Synthesis and Comparison of the Reactivity of Allyl Fluorides and ...

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Chapter Five
Workup of the reaction followed by drying over magnesium sulphate and removal of the
solvent in vacuo afforded the crude product, which was purified by column
chromatography. Initially [ethyl acetate: hexane (50:50)] was used as the solvent system
but was unsuccessful in separating the product from benzoyl chloride which was used in
excess. Therefore, [chloroform: hexane (50:50)] was used and the desired product isolated
in 27 % yield.
Scheme 5.8 Synthesis of 2,2-difluoro-1-phenylbut-3-enyl benzoate (159)
The same procedure was employed in the synthesis of (160), where 4-fluorobenzoyl
chloride was used, the reaction was conducted in the same manner and the product was
isolated in a moderate 43 % yield. Further reactions were conducted in order to synthesise
more difluoroallylic esters which were derivatised on the benzyl ring with isolated yields of
the desired products ranging from 22 – 55 %. However, whereas in the synthesis of (159)
and (160) the reaction mixture was warmed to room temperature after the addition of the
benzoyl chloride, in the subsequent syntheses of (161) and (162), the reaction mixture was
maintained at 0 ºC for 4 hours. After which the crude reaction mixture underwent workup
and purification by column chromatography. (163) and (164) were formed in the same way
as (160), however, the reaction mixture was stirred at room temperature for 4 hours instead
of 2 hours to ensure that the product formed in good conversion. Column chromatography
was carried out on the crude reaction mixture using [chloroform: hexane (30:70)], affording
the desired products in 34 % and 20 % isolated yields respectively.
Scheme 5.9 General reaction scheme for the synthesis of difluoroallyl esters
(For a definition of R see Table 5.4)