Reactivity of Polar Organometallic Intermediates

It is the aim of this chapter to provide some guidance in the selection of reaction conditions. Useful data are given about the scope, characteristics, typical conditions and trends for various derivatization reactions with sp2-polar organometallic intermediates. This information is based on a critical consideration of relevant literature and on our own experimental experience. 

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As in the previous volume, only a limited number of procedures for metallations and subsequent functionalizations are given. Although we have tried to choose the procedures as representative as possible, the user of this book may need additional information for designing reaction conditions for a particular synthetic operation. In this respect the present chapter on reactivity of polar organometallic intermediates may be helpful. It is based upon the results of several experiments carried out during the preparation of this volume and other books in this series, and on selected literature data. 

Table 1. Reactivity of polar organometallic intermediates towards butyl bromide in 1 1 mixtures of THF and hexane temperature ranges for butylation with n-C4H9Br in preparative experiments...

In our synthetic investigations we have noticed considerable differences in reactivity of the various types of polar organometallic intermediates towards alkyl halides and epoxides. Exceptionally high alkylation rates were observed in reactions of benzyllithium (or potassium) and allyllithiums (or potassium) with primary alkyl bromides in mixtures of THF and hexane. Under preparative conditions (concentration of reagents 0.5 to 0.8 mol/liter) the characteristic orange colour of benzylalkali solutions disappeared completely within a few seconds upon addition at — 90 °C of a slight excess of alkyl bromide. The allylic intermediates reacted with comparable ease. 

The thermodynamic basicity of RM is likely to determine the reactivity in these acylations. Since most polar organometallic intermediates react smoothly even at... 

A large number of experimental procedures show how polar organometallic intermediates of the broad spectrum of organic compounds can be generated and how these can be functionalized with various electrophilic reagents. All procedures (on the 0.05-0.10 molar scale) have bee preparatively checked in the laboratory of the authors. The reaction conditions recommended reflect reactivities of starting compounds and intermediates directly. This may save the user a considerable amount of time. 

It should be pointed out that these alkylations are significantly faster than those of intermediates with comparable basicities having the metal linked to sp2-carbon, e.g., phenyllithium or -potassium. Many other sp3-polar organometallic intermediates are more reactive in alkylation reactions than PhLi or PhK, though the differences are not so dramatic as in the cases of benzylic and allylic metal compounds. 

The term Y-Z bond activation is traditionally understood as a reaction that cleaves the bond [1]. Often, the term is restricted to reactions involving organometallic complexes and proceeding by Y-Z coordination to the inner sphere of metal, either via an intermediate state or as a transition state. We are inclined to use the term activation for weaker (noncovalent) binding that results in the altered reactivity of a molecule through associated changes in the relative energies of its orbitals or in verified polarity. 

You have already had considerable experience with carbanionic compounds and their applications in synthetic organic chemistry The first was acetyhde ion m Chapter 9 followed m Chapter 14 by organometallic compounds—Grignard reagents for example—that act as sources of negatively polarized carbon In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic and that this property can be used to advantage as a method for carbon-carbon bond formation... 

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