Reaction with chromium complexes

Ethyl (Z)-2-bromomethyl-2-heptenoate and aldehydes condense on reaction with chromium(II) chloride to furnish cw-3,4-disubstituted dihydro-3-methylene-2(3 //)-( uranones exclusively16, indicating that a (Z)-allylchromium complex might serve as reactive intermediate in the. mv-selec-tive addition step due to the bulky 2-substitucnt. Alternatively, an acyclic transition state for the reaction of the ( )-diastereomer, mediated by the Lewis acid dichloroaluminum hydride, has been discussed16. 

Although most of the examples of [3S+2C] cycloaddition reactions with carbene complexes are referred to as 1,3-dipolar processes, we should include in this section another kind of non-dipolar transformation dealing with the reaction of pentacarbonyl(methoxymethylcarbene)chromium with a base followed by treatment with an epoxide in the presence of boron trifluoride. This reaction gives cyclic carbene complexes in a process that can be considered a [3S+2C] cycloaddition [44] (Scheme 14). 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

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). 

Chromium(III) organocations (see previous section) have attracted a good deal of attention. The nature of the R group in Cr(H20)jR controls the reactivity. When R is a primary group, the complex is stable in Oj. A chain mechanism holds for Oj reaction with a complex containing a seondary or tertiary alkyl R group while reaction is indirect and via unimolecular homolysis with benzylchromium(lll) (Sec. 2.1.6). 

Many complexes of conjugated ketones are also known, such as the iron tricarbonyl complexes of substituted cyclopentadienones 30), although reaction with chromium hexacarbonyl occurs only if phenyl substituents are available for tt complexing 31). A common difficulty of preparing complexes of heterocyclics is the ability of the heteroatom to form o bonds with the metal. 

Complexes of 8-hydroxyquinoline and chromium(III) have been known for many years.1139 Complexes have been mercurated and the deuterated quinoline isolated.1139 The bromination of chelated 8-hydroxyquinoline proceeds about thirty-five times faster than that of the free ligand.1140 Tris(8-hydroxyquinolinato)chromium(III) absorbs large amounts of hydrochloric, hydrobromic and hydrofluoric acids. Chemical reaction with the complex was considered a more likely explanation than solid solution or clathrate formation, even though more than one mole of acid was absorbed per mole of complex.1141... 

For production of Co111 1 2 complexes, a cobalt salt is generally added after coupling and warmed to 60-90°C in a weakly alkaline medium. Reaction with chromium requires more severe reaction conditions, for example, refluxing for 20 h [67],... 

A number of allylic chlorides have been reported to yield photoproducts characteristic of the alkene moiety and not of the C—X bond123. Allyl and crotyl halides undergo photochemical reactions with chromium, molybdenum and tungsten complexes, which involve C—X bond cleavage133. 

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