Chromium benzene reductions

With the same excess of catalysts hydrogenations of the esters over Raney nickel could be carried out at temperatures as low as 25-125° at 350atm with comparable results (80% yields). However, benzene rings were saturated under these conditions [55]. In addition to nickel and copper, zinc and chromium oxides, rhenium obtained by reduction of rhenium heptoxide also catalyzes hydrogenation of esters to alcohols at 150-250° and 167-340 atm in 35-100% yields [42]. 

The synthetic potential of transition metal atoms in organometallic chemistry was first demonstrated by the formation of dibenzenechrom-ium (127). Apart from chromium, Ti, V, Nb, Mo, W, Mn, and Fe atoms each form well-defined complexes with arenes on condensation at low temperatures. Interaction has also been observed between arenes and the vapors of Co, Ni, and some lanthanides. Most important, the synthesis of metal-arene complexes from metal vapors has been successful with a wide range of substituted benzenes, providing routes to many compounds inaccessible by conventional reductive preparations of metal-arene compounds. 

Fischer and Hafner reported an alternate synthesis of bis-ir-benzene chromium using a reductive Friedel-Crafts method which probably also proceeds via a [Pg.231]

The mixture 258 was converted to the unstable benzenesulfonyl aziridine 259 by treatment with an excess of benzenesulfonyl azide in benzene. Ace-tolysis of 259 with acetic acid and sodium acetate at room temperature for several days afforded the crystalline mixture of diastereoisomers represented by the formula 260. The aziridine rearrangement was regiospecific and 260 was the only product detected during this rearrangement. Lithium aluminium hydride reduction of 260 followed by acetylation yielded the mixture 261 in 85% yield. Selective hydrolysis of 261 afforded 262 in quantitative yield. The diastereoisomeric mixture 262 was converted into the diols 263 by hydrogenolysis. The diol mixture was oxidized with chromium trioxide... 

Dehydrogenation catalysts.2 Reduction of di(benzene)chromium with potassium sand in DME generates a species that dehydrogenates 1,4-cyclohexadiene to benzene. The catalyst prepared from benzene(l,3-cyclohexadiene)iron(0) promotes dehydrogenation and disproportionation to cyclohexene and benzene. 

Pyridine is also capable of deoxygenating arene oxides 110 and 112. The deoxygenation of benzene oxides in the presence of iron, chromium, and rhodium catalysts has been studied, and a mechanism for the reduction process has been proposed. Deoxygenation of K-region arene oxides is also catalyzed... 

The third approach to obtain diarylmethylpiperazine derivatives uses the highly stereospecific chiral oxazaborolidine-catalyzed reduction, using catecholborane as the reductant of the 4-bromobenzophenone chromium tricarbonyl complex, as described by Corey and Helal [59], followed by the stereospecific displacement of the hydroxyl benzyl group by the /V-substituted-piperazine [44]. As outlined in Scheme 2, Delorme et al. [44] used this approach for the enantioselective synthesis of compound 31, (+)-4-[ (aS)-a-(4-benzyl-l-piperazinyl)benzyl]-lV,lV-diethylben-zamide. Lithiation of the readily available benzene chromium tricarbonyl with n-BuLi in the presence of TMEDA in THF at —78 °C, followed by addition of... 

The (diphenyl)(benzene)chromiuin (I) iodide can also be prepared a reductive Friedel-Crafts reaction, starting with an appropriate mixture of benzene, diphenyl, CrCls, AICI3 and Al powder. The reaction can be carried out under reflux at atmospheric pressure, but again affords (dipheny)(benzene)chromium ina mixture with dibenzenechromium andbis(diphenyl)chromium. After conversion to the iodides, they must be separated from each other in a manner analogous to that given above. 

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