage], 2nd, Munich, 1994 1996, 389-398. 

The mechanism of the formation of the silanol 9 (Scheme 3) is understood as the result of an acid-catalyzed formal elimination of dimethylaminotrimethylsilane generating the silene 7, followed by the addition of H2SO4 at the silene double bond and subsequent hydrolysis of the silylsulfate 8. To our best knowledge, this is the first case of an acid-induced sila-Peterson reaction. 

Scheme 2.157. Synthesis of silenes by the sila-Peterson reaction.

A modified Peterson reaction has been used to generate silenes which have then been converted to diols and lactones. The reaction involves the formation of the sila-Grignard reagents under standard conditions, followed by treatment with isobutyraldehyde to give the silene precursor 76. Silacyclohexene 76 was then produced by reaction with 1,3-pentadiene. High diastereoselectivity was observed and confirmed by 2D NMR experiments on the subsequent reduction and oxidation products. Thus, standard conditions converted silane 78 into diol 79 which was then oxidised to give lactone 80. The protocol can be expanded to achieve further functionalisation, for example in the synthesis of homoallylic alcohol 82. 

Since the isolation of the first example of a stable silene (1), compounds possessing silicon—carbon double bonds have been a focus of research because of their unique electronic properties and reactivities. However, the number of stable silenes remains very limited. Sila-Peterson reaction has proved to be a facile route toward the synthesis of stable silenes replacing photolysis and thermal treatments. 

A relatively new synthetic approach to silenes was established independently in the laboratories of Oehme101-110, Apeloig39,111 and Ishikawa112,113. The key-step is a base-initiated 1,2-elimination of silanolate from at-hydroxydi si lanes 157 and formation of silenes 158 analogous to the original Peterson olefination reaction (equation 39). 

Three different routes to the key compounds for the sila-Peterson elimination, the a-alkoxydisilanes 157, are described in the literature, namely A, reaction of silyllithium reagents with ketones or aldehydes B, addition of carbon nucleophiles to acylsilanes C, deprotonation of the polysilylcarbinols. In addition, method D, which already starts with the reaction of 2-siloxysilenes with organometallic reagents, leads to the same products. The silenes of the Apeloig-Ishikawa-Oehme type synthesized so far are summarized in Table 4. 

The [4 + 2] reaction with dienes has been frequently used to establish the intermediacy of a silene in the sila-Peterson olefination reaction39,107-112-1 r fulSj further examples for cycloadditions can be found in Section I.A.5. 

But till now only methods involving an in situ formation of deprotonated 1 have been described. In this paper we present a procedure leading to pure, isolated 1-hydroxyalkyl-polysilanes 1. In presence of base fliey are easily converted into transient silenes, which were characterized by various dimerization and addition reactions. The availability of isolated 1 in the synthesis of silenes according to the Peterson concept offers the possibility of a free choice of the solvent and the base used to initiate the silanolate elimination. With respect to the significance of the reaction medium for the silene generation and its subsequent reactions, this is of particular importance. 

Deprotonation of la-lc with methyllithium in ether at -78 °C leads to spontaneous elimination of lithimntrimethylsiloxide according to a modified Peterson mechanism and formation of the unstable silenes 2a-2c. In absence of trapping agents these undergo dimerization reactions, 2a leading to l-isopropenyl-2-isopropyl-l,l,2,2-tetrakis(trimethylsilyl)disilane (3) (Scheme 1). 

Reaction of adamantanone with (Me3Si)3SiLi in hexane leads, via a Peterson-type elimination, to the silene, which dimerizes spontaneously to 1,2-disilacyclobutane in 85% yield [Eq. (51)] (98a,b)-... 

Apeloig, Y., Bendikov, M., Yuzefovich, M., Nakash, M., Bravo-Zhivotovskii, D., Blaser, D., Boese, R. Novel Stable Silenes via a Sila-Peterson-type Reaction. Molecular Structure and Reactivity. J. Am. Chem. Soc. 1996, 118,12228-12229. 

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