Si-N bonds

This chapter will not cover compounds containing heteroatoms hnked to the P and/or N atom of the P-N unit, with the notorious exception of those with N-Si bonds (A/-silyl phosphinous amides) due to its particular relevance in terms of chemical reactivity. 

The electron-rich and cyclically conjugated 8 rr electron system of 1,4-dihydropyrazine can be stabilized in essentially planar conformation by organosilicon substitution at the enamine nitrogen centers [1], In addition to electron transfer [1] and triorganosilyl exchange reactions [2], we have explored the possibility of inserting heterocumulenes X=C = Y into one or both of the N-Si bonds of the compounds 1 in order to functionalize this unusual ring structure [3],... 

The insertion of CO2 into one N-Si bond occurs quite readily under standard conditions when R, R = Me formation of l,4-bis(0-trimethylsilylcarboxyl)-l,4dihydropyrazine, however, requires more drastic measures (60°C, 50 bar CO2) than previously suggested [3]. The latter compound exists as a mixture of cis and trans isomers due to restricted rotation around the N-C(=0) bonds on the NMR time scale [4],... 

Very similar Si H M agostic interactions, characterized by a red shift of the Si-H absorption (IR), a high-field shift of the Si-H resonance ( H NMR), reduced Si-H coupling constants, and acute M-N-Si bond angles, were thoroughly studied by Anwander et for a range of d lanthanide complexes. In particular, the series... 

Stable N—Si coordinated complexes generally contain a chelate ring with a covalent bond to silicon on one side, and a dative N - Si bond on the other. Only few such bidentate ligand types have been used, and the resulting families of related chelates are depicted in structures 90-10076 -95. 

The lithium salt shown in Fig. 8 crystallizes from THF as a dimer in a boat conformation with a planar (Li-N)2 ring. The partially anionic Li-N bond causes increasing electron density at the nitrogen atom N(l) and therefore a shortening of the N-Si bonds in its neighborhood. 

A catalytic asymmetric in situ reduction of N-H imines has been achieved in a sequence in which trifluoroacetophenones, ArCOCF3, are first converted to silylimines [using LiN(SiMe3)2], and then on to give trifluoromethylated amine salts, Ar-C(CF3)-NH2.HC1, in good to excellent yield and ee.5s The intermediate N-H imines can be isolated via methanolysis of the N-Si bond, while the enantioselective reduction can be carried out using a chiral borane auxiliary. 

Addition of lithium bis(trimethylsilyl)amide to perfhiorinated ketones and solvolysis of the N-Si bond in methanol resulted the formation of stable, isolable N-H imine Z-E isomer mixtures along with a methanol adduct. Enantioselective reduction of these three-component mixtures with oxazaborolidine catalysts and catecholborane provided trifluoromethylated amines in 72-95% yields and 75-98% ee 267... 

While tertiary amines often form quite stable adducts with N— Si bonds (see Section III,D), ammonia and primary or secondary amines normally cleave silicon-metal bonds intermediate adducts are either not observed or found only at low temperatures 134, 235). Typical behavior is shown by 28, 30, 31)... 

Cleavage of the N-Si bond of silylamines by acyl chloride constituted a valuable approach to the corresponding amides.179... 

Treatment of the silylene 78 with pyridine in benzene afforded the crystalline compound 131 (Scheme 47) <2004JOM(689)1350>. The formation of 131 most likely proceeded via a three-membered intermediate 130, followed by insertion of the silylene 78 into the C-Si or N-Si bond. When quinoline was used instead of pyridine, a similar adduct was formed, which in contrast to 131 was thermally stable. 

The influence of steric factors was thoroughly studied in the reaction of Ln(btsa)3 with the alcohol tritox-H. While the reaction takes place with larger lanthanides like neodymium to yield the homoleptic alkoxide complexes (Eq. 18) [264], the analogous reaction does not work with smaller metals like yttrium and thulium (Eq. 19). However, variation of the reaction conditions to a stoichiometric solid reaction yielded a fully exchanged product along with an unexpected and unusual silylamine degradation [265] (Eq. 20). This degradation reaction seems to be sterically forced and points out N-Si bond disruptions and C-Si bond formations under mild conditions [114]. 

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