Organochromium reagents synthesis

Allylic bromides can also serve as progenitors for nucleophilic organochromium reagents. An elegant example is found in Still and Mobilio s synthesis of the 14-membered cembranoid asperdiol (4) (see Scheme 2).7 In the key step, reduction of the carbon-bromine bond in 2 with chromium(n) chloride in THF is attended by intramolecular carbonyl addition, affording a 4 1 mixture of cembranoid diastereoisomers in favor of the desired isomer 3. Reductive cleav-... 

Nozaki-Hiyama-Kishi (NHK) reactions215,216 are well known and often employed as a useful method for the synthesis of natural products by coupling of allyl, alkenyl, alkynyl, and aryl halides or triflates with aldehydes. The organochromium reagents are prepared from the corresponding halides or triflates and chromium(ll) chloride, and are employed in polar aprotic solvents (THF, DMF, DMSO, etc.). Subsequently, it was found that nickel salts exhibited a significant catalytic effect on the formation of the C-Cr bond217,218 (Equation (19)). 

For a comprehensive review of carbon-carbon bond formation with organochromium reagents, see (a) Fiirstner, A., Chem. Rev. 1999, 99, 991. (b) Wessjohann, L. A., Scheid, G., Synthesis 1999, 1. 

Organochromium reagents have found varied use in organic synthesis. Aryl-chromium complexes influence significantly the reactivity of the aromatic ring and have been used widely. Other unsaturated chromium complexes, alkyl-chromium species and chromium carbenes promote useful transformations and continue to attract attention. 

Under certain circumstances, organochromium(ni) compoimds transfer their allaryl groups to aldehydes and, less frequently, to ketones. The particular selectivity and tolerance of this reaction make it particularly useful in organic synthesis. The use of these chromium reagents may be advantageous for use with acid-sensitive substrates, because of their reduced Lewis acidity relative to other transfer reagents [e.g., trichloromethyl-titanium(IV) or all[Pg.35]

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 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. 

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