Silanimines dimerization

Aminofluorosilanes 679 are formed in the reaction of difluorosilanes with lithiated amines. 679 can then be metalated by RLi (R = Me, Bu) to give the corresponding lithium salts (e.g. 680a). The structure of salts 680 depends on the basicity of the nitrogen atom and on the solvent. Representative structure types (680a-c) will be discussed in the subsection dedicated to structural features. The thermal elimination of LiF leads to silanimine dimers (e.g. 682 from 680c), if sterically possible (equation 223), or to rearrangements. 

An exception is the silaazetidine 516 prepared from 349 and iV-lhiorenylidincaniline, which decomposes slowly already at room temperature, yielding the head-to-head dimer 517 of the intermediate silanimine 518 and siloxyalkene 519 (equation 176)248. 

The synthesis of silanimines by salt elimination has been independently developed in the laboratories of Klingebiel304-307. An example of the synthesis of a stable donor adduct (681) is shown in equation 222 and equation 223 illustrates the general dimerization... 

The thus formed heterocycles 412a decompose or isomerize thermally the required reaction temperature depends on the substituents. The isomerization leads to diazomethane derivatives 413a, whereas the decomposition by [2 + 3] cycloreversion reaction gives bis(trimethylsilyl)diazomethane and short-lived silanimines 414a, which dimerize in most cases. The ratio isomerization/cycloreversion depends on R, the solvent and the temperature and more cycloreversion is observed at higher temperature. An unfavourable side reaction is the insertion of Me2Si=NR into Si—N bonds (formation of 415a and 686 see below). 

The yield of the [2 + 3] cycloadducts lies between 15 and 100%. The siladihydrotetra-zoles 344a are thermally quite stable and decompose in solution at temperatures higher than 130 °C in the reversal of their formation reaction to give silanimines and azides. The Si=N species dimerize to 687 or may be trapped by, e.g., acetone to give the ene product 688 (equation 228)310. 

The cyclization of allyl silyl amine 697 by hydrosilylation led to silaazetidine 698, which was subjected to flash vacuum thermolysis at 700-900°C at 10-4 hPa313. The silanimines 699 and 700 themselves were too reactive to be observed by high resolution mass spectrometry of the reaction mixture, but their cyclic dimers, the cyclodisilazane 701 and 702 and a trapping product with t-BuOH 703, were definitely confirmed... 

Similarly to the preparation of 739, substituted silanimine 744 was prepared and characterized along with the silyl substituted analogue 745313. HRMS showed 745 and the dimer 702 of 744, which is expected to polymerize rapidly in the condensed phase, (equation 245). 

The detection of the r-BuOH trapping product 703 and of the dimer 746 gives further evidence for the intermediacy of 744 and 745, respectively. Silanimine 735 is an intermediate in the synthesis of silaisocyanide 737 by retro-ene reaction from 747 under FVT conditions (equation 246)333. 737 was characterized by millimeter wave spectroscopy. 

The compounds resulting from the reaction of 748 were characterized by HRMS directly coupled to the reactor. The stable products 750 and 751 were analysed by GC and 111 NMR spectroscopy. The formation of the cyclodisilazane 750 is explained by dimerization of the unstable silanimine 749 only in the cold trap, as the reaction is carried out under high dilution conditions (equation 247). It was also shown that the hydrogen chloride elimination did not occur in the ion source of the mass spectrometer. 

For the formation of cyclodisilazanes by salt elimination from silylamides =Si(X)-N(M)-(X = halogen, H M = alkali metal) two routes are discussed (i) intramolecular MX elimination and dimerization of the formed silanimine, (ii) intermolecular MX elimination to an intermediate =Si(X)-N(-)-Si(=)-N(M)- that immediately cyclizes with MX elimination [1-5]. The hydridosilylamides Me2(H)SiNLiR (R = Me2(H)Si, Me2CH, CMej) and RR (H)SiNLiSiMe3 (R, R = Me R = MesSiNH, Me3SiNLi, R = Me) react with MesSiCl in THF to give A -substitution products and in -hexane to form 1,3-cyclodisilazanes, formation of which has been explained via route (i) [6, 7], However, the silanimine has not been detected experimentally, Here we describe the synthesis of new hydridosilylamides and the reaction of these compounds and known hydridosilylamides with MesSiCl. 

Simple silanimines tend to dimerize to cyclodisilazanes or to react in other ways and have only fleeting existence under most experimental conditions. Sufficient protection by... 

Previous attempts to eliminate LiF from lithium salts of aminofluorosilanes1 331 337 produced cyclodisilazanes, formal head-to-tail dimers of silanimines. Although silanimines might have been formed as intermediates in some of these processes, which include various rearrangements, this was not proven. The most common mechanism for dimer formation is probably that given in equation 158244. 

The pyrolysis of [dimethoxy(methyl)silyl]bis(trimethylsilyl)amine350 yields products which can be ascribed to an initial formation of N-(trimethylsilyl)-methoxy(methyl)silanimine (equation 165). The isolated dimers may be formed by the imine dimerization or possibly by an attack of the silanimine on the precursor instead. Additional evidence for the presence of some of the intermediates postulated to account for the results was obtained by a study of the copyrolysis of [dimethoxy-(methyl)silyl]bis(trimethylsilyl)amine and hexamethylcyclotrisiloxane, which yielded a fairly complex mixture of products351. The formation of product 88 most likely proceeds through the unsaturated eight-membered ring silanimine 89 and not by way of meth-ylsilanitrile, although the latter possibility was also considered by the authors (equation 166). 

The thermal decomposition of 90 (R = Me3Si) in solution thus offers a steady supply of N-(trimethylsilyl)dimethylsilanimine (92) at a low concentration suitable for reactions with various trapping reagents. In the absence of such reagents, the silanimine yields a dimer and oligomers. 

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