Preparation reduced halides

By contrast, ZrCl and ZrBr, also prepared by the high temperature reduction of ZrX4 with the metal, appear to be genuine binaiy halides. They are comprised of hep double layers of metal atoms surrounded by layers of halide ions, leading to metallic conduction in the plane of the layers, and they are thermally more stable than the less reduced phases. Zrl has not been obtained, possibly because of the large size of the iodide ion, and, less surprisingly, attempts to prepare reduced fluorides have been unsuccessful. 

Powdered KOH (0.17 g, 3 mmol) is added to the freshly prepared thioiminium halide [MeC(SR)NH2+Cl- or McC(SR)NMc2+C1 ] (3 mmol) and TEBA-C1 (0.12 g, 0.5 mmol) in CH2C12 (30 ml). The mixture is stirred at room temperature until the reaction is complete, as shown by TLC analysis. The organic phase is separated, washed with H20 (2 x 25 ml), dried (Na2S04), and the solvent evaporated under reduced pressure to yield the alkyl aryl thioether and the dialkyl disulphide, which can be separated by chromatography from silica. 

In the case of ortho-substituted aryl halides, which are less reactive towards Ni°(bpy)n the formation of the arylzinc intermediate likely involves the occurrence of a Ni°-bpy-Zn(II) complex, which by reduction leads directly to the oxidative addition-transmetallation process. According to this, the nickel catalyst is NiBr2bpy, without extra bipyridine. It is thus possible to prepare arylzinc halides from not easily reduced 2-chlorotoluene or 2-chloroanisole, but also, more importantly, from aryl bromides or chlorides bearing reactive functional groups (COR, C02R, CN). These compounds can then be added... 

Complexes of relatively strongly oxidizing metal ions with the more reducing halide ions are not prepared easily because the halide ion is oxidized by the metal ion. The low-temperature (< 25°) method discussed here allows the preparation of bromo and iodo complexes of oxidizing metal ions which could not be prepared by other means. The complex is formed directly as a solid salt in which crystal-lattice energy gives stability. 

The synthesis of lanthanide and actinide compounds was the subject of a book (Meyer and Morss 1991). Detailed information is given on the synthesis of lanthanide fluorides (Muller 1991), binary lanthanide halides, RX3 (X=Cl,Br,l) (Meyer 1991a), complex lanthanide(in) chlorides, bromides and iodides (Meyer 1991b), and on two alternative routes to reduced halides, the conproportionation route (Corbett 1991) and the action of alkaU metals on lanthanide(lll) halides (Meyer and Schleid 1991). Therefore, a brief outline of the main preparative routes and synthetic strat es might be sufficent. 

The lanthanide and actinide halides remain an exceedingly active area of research since 1980 they have been cited in well over 2500 Chemical Abstracts references, with the majority relating to the lanthanides. Lanthanide and actinide halide chemistry has also been reviewed numerous times. The binary lanthanide chlorides, bromides, and iodides were reviewed in this series (Haschke 1979). In that review, which included trihalides (RX3), tetrahalides (RX4), and reduced halides (RX , n < 3), preparative procedures, structural interrelationships, and thermodynamic properties were discussed. Hydrated halides and mixed metal halides were discussed to a lesser extent. The synthesis of scandium, yttrium and the lanthanide trihalides, RX3, where X = F, Cl, Br, and I, with emphasis on the halide hydrates, solution chemistry, and aspects related to enthalpies of solution, were reviewed by Burgess and Kijowski (1981). The binary lanthanide fluorides and mixed fluoride systems, AF — RF3 and AFj — RF3, where A represents the group 1 and group 2 cations, were reviewed in a subsequent Handbook (Greis and Haschke 1982). That review emphasized the close relationship of the structures of these compounds to that of fluorite. 

Several areas of halide chemistry have recently been active or significantly altered by recent results. The area of preparative methods has been surprisingly active and has expanded to include crystal growth procedures. Substantial progress has been made in the definition and refinement of structural properties and in the expansion of thermodynamic data. Other important areas of progress include the identification and characterization of both novel reduced halides and interesting gas phase species. 

The large number of recent publications on preparative methods suggests that this is an important aspect of halide chemistry. High purity materials with low levels of anionic contamination are naturally desired for property measurements. The trihalides occupy a position of special importance because they are employed as starting materials for the preparation of essentially all the tetrahalides and reduced halides. High quality anhydrous halides are commercially available, but care must be exercised in their procurement (Haschke, 1975a). The reactivity of the chlorides, bromides and iodides to atmospheric moisture virtually necessitates that they be prepared in the laboratory. Preparative procedures have been described in several comprehensive reviews (Taylor, 1962 Kiss, 1963 Brown, 1968). Additional reviews have appeared for the fluorides (Carlson and Schmidt, 1961 B tsanova, 1971) and for the chlorides, bromides and iodides (Block and Campbell, 1961 Johnson and Mackenzie, 1970). In the present review, an attempt will be made to evaluate the various methods. 

