Chromium hydrides

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... 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

The intermediate in the organochromium-mediated defluorination of 7 or 8 [e.g., PhCrCl2(THF)3/NaH, Bu3Cr, CrCl3/ LiAlH4] is believed to be a chromium hydride spe-cies.144 ,4S... 

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. 

Hydrides of the bcc group VIA elements Cr and Mo, but not W, are prepared by high-P techniques " . Chromium hydride is synthesized directly from Cr with Hj by keeping a thin Cr foil at 150°C under 2200 MPa for 4 h. The H/Cr atom ratio is 0.93 and the metal lattice structure hep . The hydride, therefore, is equivalent... 

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). 

Chromium hydride evolves hydrogen by the irreversible reaction [8] ... 

Or a chromium hydride might be formed as an intermediate, followed by initiation as a separate step. Either way each chain is expected to have a terminal vinyl group and this agrees with actual infrared measurements on the polymer. Or propagation can also occur if the coordinated monomer inserts into the active bond. 

Reaction uncertain, but believed to result in formation of divalent chromium hydride or alkyl. 

Alternatively, demetallation to give enol ethers 839 can be achieved by treatment with pyridine (Scheme 8.14). Under these conditions, as pyridine is a weak base, an equilibrium is established between the carbene 8.35 and its anion 837. The anion, however, can also reprotonate on chromium to give a chromium hydride 838. This is followed by reductive elimination. The enol ether 839 is obtained as its Z-isomer, a consequence of the carbene anion having S-geometry to keep the alkyl group away from the bulky Cr(CO)s moiety the chromium is converted into a pyridine complex. 

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