Si-Transamination

The compounds described herein were prepared by three methods. The first route involves deprotonation/substitution at the N-H sites of 1, the second consists of a cleavage reaction of an Si-N derivative of 1 with PhBCI2, and the third route is a transamination reaction between a bis(dimethylamino)boryl derivative of 1 and an aliphatic diamine. In the first approach, compound 1 is deprotonated by treatment with one equivalent of n-BuLi. Quenching of the resulting anion with various electrophiles produces the monosubstituted products 2-4 (eq 3). 

The synthesis of substituted calcium amides is generally achieved either via salt elimination between a halide (often Cal2) and an alkali metal amide or via transamination using bis(trimethylsilylamido)calcium derivatives, most commonly [Ca N(Si-Me3)2 2(thf)2]. The latter was also conveniently prepared from Ca(OSO2CF3)2 and 2Na- N(SiMe3)2 in thf. ... 

A transamination route, as shown in Equation (8.4) (SiRa = SiMes, Si(Bu )Me2), °has afforded complexes with unusual trigonal, monopyramidal coordination at aluminium. 

Thus we designed and synthesized a bicyclic pyridoxamine derivative carrying an oriented catalytic side arm (16) [11], Rates for conversion of the ketimine Schiff base into the aldimine, formed with 26 (below) and a-ketovaleric acid, indolepyruvic acid, or pyruvic acid, were enhanced 20-30 times relative to those carried out in the presence of the corresponding pyridoxamine derivatives without the catalytic side arm. With a-ketovaleric acid, 16 underwent transamination to afford D-norvaline with 90% ee. The formation of tryptophan and alanine from indolepyruvic acid and pyruvic acid, respectively, showed a similar preference. A control compound (17), with a propylthio group at the same stereochemical position as the aminothiol side arm in 16, produced a 1.5 1 excess of L-norvaline, in contrast to the large preference for D-amino acids with 16. Therefore, extremely preferential protonation seems to take place on the si face when the catalytic side arm is present as in 16. 

L-aspartate. They observed that tritium was introduced at C-4 of the cofactor from the Re face to the extent of over 90%. Thus the exposed face of the coenzyme-substrate complex in the modified enzyme is the Re face as in the holoenzyme rather than the Si face as in the complex of the normal enzyme with L-aspartate. Yet this modified enzyme is still able to undergo the half-transamination reaction with conversion of active site bound PLP to PMP [45] and does so with stereospecific protonation of the cofactor from the Si face [46]. According to this result, a change in the exposed face of the cofactor upon transaldimination or a conformation in which the Si face of C-4 is exposed in the coenzyme-substrate complex are not requirements for catalytic activity. Likewise, the face on which reactions take place in the catalytic process is not necessarily the face that is most accessible to external reagents. 

R As(NR2)3- , are routinely synthesized by the aminoly-sis of a haloarsine or transamination of another aminoarsine (eqnations 64-68). 

Transamination of Tris with Methylamine via Standard Addition. Toluene (375 mL) and HSi[N(CH3)2]3 (369.9 g, 2.29 mol) were combined under a N2 atmosphere in an apparatus similar to that described in the previous section. The mixture was stirred and then charged with p-TSA monohydrate (9.0 g, 0.05 mol). The reaction mixture was heated to reflux temperature and then sparged with CH3NH2 continuously. The reaction mixture was maintained at reflux temperature for 4 h, at which time it was determined by GC that no HSi[N(CH3)2]3 remained. The reaction mixture was cooled to room temperature, and the toluene solvent was removed under vacuum. The remaining white resinous product was washed with 200 mL of pentane, and the washing was discarded. The sample was dried further under vacuum at 13 Pa to yield 138.2 g of a white resin that hardened at <40 °C. The product was not distillable under vacuum. The structure of the product mixture was nearly identical to that obtained in the previous experiment (on the basis of H, C, and Si NMR spectroscopic analyses) but was more highly condensed and cross-linked. 

Figure 1. Representative X-ray powder diffraction spectrum of [Si(NH2)2]> which is the product of pyrolysis at 1550 °C of the transamination product of Tris with ammonia, a and P refer to a- and -phase silicon nitride, respectively.

H3C)3Si-NH-Si(CH3)3 (HMDS). It is commercially available and synthesized in large quantities by ammonolysis of trimethylchlorosilane. Costs per kilogram are less than US 10. HMDS cannot be directly used as a precursor for ceramics because it is a nonprocessable volatile liquid (boiling point 125°C). Nevertheless, it is an important source for the synthesis of polysilazanes by transamination reactions (see below) and also a valuable single-source precursor for the preparation of Si-C-N coatings by CVD processes. ... 

In all the examples studied to date, during half-transaminations, physiological or artificial , the medium derived proton has been shown experimentally to enter the Si face at C-4 of the coenzyme. This surprising stereochemical consistency in diverse types of pyridoxal-P-dependent enzymes has been interpreted by Dunathan and Voet [111] as evidence for the evolution of this family of enzymes from a common ancestor whose distribution of binding groups at the active site has remained essentially unaltered throughout the course of evolution. 

There have been several reviews that provide a comprehensive coverage of the synthesis and reactivity of organoarsenic compounds that contain tricoordinate arsenic with an As-N bond. The acyclic aminoarsines, R As(NR2)3 , are routinely synthesized by the aminoly-sis of a haloarsine or transamination of another aminoarsine (equations 64-68). 

Subsequent to this work, apoaspartate transaminase was used to assay the stereospecificity of a variety of other transaminases, all of which were shown to involve protonation/deprotonation at the C-4 Si face of the cofactor. These enzymes included pyridoxamine-pyruvate transaminase (EC 2.6.1.30) (26) and a-dialkylamino acid transaminase (27). L-Glutamate decarboxylase (EC 4.1.1.15) catalyzes an abortive transamination reaction when oc-methylglutamate is used as substrate, and this too was shown to occur with protonation at the Si face of C-4 in the intermediate 4d (28) as was the abnormal transamination of D-alanine by serine hydroxymethyltransferase (29). 

Tryptophan synthase (EC 4.1.2.20) normally catalyzes the synthesis of tryptophan from serine by the oc,p elimination-addition reaction outlined in Scheme 5 where X = OH and Z = indole. The B protein of the oligomeric enzyme will catalyze the dehydration of serine, and in the presence of PLP and mercaptoethanol, the intermediate 15 will form adduct 25. This will then react as in Scheme 9 to yield the ketoacid 26 and pyridoxamine-phosphate 6. The net transamination has been shown to involve protonation at the 4 -Si face in yielding PMP (30). When the apoenzyme of tryptophan synthase is reconstituted with the unnatural substrates (4 / )- or (4 S)-[4- H,]pyridox-amine-phosphate and indole-3-pyruvic acid, an unnatural transamination... 

A niimber of B-N-Si cyclic systems have been prepared for the first time, by a variety of routes, chiefly (i) the reaction of N-lithio-aminoboranes with bis-(chlorosilylamines), (ii) transamination of (aminoborylamino)silanes, and (iii) condensation of Me2Si(NHMe)2 and alkylbis(dimethylamino)boranes. H, and n.m.r. data suggest that the silaborazine ring in, c.g., (39) is non-planar/ " The crystal structure of (40) shows that the molecule possesses Da symmetry. 

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