Silenes deprotonation

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. 

Summaiy Deprotonation of l-hydroxyalkyltris(tnmethylsilyl)silanes, (Me3Si)3Si-C(OH)r R (1) with methyl lithium in ether at low temperature leads to transient silenes, (Me3Si)2Si=CR R (2a R = R = Me 2b R = H, R = u 2c R = H R = Mes), which dimerize in absence of trapping agents to give l-isopropenyl-2-isopropyl-l,l,2,2-tetrakis-(trimethylsilyl)disilane 3, ( )-3,4-di-tbutyl-1,1,2,2-tetrakis(trimethylsilyl)-1,2-disilacyclo-... 

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). 

Compoimd 3 has also been obtained by Ishikawa et al. by interaction of acetyltris(trimethyl-silyl)silane with methyllithium [3]. Deprotonation of lb under the same conditions gives ( )-3,4-di-rt)utyl-l,1.2,2-tetrakis(trimethylsilyl)-l,2-disilacyclobutane 4, the head-to-head [2+2] cycloaddition product of silene 2b (Scheme 1). The constitution of 4 is proved also by an X-Ray crystal structure analysis (Fig. 1). 

An unexpected und unusual dimerization behaviour is observed for 2-mesityl-1,1-bis(trimethylsilyl)-silene (2c), formed by deprotonation of Ic. The structure of the product, obtained in more than 70 % yield, was revealed as ( )-l,2,3,8a-tetrahydro-l-mesityl-5,7,8a -trimethyl-2,2,3,3-tetrakis(trimethylsilyl)-2,3-disilanaphthalene (5) (Scheme 1). The formation of this unexpected structure is interpreted as the result of a head-to-head-dimerization of the transient silene 2c, which in an unusual [2+4]-cycloaddition reaction is formally acting as the monoene and - involving the aromatic substituent - also as the diene (Eq. 3). 

Deprotonation of lb or Ic with methyllithium in presence of 2,3-dimethyl-l,3-butadiene gives the silacyclohexenes 9a and 9b, the expected [2+4] cycloaddition products of the respective silene and the diene (Eq. 6). Attempts to perform the same reaction with la failed. 

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