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

91
Chapter Three
Though the reaction was conducted on a small scale, this experiment clearly shows that a �-
allyl complex is formed (peaks at 3.0, 3.8, 4.1 and 4.5 ppm in Figure 3.8) and subsequently
reacts to form the elimination product (133) (peaks at 5.1, 5.3, 6.3 and 6.8 ppm in Figure
3.8). Although the cationic Pd(II) species proved difficult to separate from (133), dba and
any excess PPh3, 2-(buta-1,3-dienyl) isoindoline-1,3-dione (133) was obtained as a yellow
solid in 50 % yield.
Following on from the NMR experiment undertaken with (74), the same experiments were
conducted with 2-chlorobut-3-enyl benzoate (108) and 2-fluorobut-3-enyl benzoate (75) in
order to compare whether an allylic chloride and fluoride would form the same �-allyl
palladium complex. Both (75) and (108) were reacted with 0.5 equivalents of Pd(dba)2 and
1 equivalent of PPh3, and small aliquots of the reaction mixture transferred into an NMR
tube. The progress of the reactions were then monitored by 1 H, 19 F{ 1 H}, 31 P{ 1 H} NMR and
electrospray mass spectrometry. Unfortunately, due to the excess of PPh3 in the reaction
mixtures the 1 H NMR spectra were largely uninterpretable.
Scheme 3.17 Synthesis of (134) and (135)
Therefore, with 2-fluorobut-3-enyl benzoate (75), the reaction was predominantly monitored
by 19 F NMR spectroscopy. Over 80 minutes the allyl fluoride peak at – 187.5 ppm was
observed and seen to decrease in height and broaden before finally disappearing, whilst
simultaneously a new signal at – 170.8 ppm developed from a small peak to a large broad
peak. After 80 minutes it was apparent that (75) had disappeared and this was confirmed by
the electrospray mass spectrum, where a signal was seen at m/z 805 attributable to product
(134).
With the reaction of 2-chlorobut-3-enyl benzoate (108) as only 1 H and 31 P{ 1 H} NMR
spectroscopy could be used to monitor the reaction, they were closely observed. However,
the 1 H NMR spectra were dominated by triphenylphosphine and the 31 P{ 1 H} NMR spectra
contained several peaks which were difficult to assign. Therefore the reaction was