The trifunctional ligands
(R)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (2R),
(S)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane
(2S),
and
(S)-(phenyl(2-anisyl)phosphino)(2-pyridyl)methane
(3) have been synthesized, as well
as the corresponding RuCl2(PPh3)(L)
complexes. The complexes
RuCl2(PPh3)(2R)
(5) and
RuCl2(PPh3)(2S)
(6) were isolated as mixtures of two isomers, 5a
and 5b and 6a and 6b,
respectively. In each of these isomers, the ligands 2
are η3-(P,N,O) bound. They differ by
the position of the triphenylphosphine, which is either in a trans
position relative to the
pyridyl ring (5a or 6a) or in a trans position
relative to the ether function (5b or 6b).
Variable
temperature NMR experiments have shown that the hemilabile character in
solution of the
ligands 2 is through their pyridyl arm in the isomers
a or their ether arm in b. The
complexes
6b and
RuCl2(PPh3)(3)
(9) were characterized by X-ray diffraction. The
complexes 5, 6, and
9 are very active catalysts for the transfer hydrogenation
of acetophenone by propan-2-ol in
basic media, the higher activity being observed for 9
(turnovers frequency 48 900 h-1).
The
enantioselectivity is modest and dependent on the reactions conditions.
The best result has
been observed for 5, with an ee of 60%.
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