o-ALKYL LIGANDS: PROPENE HYDROFORMYLATION
307
�
cone angle 151 , the same as in MeP), but having a higher temperature, and deactivation) induce a pattern of changes
31
P-NMR shift (� 6.3 vs � 10.7). Indeed, the regioselectivity similar to those induced by PPh3.
achieved with pyrMeP (Sisobutanal: 42% ) was similar to that
achieved with MeP (Sisobutanal:43% ), but the initialaldehyde
formation activity was even lower than that of MeP (20 and
ACKNOWLEDGMENTS
�
1
� 1
0 mol mol s , respectively). Usually, the reaction rates
Rh
3
The authors wish to thank Ms. Johanna Suutari for performing part
of the hydroformylation experiments. The funding from Neste Chemicals
Oy and the National Technology Agency of Finland (Tekes) is gratefully
obtained with pyridyl-modified compounds are higher than
the rates of their phenyl analogues, which has been at-
tributed to the electron-withdrawing capasity of the pyridyl acknowledged.
ring causing the CO molecule to be less strongly bonded
to the rhodium centre and thus facilitating alkene coordi-
nation (26). However, in the case of o-pyridyl groups the
reaction rates have been reported (27, 28 ) to be lower com-
REFERENCES
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. Beller, M., Cornils, B., Frohning, C. D., and Kohlpaintner, C. W.,
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withdrawing capasity of the ortho nitrogen should be
stronger than that of meta or para nitrogen. Thus, it seems
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group would have been a better alternative than the tested
2
. Bizzari, S. N., Fenlon, S., and Ishikawa-Yamaki, M., “CEH, SRI Inter-
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(
4
5
2
-pyridyl (pyrMeP) where the ortho nitrogen (27) is not as
(
inert as the para or meta nitrogens.
Changes in the process conditions for the o-alkyl-
substituted ligands induced changes in regioselectivity sim-
ilar to those found in the PPh3-modified reaction. Although
the ligand-to-rhodium ratio has an influence on the regio-
selectivity, as also shown by Di e´ guez et al. (13), within
the range of L/Rh ratios we tested the selectivities of the
MeP- and EtP-modified catalysts to isobutanal were con-
stantly higher than the selectivity of PPh3. The deactiva-
tion pattern of the MeP and EtP ligands was similar to that
of PPh3: the activity decreased but the regioselectivity re-
mained unaltered. Apparently, the o-alkyl-substituted lig-
(
7
. Rode, E. J., Davis, M. E., and Hanson, B. E., J. Catal. 96, 574
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(
8. Bahrmann, H., Bach, H., Frohning, C. D., Kleiner, H. J., Lappe, P.,
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1
1
0. Wilkinson, G., J. Chem. Soc. A 3133 (1968).
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J. A. Jr., and Powell, D. R., J. Am. Chem. Soc. 114, 5535 (1992).
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2
9, 30) for PPh3. To summarise, it appears that even though
1
1
the process conditions have an effect on the regioselec-
tivity, the conditions selected for comparison are not the
cause of the higher selectivity to isobutanal with the MeP-
4. Fuchikami, T., and Ojiama, I., J. Am. Chem. Soc. 104, 3527
(
1982).
and EtP-modified catalysts when compared to those with 15. Suomalainen, P., Reinius, H. K., Riihim a¨ ki, H., Laitinen, R. H.,
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1
1
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(
CONCLUSIONS
We have shown that, in the rhodium-catalysed hydro-
formylation of propene, o-alkyl-substituted triphenylphos-
phine ligands enhance the selectivity to isobutanal. Unfor-
tunately, the activity decreases drastically as the selectivity
increases. A correlation was found between the P-NMR
shifts of these closely related ligands and the regioselectiv-
ity and activity: as the shift decreases the regioselectivity
to isobutanal increases, while the activity decreases. The
regioselectivity and activity seem to be controlled through
the steric properties of the active complex in which one side
of the complex is blocked by the alkyl substituents. The be-
haviour of the o-alkyl-substituted ligands closely resembles
that of PPh3: changes in the process conditions (L/Rh ratio,
1
1
8. Drent, E. I. T., GB Patent 2 240 545 A, 1991.
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Gases and Liquids,” McGraw-Hill, New York, 1988.
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New York, 1980.
31
2
2
2