Silicon complexes imines

Only recently, nitriles were shown to increase the silicon coordination number to up to five in cationic complexes (18) [95], even though nitriles had already been shown to form adducts with sihcenium ions, thus functioning as a donor moiety in a tetrahedral Si coordination sphere [96]. Amines are scarcely encountered in silicon coordination compounds, only few examples of crystallographically evidenced silicon complexes with monodentate amines have been reported so far, which include the adduct SiF4(NH3)2 [97-100], Imines, however, are well known to add to various halosilanes. Especially IV-heterocycles with imine functionahty, such as pyridines [101-103], imidazoles [104, 105], pyrazoles [106], and related compounds, can be found as ligands in various silicon complexes (e.g., in 19 and 20). 

In a similar maimer, hydride (43) [169,170], silyl groups (44) [17 l],allyl groups (45) [169,172], as well as alkyl (47a-c) and aryl groups (47d) [173-176] can migrate to imine carbon atoms of the ligand, either during the metathesis reaction or upon thermal or photochemical activation of an intermediate hypercoordinated silicon complex (46a-d). 

The reaction conditions for the synthesis of [Si2(pz )g] and [Si(pz )4] have been investigated in detail in order to obtain suitable precursor compoxmds for the synthesis of the hitherto unknown Janus-head ligand tris(3,5-dimethylpyrazolyl) silanide ([Si(pz )3] [241]. The X-ray stmctures of several pentacoordinated disilanes of type 95 with X = Cl or 3,5-dimethylpyrazolyl have been presented in this work. The chemistry of 3,5-dimethylpyrazolyl silicon complexes has been further explored. Although pyrazolide is an anionic substituent, its second N atom (the imine N atom of N-silylated pyrazole) also coordinates to silicon, thus exhibiting features of a neutral nitrogen donor as well. 

A few publications deal with silicon complexes of chiral 0,N,0-ligands. Tridentate imine ligands were obtained from chiral amino alcohols by Schlecht et al. [278]. These ligands serve for the formation of bis-chelated silicon complexes. Whereas the complex based on 2-amino-l,L2-triphenylethanol is obtained as a diastereomeric... 

By contrast, the isomerization of silyl olefins and addition of silylacetylenes =C—H bond into imines catalyzed by iridium complexes appears to serve as a suitable route for the synthesis of silylfunctionahzed organic compounds. Hence, the acquisition of experimental data on catalysis by iridium complexes in silicon chemistry may be regarded as an initial stage in the quest for catalytic processes leading to the synthesis of other p-block (e.g. B, Ge, Sn, P)-carbon bond-containing compounds. 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

In the presence of water-free late transition metalphosphine cation complexes as Lewis acids, glyoxylatetosylamine imine reacted with silicon enolates stereoselectivity [23-26]. It was proposed that imine coordinated to the metal such as Ag(I), Pd(II), and Cu(I) in a bidentate manner [23]. The copper-based catalyst was the most effective, and the desired product was obtained in high yields with high enantioselectivities [(Eq. (4)]. 

In 1998, a new type of Pd(II) binuclear complex was reported which was effective for Mannich reactions of an imine derived from glyoxylate and anisidine with silicon enolates [38,39]. In these reactions, use of solvents including a small amount of water was essential. It was shown that water played an important role in this system water not only activated the Pd(II) complex to generate a cation complex, but also cleaved the N-Pd bond of the intermediate to regenerate the chiral catalyst. This reaction reportedly proceeded via an optically active palladium enolate on the basis of NMR and ESIMS analyses. A unique binuclear palladium-sandwiched enolate was obtained in the reaction of the p-hydroxo palladium complex with the silyl enol ether [(Eq. (9)]. 

The allylation of a-iV-tosylimino esters, highly activated imines, proceeds by catalytic use of a Lewis acid. Tol-BINAP-Cu(l) complexes are effective catalysts of asymmetric allylation of these imines with allylsilanes (Equation (28)).124 125 The presence of an aryl group at the position j3 to silicon improves the enantioselectivity. 

Perfluoroalkyllithium reagents react with electrophiles such as aldehydes, ketones, silicon and tin halides, alkyl halides, carbonates, esters, boron trifluoride-imine complexes, and sulfur dioxide. ... 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

The same trend is evident for the more Lewis-acidic silicon halides or pseudo-halides, " when the coordination number is increased from 4 to 6 (Scheme 8). Variable temperature Si NMR studies show the equilibrium between four-, five- and six-coordinate silicon atoms in imine and enaminechelate complexes. There are numerous Si NMR spectroscopic studies of hypervalent silicon... 

An assumed, simplified catalytic cycle is shown in Scheme 5.91. First, the precursor complex 329, which has been identified and characterized by NMR spectroscopy, has to be stereomutated inasmuch as one of the four axial BINOL oxygen atoms has to flip into an equatorial one in complex 334 in order to enable the imine ligand to replace the imidazoles L. The assumption of a bidentate character of the ort/ro-hydroxyanilide has been supported indirectly by the fact that, with the corresponding anilide or ort/ro-methoxyanilide derivatives, almost no stereoselectivity was observed. In the loaded complex 335, the ortho-hydroxyanilide ligand occupies the axial and equatorial positions. Upon reaction with the nucleophilic silicon enolate 336, the carbon-carbon bond formation... 

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