Organometallic compounds, 1,4-addition reactivity

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Both overt carbanions and organometallic compounds, such as Grignard reagents, are powerful nucleophiles as we have seen in their addition reactions with C=0 (p. 221 et seq.) they tend therefore to promote an SN2 pathway in their displacement reactions. Particularly useful carbanions, in preparative terms, are those derived from CH2(C02Et)2, (3-ketoesters, l,3-( 3-)diketones, e.g. (55), a-cyanoesters, nitroalkanes, etc.—the so-called reactive methylenes ... 

Reactive and moderately reactive organometallic compounds will react with all functional groups two major types of reaction in which they are involved are oxidation and cleavage by acids, Probably the most important organometallic reactions are those involving addition to an... 

The strong similarities between HDS and DHN results suggest that the type of catalytic sites involved in these two reactions are similar. This would mean that the routes for DHN of THT and HDS of thiophene include some identical reactional step(s), at least for the kinetically limitative one(s). Angelici and coworkers, in a detailed work [10] concerning the HDS of thiophene, THT, 2,3- and 2,5-dihydrothiophene showed that the dihydrothiophenes are much more reactive than the other molecules. On this basis and an additional study [11] on organometallic compounds, these authors have suggested that dihydrothiophenes are reaction intermediates and proposed a mechanistic pathway including this step. 

In addition, organometallic compounds produce metallic products that influence the reactivity of the catalyst by deposition on its surface. The transfer of metal from the feedstock to the catalyst constitutes an irreversible poisoning of the catalyst. After combustion to remove the carbonaceous deposits, the catalysts are treated to redisperse active metals. 

With the usual reaction of organometallic reagents with acid derivatives (ester or acid chloride), the starting materials can add two equivalents of organometallic compound. The ketone generated after the first addition is quite reactive, and there is quite no selectivity between it and the starting acid derivative ... 

The accurate determination of rate constants for the reactions of 19F atoms is often hampered by the presence of reactive F2 and by the occurrence of side reactions. The measurement of the absolute concentration of F atoms is sometimes a further problem. The use of thermal-ized 18F atoms is not subject to these handicaps, and reliable and accurate results for abstraction and addition reactions are obtained. The studies of the reactions of 18F atoms with organometallic compounds are unique, inasmuch as such experiments have not been performed with 19F atoms. In the case of addition reactions, the fate of the excited intermediate radical can be studied by pressure-dependent measurements. The non-RRKM behavior of tetraallyltin and -germanium compounds is very interesting inasmuch as not many other examples are known. The next phase in the 18F experiment should be the determination of Arrhenius parameters for selected reactions, i.e., those occurring in the earth s atmosphere, since it is expected that the results will be more precise than those obtained with 19F atoms. 

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