Transition metal halides, reduction

Mechanism of Meta Hydride-Transition Metal Halide Reductions... 

The utility of metal hydride-transition metal halide reductions is limited only to alkenes and alkynes which do not contain functional groups which can react with the metal hydride, or with potential low-valent transition metal species. This severely limits the use of this reaction. 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

The reduction of transition metal halides with Li has been recently extended by Boudjouk and coworkers for Ullman coupling (benzyl halide to bibenzyl) by Cu or Ni, using a low intensity cleaning bath (5J.). Ultrasound dramatically decreased the time required for complete reduction of the metal halides ( 12 h without, <40 minutes with ultrasound). The subsequent reactivity of the Cu or Ni powders was also substantially enhanced by ultrasonic irradiation. This allowed significant increases in the yield of bibenzyl (especially for Ni) at lower temperatures, compared to simple stirring. 

In the reduction of dienes and polyenes, combinations of a metal hydride and transition metal halides can also be used. Sodium borohydride and cobalt(II) halides were applied in the selective reduction of unsaturated carbon-carbon double bonds93. LiAlH4, in the presence of Zrlv-, TiIV- or Vlv-halides, is a selective reducing agent of dienes94,95. The following reactions were carried out with sodium borohydride and iodine (equation 28)96. 

The metal hydride-transition-metal halide combinations predominantly produce cis alkenes however, significant amounts of tram alkenes are also formed.34 As a notable exception, cis alkenes are the sole products in the reduction with MgH2 in the presence of Cu(I) ions 200... 

Similar arguments are also valid for the successful preparation of hydridophosphinemetalates (shown in Figure 5) from the reaction of transition metal halide complexes with KH and suitable crown ether reagents <1998JA13138>. These materials are quite interesting for their reactivity as strong nucleophiles and reductants. 

Boroles are readily reduced to the respective dianions owing in part to the empty p orbital on the boron atom. The reduction of 6-(4-/i< t/-butylphenyl)-3-/i t/-butyl-5-(bis-2,6-(4-/< /r-butylphenyl)phenyl)dibenzoborole 37 with excess lithium powder in diethyl ether suspension at — 10°C led to deep red crystalline dianionic bis(diethylether)dilithium salt 38. An attempt to obtain transition metal complexes by metathesis reactions of transition metal halides SnCU,... 

Kruck and co-workers (174) have shown that reductive fluorophos-phination of transition metal halides with high pressures of PF3 in the... 

A classical catalyst for metathesis reactions 1 reminiscent of a polymerization Ziegler catalyst it is essentially a combination of a transition metal halide (WCU. MoCl ) and an alkyl metal derivative (AIR). SnK. etc). It is noteworthy that a reduction step occurs during the constitution of the active center, since an elTicieni metathesis of terminal olefins has been achieved under electro-catalytic conditions [40]. (CI4 being the active carbenic entities in... 

Electropositive metals characterized by low standard reduction potentials (alkali metals. Mg, Zn) have been frequently used for the reduction of transition metal halides in the presence of carbon monoxide. The finely divided reducing metal is previously activated by one of the conventional methods. Ethers are frequently used as reaction media. 

REDUCTION OF C=C AND CsC BY METAL HYDRIDE-TRANSITION METAL HALIDE COMBINATIONS... 

All of the proposed mechanisms for the reduction of alkynes with metal hydride-transition metal halide combinations involve an initial hydrometallation of the ir-system by the transition metal hydride, formed by the reaction of the original metal hydride with the transition metal halide, to form the vi-nylmetallic intermediate (99 equation 38). For the reduction of alkenes, similar alkylmetallic intermediates are implied to be formed. In the case of the reduction of alkenes with NaBH4 in the presence of Co" in alcohol solution, the hydrometallation reaction appears to be reversible as evidenced by the incorporation of an excess of deuterium when NaBD4 was used in the reduction. ... 

Ashby et al. have studied reductive dehalogenation by the combined use of LAH and first row transition metal halides (Co", Ni", Fe", Fe" , Mn", Ti " and V" ). - Table 2 shows that C0CI2, NiCh and TiCb notably elevate the reducing ability of LAH, and the added salts need only be present in catalytic amounts (10 mol %). Among them, the reagent system LAH-NiCh shows the highest reactivity that is, primary, secondary and cyclic halides (I, Br, Cl) are reduced in essentially quantitative yield at room temperature. 

Reduction of transition metal halides by organometal compounds... 

Transition metal halides LmMX (L = ligand, X = halogen, m = 0, 1, 2. . . ) undergo multiple reduction with BSD. The reaction products are transition metal halides LmMX p in lower oxidation states, complexes such as LmM, or the metal itself (49). The products, LmMX p, LmM, or M can, in some cases, react further with BSD to form complexes (cf. Section VII). In this way, BSD in methylene chloride transforms titanium tetrachloride, TiCl4, to titanium dichloride which, being a very mild oxidizing agent, is incapable of further reduction (with BSD) to the metallic state (50) [Eq. (90)]. Complete reduction to the metallic state, on the other hand, has been observed... 

Where the nature of the metal or experimental conditions render such auxiliary complexation with either the substrate or the solvent less favorable, there is an increased tendency towards homolysis of the carbon-metal bond. Treatment of organolithium and organomagnesium alkyls with transition metal halides leads to metal halide reduction and hydrocarbon, suggestive of the transitory formation of metal alkyls (45), decomposing thereupon into a lower salt and free alkyl radicals ... 

Since Watanabe s synthesis of 4(3H)-quinazolinones in 1993 via transition-metal catalyzed reductive N-heterocychzation [ 181 ], several catalytic methods for quinazoline synthesis have been developed [182-186]. For example, palladium-catalyzed cyclocarbonylations of halides with appropriate reactants provided regioselective synthesis of 4(3H)-quinazolinone derivatives [182] and indoloquinazolines [184]. Also selenium-catalyzed reductive N-heterocyclization to quinazolinones has been developed by Sonoda et al. [183]. Copper-catalyzed heteroannulation with alkynes has been developed as highly region- and stereoselective route to 2-(2-arylvinyl)-l,2,3,4-tetrahydroquinazolin-4-ones 64 by Kimdu et al. [ 185] (Scheme 12). Recently, condensation of anthranylamide with various aldehydes to 4-quinazotinones has been found to give excellent yields in the presence of cupric chloride [186]. 

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