Silyl hydrazines, reactions

Compound 21 is formed upon reaction of Bu2Mg with a silyl hydrazine (Scheme 4). In this instance the base removes a proton from the hydrazine followed by an unexpected migration of the benzyl group, and subsequent deprotonation of the trimethylsilyl... 

Some other mono(silyl)hydrazines have been synthesized in the reaction of fluorosilanes with lithiated hydrazine ... 

Their stability allows a directed synthesis of asymmetrical bis(silyl)hy-drazines by the reactions of lithiated mono(silyl)hydrazines with halosi-lanes (Section B,2). These reactions often lead to the formation of isomeric products. 

These condensation reactions often lead to the formation of isomers. Structural isomerism of bis(silyl)hydrazines was first observed in 1964.19-24 In the absence of strong steric or electronic constraints, the bis(silyl)hy-drazines such as bis(trimethylsilyl)hydrazine give in a thermoneutral reaction essentially equal amounts of the N,N- and N,N -isomers at equilibrium.7 23 Wannagat et al. found that the reaction of hydrazine with trimethylchlorosilane at room temperature results only in the formation of N,N -bis(trimethylsilyl)hydrazine, whereas the same reaction in boiling solvents leads to a mixture of N,N- and /V,/V -bis(trimcthylsilyl)hydrazine. Both could be separated by preparative gas chromatography. Their struc-... 

Tris(silyl)hydrazines are obtained by different methods they can be obtained in the reaction of chlorosilanes with hydrazines in the presence of Et3N as in Eq. (14)12 ... 

Isomerism is observed also in the formation of tris(silyl)hydrazines (56a/b). The reaction of 46 with fluorosilanes leads to the formation of the isomeric products 67 and 68 or 69 and 70 [Eq. (16)]. 

The condition for the formation of isomers is a side-on coordination of the lithium in 47, the lithium derivative of 27. Depending on the bulkiness of the fluorosilane, this side-on coordination makes possible a substitution that is suitable for the system. In comparison, the reaction of lithiated 27 with iPr2SiF2 leads only to the formation of the tris(silyl)hydrazine 75,16 so that kinetic control of the reaction must be supposed ... 

When tris(silyl)hydrazines contain an NH and an SiF group in the molecule, disilatriazoles can be obtained in the reaction of these silylhydrazines with metalorganic bases. When 67 and 68 are treated with n-butyllithium, the five-membered ring 119 is isolated as a cross dimer of the two isomers39 [Eq. (33)]. 

Mono(silyl)hydrazines can also be used as precursors for the synthesis of five-membered silylhydrazine ring systems. In the reaction of the mono(silyl)hydrazine 7 with n-butyllithium in a 1 1 molar ratio, colorless crystals with a melting point of 184°C were obtained from n-hexane at 0°C. The reaction product did not contain the expected six-membered ring, but, as proved by NMR and IR investigations, consisted of the disilatriazole 121,15 as shown in Eq. (34). 

In the reaction of difluorodiisopropylsilane with lithiated hydrazine, the six-membered ring 123 is formed.32 No formation of HF is observed, which means that the reaction proceeds via the mono(silyl)hydrazine that is then lithiated as shown in Eq. (36). 

The bicyclic six-membered ring system 132 was obtained according to Eq. (41) by the reaction of bis(Af,./V -fluorodimethylsilyl-)V,)V -trimethyl-silyl)hydrazine 87 with dilithiated hydrazine.39... 

These condensation reactions often lead to the formation of isomers. Structural isomerism of bis(silyl)hydrazines was first observed in 1964. 

Summary Mono(silyl)hydrazines condense to bis(silyl)hydrazines at higher temperature. The degree of oligomerization of mono- and dilithiated silylhydrazines in the crystal depends on the bulkiness of the substituents. Reactions of lithiated silylhydrazines with fluorosilanes lead, for example, to the formation of tetrakis(silyl)-hydrazines, six-, four- and five-membered rings. Formaldehyde derivatives of mono-(silyl)hydrazones are obtained in a reaction of mono(silyl)hydrazines with aqueous formaldehyde solution. 0-Silylpyrazolones can be synthesized by treating mono(silyl)-hydrazines with acetoacetic ester. A dipyrazolonesilane is formed in the reaction of an O-silylpyrazolone with dichlorodimethylsilane. 

Stable mono(silyl)hydrazines are formed in reactions of lithiated hydrazine with bulky fluorosilanes [1,2]. They condense to form bis(silyl)hydrazines above 200 °C [2] ... 

The first stable monomeric formaldehyde derivatives are formed by reaction of mono(silyl)hydrazines with aqueous formaldehyde solution. 

Summary Condensation of stable mono(silyl)hydrazines leads to the formation of bis(silyl)hydrazines above 200°C. The degree of oligomerization of mono- and dilithiated silylhydrazines depends on the bulkiness of the substituents. Lithiated silylhydrazines react with fluorosilanes forming, for example, tris- and tetrakis(silyl)hydrazines, and six-membered rings. In oxidation reactions of bulky bis(silyl)hydrazines bis(silyl)diazenes are obtained. Lithiumderivatives of fluoro functional bis(silyl)hydrazones are precursors for l,2-diaza-3-sila-5-cyclopentenes. Further lithiation with iBuLi leads to tricyclic compounds and LiF. 

Isomerism is observed in the formation of tetrakis(silyl)hydrazines. The reaction of dilithiated bis(dimethylphenylsilyl)hydrazine with two equivalents of trifluoromethylsilane leads to the formation of the isomeric products 7 and 8 (Eq. 6) [2],... 

Bis(silyl)diazenes are obtained in oxidation reactions of bulky bis(silyl)hydrazines, e.g. bis(di-/er/-butylmethylsilyl)hydrazine reacts with bromine in THE forming the first thermally stable bis(silyl)-diazene 9 (Eq. 7) [6-8],... 

This silyl hydrazone formation-oxidation sequence was originally developed as a practical alternative to the synthesis and oxidation of unsubstituted hydrazones by Myers and Furrow [31]. The formation of hydrazones directly from hydrazine and ketones is invariably complicated by azine formation. In contrast, silyl hydrazones can be formed cleanly from /V,/V -bis(7< rt-butyldimethylsilyl)hydrazine and aldehydes and ketones with nearly complete exclusion of azine formation. The resulting silylhydrazones undergo many of the reactions of conventional hydrazones (Wolff-Kishner reduction, oxidation to diazo intermediate, formation of geminal and vinyl iodides) with equal or greater efficiency. It is also noteworthy that application of hydrazine in this setting may also have led to cleavage of the acetate substituents. 

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