REACTION OF PHENYLLITHIUM WITH E-CINNAMALDEHYDE - SURVEY OF SEVERAL VARIABLES AND THEIR INFLUENCE ON THE MECHANISM OF REACTION AND IN ORGANIC-SYNTHESIS

The reaction of phenyllithium (PhLi) with E-cinnamaldehyde (1) has bee n fully examined. Besides the main product E-1,3-diphenyl-2-propen-1-o l (2), three other by-products were detected: E-cinnamyl alcohol (3), E-chalcone (4) and E-1,3-diphenylpropanone (5). The effect of several variables on the nature and relative yields of products was examined. In all the solvents studied, the selectivity of the reaction was highe r at higher temperatures, probably owing to aggregation effects; at lo wer temperatures the reaction is slower and the amount of by-products increases. The addition is complete in 1 h at 0 degrees C in THF for a [PhLi]:[l] ratio of 1:1, and longer reaction times have almost no eff ect, while for the reaction in toluene the amount of by-products incre ases when the reaction mixture is allowed to stand. The concentration of the reagents has no important effect on the reaction as long as the ratio is kept equal to 1:1. The influence of light was examined and a marked decrease in the selectivity of the reaction was observed. When the reaction was carried out in the presence of radical traps, no by- products were detected. Finally, for a [PhLi]:[1] ratio of 3:1 the mai n product is the dihydrochalcone 5, especially for long reaction times . All the above results could be interpreted in a reaction scheme invo lving electron transfer from PhLi to 1, and further reaction of the ra dical ions formed as well as reaction of dimeric PhLi without previous deaggregation when the ratio is 3:1. (C) 1998 John Wiley & Sons, Ltd.