Allyl additions allyllithium reagents

From Hard Allylic Organometallics. The most common preparation of allylic boranes and boronates is the addition of a reactive allylic metal species to a borinic or boric ester, respectively (Eqs. 10 and 11). Preparations from allyllithium, " " allylmagnesium, and allylpotassium " ° reagents are all well known. These methods are popular because the required allylic anions are quite easy to prepare, and because they generally lead to high yields of products. 

Gajewski has examined the secondary deuterium isotope effects in the addition of allyllithium and allyl Grignard additions to benzaldehyde [157]. With allyl-lithium and allylmagnesium halides a normal secondary deuterium isotope effect was observed. The results indicate that rate-determining single-electron transfer occurs with the allyl reagents. 

Hoffmann et al have reported the addition of allyl(dimethoxy)borane to linear and branched a-aryl-aldimines (entries 16-19, Table 2). The absence of a-deprotonation may be explained by a delicate balance between the basicity and the reactivity of the allylboronate. Allyl(dimethoxy)borane should thus be considered the reagent of choice in reactions with enolizable aldimines. Reactions are conveniently carried out at 25 °C in CH2CI2 and work-up is performed using triethanolamine to break up amine-boronate complexes. Allyl(dimethoxy)borane also adds to the cyclic imine, A -piperideine, in 90% yield. The reported yields for the addition of allyllithium and allylmagnesium chloride to A -piperideine are low. 

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. 

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