Hydride complexes chromium

Anionic chromium hydride complexes proved to be efficient hydrogen atom donors. Newcomb determined PPN+ HCr(CO)5 to be an efficient radical initiator and reducing agent for radicals and determined the kinetics of the hydrogen abstraction reaction [214]. In line with the observation that 3d metal complexes are much more prone to radical pathways than the corresponding 4d and 5d complexes, an increase of the extent of competing S -pathways for the bromide abstraction was found for molybdenum and tungsten complexes compared to the chromium complex. 

Our attempts to prepare chromium hydrides and to evaluate their role in polymerization catalysis eventually led to the isolation of a series of alkyls and hydrides lacking any ancillary ligands besides the cyclopentadienyl moiety (see below).[6] Reduced to the essence of alkyls, these complexes provided another piece of evidence in the growing case against polymerization activity of divalent chromium none of the alkyls even reacted with ethylene. The hydride underwent one insertion and stopped at the stage of an ethyl group. 

In reactions carried out for 24 h at room temperature, a 95% yield of cyclo-hexanol from cyclohexanone was obtained. Other ketones and aldehydes were also hydrogenated under identical conditions, but with slower rates (38% conversion for hydrogenation of 2-hexanone, 25% conversion of acetophenone, 45% for 3-methyl-2-butanone). Insertion of the C=0 bond of the ketone or aldehyde into the Cr-H bond was proposed as the first step, producing a chromium alk-oxide complex that reacts with acid to generate the alcohol product. The anionic chromium hydride [(COJsCrH]- is regenerated from the formate complex by... 

Rate Constants (10 3 k/M 1 s ) for the Reactions of Chromium(IV) and Chromium(V) Complexes with Rhodium Hydrides ... 

HETEROBINUCLEAR NONACARBONYL COMPLEXES AND HYDRIDE COMPLEXES OF IRON-CHROMIUM, IRON-MOLYBDENUM, AND IRON-TUNGSTEN... 

The cocondensation of chromium vapour and the phosphorus ligand is used to prepare [Cr(dmpe)3] and the trimethyl phosphite complex [Cr P(OMe)3 6] (Table 8). The former has a distorted octahedral structure, and the latter forms a seven-coordinate hydride CrH2 P(OMe)3 5 which, with other chromium(II) complexes of P donor ligands, is considered in Section 35.3.4. 

Chromium carbene complexes, 82 Chromium carbonyl, 51 Chromium(II) chloride, 84 Chromium(III) chloride-Lithium aluminum hydride, 84 Chromium(VI) oxide, 338... 

Chromium tricarbonyl complexes of arylacetic esters can be alkylated by the use of phase-transfer catalysis or sodium hydride in Af, /V-dimethyl-formamide (77). However, the yields obtained by the latter method are as good as or superior to those realized using phase-transfer techniques. Both methods give similar stereochemical results (e.g., 68 — 69 and 70). 

It is speculated that a chromium hydride or alkyl species is formed, but the exact mechanism for its formation is not known, though approximately six decades have elapsed since the basic discoveries of Hogan and Banks. The hydride (or alkyl) initially forms a n-complex with ethylene. The ii-complex collapses to an alkylchromium moiety that serves as the active center for the polymerization (eq 5.2). 

Halogen-substituted arene chromium tricarbonyl complexes undergo nucleophilic substitution by alkoxide ions at a considerably enhanced rate over the free arene 39, 327, 444). The effect of the chromium tricarbonyl moiety on the rate of substitution of the arene is approximately equal to that of a -nitro group (39). Treatment of ethylbenzene chromium tricarbonyl with fcrf-butyl lithium followed by hydrolysis and decomposition of the resulting complexes with Ce(IV) yielded m- and p-ethyl-tcrt-butylbenzene along with some unsubstituted ethylbenzene (55). The reaction represents a novel nucleophilic displacement of a hydride ion and contrasts with the metalation reaction observed with w-butyl lithium (304). 

Amido-substituted methylenecyclopropanes can be used in [2 + 2] photocycloadditions with chromium-alkoxycarbene complexes. (5)-3-(2-Methylenecyclopropyl)-4-phenyloxazolidin-2-one (14) was prepared from pentacarbonyl[A-phenylglycinyl(cyclopropyl)carbene]chro-mium(O) by ring closure with sodium hydride and diphenyl carbonate. Photolysis of a variety of chromium-alkoxycarbene complexes 15 in the presence of two equivalents of optically active enecarbamate under carbon monoxide produced optically active cyclobutanones. The cis- and trani-products, 16 and 17, were formed with a high degree of asymmetric induction at the position a to the oxygen. ... 

In contrast to CraqO ", the chromium(V) complex (saIen)CrO reacts by hydride transfer and yields no L(H20)Rh00 . The result... 

Allenes 169 and alkynes 170 are prepared by hydrogenolysis of propargyl compounds with several hydrides. Triethylammonium formate is used most conveniently under mild conditions [45]. Chromium tricarbonyl-complexed phenylallene 172 was prepared from the carbonate 171 [46]. The alkyne 174 was obtained selectively from the propargyl formate 173 having an amino group [47]. 

Semmelhack et aL (24) found that the halo leaving group is unnecessary, and a hydride ion can be formally displaced from the benzene ring of the chromium tricarbonyl complex. Evidence seems to be in favor of initial attack on the chromium atom which is also a soft-soft interaction. 

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