Organometallic Compounds of the Group I, II, and III Metals

The compounds of lithium and magnesium are the most important of the group IA and IIA organometallics from a synthetic perspective. The metals in these two groups are the most electropositive of the elements. The polarity of the metal-carbon bond is such as to place high electron density on carbon. This electronic distribution is responsible for the strong nucleophilicity and basicity of these compounds. 

The reaction of magnesium metal with an alkyl or aryl halide in diethyl ether is the classical method for synthesis of Grignard reagents. 

One test for the involvement of radical intermediates is to determine if cyclization occurs in the 6-hexenyl system, in which radical cyclization is rapid (see Section 12.2.2 in Part A). Small amounts of cyclized products are formed upon preparation of the Grignard reagent from 5-hexenyl bromide.8 This indicates that cyclization of the intermediate radical competes to a small extent with combination of the radical with the metal. A point of considerable discussion is whether the radicals generated are free or associated with the metal surface.9 

If such bridged intermediates are involved, the larger steric bulk of secondary systems would retard the reaction. Steric restrictions may be further enhanced by the fact that organomagnesium reagents are often present as clusters (see below). 

The usual designation of Grignard reagents as RMgX is a useful but incomplete representation of the composition of the compounds in ether solution. An equilbrium exists with magnesium bromide and the dialkylmagnesium. 

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The chemical reactivity of organometallic compounds, in particular those of the elements of Groups I, II, and III, renders them well-suited to study by reaction calorimetric methods. Examples reported include measurements of the heats of hydrolysis of alkyls of Li, Zn, Cd, and Al, the heats of hydroboration of olefins, and the heats of halogenation of alkyls of Zn, Cd, Hg, Sn, and Ga. The heat of formation data derived by use of these methods are discussed in the following Section, in which best AHf° values are selected from the available published data on compounds containing metal-carbon bonds. 

Alkyllithium compounds interact with organometallic compounds of different metals (MtR ), most notably those of Groups I, II, and III, which behave like Lewis adds, to form mixed organometallic compounds, referred to as ate complexes, schematically represented in the following equation ... 

There are a number of reactions known involving metal alkoxides and organometallic compounds of main group elements (Li, Mg, Al, Sn, or Zn). The fundamental reaction generally involves elimination of a main group metal alkoxide. The composition of the reaction products is considerably influenced by (i) the nature and chemistry of the reactants as well as their stoichiometry, (ii) the reaction conditions, and (iii) the presence of other reactive substrates in the reaction medium. Some typical reactions are illustrated by Eqs (2.301)-(2.319). 

Three methods are commonly employed for the in situ preparation of organopalladium derivatives (i) direct metallation of an arene or heterocyclic compound with a palladium(II) salt (ii) exchange of the organic group from a main group organometallic to a palladium(II) compound and (iii) oxidative addition of an organic halide, triflate or aryldiazonium salt to palladium(O) or a palladium(O) complex. 

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