Anthony KERMAGORET - THESES ET MEMOIRES DE L'UDS

Chapitre VI
Noteworthy is the contrasting H-bonding behaviour of the P,O ligand in 13 and 14 which
functions as both a donor and an acceptor in the former and acceptor in the latter. The Ni-O(1)
bond of 13 should therefore be less perturbed by the H-bonding interaction involving O(2)
than in 14, and the catalytic properties of the complex should remain similar to those of the
SHOP-type catalyst 2.
The reaction of ylids 8-10 with equimolar amounts of [Ni(COD)2] and PPh3 at room
temperature only afforded the desired complexes 15-17 in low yield [eqn (4)].
Ph
O
8-10
PR 2R' + Ni(COD) 2 + PPh 3
Ph
R R'
P
O
toluene
Ni
Ph
PPh 3
15 R = Ph, R' = t-Bu
16 R = R' = n-Bu
17 R=Ph,R'=n-Bu
A significant fraction of the PPh3 reacted with the nickel precursor to form [Ni(PPh3)4], which
gave rise in 31 P NMR spectroscopy to a broad peak around 26.1 ppm, and it was therefore not
available to form the desired product. This led to only partial reaction and the low yield was
evaluated by 1 H NMR monitoring of the chemical shifts of the C-H proton α to the
phosphorus in the ligand and in the complex, or by evaluating unreacted [Ni(COD)2]. 38 In
31 P{ 1 H} NMR spectroscopy, the two trans-coordinated phosphorus atoms of complexes 15-17
form the expected AB pattern with 2 JPP coupling constants of 278, 280 and 282 Hz,
respectively. All attempts made to isolate these nickel complexes led to increased
decomposition. In the case of the β-keto phosphorus ylids 7 and 11, the formation of the
complexes was not observed. Performing the complexation reactions at 60 °C with the hope
to increase the yield resulted only in decomposition of [Ni(COD)2].
To avoid the competition between the formation of [Ni(PPh3)4] and of the desired
complexes by oxidative addition of the ylid, we used a pyridine ligand which is less
nucleophilic than a phosphine. However, pyridine was not as reactive as PPh3 and a large
excess had to be used. The reaction of the ylids 3 and 8-11 with one equiv. of [Ni(COD)2] and
an excess of pyridine at room temperature led to the formation of the desired complexes 18-
22, which have been characterised in 1 H and 31 P{ 1 H} NMR [eqn (5)].
RT
(4)
9