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

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Chapter Three
Therefore, from the range of different reaction conditions tested it is evident that in the
reactions conducted in toluene at -78 ºC with Pd(PPh3)4, the substrate clearly undergoes
oxidative addition to Pd(0), as demonstrated by the formation of the elimination product in
entries 13 and 14. Whilst those reactions conducted with Pd(dba)2 (both at low and elevated
temperatures) were unsuccessful, affording only the starting allylic fluoride.
3.2.2.2 Activation of Allylic Fluoride with Pd(0) in the presence of a nucleophile-
Trapping the Allyl Pd Intermediate
During this time, work was published by Gouverneur et. al. which also examined the
oxidative addition of Pd(0) to allylic fluorides. [34] Whereas the work in this thesis focussed
on isolation of the �-allyl palladium species, Gouverneur et al. concentrated on the
formation of the �-allyl species in situ and its subsequent reaction with a nucleophile such
as dimethyl malonate, in order to quantify the reactivity of fluoride as a leaving group in
comparison with OAc, OBz and OCO2Me. It was reported that NMR experiments were
conducted in order to monitor the reaction and observe the changes occurring.
In light of the scoping experiments discussed previously, this methodology was utilised in
the reaction of (74) with 20 mol % Pd(dba)2 and 2 equivalents of PPh3 in anhydrous CDCl3.
The reaction mixture was stirred for 1 minute under nitrogen before transferring a small
aliquot to a Young’s NMR tube. The progress of the reaction was then monitored by 1 H,
19 F{ 1 H}, and 31 P{ 1 H} NMR spectroscopy (see Tables 6.1 and 6.2 in Chapter 6). Initially
only the allylic fluoride (74), PPh3 and dba were evident in the 1 H NMR spectrum, though a
new small broad peak was observed at – 157.9 ppm in the 19 F NMR spectrum. Over time,
(74) (see Figure 3.7 for initial spectra ) began to disappear from both the 1 H and 19 F NMR
spectra and peaks became apparent that were attributable to the desired �-allyl species (132)
(3.0, 3.8, 4.1 and 4.5 ppm) and the elimination product (133) (5.1, 5.3, 6.3 and 6.8 ppm) (see
Figure 3.8). The initially small peak at -157.9 ppm in the 19 F NMR spectrum grew
becoming a very large broad peak and the allylic fluoride peak at -184.7 ppm was barely
visible (see Figure 3.9). Electrospray mass spectrometry of the reaction mixture confirmed
formation of the allylpalladium cationic complex (132) with a strong signal for the cation at
m/z 830.