Chromium complexes photochemistry

For a review on chromium carbene complex photochemistry in organic syntheses see Hegedus LS (1997) Tetrahedron 53 4105... 

It will become evident from this discussion of cobalt(III) and chromium(III) photochemistry that a fortunate combination of many factors makes these systems especially amenable to mechanistic investigation this is not the case for most other inorganic systems. For this reason, the progress made toward understanding the photochemical reactions of cobalt(III) and chromium(III) complexes may not be easily duplicated in other areas and it is useful to inquire whether conclusions reached here can be generalized. It is my belief that they can, and this and the next section are intended to place the subsequent discussion in the perspective of inorganic photochemistry. 

Much less is known about photoinitiated ligand exchange reactions of coordination complexes of cobalt(III) and other de ions than is the case for chromium(III). With the exception of the cobalt(III) ammines, however, available data suggest that photochemical ligand exchange reactions of cobalt(III) and d6 systems involve the thermally equilibrated 17 g and/or 37, states of the complex as intermediates. The reasoning is completely analogous to that described earlier with respect to chromium(III) photochemistry. 

L. S. Hegedus, Synthesis of Amino Acids and Peptides Using Chromium Carbene Complex Photochemistry, Acc. Chem. Res. 28, 299-305 (1995). 

Chromium, tris(hexafluoroacetylacetonato)-optical isomerism, 28 Chromium, tris(l,10-phenaiithroline)-ligand field photochemistry, 398 Chromium, tris(trifluoroacetylacetonato)-isomerism, 28 Chromium complexes geometric isomerism, 11 Chromium (0) complexes magnetic behavior, 273 Chromium(I) complexes magnetic behavior, 272 Chiomium(II) complexes magnetic behavior, 272 spectra, 252 thiocyanate... 

Another applieation of photogenerated metal coordinated ketenes is based on the addition of protie nucleophiles and has been exploited in the synthesis of amino aeids and peptides. [66] As usual, the reactive intermediate is generated by photolysis of an aminoearbene complex 46, which may be a-alkylated in a previous step. The oxazolidine auxiliary applied successfully inducing asymmetry in the P-laetam formation, allowed an enantioselective synthesis of amino aeids. Sinee both enantiomers of the auxiliary may be obtained from the corresponding phenyl glycine enantiomer, natural (5) and non-natural R) amino acid esters 47 are accessible via this route (Scheme 25). A recent review on synthetical applications of chromium carbene photochemistry has been published, [li]... 

Harriman, A. Photochemistry of Manganese Complexes, Coord. Chem. Revs. 1979,28,147. Jamieson, M. A. Seipone, N. Hoffinan, M. Z. Advances in the Photochemistry and Photophysics of Chromium(ni) Polypyridyl Complexes in Fluid Solution, Coord. Chem. Revs. 1981,39,121. Kirk, A. D. Chromium(ni) Photochemistry and Photophysics, Coord. Chem. Revs. 1981,39,225-293. Kutal, C. Spectroscopic and Photochemical Properties of d Metal Complexes, Coord. Chem. Revs. 1990,99,213. 

Hegedus, L.S. (1995) Synthesis of amino adds and peptides using chromium carbene complex photochemistry. Accounts of Chemical Research, 28, 299-305, and references therein. 

H. PHOTOCHEMISTRY OF CHROMIUM(HI) COMPLEXES A. Survey of Thermal and Photochemistry of Chromium(III) Complexes... 

Robert Plane I would like to comment on the photochemistry of these complexes, particularly the chromium which I think are well chosen for at least two of Dr. Gray s reasons. First, in the case of chromium, unlike cobalt, the charge transfer band is well separated, so that one can study the d-d transitions, at least in certain systems where one has six ligands all alike, and there is no Jahn-Teller splitting, and one has a fairly good idea as to the assignment of bands. 

Table 2 Photochemistry of Mixed-ligand Chromium(III) Complexes upon Irradiation into the Lowest Quartet Ligand...

Both chromium forms are photochemically active, and the metal-centred (MC) photochemistry of the Crm complexes has been known for a long time [88]. Photo-substitution or photoisomerization is, however, of minor environmental relevance to facilitate or accelerate the mutual conversion of Cr111 and CrVI only photoredox behaviours are essential. 

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