In general, the same precautions regarding crucible materials must be observed for the dihalides as for the trihalides, cf. ch. 32, section 2.1. Although noble metal containers were employed in much of the early work, platinum is particularly unsuited because of the formation of stable R-Pt alloys (Brewer et al., 1950 Bedford and Catalano, 1970). Several of the dihalides melt and vaporize congruently, cf. ch. 32, section 4.4.3, and can be purified by sublimation or distillation and single crystals can be grown from the melts. The majority of the reduced halides are thermally unstable and preparation of high purity samples is difficult. 

On the other hand, the consumable anode process does not allow access to less-reactive organometallic compounds such as zinc species. Indeed, the metallic ion generated by the oxidation of the anode has to be reduced at more negative potential than the halide. The use of a zinc anode indeed produces 7.n ions, which are in most cases more easily reduced than organic halides. Consequently, organozinc species can only be obtained by this method from easily reduced halides. The same remarks apply to other metals such as cadmium or copper. For example, the preparation of cadmium, zinc [5], and copper compounds [6] can be performed from CF3Br, which is easily reduced. (Scheme 15.2). 

The reactions with water are summarised in Table 6.3. Since the metals are powerful reducing agents (p. 98) they cannot be prepared in aqueous solution electrolysis of the fused anhydrous halides is usually employed using a graphite anode. 

Gattermann (1890) found that the preparation of the cuprous halide may be avoided by making use of the fact that finely-divided copper (e.g., freshly-precipitated or reduced by hydrogen or copper bronze) acts catal3d.ically in the decomposition of solutions of diazonium salts, for example ... 

Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament. 

A special problem arises in the preparation of secondary amines. These compounds are highly nucleophilic, and alkylation of an amine with alkyl halides cannot be expected to stop at any specifle stage. Secondary amides, however, can be monoalkylated and lydrolyzed or be reduced to secondary amines (p. 11 If.). In the elegant synthesis of phenyl- phrine an intermediate -hydroxy isocyanate (from a hydrazide and nitrous acid) cyclizes to pve an oxazolidinone which is monomethylated. Treatment with strong acid cleaves the cyclic irethan. 

Alkyl azides prepared by nucleophilic substitution of alkyl halides by sodium azide as shown m the first entry of Table 22 3 are reduced to alkylammes by a variety of reagents including lithium aluminum hydride... 

Alkyl azides prepared by nucleophilic substitution by azide ion in primary or secondary alkyl halides are reduced to primary alkylamines by lithium aluminum hydride or by catalytic hydrogenation... 

Preparation of Uranium Metal. Uranium is a highly electropositive element, and extremely difficult to reduce. As such, elemental uranium caimot be prepared by reduction with hydrogen. Instead, uranium metal must be prepared using a number of rather forcing conditions. Uranium metal can be prepared by reduction of uranium oxides (UO2 [1344-59-8] or UO [1344-58-7] with strongly electropositive elements (Ca, Mg, Na), reduction of uranium halides (UCl [10025-93-1], UCl [10026-10-5] UF [10049-14-6] with electropositive metals (Li, Na, Mg, Ca, Ba), electro deposition from molten... 

Finally, Vogtle and his coworkers have prepared a number of cascade molecules which are structurally related to the aforementioned systems. These are repeating ring units of increasingly large cavity size and are prepared by repetitive synthetic procedures. Typically, an amine is cyanoethylated, the nitrile reduced to an amine which may then be further cyanoethylated and reduced or cyclized with a diacid halide. The rather elaborate scheme is illustrated in ref. 61 and examples of the structural type are shown in Table 8.4. 

The 17-ethylene ketal of androsta-l,4-diene-3,17-dione is reduced to the 17-ethylene ketal of androst-4-en-3,17-dione in about 75% yield (66% if the product is recrystallized) under the conditions of Procedure 8a (section V). However, metal-ammonia reduction probably is no longer the method of choice for converting 1,4-dien-3-ones to 4-en-3-ones or for preparing 5-en-3-ones (from 4,6-dien-3-ones). The reduction of 1,4-dien-3-ones to 4-en-3-ones appears to be effected most conveniently by hydrogenation in the presence of triphenylphosphine rhodium halide catalysts. Steroidal 5-en-3-ones are best prepared by base catalyzed deconjugation of 4-en-3-ones. ... 

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