Chromium dihalides

Anhydrous chromium dihalides are conveniently prepared by reduction of the trihalides with H2 at 300-5()0 C, or by the action of HX (or h for the diiodide) on the metal at temperatures of the order of 1000°C. They are all deliquescent and the hydrates can be obtained by reduction of the trihalides using pure chromium metal and aqueous HX. All have distorted octahedral structures as anticipated for a metal ion with the d configuration which is particularly susceptible to Jahn-Teller distortion. This is typified by Crp2, which adopts a distorted rutile structure in which... 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

The first step is a Takai reaction.4 Geminal dihalide 12 is presumably converted in the presence of ehromium(H) chloride into the di-chromium complex 13, which reacts with aldehyde 5 to give product 14. 

Digermanes, unsymmetrically substituted, 3, 787 Digermenes, preparation, 3, 796 Digestion studies, stability, 12, 612-613 DIGISIM, in cyclic voltammetry, 1, 282-283 Digital simulations, in cyclic voltammetry, 1, 282 Dihalides, in chromium mononitrosyl complexes, 5, 302 Dihaloarenes, cross-coupling polymerization, 11, 659... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

The mechanism of the TaA az-olefination is not yet fully understood. A possible mode of action proceeds via the formation of a geminal chromium(III) species 48 which is formed from the geminal dihalide 47. Nucleophilic attack of this intermediate to the aldehyde moiety in 49 leads to p-oxychromiumspecies 50. A formal elimination of the two cromium-containing substituents via antiperiplanar conformation depicted in 50 leads to the -alkene 51 as the major product. 

Gaseous diffusion, with subsequent decomposition on a hot wire, has been used for the purification of zirconium , uranium , chromium, niobium and tantalum . The metal is transported as a volatile iodide, which then decomposes on the hot wire or other heated receiver. In addition to diffusion, convection is observed in large diameter tubes (> 2 cm) at several atmospheres pressure. The tube is placed in an inclined position with the hot end downward. Silicon in the form of its dihalides has been transported in this manner . 

It is not always necessary to start with gem-dihalides for the synthesis of aldehydes or ketones. There is a process by which carbonyl compounds can be obtained from monohalides, sometimes in excellent yield for instance, when 3-chlorocyclopentene is stirred vigorously with aqueous sodium dichromate solution at 0°, a chromium complex of 2-cyclopenten-l-one is formed, and this is decomposed to give a 60-68% yield of the ketone if 50% sulfuric acid is dropped in carefully with cooling 513 and 1,2-cyclopentanedione can be obtained in 80% yield by dropwise addition of aqueous iron(m) chloride solution to 2-chlorocyclopentanone in water with rapid stirring at 100°.539... 

The dihalides of Mn, Fe, Co and Ni are readily available. Trihalides of vanadium and chromium can be used, but an excess of NaCgHg is then required as a reducing agent. Alternatively MX, can be reduced in situ to MX, using zinc or magnesium before NaC5Hg is added. 

The reactivities of organotin hydrides [24] and chromium (II) complex [25] toward alkyl halides are also in the order of tertiary > secondary > primary alkylhalides. However, this trend is much stronger with the ate complex of 9-BBN than that with these reagents. Consequently, the high selectivity, gentleness, and convenience exhibited by the reagent has the practical synthetic application (Eq. 25.20) Benzylic halides are also reduced easily (entries 11-13, Table 25.10), whereas aryl and vinylhalides are inert (entries 19,20). Benzylic geminal dihalides are reduced stepwise (entries 16, 17). The reaction of 1-phenylallyl-chloride [26] which also contains 33% of cinnamyl chloride with an equiv of ate complex of 9-BBN affords mixture of allylbenzene (36%) and 3-metliylstyrene (66%). 

The chromium(II) reductions of a number of trans-dihalide ruthenium (III) complexes are inner sphere in nature with halide bridging groups. Reaction rates are sensitive to steric effects, whereas the corresponding reductions by vanadium(II) show little steric variation, exceed the V " ligand substitution rate, and must be considered outer sphere. 

Chromium(II) chloride also mediates Wittig-type reactions of a-heteroatom-substituted gem-dihalides and aldehydes. In Scheme 5.27, representative examples of the preparation of alkenylboranes [38], -silanes [39], and -stannanes [40] are shown. In each case, high -selectivity was observed. As these compounds are very important substrates for Suzuki, Hiyama, and Stille couplings, their stereoselective formation enhances the value of the chromium(II) chloride mediated reactions. 

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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