Allyl bromide, reaction with dianion

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

When 5-methylisoxazole (474) was exposed to two equivalents of LDA in THF at -10 °C the dianion of acetoacetonitrile was generated (78TL4221). Reaction of this dianion (475) with 1.1 equivalents of allyl bromide gave the alkylated product (476) in 60% yield (Scheme 105). The reaction of the dianion with nitriles and aldehydes was also studied. 

Selective /3-allylation and /3-alkylation was observed when Goswami s dianion was reacted with allyl bromide and propyl iodide (Scheme 16)9. The dianion undergoes isomerization to the a,a -dianion at 0 °C and, as a result, the reaction must be conducted carefully at low temperatures. 

Table 2 lists examples of monoalkylation reactions of ketone a,p-dianions. The dianions always react with alkylating agents at the /3-carbon atom to generate the corresponding lithium enolates, which can be quenched by water or electrophiles such as TMSC1 and PhSSPh. The last two examples demonstrate the results of the single alkylation reaction of a ketone a,5-dianion with hexyl and allyl bromides, which occurs at the 5-position to give lithium Z-enolates. 

The more reactive ft -carbon atom of ketone a,/ -dianions can be regiospecifically coupled with alkyl halides to give first lithium enolates, which are then trapped by more reactive carbon electrophiles such as allylic halides. The first example shown in Table 8 deals with the sequential /1-alkylation and ce-allylation of a ketone a,/1-dianion1 13. Thus, the dianion underwent regioselective alkylation at the ft carbon with //-pentyl bromide and then allylation with allyl bromide at the a carbon. When an excess of allyl bromide is reacted with the a, ft -dianion, the diallylated product is obtained in a good yield, whereas a threefold excess of pentyl bromide only resulted in the formation of the ft-alkylation product. Similar consecutive alkyl/allyl-type reactions are also possible for ketone a,5-dianions14. 

Diastereoselective asymmetric thio-Claisen rearrangement has been carried out by the reaction of thioamides with allyllic bromide (Scheme 37). Thioamide dianions (generated by the highly efficient reaction of A -benzyl thioamides with 2 equiv. of BuLi) take place alkylation, allylation and silylat-ion selectively at the carbon atom adjacent to the nitrogen atom of the thioamide dianions (Scheme 38). ... 

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