Organochromium compounds bonding

Early attempts to prepare stable, isolable, cr-bonded organic derivatives of the transition metals were almost uniformly unsuccessful, except for certain organometallic derivatives of platinum and gold. It has already been mentioned that the V-bonded organochromium compounds of Hein and co-workers were in fact 7r-bonded arene-chromium derivatives. A detailed and critical account of these earlier studies was written in 1955 by Cotton (53). 

Although there are many differences between chromium oxide catalysts and the organochromium catalysts, when they are bonded to the support, organochromium catalysts usually display a similar, but exaggerated, MW response in the polymer produced relative to what is observed with chromium oxide catalysts. For example, the MW of polymer produced with each type of catalyst usually decreased as the support calcination temperature was raised. Similarly, when both chromium oxide and the organochromium compounds were deposited onto aluminophosphate supports, they always yielded lower-MW polymer as the amount of phosphate in the support was raised. 

A catalyst based on chromocene, Bis(cyclopentadienyl)chromium, was developed in the 1960s at Union Carbide by G. L. Karapinka and cowork-ers and was the first commercial chromium-based catalyst that was prepared with an organochromium compound containing Cr-carbon bonds in the starting material. In addition, the starting material based on Cr(II), did not need to be oxidized to a Cr(VI) species to obtain a high activity ethylene polymerization catalyst. 

The practical use of chromium(II) chloride in organic synthesis was begun by Hiyama and Nozaki in 1976. They used anhydrous chromium(II) chloride for the reduction of allylic halides to get allylic chromium reagents [47]. Since then, useful C-C bond formation reactions between organic halides and carbonyl compounds, which were mediated by chromium(II) salt have been developed. The most important features of these reactions were chemoselectivity and stereoselectivity. In these transformations, treatment of organic halides with chromium(II) salt was considered to afford the intermediary organochromium compounds although these compounds have not been isolated. As described in the previous section, reduction of gem-dihalides with chromium(II) salt may afford gem-dichromium species. 

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

The one-pot Barbier-type addition of alkenyl, aryl, allyl, vinyl, propargyl, alkynyl, or allenylchromium compounds to aldehydes or ketones is known as the Nozaki-Hiyama-Kishi (NHK) reaction. An excellent review by Furstner published in 1999 detailed the exhaustive literature on the carbon-carbon bond formations involving organochromium(III) reagents. This chapter will present major developments and examples of recent carbon-carbon bond formation methodology and improvements as well as their use in natural products synthesis since 1999. 

The Nozaki-Hiyama-Kishi (NHK) reaction, first reported in the late 1970s, has become an important and versatile carbon-carbon, bond-forming process, involving the nucleophilic addition of organochromium(III) reagents to carbonyl compounds (Scheme 12.1). ... 

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