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

56
Chapter Two
purified using column chromatography [hexane: diethyl ether (95:5)], however, the product
was obtained as a mixture with the starting material. Therefore, the reaction was repeated
on a 2 mmol scale, after 48 hours the reaction had reached 65 % conversion. The reaction
mixture was then purified via column chromatography [hexane: 100] followed by [hexane:
diethyl ether (95:5)], however, no pure product could be isolated.
The reaction of dimethylallyl benzoate (86) with allyltrimethylsilane proceeded in a similar
manner, after 48 hours there was only 33 % conversion, therefore the reaction mixture was
refluxed for an additional 48 hours, however, there was no improvement of the conversion.
Purification of the mixture was attempted by column chromatography [hexane: diethyl ether
(97:3)], however, no desired product was recovered only unwanted starting material.
Hence, the reactions of methyl and dimethylallyl benzoate proved unsuccessful in affording
the corresponding silyl products. This could be attributable to the methyl substituents
hindering the metathesis mechanism.
2.2.1.3 Synthesis of Silyl Derivatised Allyl Benzoates
Once all the derivatised allyl benzoates had been synthesised they were reacted with 3
equivalents of allyltrimethylsilane and 5 mol % 2 nd generation Grubbs catalyst in DCM, as
previously described (Scheme 2.20). The yields obtained range from low to very good (34-
83 %) (Table 2.12). Both the 4-fluoro (81) and 3-fluoro benzoates (82) exhibited good
conversion to (88) and (89) in the crude 1 H NMR spectra, 91 % and 77 % respectively, after
48 hours. Both reactions were then worked up by removing the solvent in vacuo and
purifying by column chromatography [hexane (100)] followed by [hexane: diethyl ether
(96:4)]. However, despite the fact that very small fractions were collected both the product
and unreacted starting material came out together. The unreacted starting material does not
affect the fluorination step, therefore most of the product: starting material mixture was used
for the next synthetic step, whilst a small amount was retained so that it could be purified
fully in order for full characterisation to be conducted.
After testing several solvent systems, it was found that chloroform: hexane (70:30) could be
used to successfully separate the desired product from the starting material. This solvent
system was then also used to separate (89) from starting material, affording the product in
71 % yield. Allyl 2-fluorobenzoate (83) was converted to the corresponding silyl product
(90) in 77 % yield after 48 hours. The product mixture was purified in the same way
affording the silyl product in a 56 % isolated yield.