Reviews chromium complexes

Three reviews in the A. W. Adamson 80th birthday commemoration volume deal with mechanisms of photochemical reactions of chromium complexes (90) rate constants for photochemical and thermal solvolyses have been compared for [Cr(NCS)6]3- and for [Cr(NCS)5(H20)]2- in DMFand in MeCN (91). 

Although slightly outside the scope of this review, an interesting case of stereoselection should be presented here. It has been observed by Gibson (nee Thomas) and coworkers during the deprotonation of tricarbonylchromium complexes of benzyl alkyl ethers by means of the chiral bis(lithiumamide) base 234 (equation 54) . The base removes the benzylic pro-R-H atom in 233 from the most reactive conformation to form the planary chiral intermediate 235. The attack of the electrophile forming 236 proceeds exclusively from the upper face in 235, because the bulky chromium moiety shields the lower face. Simpkins and coworkers extended the method to the enantioselective substitution of the chromium complexes of 1,3-dihydroisobenzofurans . 

Uemura and Kamikawa have presented a review on the stereoselective synthesis of axially chiral biaryls utilizing planar chiral (arene)chromium complexes [44]. 

General reviews have appeared in the literature [1, 2] concerning arene chromium complexes as well as iron [2] and manganese [3,4] complexes, and nucleophilic additions followed by iodine oxidation and loss of the metal have been well reported, for example by Semmel-hack [lc] and by Morris in 1995 [le]. The purpose of this chapter is to focus only on ipso,... 

A variety of transition metals, for example, chromium, molybdenum, tungsten, iron, vanadium, manganese, and rhodium can be used to prepare relatively stable j -arene complexes (see Arene Complexes). Reactions of j -arene chromium tricarbonyl complexes have been extensively examined, and numerous reviews are available. Although chromium complexes are by far the most utilized in organic synthesis, complexes of iron and manganese are emerging as potentially useful alternatives. 

The role of photocatalysis by transition metal complexes in the environment is reviewed, and its influence on composition of the environmental compartments, transport between them, and activation of the environmental self-cleaning behavior is characterized. In description of atmospheric processes, the attention is paid to coordination compounds as photocatalysts of the transfer and redox reactions of nitrogen oxides. In the case of hydrosphere and soils, various mechanisms of organic pollutant photodegradations are presented in which the iron, copper, and chromium complexes play... 

A comprehensive treatment of the benzannulation of Fischer carbene complexes with alkynes is not possible in this review, and thus instead the material presented here will hopefully serve to give the reader an overview of its scope and limitations. The first report of this reaction was in 1975 by Dotz in which he describes the formation of the naphthol chromium tricarbonyl complex (236) from the reaction of the phenyl chromium complex (la) with diphenylacetylene. In the intervening years over 100 papers have been published describing various aspects of this reaction.The reaction of the generic cartene complex (233 Scheme 34) with alkynes will serve to focus the organization of the scope and limitations of the benzaimulation reaction. The issues to be considered are (i) the regioselectivity with unsymmetri-cal alkynes (ii) possible mechanisms (iii) applications in natural product syntheses (iv) the effect of substitution on the aryl or alkenyl substituent of the carbene carbon (v) functionality on the alkyne (vi) effects of the solvent and the concentration of the alkyne (vii) tandem applications with other reactions of carbene complexes (viii) reactions where aromatization is blocked (cyclohexadienone annulation) (ix) annulation of aryl versus alkenyl carbene complexes (x) the effect of the ligands L on the metal (xi) the effect of the ancilliary substituent RX and (xii) reactions with —C X functionality. 

The reactivity of carbene-metal complexes, amongst others the reactivity with respect to alkenes and alkynes, has been reviewed by Dotz Just like free carbenes the coordinated carbenes add to triple bonds to give cyclopropene derivatives. Other reaction products, however, are also possible. For instance, the carbene ligand of chromium complex 23 reacts with diphenylacetylene to a mixture of products, including naphthalene derivative 24 and furan derivative 25 (equation 18). A carbonyl ligand has participated. Molecular orbital calculations by Hofmann and Hammerle " on this system reveal that the reaction would pass through an y-vinylcarbene type of complex (26) instead of through a planar chromacyclobutene 27. The subsequent steps to yield either phenol or furan could involve vinylketene 28, but this still is a matter of debate. Similar, but more selective, furan syntheses have been observed for carbene complexes based on iron and cobalt. ... 

The synthesis of planar-chiral arene chromium complexes has been reviewed several times and has also been discussed in an earlier article [9,10]. Apart from stereoselective complexation reactions, diastereoselective and enantioselective... 

As a consequence of the unsymmetric substitution pattern encountered in the arene ring (OH versus OR) formed upon benzannulation the chromium complexes bear a plane of chirality. Complexes of this type are powerful reagents in stereoselective synthesis, and approaches directed towards diastereoselective benzannulation will be addressed in section 2.1 of this review. 

For a review of palladiumcarbon-carbon bond formation using tricarbonyl(T chromium complexes, see Carpentier, J.F., Petit, F., Mortreux, A., Dufand, V., Basset, J.-M. and Thivolle-Cazat, J. (1993) J. Mol. Cat., 81,1-15. 

The control of stereochemistry with chromium carbine complexes has been reviewed. The DBR can create a new stereocenter in three ways. First, the arene tricarbonyl chromium complex contains a plane of chriality, thus the complexes 35 and ent-35 are enantiomers when RL Ri and RS R2. Second, when phenyl substituents are included in the reactants the resulting biaryls can posess axial chirality if there is hindered rotation about the new aryl-aryl bond as in 36. Finally, all DBRs with differentially p-disubstitued alkenes give rise to cyclohexadienones 37 with a new stereocenter adjacent to the carbonyl. When Ri and R2 are not hydrogen, tautomerization cannot occur and the final product possesses a chiral center. 

A number of reviews contain relevant material such as those on dimolybdenum and ditungsten complexes, substitution reactions of chromium complexes, the reactivity of complexes containing multiple bonds between metal atoms, model reactions of coupling ieactions2 5, and in a review principally on ruthenium chemistry.216... 

Chromium. The history of the investigations estabHshing the essentiaHty of chromium has been reviewed (136). An effect of brewer s yeast in preventing or curing impaired glucose tolerance in rats was revealed, and the active factor was identified as a Cr(III) organic complex, glucose tolerance... 

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