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

51
Chapter Two
completion. This enabled the reaction mixture to form a little more product but the reaction
was not found to go to completion even after an extra 72 hours, and the addition of more
catalyst. Therefore purification was conducted using column chromatography [hexane:
diethyl ether (97:3)], affording the desired product as a cloudy oil, in a 29 % yield.
After the disappointing yields obtained in the synthesis of (68), (3-[1,3]dioxalan-2-yl-allyl)
trimethylsilane (69) was prepared with relative ease. The reaction was conducted as
described by Gouverneur et al., yielding the pure product in 74 % yield as an oil.
2-(4-trimethylsilanyl-but-2-enyl)isindole-1,3-dione (70) was synthesised after firstly
preparing 2-allyl-isoindole-1,3-dione (66), following the protocol by Abulikemu et al., [49]
and was obtained as white crystals in a moderate 51 % yield. The subsequent reaction with
allyltrimethylsilane afforded the desired product as a white solid in 79 % yield.
Benzoic acid 4-trimethylsilanyl-but-2-enyl ester (71) was synthesised by firstly forming the
starting allyl benzoate (67), which was prepared following the protocol by Yasui et al. [50]
Allyl alcohol and benzoyl chloride were stirred in the presence of triethylamine, after
workup, the desired product was afforded in 90 % yield. Therefore, allyl benzoate was
reacted with 3 equivalents of allyl trimethylsilane in DCM with 5 mol % Grubbs catalyst.
The reaction mixture was refluxed for 48 hours under an atmosphere of argon, after which
column chromatography [hexane: diethyl ether (95:5)], afforded the product (71) as an oil in
65 % yield (Table 2.8).
The fluorination step for (69) proved problematic. The reaction was conducted as described
in the literature, (69) was stirred with 1 equivalent of Selectfluor in MeCN, under an
atmosphere of argon for 48 hours at room temperature. The crude reaction mixture was then
purified via column chromatography [hexane: diethyl ether (96:4)] However, only a single
fluorine peak at δF -150 ppm was observed in the 19 F{ 1 H} NMR spectrum, indicative of the
Selectfluor BF4 - counter ion. The reaction was repeated, using different solvent
combinations for purification by column chromatography, however, the desired product (72)
was not formed.
In contrast, both (68) and (71) showed excellent conversion to the corresponding allylic
fluorides ((73) and (75)) and were isolated by column chromatography [hexane: diethyl
ether (95:5)] as oils in 33 % and 55 % yield. The fluorination of (70) also proceeded
smoothly and the desired allylic fluoride (74) was afforded as a white solid in 77 % yield