Common Organometallic Reactions

There are two low-valent oxidation states available to the lanthanides under normal conditions the +2 oxidation state and the formally zero oxidation state found in the elemental metals. The zero oxidation state is available to all the lanthanides, but only three members of the series have +2 oxidation states accessible under common organometallic reaction conditions Eu (4/ ), Yb (4/ ), and Sm (4/ ). The Ln VLn" reduction potentials [vs. normal hydrogen electrode (NHE)] (12), - 0.34 V for Eu, - 1.04 V for Yb and - 1.50 V for Sm, indicate that Eu is the most stable and Sm the most reactive of these divalent ions. Sm is also the most reactive based on radial size considerations, since it is the largest and most difficult to stabilize by steric saturation. 

The currently accepted mechanism involves attack of a free water molecule from the solvent on the coordinated ethylene with inversion of the stereochemistry at one of the carbons, a common organometallic reaction type. The loss of two Cl ions from [PdC ] is needed to provide a site for the alkene to bind and to remove the anionic charge from the metal, which would otherwise prevent the nucleophilic attack of water from taking place. Figure 6 shows the sequence of events as now understood. This mechanism implies that an [H2 O] term should be present in the rate equation, and if it could have been seen, the mechanistic problem would have been solved earlier, but one cannot normally alter the concentration of a solvent and get meaningful rate data, because changing the solvent composition leads to vmpredictable solvent effects on the rate. 

One of the important new directions in the study of addition reactions of organozinc compounds to aldehydes is the use of ionic liquids. Usually, application of these compounds in reactions with common organometallic reagents has a serious problem ionic solvents are usually reactive toward them, particularly Grignard and organolithium derivatives. It has been recently reported that carbonyl compounds react with allylzinc bromide formed in situ from allyl bromide and zinc in the ionic liquid 3-butyl-l-methylimidazolium tetrafluoroborate, [bmim][BF4].285 Another important finding is that the more reactive ZnEt2 alkylates aldehydes in a number of ionic liquids at room temperature.286 The best yields (up to 96%) were obtained in A-butylpyridinium tetrafluoroborate, [bpy][BF4] (Scheme 107). 

The first of these problems appears to have a reasonable expectation of solution. There are a few publications which state that supported metals can be used to promote some of the organometallic reactions commonly run with soluble catalysts (refs. 2-4). In these reports, though, the use of the supported metal is generally included only as an entry in a table describing the effect of changing reaction parameters on product yield and/or selectivity. The fact that a heterogeneous catalyst was used is seldom discussed. When it is mentioned, it is... 

An interesting organometallic reaction is the Heck Arylation (Eqn. 1) (ref. 5), which is commonly run using a Pd(OAc>2 catalyst. This reaction is used to prepare aryl enol ethers which can be valuable synthetic intermediates in that they can be hydrolyzed to aldehydes or ketones, species which can be useful themselves or as intermediates in further reactions. The influence of reaction parameters on the rate and selectivity of this reaction was reported in a series of papers (refs. 2, 3). In these a brief mention in some tables was made that Pd/C was able to catalyze this reaction but no discussion of the use of this catalyst was included. We have found, though, that this reaction is readily promoted over dispersed Pd catalysts. When run with Pd(OAc)2 as the catalyst, the Heck reaction gives as the primary products the E (1) and Z (2) aryl P enol ethers in about a 2 1 ratio. The a isomer, 3, and ester, 4, are also produced but in much smaller amounts. When the reaction is run over Pd/A Oj, the same products are obtained but the P enol ethers 1 and 2 are produced in nearly a 3 1 ratio. Table 1 lists the product compositions of these reactions. 

The Oppenauer oxidation is a common side reaction during the condensation of organometallic compounds with aldehydes and ketones, something that very often comes as a surprise for the unaware chemist. This has been observed in condensations of diverse organometallic species, for example chromium,61 Zr62 and Mg63 organometallics. This side reaction... 

This type of US-assisted reaction, which is very common in organic chemistry (particularly organometallic chemistry), has not yet been used as such in the analytical field, where US could be applied to common organic reactions preceding GC separation that could be implemented in a continuous manner by placing the solid catalyst in a minicolumn subjected... 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

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