Chromium derivatives

Tungsten alkynyl Fischer carbene complexes are excellent dienophile partners in the classical Diels-Alder reaction with 1-azadienes (see Sect. 2.9.2.1). On the contrary, the chromium-derived complexes exhibit a different behaviour and they react through a [4S+3C] heterocyclisation reaction to furnish azepine derivatives [116] (Scheme 68). The reaction is initiated by a 1,2-addition of the nitrogen lone pair to the carbene carbon followed by a [l,2]-Cr(CO)5 shift-pro-... 

Apart from chromium and chromium (II) salts, which are reducing, the table of accidents involving chromium derivatives is mainly one of very reactive oxidants. The effect of this dangerous property is worsened by the fact that the cited substances are commonly used, not only in industry (especially in sur ce treatment) but also in analysis, research and training laboratories. 

H.B. Gray, California Institute of Technology The rate of Co(phen) 3 oxidation of the chromium derivatives of the reduced blue copper protein is surprisingly close to the rate for the native protein, in my view. Could you comment further on what you think this result means in terms of the proposed interaction sites in the various mechanisms ... 

The starting Fischer-type carbene complexes 1 were obtained by Michael addition of dimethylamine to the carbon-carbon triple bond of the corresponding ethoxy-(phenylethynyl)carbenes. In this regard, de Meijere and co-workers observed that the reactions of several primary and secondary amines with this sort of carbenes, in particular chromium derivatives 3 containing bulky substituents at the terminal carbon of the acetylenic unit, result in formation of the aminoallenylidene derivatives 5 as by-products of the expected Michael adducts 4 (Scheme 2) [20-24]. 

The ammino-rhodium compounds resemble closely the corresponding cobalt and chromium derivatives. 

The application of lignosulfonate-chromium derivatives in oil well drilling mud has already been mentioned as a typical example of the above effects in combination with crosslinking. The use of lignosulfonates as active extenders in concretes is a further example of the applicability of the above effects (61). Another is the use of lignins as extenders for asphalt cement (70). 

Sulfoxide complexes can be formed for a wide range of metal oxidation states, as shown by the chromium derivatives Cr02Cl2 3DMSO, CrCl3 3DMSO, CrCl2-2DMSO and Cr(CO)s DMSO. 

The redox reactions have been quite well studied for the chromium deriv-... 

In the polymeric structures of the first members of the homologous series of trivalent chromium derivatives, the metal atoms possess octahedral coordination [1366], while the derivatives of branched radicals are often monomeric [Cr(OR)2L2] molecules with square planar coordination [1571] or dimeric [Cr(OR)2]2 with triangular cordination of the metal atoms [795, 1185]. The monomeric [Cr(OR)3L2] are trigonal bipyramids [1571], while the derivatives of chromium (IV) and (VI) are tetrahedral [167, 1573, 1527]. 

The major practical application has been developed so far for Cr(OR)4. They are used in combination with metalalkyls as the catalysts of stereoregular polymerization by Ziegler [696]. They can also serve as versatile precursors of otherwise hardly accessible chromium derivatives — for example,... 

Intramolecular nucleophilic aromatic substitution of a tricarbonyl chromium derivative in the presence of sodium hydride and dimethoxyethane gave 33 which on photolysis yielded bicyclo[3.2.0]hept-3-en-7-one derivative 462 with ee>98% (Scheme 16). The mechanistic pathways of the reaction have been discussed <2000TA1927, 2005TA971>. 

Scheme 8.4 Stepwise synthesis of the tetrapeptide 18, containing the sequence 14-17 of the human proinsulin C-peptide, from the methoxy(phenyl)carbene complex 16 via the carbene chromium derivative 17 as the intermediate (Gly-OMe and Pro-OMe were the methyl esters of glycine and proline). The C—N coupling reactions in steps (1)—(4) were carried out using the DCCD/HOSU method, explained in [35]...

The ammonia and amine complexes are the most numerous chromium derivatives and the most extensively studied. They include the pure ammine [CrAm6]3+, the mixed ammine-aqua types, that is, [CrAm4 (H20) ]3+ (n = 0-4, 6), the mixed ammine-acido types, that is, [CrAm6 X ](3" )+ (n = 1-4, 6), and mixed ammine-aqua-acido types, for example, [CrAm6 m(H20) Xm](3 m7+ (here Am represents the monodentate ligand NH3 or half of a polydentate amine such as ethylenediamine, and X an acido ligand such as halide, nitrite, or sulfate ion). These complexes provide examples of virtually all kinds of isomerism possible in octahedral complexes. 

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