Silyl hydrazines

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... 

The formation of the silylhydrazines depends on the reactivity and the bulkiness of the halosilanes. The condensation increases with increasing number of the hydrogen atoms. While iodosilane or bromo(methyl)-silane yield the tetrakis(silyl)hydrazines,912... 

The only mono(silyl)hydrazine known until 1993 was the triphenylsilyl-hydrazine 1, described by Wannagat and Liehr in 1958 ° [Eq. (3)]. 

Primary silylhydrazines with R = Me, Et, or Pr cannot be isolated, as they immediately undergo further condensation to bis(silyl)hydrazines with elimination of hydrazine.6 10 For the triphenylsilylhydrazine this condensation only occurs under more drastic conditions at 90°C [Eq. (4)]. 

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

This class of compounds is kinetieally stabilized by bulky tm-butyl groups (3-6) or amine groups (7,8). In 3, 7, and 8, HF elimination should be possible, but it is hindered by the very strong SiF bond energy. These mono(silyl)hydrazines are very stable molecules and show no tendency to undergo condensation at room temperature. 

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. 

When mono(silyl)hydrazines are not stable at room temperature, they condensate to give the symmetrical bis(silyl)hydrazines 10-15 ... 

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-... 

Another possibility for the formation of symmetrical bis(silyl)hydrazines is the condensation of the corresponding mono(silyl)hydrazines under more drastic conditions, as shown for the formation of the triphenylsilylhy-drazine 2 by heating 1 above its melting point at 90°C. In order to synthesize the compounds 16-19, the mono(silyl)hydrazines have to be heated for seven days at 220°C [Eq. (7)]. 

The first asymmetric bis(silyl)hydrazines 20-22 were obtained in the 1950s by treating hydrazine with a mixture of different halosilanes [Eq. (8)]. 

Because of the stability of the mono(silyl)hydrazines 3-8, their lithium derivatives can be used to synthesize asymmetric Af,AT -bis(silyl)hydraz-ines (23-40), as shown in Eq. (9). 

Depending on the steric effects, either Ar,A -bis(silyl)hydrazines 41 or isomeric products 42a/b-45a/b are formed [Eq. (10)]. 

Four crystal structures of lithiated bis(silyl)hydrazines are described.16,32-35 The Li centers exhibit tri- and tetracoordination, but because of Li-C interactions, the Li atoms may also adopt higher coordination numbers. All lithiated bis(silyl)hydrazines show side-on and end-on coordinated lithium. 

In contrast to 46, no formation of dilithiated hydrazide units is observed in 47, which is the only purely monolithiated bis(silyl)hydrazine. Similar to this structure, a monolithiated iV-/m-butyl-iV -trimcthyIsilylhydrazine has been described.16... 

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 ... 

In 1959, lithiated silylhydrazines were first used in the preparation of tris(silyl)hydrazines by Wannagat el al.u ... 

By the second method, tris(silyl)hydrazines can be substituted not only with two, but also with three different silyl groups (72-74). 

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 ... 

Four different methods have been employed for the synthesis of tetrakis (silyl)hydrazines.9-12-16-40-42-45... 

The symmetrical tetrakis(silyl)hydrazines (SiR3)4N2 77-81 have been synthesized by this method. 

According to method (d), unsymmetrical tetrakis(silyl)hydrazines 82-92 were synthesized ... 

The first unsymmetrically substituted tetrakis(silyl)hydrazine that was determined by a crystal structure is the IV,jV -[bis(difluorophenylsilyl)]-/V,Ai -[bis(trimethylsilyl)]hydrazine 83 [see Eq. (22)].16 Like those of 77 and 79, the N atoms of 83 show a planar coordination geometry, as the angle sum is found to be 360°. Because of the electron withdrawing effect of the fluorine atoms, these Si—N bonds are shortened compared to the other Si—N bonds of this molecule (Table XIV). The fluorine atoms of... 

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