Chromium trichloride reduction

As is mentioned in the introductory material, chromium exhibits a propensity to form quadruple bonds that is unmatched by any other first-row transition metaL Although Cr2(02 CQl3)4(H20)2 was first reported over 100 years ago (210), it was not promoted as a quad ruple-bond-containing unit until Cotton and co-workers carried out a redetermination of the x-ray structure and reported a change in the chromium atom separation from the accepted value of 2.64 A initially reported in 1953 (251) to 2.3855(5) A (66, 67). Reduction of chromium trichloride to chromium(II) in aqueous solution with zinc followed by addition of sodium acetate is a convenient route to Cr2(02CCH3)4(H20)2 (204), which can be dehydrated by heating under vacuum to form Cr2(02 CCH3)4. 

The first well-authenticated preparation of the [Cr(C0)5] anion was carried out by Behrens and Weber (41) and involved the reduction of chromium hexacarbonyl with elemental sodium, lithium, calcium, or barium in liquid ammonia solution. It is not surprising that such a powerful reducing agent is necessary to effect the reduction of the very stable hexacoordinate chromium hexacarbonyl to the less stable pentacoordinate [Cr(CO)s] anion. In a subsequent report by Podall and associates the reduction of chromium hexacarbonyl with sodium amalgam in tetrahydrofuran or diglyme solution (42) is described. The same report also describes the direct preparation of the [Cr(CO)s] anion from chromium trichloride by treatment with elemental sodium in diglyme solution under carbon monoxide pressure. 

Chemical deoxygenation of sulfoxides to sulfides was carried out by refluxing in aqueous-alcoholic solutions with stannous chloride (yields 62-93%) [186 Procedure 36, p. 214), with titanium trichloride (yields 68-91%) [203], by treatment at room temperature with molybdenum trichloride (prepared by reduction of molybdenyl chloride M0OCI3 with zinc dust in tetrahydrofuran) (yields 78-91%) [216], by heating with vanadium dichloride in aqueous tetrahydrofuran at 100° (yields 74-88%) [216], and by refluxing in aqueous methanol with chromium dichloride (yield 24%) [190], A very impressive method is the conversion of dialkyl and diaryl sulfoxides to sulfides by treatment in acetone solutions for a few minutes with 2.4 equivalents of sodium iodide and 1.2-2.6 equivalents of trifluoroacetic anhydride (isolated yields 90-98%) [655]. 

One final interesting application of CAN which exemplifies one of the many possible reactions of qui-nones comes from Hassall and coworkers in their synthesis of 4-demethoxydaunomycinone (47). Thus, oxidation of the boronate (42) with CAN gave the crude quinone (43) which was reacted with trans-1,2-bis(acetoxy)-l,2-dihydrobenzocyclobutene (44) to give the tetracyclic quinone (45) in an impressive 79% overtdl yield. Deacetalization and reductive acetylation to the naphthacene (46), followed by oxidation with anhydrous chromium trioxide and deprotection with boron trichloride afforded the target compound (47 Scheme 10). 

TiCU is produced by the reduction of the tetrachloride with hydrogen or a metal like silver or mercury. When heated in the air it breaks up, giving the volatile tetrachloride and the solid dichloride. TiCl is deli quescent, forms a reddish violet solution with water, and violet crystals, TiCU 6 H20, from a hydrochloric acid solution. An unstable green hydrate of the same composition is formed when an aqueous solution of the trichloride is covered with ether and saturated at 0° with HC1. From the violet form all the chlorine may be removed by AgNO , but this is probably not true of the green modification. The trichlorides of chromium and vanadium likewise are known in two forms. TiCla forms double salts with the chlorides of rubidium and caesium. It is a more powerful reducing agent than stannous chloride and on this account finds extensive application in both qualitative and quantitative analysis. 

The reduction of a metal salt by aluminium and aluminium trichloride in the presence of an aromatic hydrocarbon, which is illustrated in Table XIII, provides the most general method known for the preparation of bis-ff-arene complexes. It was first used by Fischer and Hafner for the synthesis of bis-w-arene chromium complexes (Fischer s reducing Friedel-Crafts proceditfe). 

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