Organochromium compounds types

Although organochromium catalysts are not well characterized, organochromium compounds are thought to bind to the support by reaction with surface hydroxyls as other types do. When Cr(allyl)3 or Cr(allyl)2 is used, propylene is released (59,60). Chromocene loses one ring (52-55), and / -stabilized alkyls of chromium lose the alkane (81). 

Hein wrote (translation in English) It should be mentioned, and briefly explained, why I considered the organochromium compounds, at the time of their discovery, to be tetraphe-nylchromium salts, and not formulations of the type now suggested by H. Zeiss and E. O. Fischer. The main reason was that at that time, there were no precedents for such a view, and I firmly believe that if I had brought such a formulation of this type into consideration, I would not have taken seriously in the context of the state of knowledge at the time . 

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. 

Two types of OF groups (P-OF1 and A1-OF1) have been identified on these supports as available for reaction with the organochromium compound. Therefore, the two peaks in the MW distribution are identified as emerging from P-O-Cr-DMPD and Al-O-Cr-DMPD species. This chemistry is parallel to that identified by the silane probes as summarized in Table 51. Clearly, it is the phosphate-associated site that produces the low-MW polymer peak and the Al-associated site that produces the high-MW peak. 

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. 

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