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

6
Chapter One
2,10 (3,3-dichlorocamphorsultam) (6), N-fluoro-2,10-(3,3-dimethoxycamphorsultam) (7) in
addition to N-fluoro-2,10-(camphorsultam) (4).
Figure 1.4 N-fluoro-2,10-(3,3-dichlorocamphorsultam) (6) and
N-fluoro-2,10-(3,3dimethoxycamphorsultam) (7).
N-fluoro-2,10-(3,3-dichlorocamphorsultam) (6) was found to give greater yields and
enantioselectivities than N-fluoro-2,10-(camphorsultam) (4) and this was attributed to the
greater reactivity of (6) in comparison with that of (4), with fluorinations occurring at lower
temperatures (-78 ºC vs. r.t. for (4)). Since the reactions occur at -78 ºC with (6), the rate
of HF elimination is decreased, which in turn increases the yield of the fluorinated product.
The best result observed for N-fluoro-2,10-(3,3-dichlorocamphorsultam) (6) was the
fluorination of the sodium enolate of 2-methyl-1-tetralone, affording the product 2-fluoro-2-
methyl-1-tetralone in 53 % yield with 76 % ee. However, with fluorinations of �-ketone
ester enolates, enantiomeric excesses were in the range of 34-46 %. The only secondary
enolate studied, the enolate of propiophenone, gave a moderate yield but racemic product,
due to base-catalysed epimerisation under the reaction conditions, owing to the enhanced
acidity of the �-fluoro proton. Fluorination yields for N-fluoro-2,10-(3,3-dimethoxy-
camphorsultam) (7) were good (55-83 %), however, the enantiomeric excesses were very
low (< 5 %).
Takeuchi and Shibata synthesised three enantiomeric N-fluorosulfonamides, (8, 9, 10),
which comprised of a stable but reactive N-F bond and also steric factors that were expected
to favour asymmetric induction (Figure 1.5). [21-23]
Figure 1.5 The three N-Fluorosulfonamides synthesised by Takeuchi and Shibata