have to be unidentate in some of the crucial intermediates, or
because both catalysts are present in the reaction. The drop
in l : b ratio with continuous use suggests that the contribution
from complexes containing 5 diminishes, but no catastrophic
loss occurs. 31P NMR studies on the solution recovered after
the 30 h run showed that it contained 5 (d -18.7), its ox-
ide (d -22.7, 28.2), and dioxide (d 28.5), [PrMIM][TPPMS]
(d -4.0), its oxide (d 30.0), and a complex resonating as a doublet
at d 31.9 (JRh-P = 131 Hz), which is not present in solutions
containing Rh and 5 under CO/H2, but which has been observed
before in Rh/[PrMIM][TPPMS], systems under CO/H2. It
appears that oxidation of the xantphos ligand is reduced,
but not eliminated by the presence of the cheaper unidentate
phosphine.
Although the lower l : b ratio observed when using the two
ligands, 5 and [PrMIM][TPPMS], together suggests that both
xantphos and TPPMS complexes might be present, the very
similar rates observed in the two systems are rather surprising
since the TPPMS catalyst is known to be about twice as
active as that containing 5 under the flow conditions used
in this study¶ and so the rate might be expected to increase.
We have recently reported39 that hydroformylation reactions
catalysed by Rh/xantphos complexes can be greatly affected by
adding PEt3 so that the chemoselectivity alters to give alcohol
rather than aldehyde products, whilst the linearity obtained
with xantphos is preserved. It appears that this cooperative
behaviour arises because PEt3 is not coordinated during the
selectivity determining step (hydride migration), but coordinates
to the chemoselectivity producing acyl intermediate. In principle
similar chemistry could occur here, but PPh3 would not affect
the chemoselectivity, so the effect would probably go undetected.
One of the major factors affecting the reaction rate is the amount
of the catalyst in the active form (6, Scheme 4) rather than the
resting state (12, Scheme 4). One possible explanation for the
rates being similar is that, in the presence of [PrMIM][TPPMS],
TPPMS can coordinate to give 13, this removing even more
catalyst from the cycle when 5 is coordinated. This would reduce
the rate of this cycle and by coincidence, this rate reduction may
exactly balance the rate increase expected from the presence of
a catalyst derived from [RhH(CO)2(TPPMS)2]2-.
Conclusion
The use of rhodium complexes of ligand 5 dissolved in
[OMIM]NTf2 in a continuous flow system with scCO2 as
the transport medium, provides the first demonstration of a
continuous flow hydroformylation process for long chain alkenes
in which rates and selectivities to the desired linear aldehyde
are of commercial interest. The presence of oxygen in the feed
eventually leads to much reduced l : b ratios and increased
leaching, but these can be ameliorated to some extent by adding
excess [PrMIM][TPPMS].
Acknowledgements
We thank Professor Mike Green for his help and support
throughout the work, Sasol Technology (pty) Ltd for funding
a studentship and secondment (T. E. K.) and the EPSRC for
Postdoctoral (P. B. W.) and other support.
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1480 | Green Chem., 2011, 13, 1476–1481
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