Reduced halides

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Table 30.5 Stoichiometries and structures of reduced halides (X/M < 2) of scandium, yttrium, lanthanum and the lanthanides...

Freshly distilled decahydronaphthalene was used. With the more easily reduced halides, and where the boiling point of the neutral reduction product was close to that of decahydronaptha-lene, an excess of 2-propanol was used as the reaction medium. Other hydrocarbons and secondary or tertiary alcohols may be employed for convenience in particular reductions. Diethyl ether and tetrahydrofuran were not found to be generally suitable media. 

When there is no sustained reaction after 10 minutes, initiation can often be accomplished by the addition of another crystal of iodine (no stirring) and/or a small amount of an easily reduced halide such as 1-bromobutane. 

Unlike the di-f dihalides, such compounds differ little in energy from both the equivalent quantity of metal and trihalide, and from other combinations with a similar distribution of metal-metal and metal-halide bonding. So the reduced halide chemistry of the five elements shows considerable variety, and thermodynamics is ill-equipped to account for it. All four elements form di-iodides with strong metal-metal interaction, Prl2 occurring in five different crystalline forms. Lanthanum yields Lai, and for La, Ce and Pr there are hahdes M2X5 where X=Br or I. The rich variety of the chemistry of these tri-f compounds is greatly increased by the incorporahon of other elements that occupy interstitial positions in the lanthanide metal clusters [3 b, 21, 22]. 

Halomet [Halogen metal] A process for reducing halides to metals by reaction with metallic aluminum or magnesium in a closed vessel. Invented in 1968 by R. Nowak and W. Schuster at Halomet, Basle. 

In an investigation of Mn5Si3-derivative phases (Mn5Si3-type hosting structures), Zheng et al. (2002) remarked that extended structures featuring polyhedra or condensed polyhedra are ubiquitous in chemistry. Octahedron-based frameworks of reduced halides are known to be very versatile hosts, as they can accommodate... 

Only a small number of zirconium(III) and hafnium(III) complexes are known. Nearly all of these are metal trihalide adducts with simple Lewis bases, and few are well characterized. Just one zirconium(III) complex has been characterized structurally by X-ray diffraction, the chlorine-bridged dimer [ ZrCl PBu,) ]- Although a number of reduced halides and organometallic compounds are known in which zirconium or hafnium exhibits an oxidation state less than III, coordination compounds of these metals in the II, I or 0 oxidation states are unknown, except for a few rather poorly characterized Zr° and Hf° compounds, viz. [M(bipy)3], [M(phen)3] and M Zr(CN)5 (M = Zr or Hf M = K or Rb). 

The niobium and tantalum tetrahalides are all known, except TaF4, whose non-existence is consistent with the fact that [TaF5]4 should be the least reducible halide of the group VA... 

Beryllium reacts with fused alkali halides releasing the alkali metal until an equilibrium is established. It does not react with fused halides of the alkaline-earth metals to release the alkaline-earth metal. Water-insoluble fluoroberyllates, however, are formed in a fused-salt system whenever barium or calcium fluoride is present. Beryllium reduces halides of aluminum and heavier elements. Alkaline-earth metals can be used effectively to reduce beryllium from its halides, but the use of alkaline-earths other than magnesium [7439-95 4] is economically unattractive because of the formation of water-insoluble fluoroberyllates. Formation of these fluorides precludes efficient recovery of the unreduced beryllium from the reaction products in subsequent processing operations. 

Many highly reduced halides of scandium, yttrium, and zirconium have been found to have infinite metal-metal bonded chains.169 Zirconium chloride, for example, contains double metal layers alternating with double chlorine layers (Fig. 16.68). It was dis-... 

A recent study showed that 152 behaves mechanistically different from other catalysts in addition reactions of more activated halides 140, such as trichloroacetate to styrene [222]. After initial reduction to Ru(II), chlorine abstraction from substrates 140 is in contrast to all other ruthenium complexes not the rate limiting step (cf. Fig. 36). ESR spectroscopic investigations support this fact. The subsequent addition to styrene becomes rate limiting, while the final ligand transfer step is fast and concentration-independent. For less activated substrates 140, however, chlorine abstraction becomes rate-determining again. Moreover, the Ru(III) complex itself can enter an, albeit considerably slower Ru(III)-Ru(IV) Kharasch addition cycle, when the reaction was performed in the absence of magnesium. This cycle operates, however, for only the most easily reducible halides, such as trichloroacetate. 

Regeneratable polymeric organotin dihydride beads have been used to reduce halides selectively (e.g. 3-bromocamphor to camphor).52 /3-Unsubstituted cyclohex-enones undergo exclusive and almost quantitative 1,4-reduction by potassium tri-s-butylborohydride to the corresponding saturated ketone [e.g. (16)] 53 reductive alkylation of carvone proceeds similarly to (17 R = Me, X = H).53 In contrast,... 

Nitriles may reduce halides in higher oxidation states [e.g., interaction of ReCl5 or WCls gives ReCL,(MeCN)2 and WCl4(MeCN)2, respectively] together with chlorinated organic N compounds. 

The ability of reducing agents, such as lithium aluminum hydride, to reduce halides and sulfonates to their corresponding alkyl derivatives is well known. Lithium aluminum hydride is also selective for the reduction of primary halides and sulfonates over secondary analogs. As shown in Scheme 6.70, this reaction was applied to a bis-tosylate with the major isolated product being the mono-tosylate with deoxygenation at C-6 [109]. 

Complex halides of scandium and yttrium have been discussed in CCC (1987).1 Most halides and halide complexes reported are of Sc111 and Y111 but there are some reduced halide species such as the diiodide.229... 

Sodium and potassium borohydrides are above all used for reducing aldehydes and ketones (Sections 3.2.1, 3.2.2) a,p-ethylenic ketones are converted to mixtures [W3]. In alcoholic media or THF, they leave epoxides, esters and lactones, acids, amides, and most nitro compounds unreacted, but they reduce halides (Section 2.1), anhydrides (Section 3.2.6), quartemary pyridinium salts (Section 3.3), double bonds conjugated to two electron-withdrawing groups (Sections 3.2.9, 4.4), and CUPd... 

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. 

Reductions. Lane has reviewed reductions with BMS. He notes that this reagent is somewhat less reactive than BH3 THF and usually requires a temperature of 20-25°. He recommends that the reagent be added at this temperature. Reduction of aliphatic carboxylic acids proceeds readily, but reduction of benzoic acids is slow unless trimethyl borate is added. The reagent reduces acids, esters, oximes, nitriles, and amides, but does not reduce halides or nitro groups. 

Tantalumasa High-Temperature Container Material for Reduced Halides 15... 

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