Hydrazines, silylated synthesis

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. 

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. 

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

Bis(hydrazino)silanes are better stabilized with silyl groups bound to the nitrogen atoms. Although Zn-, Cd-, and Hg- connected silylhydrazines were described in 1970,52 silyl-substituted bis(hydrazino)silanes were not known until later. The first bis(hydrazino)silane that was silyl substituted at the terminal nitrogen atom and that showed no tendency to condensation was the terf-butyl-bis[iV,iV -bis(trimethylsilyl)hydrazino]phenylsilane 106, described in 1981.53 A decade later the synthesis of other stable bis(hydrazino)silanes was reported via different preparative methods using (a) mono-, (b) bis-, (c) tris-, and (d) tetrakis(silyl)hydrazines as precursors. 

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). 

Mono(silyl)hydrazines can also be used as precursors for the synthesis of six-membered silylhydrazine ring systems. When 3 is lithiated it cyclizes above 5°C to give the six-membered ring 12413 [Eq. (37)]. 

Aliphatic amines are mainly converted to a-substituted products [99,100], whereby especially the a-methoxylation leads to valuable reagents for synthesis. The intermediate iminium salts can be directly trapped by silyl enol ethers to form Mannich bases [108]. If the a-position is blocked or steric conditions favor it, N,N coupling to hydrazo or azo compounds occurs (Table 5, numbers 17-19). 1,1-Disubstituted hydrazines are dimerized to tetrazenes in fair to excellent yields (Table 5, numbers 20-24). The intermediate diaze-nium ions can attack enolizable carbonyl compounds to form aza-Mannich bases [109]. Arylazonaphthols undergo anodic oxidation, producing radical cations. These couple to biphenylbisazo compounds (up to 34%) or can be trapped by anisidine to form azodiphe-nylamines (up to 74%) [110a]. 

Finally, method B (Scheme 1) may also be applied for the synthesis of silyl- and germyldiazenes (29). Thus, hydrazine (Me3Si)NH—NMe(S02R ) in ether at — 78°C decomposes on addition of butyllithium [Eq. (11)]. 

Niobium and tantalum belong to the metals that are able to bind dinitrogen. The development of this area was first due to Schrock and coworkers, who in 1982 could obtain stable tantalum /U.-N2 compounds in high yields by reducing Alkylidene complexes under dinitrogen at normal pressure. This route can be circumvented in a single pot synthesis by reacting the pentahalides with silylated hydrazines (equation 12). 

New methods or variations of existing methods for pyrazole synthesis continued to be reported. Notable are several hydrazine-based annelations which afford highly substituted pyrazoles for example, the perfluorinated silyl alcohol (7) with hydrazines affords the tluorinated pyrazoles (8) [94TL(35)409]. In contrast, the ketene dithioacetal... 

A recent synthesis of this building block has been published by Eastham et al. in 2006 (48). Their key step is a Dotz benzannulation reaction and is shown in Scheme 2.10. The bromohydrin 66 was formed from dihydrofuran (50). Cobalt-mediated cyclization, followed by ozonolysis with reductive work-up yielded 68 after hydrazine formation. Reductive removal of the hydrazine function, followed by chromium-carbonyl formation gave the Dotz reaction precursor 69. This reacted with an alkyne in the Dotz reaction, and was then oxidized and hydrogenated 70). Pyrolysis gave the protected alcohol and the remaining free alcohol was protected as a triflate (—> 71). Reductive removal of the triflate and deprotection of the silyl ether yielded the desired 33 in 1.2% overall yield. 

---