C O MMU N I C A T I O N S
in the alkene isomerization side reaction. Van Leeuwen and co-
workers have characterized cationic rhodium complexes where the
central oxygen atom of the Xantphos ligand is coordinated to the
metal and have shown that these are very poor hydroformylation
catalysts.14 We believe that the oxygen atom of the Xantphos ligand
is also weakly coordinating to the neutral HRh(CO)(Xantphos)
catalyst, leading to partial inhibition via the formation of a saturated
five-coordinate complex (Supporting Information). The addition of
water should engage in hydrogen bonding to the oxygen atom of
the coordinated Xantphos ligand, inhibiting the internal Rh-O
acetone is further indicated by the fact that one can easily perform
10 000 turnovers using 0.1 mM catalyst and 1.0 M 1-hexene (initial
TOF ) 60(3) min , L:B ) 29:1, 2% alkene isomerization, > 0.1%
alkene hydrogenation).
-1
Phase separation of the product heptaldehyde does occur for the
catalytic runs using water at or in excess of 20% of the acetone
volume, which was an important aspect of why a polar-phase
solvent system was initially studied. Unfortunately, the dirhodium
catalyst is more soluble in the heptaldehyde organic layer than in
the water-acetone solvent. New tetraphosphine ligand systems that
impose a considerably stronger chelate effect combined with higher
polar (or ionic) solvent compatibility are being prepared to generate
even more active and robust dirhodium catalysts for hydroformy-
lation and related reactions.
Acknowledgment. We thank NSF (CHE-0111117) and the
Louisiana Board of Regents for financial support, Celanese for
generous gifts of Rh, Bisbi, and Naphos, and Professor Piet van
Leeuwen (University of Amsterdam) for samples of Xantphos.
Supporting Information Available: Experimental details and
additional schemes, table, and figures (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
interaction and generating more of the active unsaturated HRh-
2
(
CO)(η -Xantphos) catalyst that can react with alkene to start
hydroformylation.
A key question is why does the added water have such a huge
effect on the dirhodium catalyst 1? This is proposed to be mainly
due to effective inhibition of the fragmentation of bimetallic 1 into
inactive complexes. In situ NMR spectroscopic studies have
indicated that when 1 sits under H /CO, complexes 2 and 3 are
2
formed, both of which are very poor hydroformylation catalysts.
References
(
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(
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(
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1
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(
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(
(
(
9) (a) Herrmann, W. A.; Kohlpaintner, C. W.; Manetsberger, R. B.;
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(
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HRh(CO)(TPPTS)
dissociation equilibria in water relative to HRh(CO)(PPh
organic solvents.16 A similar effect inhibiting the dissociation of
the “nonpolar” PEt -like chelate arm into the highly polar water-
3
, for example, has considerably slower phosphine
(
10) (a) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
3
)
3
in
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1
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3
31
(11) (a) Billig, E.; Abatjoglou, A. G.; Bryant, D. R.; U.S. Patent 4,668,651,
acetone solvent is proposed here. Preliminary P NMR in situ
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products 2 or 3 in 30% water-acetone.
Allowing the bimetallic catalyst solution to sit in pure acetone
2
at 90 °C under 5.4-6.1 bar of H /CO, prior to alkene addition,
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min there is essentially no hydroformylation activity. However, with
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(
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the initial studies, leading to partial catalyst deactivation.
(
(
14) (a) Sandee, A. J.; van der Veen, L. A.; Reek, J. N. H.; Kamer, P. C. J.;
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2
30% water in acetone, the catalyst can sit under H /CO at 90 °C
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deactivating in acetone. Thus, different initial heating times can
lead to considerable fluctuations in the initial TOF for 1 when using
acetone. The improved stability of the catalyst in 30% water-
(
(
b) Hunt, C., Jr.; Fronczek, F. R.; Billodeaux, D. R.; Stanley, G. G., Inorg.
Chem. 2001, 40, 5192.
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