Other Transition Metal Initiators

Titanium and vanadium nitrides may be prepared by a metathesis reaction of their tetrachlorides with the nitride, initiated by heat or friction. The reaction is potentially explosive. Other transition metal halides may cause ampules to explode after thermal initiation when anhydrous and were invariably found to do so when the hydrates were used. 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

In 1988 Bast and Haenen [201] reported that both LA and DHLA did not affect iron-stimulated microsomal lipid peroxidation. However, Scholich et al. [202] found that DHLA inhibited NADPH-stimulated microsomal lipid peroxidation in the presence of iron-ADP complex. Inhibitory effect was observed only in the presence of a-tocopherol, suggesting that some interaction takes place between these two antioxidants. Stimulatory and inhibitory effects of DHLA have also been shown in other transition metal-stimulated lipid peroxidation systems [203,204]. Later on, the ability of DHLA (but not LA) to react with water-soluble and lipid-soluble peroxyl radicals has been proven [205], But it is possible that the double (stimulatory and inhibitory) effect of DHLA on lipid peroxidation originates from subsequent reactions of the DHLA free radical, capable of participating in new initiating processes. 

Ions of the later transition metals such as Pt+ may not form [MO]+ ions with water and alcohols as shown in Table I for the reaction of Pt+ with methanol, where the formation of Pt+-CO or Pt+-H2 ions are preferred (102). As previously mentioned, Cr+ and Mn+ appear to be much less reactive than any of the other transition metals. The Cr+ ion was reported to be unreactive to primary alcohols but initiated dehydration of branched-chain alcohols it was also described as being unreactive toward propanal and acetone (9). The Mn+ ion has received scant attention due to its reduced activity. The reactions of Fe+, Co+, and Ni+ with alcohols, ethers, aldehydes, and ketones have been extensively covered (9). These ions are more reactive than Cr+ and Mn+ and generally react with alcohols causing dehydration. 

The reactions between cyclopropenes and carbon monoxide in the presence of transition metals have been of some use in synthesis,93 and in 1978 Binger initiated a study of the reactions between metal carbonyls and cyclopropenes in order to elucidate the generality of these reactions.75 It was found that dicarbonyl 775-cyclopentadienyl(tetrahydrofuran)manganese(I) reacted with 3,3-dime thy Icy clopropene at 0°C to produce -(vinylketene) complex 81 in fair yield. The only other transition metal in Binger s study that was found to react with 3,3-dimethylcyclopropene in this manner was iron (see Section VI,B). 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

Several cases of isomerisation have been noted on reaction of olefinic phosphines with transition metals. The ligand ap reacts with (norbom-adiene)M(CO)4 (M = Cr, Mo, W) to give initially (cts-pp)M(CO)4. This may be followed by a slow equilibration to give a mixture of (cis-pp)M-(CO)4 and (fraMS-pp)M(C0)4. This ligand has not yet been found to isomerise with other transition metals. 

Alkali leach methods axe exemplified by the Bayer process for the preparation of pure a-A C for electrolysis (Section 17.5) from the mineral bauxite. Bauxite consists mainly of a-AlO(OH) (diaspore) and/or 7-A10(0H) (boehmite), the difference between these being essentially that the oxygen atoms form hep and ccp arrays, respectively. The chief contaminants are silica, some clay minerals, and iron(III) oxides/hydroxides, which impart a red-brown color to the mineral. Aluminum (III) is much more soluble than iron(III) or aluminosilicates in alkali, so that it can be leached out with aqueous NaOH (initially 10-15 mol L 1) at 165 °C under approximately 0.6 MPa pressure, leaving a red mud of iron (and other transition metal) oxides/hydroxides and aluminosilicates ... 

Since our initial report we have noted three other transition metal mercury complexes in which the mercury bridges two or more metal atoms, which have been reported as products from sodium amalgam reductions, (a) D. Duffy, K. Mackay, and B. Nicholson, J. Chem. Soc. Dalton Trans., 1981,381 (b) R. Jones, F. Real, and G. Wilkinson, J. Chem. Soc. Dalton Trans., 1981,126 (c) J. Deutsche, S. Fadel, and M. Ziegler, Angew Chem. Int. Ed. Engl. 16,704 (1977). 

Redox Reactions. Aquatic organisms may alter the particular oxidation state of some elements in natural waters during activity. One of the most significant reactions of this type is sulfate reduction to sulfide in anoxic waters. The sulfide formed from this reaction can initiate several chemical reactions that can radically change the types and amounts of elements in solution. The classical example of this reaction is the reduction of ferric iron by sulfide. The resultant ferrous iron and other transition metals may precipitate with additional sulfide formed from further biochemically reduced sulfate. Iron reduction is often accompanied by a release of precipitated or sorbed phosphate. Gardner and Lee (21) and Lee (36) have shown that Lake Mendota surface sediments contain up to 20,000 p.p.m. of ferrous iron and a few thousand p.p.m. of sulfide. The biochemical formation of sulfide is undoubtedly important in determining the oxidation state and amounts of several elements in natural waters. 

We have also used free radical initiators and other transition metal complexes besides Speier s catalyst to attach some olefins to the silica hydride intermediate [26,28]. 

The homocoupling of aryl Grignard reagents triggered by organic halides in the presence of catalytic quantities of cobalt chloride or other transition metal halogenides was studied initially by Kharasch and Fields in 1941 [36]. They... 

Wunderlich and Knochel recently published the alkylation of diaryliron compounds by alkyl iodides or benzyl chloride 1 (entry 33) [77]. The reaction works well with 98% pure FeCl2 2LiCl but not with 99.998% pure metal salt. Addition of other transition metal salts showed that nickel contained in the FeCl2 of 98% purity is the likely catalyst. Alkylarenes 3 were obtained in 65-88% yield. The method tolerates ester, nitrile, fluoride, or chloride substituents. Although the use of 5-hexenyl iodide did not provide a cyclized product, the initial formation of radicals cannot be excluded safely. 

The method has been used most often for cyclization reactions. Five- and six-membered rings may be formed efficiently in the common cases and, when fused to an azetidinone, seven-membered rings are formed in synthetically useful yields (equation 168)327. Perhaloalkane-olefin additions have focused on intermolecular cases328 329 in some cases, yields are enhanced by the additional use of trimethylaluminum as a radical initiator328. Several other transition metal complexes are known to initiate the same processes, particularly RuCl2(PPh3)2330. 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

Coordination chemistry has become a powerful tool for the control and the living nature of radical polymerization [79,80]. Various examples show that the role of initiator and counter radical can be played by organometallic species with an even number of electrons. Besides aluminum complexes used by Matyjaszewski, several other transition metals, metallocenes, and organolan-thanides with various ligands have been studied in controlled radical polymerization [79-97]. In some cases, a controlled polymerization was achieved [81,83-85,87,90-94,97]. However, the mechanism of the polymerization is not always known and it may happen that heterolytic cleavage of the active bond... 

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