Titanium amides bonding

Examination of the enantioselectivities in Table 7.5 indicates a striking difference in selectivity achieved in the reduction of cyclic (entries 1-8) vs. acyclic imines (entries 9-11). The former is very nearly 100% stereoselective. The simple reason for this is that the acyclic imines are mixtures of E and Z stereoisomers, which reduce to enantiomeric amines vide infra). The mechanism proposed for this reduction is shown in Scheme 7.11 [86]. The putative titanium(III) hydride catalyst is formed in situ by sequential treatment of the titanocene BINOL complex with butyllithium and phenylsilane. The latter reagent serves to stabilize the catalyst. Kinetic studies show that the reduction of cyclic imines is first order in hydrogen and first order in titanium but zero order in imine. This (and other evidence) is consistent with a fast 1,2-insertion followed by a slow hydrogenolysis (a-bond metathesis), as indicated [86]. Although P-hydride elimination of the titanium amide intermediate is possible, it appears to be slow relative to the hydrogenolysis. 

Figure 1 (A) Schematic section through a chemically microstructured TSG sample after pCP with reactive DSU, formation of a HUT SAM and immobilization of proteins. Primary amines of proteins react with the end groups of the DSU crosslinker (11, ll -dithio-bis(succinimi-dylundecanoate), Fig. 1C bound DSU (thiolate form) with the formation of an amide bond. (B) Schematic section through a microstructured silicon/gold structure after pCP, etching the gold, titanium and silicon, removal of the etch-resistant monolayer, formation of a reactive monolayer and finally reaction with protein.

Titanium(II) compounds are very strong reducing agents many react with various small molecules. In the presence of N2, /rans-[TiCl2(TMEDA)2] reacts with the sterically demanding lithium amide LiN(SiMe3)2, in pyridine, to give 17-A-X and similar complexes 78 these N2 complexes contain very short (ca. 1.7 A) Ti=N bonds and are best considered as TP compounds. 

The reductive coupling of carbonyl compounds with formation of C-C double bonds was developed in the early seventies and is now known as McMurry reaction [38, 39]. The active metal in these reactions is titanium in a low-valent oxidation state. The reactive Ti species is usually generated from Ti(IV) or Ti(III) substrates by reduction with Zn, a Zn-Cu couple, or lithium aluminum hydride. A broad variety of dicarbonyl compounds can be cyclized by means of this reaction, unfunctionalized cycloalkenes can be synthesized from diketones, enolethers from ketone-ester substrates, enamines from ketone-amide substrates [40-42], Cycloalkanones can be synthesized from external keto esters (X = OR ) by subsequent hydrolysis of the primary formed enol ethers (Scheme 9). 

Tetrakis(dimethylamino)titanium converts DMF to tris(dimethylamino)methane (529 equation 236). This compound is also formed in the reaction of V-tetramethyloxamide with the titanium reagent, whereas amides which have a-CH bonds are converted to ketene aminals. Cyclic and spitocyclic compounds, containing the substitution pattern of a trisamino compound, are formed in the reaction of IVA -dialkylformamides or A/ -aryl-(VJV-dialkylformamidines with aryl isocyanates. Similar compounds, e.g. (530 equation 237), are produced in the reaction of imidazolines or lactamid-ines with isocyanates. 

As for the reduction of the ketones, the amphoteric catalysts featuring acidic-basic sites have been found to be very effective for the enantioselective catalysis of C-C bond formation. Thus, Soai et al. were the first to report the enantioselective addition of dialkylzincs to aldehydes using enantiomerically pure phosphin-amides and analogues as chiral catalysts in the presence of titanium tetraiso-propoxide. Numerous chiral organophosphorus compounds have been prepared and applied in a test reaction between benzaldehyde and diethylzinc [48, 49]. An important difference in terms of enantioselectivity was observed between the behavior of P=S (47-48) and P=0 (49) groups. Thus, the enan-... 

Biomaterials in general are based on the materials groups metals, polymers and ceramics [3]. Typical metallic biomaterials are based on stainless steel, cobolt based alloys and titanium or titanium alloys and amalgam alloys. Polymeric biomaterial composites from monomers are based on amides, ethylene, propylene, styrene, methacrylates, and/or methyl methacrylates. Biomaterials based on ceramics are found within aU the classical ceramic families traditional ceramics, special ceramics, glasses, glass-ceramics, coatings and chemically bonded ceramics (CBC). 

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