Anionic Transition Metal Reagents

Anionic reagents suitable for nucleophilic substitutions are formed from alcoholates and metal carbonyls. 

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Dialkylcopperlithium reagents give only low yields when used for the cycliza-tion of saturated iodoketones of type (8) to give (9). Various anionic transition metal complexes were examined and the anion derived by treatment of nickel... 

A variety of complexes of the thionyl imide anion [NSO] with both early and late transition-metal complexes have been prepared and structurally characterized. Since both ionic and covalent derivatives of this anion are readily prepared, e.g., K[NSO], McsMNSO (M = Si, Sn) or Hg(NSO)2, metathetical reactions of these reagents with transition-metal halide complexes represent the most general synthetic method for the preparation of these complexes (Eq. 7.10 and 7.11). ... 

One-electron reduction or oxidation of organic compounds provides a useful method for the generation of anion radicals or cation radicals, respectively. These methods are used as key processes in radical reactions. Redox properties of transition metals can be utilized for the efficient one-electron reduction or oxidation (Scheme 1). In particular, the redox function of early transition metals including titanium, vanadium, and manganese has been of synthetic potential from this point of view [1-8]. The synthetic limitation exists in the use of a stoichiometric or excess amount of metallic reductants or oxidants to complete the reaction. Generally, the construction of a catalytic redox cycle for one-electron reduction is difficult to achieve. A catalytic system should be constructed to avoid the use of such amounts of expensive and/or toxic metallic reagents. 

In a series of papers, metal sulfide cluster anions of first-row transition metals, principally copper, have been reacted with a variety of reagents including thiols, sulfur, phosphorus, and phosphines (99, 145, 256, 257). 

Boron-bonded p -borazine complexes of transition metals have been prepared by two different approaches (a) nucleophilic substitution of B,fi, fi"-trichloro-borazine with an anionic metal carbonyl reagent and (b) oxidative addition of a B-Br bond of 5,5, B"-tribromoborazine to a zerovalent group 10 complex (see examples in Scheme 9.2). 

The transition metal catalysed formation of five membered heterocycles through the insertion of a triple bond has also been explored. o-Halophenyl-alkynylamines, propargylamines and propargyl-ethers have been subjected to ring closure reactions. These processes, however also require the presence of a second, anionic reagent, which converts the palladium complex formed in the insertion step to the product. 

A number of authors have studied the reductive termination step. The classical work by Hermann and Nelson (121) showed that the reduction of the titanium in alkyltitanates occurred readily with the more alkylated species. The same authors (122) showed that the stability of the alkyl metal decreased markedly when the number of alkyls per metal atom increased. This is parallel to increased anionicity of the alkyl. Vanheerden (123) pointed out that the disproportionation of alkyl-titanium was bimolecular. Cotton (124) showed that the coupling occurs when phenyl Grignard reagents are mixed with various transition metal salts. This is through disproportionation or combination of the alkyl radicals. 

Solid-liquid phase-transfer catalyst.1 The reagent represents a new class of catalysts, acyclic cryptands or tridents. It is singled out of a group as the best compromise of efficiency/price/toxicity. It solubilizes salts of alkali metals as well as of transition metals such as RuC13 and PdCl2, probably because of the flexibility of the molecule. In addition the trident is sensitive to the nature of the anion, but anionic activation is less than that obtained with cryptands. 

Unfortunately, formic acetic anhydride is not a general reagent for formyl complex synthesis (29). One reason is that formylation of a transition metal monoanion would afford a neutral formyl complex. Insofar as comparisons are valid, neutral formyl complexes tend to be kinet-ically less stable than anionic formyl complexes. In cases where neutral formyl complexes are stable (vide infra), the corresponding transition metal monoanions are unknown. Whereas formic acetic anyhydride might be of greater use for the preparation of anionic formyl complexes from transition metal dianions, only a limited number of transition metal dianions [i.e., (CO)5Cr2", (t7-C5H5)(CO)3V2 J are known (49). These appear to... 

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