Anthony KERMAGORET - THESES ET MEMOIRES DE L'UDS

Chapitre VI
The synthesis of β-keto phosphorus ylids from mixed alkyl/phenyl phosphonium salts
could provide an access to SHOP-type nickel complexes having alkyl or mixed alkyl/phenyl
substituents on the P atom of the P,O chelate. Such mixed salts were prepared by reaction of
PPh2Cl with one equiv. of n-BuLi or t-BuLi or by reaction of PPhCl2 with two equiv. of n-
BuLi or t-BuLi at low temperature, 55 followed by reaction with α-bromoacetophenone and
deprotonation. The β-keto phosphorus ylids 7 and 8, with t-butyl substituents, precipitated as
white powders but 9 and 10, with n-butyl substituents, formed a pale red oil.
We did not succeed in deprotonating methyl-substituted phosphonium salts using a
MeOH/H2O (1/1) solution of NaOH but the β-keto phosphorus ylid 11 was obtained in
toluene using NaH.
The intermediate 2-bromo-1-(2-methoxyphenyl)ethanone was synthesized by reaction
of 1-(2-methoxyphenyl)ethanone with Br2 in CCl4 and used to prepare the β-keto phosphorus
ylid 12 by reaction with PPh3 and deprotonation with Na2CO3 [eqn (3)]. 56
2.2 Synthesis of the complexes
The nickel complex [NiPh{Ph 2PCH⋅⋅⋅C(⋅⋅⋅O)(o-OMe-C 6H 4)}(PPh 3)] 13 has been briefly
described earlier 56 and was obtained in high yield by reaction of the β-keto phosphorus ylid
12 with equimolar amounts of [Ni(COD)2] and PPh3 in toluene at room temperature (see
Experimental). The 31 P{ 1 H} NMR spectrum of this complex should display a typical AB
pattern for the trans coordinated phosphorus atoms but the small chemical shift difference
between the two P resonances resulted in two intense signals at 23.6 and 23.4 ppm instead of
the two doublets expected. The hydrogen atom of the C-H group α to the phosphine moiety is
characterized in 1 H NMR spectroscopy by a chemical shift of 5.97 ppm, at significantly
higher value than in similar complexes. 57,58 This suggested an intramolecular bonding
interaction between this hydrogen atom and the ether function in ortho position to the phenyl
group. A 2D NOESY spectrum confirmed this hypothesis by showing a spatial interaction
between the hydrogen atom and the methyl hydrogens of the ether function.
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