Silenes nucleophilic additions

In one series of experiments the a-lithiated silanes were produced by a nucleophilic addition of tert-butyllithium to the C=C double bond of a vinylhalosilane so that the 1,2-elimination led to a silene carrying a neopentyl substituent on the unsaturated carbon... 

Some reactions considered as nucleophilic additions to silenes may in actuality proceed by radical mechanisms, such as the addition of Br2 and possibly even SiCT 113,292, as well as reactions with halogen-containing solvents. There has also been a report of an addition of the elements of acetonitrile to a silene under high-temperature conditions, and this too may involve radical processes292. 

With respect to nucleophilic attack, silaaromatics appear to behave in a manner analogous to silenes. The addition of methanol to 1-methylsilabenzene290,326 and to hexamethyl-l,4-disilabenzene314 yields the expected adducts [exclusively 1,4 in the latter case (equation 154)], as does the addition of methanol and tert-butyl alcohol to 1-methyl-2,3,4,5-tetraphenylsilabenzene128. It is possible that the addition occurs on the Dewar isomer of the silaaromatic. The lack of stereospecificity is presumably due to subsequent isomerization of the alkoxysilane in the presence of alcohol. 

In the first stereochemical study, Brook has observed nonstereospecific addition of methanol to certain isolable silenes65,66. Although the precise stereochemistry of the products was not established, a 1/3 mixture of syn/anti or anti/syn isomers was obtained (equation 13). The results indicate a nonconcerted process for the addition of alcohol to silenes. Wiberg has proposed a two-step mechanism involving an initial formation of a silene-alcohol complex, in accord with the formation of nucleophilic adducts, followed by proton migration from the alcohol to the carbon of the silene (equation 14)59,61. This mechanism may be compatible with the results obtained by Brook and coworkers, if rotation around the silene s Si—C bond occurs faster than the proton migration. 

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. 

Addition reactions of lithium organyls have been investigated for 9763-201. The reaction is not a nucleophilic substitution of Br by R in 97-LiBr, but rather a two-step mechanism with an initial dissociation via 97 (equation 124). This was proven by the addition of the very efficient silene trap t-Bu2MeSiN3, that competes with the lithium organyl for the silene. 

Over the past ten years, absolute rate data have been reported on the kinetics of several bimolecular silene reactions in solution, including both head-to-tail and head-to-head dimerization the [l,2]-addition reactions of nucleophilic reagents such as water, aliphatic alcohols, alkoxysilanes, carboxylic acids and amines and the ene-addition, [2 + 2]-cycloaddition and/or [4 + 2]-cycloaddition of ketones, aldehydes, esters, alkenes, dienes and oxygen. The normal outcomes of these reactions are summarized in Scheme 1. 

Wiberg determined the relative rates of addition of various alcohols and amines to silene 6, the results of which have been summarized above in Table 26. The fastest rates of addition were observed with aliphatic alcohols and amines, leading to the hypothesis that the first step of the reaction involves complexation of the neutral nucleophile at silicon, followed by proton transfer to the silenic carbon. Subsequent reports of the X-ray crystal... 

The rate constants for addition of aliphatic alcohols and water to 19a decrease in the order MeOH > EtOH > H2O i-PrOH > t-BuOH in acetonitrile at 23 °C45, due to a combination of steric effects, nucleophilicity and acidity, all of which can be expected to affect the magnitudes of the individual rate constants involved in the mechanism for the reaction. The Arrhenius activation energy for addition of Z-BuOH to this silene, Ea = —1.7 0.4 kJmoP189, is closer to zero than that for MeOH addition, suggesting that the intracomplex product partitioning ratio is closer to 0.5 than is the case for the more reactive alcohol over the temperature range examined. It thus follows that the factor of ca 10 lower reactivity of t-BuOH compared to MeOH is mainly due to a reduction in the rate constant (kcj for initial complexation of the alcohol with the silene. 

Historically it was the gas-phase process which first unequivocally proved the existence of species with a 7rbond to silicon [Eq. (37)] (198). In the absence of nucleophiles, the dimethylsilene gives a head-to-tail dimerization product, l,l,3,3-tetramethyl-l,3-disilacyclobutane. On the other hand, addition products across the double bond of the intermediate silene are formed in the presence of various trapping agents. 

Besides its synthetic significance, this result is at the same time a valuable proof for the intermediate existence of transient silenes also in the formation of 3-5. Whereas the addition of the organolithium compoimds to the silicon carbon double bond with the observed regiospecifity is obvious, the formation of 6a-6e by interaction of the nucleophilic organolithium derivatives with any precursor of the silenes 2a-2c is hardly conceivable. 

The only computation of an addition path of this type dealt with the addition of HCl to silene and identified a transition state for electrophilic addition296. It seems likely to us that a more thorough search will identify a nucleophilic attack path with an even more favorable transition state. 

The same conclusion can be reached from a detailed examination of relative rates of addition of nucleophiles to 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54) in ether solution at 100°C, obtained from competition experiments in which the silene was liberated by thermal decomposition of the silazetidine 56 (Table 3)244,246. The rate of this decomposit-... 

The relatively high addition rate of the rather nonnucleophilic reagent, acetic acid, may be due to an involvement of its carbonyl oxygen in the nucleophilic attack on silicon, perhaps in a six-membered cyclic transition state with a nearly simultaneous proton transfer to the silene carbon. 

---