Chromium carbonyl, effect

To test the validity of this mechanism, chromium carbonyl (1.0 g) was photolyzed under Ar at ambient temperature in a solution of methanol and hexamethylphosphoramide in the apparatus shown in Figure 5. The lamp was turned off periodically to check for the disappearance of slightly soluble Cr(C0) . Several photolyzing cycles were necessary to effect nearly complete conversion to the solvent-stabilized coordinately unsaturated species (equivalent to in Figure 4),... 

Italogenation catalyst. Chromium carbonyl catalyzes the monohalogenation of cyclohexane by CC14 (78% yield). Other cycloalkanes undergo the same reaction, liioniination can be effected in this way with CBrCl,. Other metal carbonyl complexes arc less active. Cr(CO)f, is actually more efficient than di-/-butyl peroxide. A free indical mechanism is involved. 

A systematic investigation of ligand dissociation from chromium carbonyl complexes provided evidence regarding electronic effects. The complexes Cr(CO)sT, trans-Cr(CO)4T2, and trans-Cx CO)AlAJ have been investigated for their reaction with carbon monoxide. The order of Cr-T bond stabilities in Cr(CO)sT is ... 

Decomplexation of ArCr CO)3. The chromium carbonyl complexes of arenes are useful for activation of the aryl group to nucleophilic attack (6, 28, 125-126 7, 71-72). Decomplexation has been effected with iodine or by photochemical oxidation with destruction of the expensive Cr(CO)3 unit. A more recent method involves reflux with pyridine to form Py3-Cr(CO)3 in yields of 70-100%. The pyridine complex in the presence of BF3 can be reused for preparation of ArCr(CO)3. Isomerization of 1,3-dienes. Ergosteryl acetate (1) is isomerized by chromium carbonyl to ergosteryl 83 acetate (2) in 81% yield. Under the same conditions ergosteryl 83 acetate (3) is isomerized to ergosteryl 81 acetate (4). 80th reactions involve isomerization of a cisoid diene to a transpid diene. In contrast iron carbonyl isomerizes steroidal transoid 3,5- and 4,6-dienes to 2,4-dienes. ... 

In some cases, aniline or indole derivatives can be substituted meta to the nitrogen by lithiadon of the appropriate chromium tricarbonyl complexes. °" Examples are given in Scheme 3. One of the Cr—C=0 bonds eclipses the C—N bond, and therefore the other carbonyls eclipse the meta C—H bonds. Two suggestions have been offered for the meta selectivity (i) the butyllithium coordinates the chromium carbonyl oxygen and then removes the proximate proton (a kinetic effect) (ii) the eclipsed conformation produces a lower electron density at the meta position, which in turn renders the meta protons more acidic (a thermodynamic effect). ... 

An overview of the effect of catalyst in the reaction of arenes with chromium hexacarbrmyl has been published. The reactivity of 17-, 18-, and 19-etectron catkMis generated electrochemically from mesitylene-tungsten tricarbonyl has been examined. The gas phase ion chemistry of a range of arene tricarbonylchromium complexes has been investigated by F.T. mass spectrometry. An improved synthesis of substituted naphthalene chromium carbonyls has appeared. ... 

Many of the syntheses we have seen within this review depend on the carbonylation of a vinylcarbene complex for the generation of the vinylketene species. The ease of this carbonylation process is controlled, to some degree, by the identity of the metal. The electronic characteristics of the metal will clearly have a great effect on the strength of the metal-carbon double bond, and as such this could be a regulating factor in the carbene-ketene transformation. It is interesting to note the comparative reactivity of a (vinylcarbene)chromium species with its iron analogue The former is a fairly stable species, whereas the latter has been shown to carbonylate readily to form the appropriate (vinylketene)iron complex. 

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

Electro-generated chromium(II) is also very effective in the pinacolization of otherwise unsatisfactory dimerizing carbonyl compounds (Table 5, No. 13)220-222) this case the chromium(II) ion does not act as redox agent but catalyzes the formation of a chromium(III) complex of the carbonyl compound which subsequently is reduced to the pinacol directly at the cathode (Eqs. (73)-(77)). 

Consequently, these charge effects are reflected in the carbonyl stretching frequencies (87, 88). It has recently been found from studies of the far infrared spectra that the metal-carbon stretching frequencies also support the theory (89). These charge-distribution effects are supported further by the observed dipole moments (90-92). Thus the dipole moments of the chromium tricarbonyl complexes of hexamethylbenzene, benzene, and methylbenzoate lie in the order 6.22, 4.92, and 4.47 /x, respectively. The relationship of charge effects to chemical reactivity is described below. 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

---