Organometallic reagents from alkyl halides

This chapter is devoted exclusively to the alkylation of the above mentioned organometallic reagents with alkyl halides, epoxides and oxetanes. It includes a large variety of organosulfur- and organoselen-ium-stabilized carbanions derived from saturated and unsaturated sulfides, selenides, sulfoxides, selen-oxides and sulfones as well as those carbanions bearing another heteroatomic moiety. The chapter excludes, however, those organometallics which can be viewed as a-thio and a-seleno enolates. 

It might be mentioned that matters are much simpler for organometallic compounds with less-polar bonds. Thus Et2Hg and EtHgCl are both definite compounds, the former is a liquid and the latter is a solid. Organocalcium reagents are also known, and they are formed from alkyl halides via a single electron transfer (SET) mechanism with free-radical intermediates. "... 

The disconnection strategies summarised on p. 586 reveal that the nucleophilic organometallic reagents derived from alkyl halides (e.g. R MgX, RLi) require... 

The simplest unfunctionalised carbon nucleophiles (15 and 17) are made from alkyl halides with various metals such as Li(0) or Mg(0) or by exchange with available organometallic reagents such as butyl-lithium (BuLi) in anhydrous coordinating solvents like ether (Et20) or THF (tetrahydro-furan 16). Enolates 11 are very important and will be discussed at length in later chapters. 

Chiral centers can be converted to achiral intermediates by loss of a leaving group to give a planar carbocation (as in the SnI mechanism). Attempted formation of an organometallic reagent from a chiral alkyl halide often results in significant racemization and is generally not synthetically useful. Similarly, radical mechanisms result in racemization since a radical intermediate is essentially planar and thus achiral. 

Grignard reagents react with ethylene oxide to yield primary alcohols containing two more carbon atoms than the alkyl halide from which the organometallic compound was prepared... 

Grignard reagents are a very important class of organometallic compounds. For their preparation an alkyl halide or aryl halide 5 is reacted with magnesium metal. The formation of the organometallic species takes place at the metal surface by transfer of an electron from magnesium to a halide molecule, an alkyl or aryl radical species 6 respectively is formed. Whether the intermediate radical species stays adsorbed at the metal surface (the A-modelf, or desorbs into solution (the D-model), still is in debate ... 

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