Chromium complex hydrogenation

After penetration of the hide by the chromium the pH is raised to about 3.5—4.0. At this higher pH a change occurs in the chromium complexes as the basicity of the chromium increases and binding to the protein becomes possible. Chromium binds firmly to the protein forming a cross-link species, and as the pH increases the hydrogen is removed from the complex forming a stable stmcture. 

As expected, under a hydrogen atmosphere in the presence of Pd/C in ethanol, the benzannulated pyrrolizine 48 leads to the dihydropyrrolizine derivative 49. However, semireduction of the pyrrole ring could be performed via the tricarbonyl chromium complex of 49 with various hydrides. Use of cyanoborohydride in trifluoroacetic acid (TFA) gave the best results for compound 50, both in terms of chemical yield (92%) and diastereoselectivity (90% of the trans-isomer) <2000TL1123>. 

Hagen, A. P., and Beck, H. W., Inorg. Chem. 15, 1512 (1976) (Synthesis of arylsilane-(tricarbonyl)chromium complexes and their high pressure reaction with hydrogen chloride). 

Certain chromium complexes also exhibit similar selectivities in the hydrogenation of conjugated dienes.139 140 Coordination of the diene in the cisoid conformation and 1,4 addition can account for the selectivity. 

Sodium hydrogen telluride, 282 Tin(II) chloride, 298 By other methods Acetic-formic anhydride, 1 Arene(tricarbonyl)chromium complexes, 19... 

Hydrogenation catalysts Arene(tricarbonyl)chromium complexes, 19... 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

Chromium carbonyl hydrogen complexes, preparation and characteristics, 5, 206 Chromium complexes... 

Anionic chromium hydride complexes proved to be efficient hydrogen atom donors. Newcomb determined PPN+ HCr(CO)5 to be an efficient radical initiator and reducing agent for radicals and determined the kinetics of the hydrogen abstraction reaction [214]. In line with the observation that 3d metal complexes are much more prone to radical pathways than the corresponding 4d and 5d complexes, an increase of the extent of competing S -pathways for the bromide abstraction was found for molybdenum and tungsten complexes compared to the chromium complex. 

Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex shown in Fig. 2.4, are often referred to as Fischer carbenes. The other type of alkylidene complexes has the metal ion in a high oxidation state. The tantalum complex is one such example. For both the types of alkylidene complexes direct experimental evidence of the presence of double bonds between the metal and the carbon atom comes from X-ray measurements. Alkylidene complexes are also formed by a-hydride elimination. An interaction between the metal and the a-hydrogen atom of the alkyl group that only weakens the C-H bond but does not break it completely is called an agostic interaction (see Fig. 2.5). An important reaction of alkylidene complexes with alkenes is the formation of a metallocycle. 

M. 1-Aza-t,3-butadiene Complexes via 1,4- and N-Atkenyl(amino)carbene Chromium Complexes via 1,5 Hydrogen Shifts... 

In contrast, the chemistry of the oxidation of a primary alcohol to an aldehyde differs sharply from the oxidation of an aldehyde to a carboxylic acid (case (b)). Advantage, in this case, must be taken of the difference in the mechanisms of these steps. Among the reagents which can effectively oxidize alcohols and remain rather inert toward aldehydes are pyridinium chlorochro-mate (a chromium trioxide-hydrogen chloride complex of pyridine) or dimethyl sulfoxide-Lewis acid. 

Bisbenzene chromium has been found to be a catalyst for the polymerization of ethylene at 200°-250°C. Chromium metal was postulated as the active catalyst in the system 410, 411). The polymerization of ethylene by bisarene chromium(I) salts in the presence of (i-Bu)3Al has also been studied (406). The catalytic activity was found to be a function of both the arene and the anion present. When bisarene chromium complexes are air oxidized in water, hydrogen peroxide is produced ... 

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