Si—N bond formation

The active catalytic species appears to involve a metal hydride complex, as added H2 enhances the reaction rate. Additionally, H4Ru4(CO)12 is observed during the course of the reaction and may also be used as an effective entry into the catalytic system. The oligomerization mechanism may proceed via a nucleophilic attack of the amine on a metal-activated Si-H bond, resulting in Si-N bond formation. 

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). 

Quantum chemical calculations Indicate that Si-N bond formation nay be preferred because of the lower energy of SlNC compared to SICN. ... 

Scheme 39 Si-N bond formation in the coordination sphere of [(q -CsMesjldppf )Mo].

The formation of the hetero-Diels-Alder product 3 is remarkable, since the Si=N bond in iminosilanes does not yield analogous adducts. 

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

There appear to be considerable steric demands involved in formation of the Si-N bond in reaction (28). Support for the importance of steric constraints in dehydrocoupling comes from studies on the synthesis of oligosilazanes from PhSiHj and NH3. When reaction (29) is run at 60°C, NMR and elemental analysis... 

A unique system for catalytic silaboration of allenes, in which a catalytic amount of organic halide is used as a crucial additive, has been reported (Equation (86)).232 In the presence of Pd2(dba)3 (5 mol%) with 3-iodo-2-methyl-2-cyclohexen-l-one (10mol%), reactions of terminal allenes with a silylborane afford /3-silylallylboranes in good yields with excellent regioselectivity. It is worth noting that the addition takes place at the terminal C=C bond in contrast to the above-mentioned palladium-catalyzed silaboration. The alkenyl iodide can be replaced with iodine or trimethylsilyl iodide. The key reaction intermediate seems to be silylpalladium(n) iodide, which promotes the insertion of allenes with Si-C bond formation at the central -carbon. 

Attempts have been made to prepare polysilanes containing the 8-dimethylaminonaphth-l-yl ligand.863 The coupling reaction of 8-dimethylaminonaphth-l-yl lithium with MeSiClj has given the dichlorosilane 909, whose mild reduction with Mg has surprisingly yielded the disilane 910 rather than the expected polysilane (Scheme 128). The formation of the disilane may be rationalized by the insertion of a transient silylene 904 into an Si-N bond of... 

Other reactions include insertion of S03 into the Si—N bond and formation of di-... 

The N-protonated form of HNSi, the H2NSi+ cation 69 and its possible isomers have been studied extensively by theoretical methods230-234. Bohme47 suggested that the ion 69, which is formed upon reaction of silicon cations with ammonia and subsequent dissociation of 69 upon electron capture (equation 33)235, is of prime importance for the formation of Si—N bonded species such as 67 in the chemistry of interstellar matter. 

It should be emphasized that these reactions involving two trifunctional reagents afford monomeric compounds in high yields instead of polymeric products. There is no question that the driving force of these reactions which generate the tricyclic silatrane skeleton is the formation of the transannular Si- —N bond. 

Reaction 24 does not occur with the protonated amine (i.e. with the zwitterionic form of the dihydroxyethylated amino acid) in the absence of pyridine. It seems most likely that the catalytic role of pyridine involves release of the lone-pair electrons of the nitrogen atom, that facilitates the formation of the transannular Si- —N bond and, consequently, the silatrane ring110. 

By use of the X-ray data and the method of bond moments, the contribution of the Si — N bond to the molecular dipole moment of 1-organylsilatranes was calculated to be 1.5-3.1 D12,206. These values correspond to a transfer of approximately 0.2e from the nitrogen to the silicon atom. Use of the known empirical relationship (equation 45) between the heat of formation of the donor-acceptor (DA) bond (AHda) in DA complexes, the dipole moment (/xda) and the inverse value of the length of the DA bond... 

Dipole moment studies207,378,380 found a lower dipole moment of the Si — N bond in 3-homosilatranes (1-2 D) than in silatranes (1.5-3.1 D). The enthalpy of formation of this bond was estimated to be 3-11 kcalmol-1 depending on the substituent X and on the method of the bond dipole calculation207. The observation of an absorption band at 570-600 cm-1 in the IR spectra suggests a tricyclic structure of 3-homosilatranes in solution378. 

Position of the Si-N-Bond Cleavage, Formation of the Silyl-Bridged... 

Indeed, almost the reverse process of C-M bond breaking and Si-M bond formation may be seen in entry 10. (4) Germanium and tin analogs of Cl3SiCo(CO)4 react with tri-n-butylphosphine to give ionic products (341), as in... 

The mechanism of reaction according to Eq. (14) is discussed by the authors 57>. They postulate the formation of the adduct (RsSi)aN(Cl)BCl3 which is followed by rupture of the Si—N bonds resulting in the formation of halosilanes. A similar reaction has been reported elsewhere (see ref. 58> and lit. cited therein). However, in the reaction of N-chlorohexamethyl-disilazane with trichloroborane substantial side reactions seem to occur. Several liquid products have not yet been identified. 

Catalytic reactions of triorganylsilanes with ammonia and amines lead to the formation of the Si—N bond. In contrast, reaction of triphenylstannane with primary amines gives an Sn—Sn bond, i.e. Ph3SnSnPh3. 

The diaminodisilyldisilene 8 has a most unusual structure it not only exhibits the longest Si=Si formal double bond length observed to date but also the largest tram-bent and twist angles. These structural parameters are probably attributable to the substitution patterns in 8. Although the Si—Si bond in the intermediate 7 promotes the formation of an Si=Si double bond, the Si—N bond also present hinders electronically Si—Si contacts, with the overall result that the structural parameters described above represent a compromise between these opposing effects28. It is important to note that theoretical calculations fully reproduce the experimentally determined structure of 827. 

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