Anthony KERMAGORET - THESES ET MEMOIRES DE L'UDS

4. Conclusion
Chapitre VI
The β-keto phosphorus ylids 6-12 with alkyl substituents on the phosphorus atom have been
synthesized in high yield. The reaction of 8-10 and 12 with equimolar amounts of [Ni(COD)2]
and PPh3 yielded the complexes 13 and 15-17 in which an aryl substituent of the ylid
phosphorus atom has migrated to the metal centre. Migration of an alkyl group to zerovalent
nickel is much less favourable than that of an aryl group and was never observed. Thus, the
presence of three alkyl groups on the P atom of β-keto phosphorus ylids did not allow the
formation of the desired Ni(II) complexes. In such cases, alternative synthetic approaches to
such SHOP-type catalyst are necessary (e.g. reaction of a sodium phosphinoenolate with a
aryl, halide Ni(II) complex). 54 Such an approach has been used in the case of Pd(II) analogues
to the SHOP-type catalysts since the β-keto phosphorus ylid route does not apply to other
metals but nickel. 61
1 H NMR studies on 13 indicated an intramolecular bonding interaction between the H
atom of the C-H group α to phosphorus and the ether function, which was confirmed by X-
ray diffraction (calculated O-H distance of 2.28 Å). Complexes 18-22 have been synthesized
by reaction of β-keto phosphorus ylids 3 and 8-11 with an equimolar amount of [Ni(COD)2]
and an excess of pyridine and have been characterized by 1 H and 31 P{ 1 H} NMR spectroscopy.
Active catalysts for ethylene oligomerization were prepared in situ by stirring
equimolar amounts of β-keto phosphorus ylids 3, 8, 10 or 12, [Ni(COD)2] and PPh3 under 30
or 60 bar of ethylene. TON up to 12700 mol C2H4/(mol Ni) were obtained with 12 under 30
bar of ethylene and the selectivity for linear α-olefins was between 95% and 98%. The
influence of the o-methoxy group on the enolate-bound aryl appears beneficial for catalyst
activity. Catalysis with one equiv. of 8 and PPh3 and three equiv. of [Ni(COD)2] under 30 bar
of ethylene led only to the formation of polyethylene with a TON of 5500 mol C2H4/(mol Ni).
We also noted that the presence of two alkyl groups on the P atom of the P,O chelate
in the complexes is detrimental with regard to catalytic activity. Therefore, a subtle balance
has to be found concerning the nature of the P-substituents in order to optimize the catalytic
properties of these catalysts.
12