Paper
Catalysis Science & Technology
Conclusions
and the heating jacket set to 90 °C while stirring. Once the
temperature reached 90 °C, the reaction was stirred for 1
hour to fully activate the catalyst. Then pressure was then
slowly released and replaced with propene/CO/H2 (20 bar).
The reaction was then run for 1 hour before immediate anal-
ysis by GC.
Four novel phosphine-phosphoramidites have been prepared
and fully characterised. The active Rh hydroformylation cata-
lysts derived from these ligands have been studied using
NMR and IR. While we envisaged that this ligand structure
may well be finely poised between bis-equatorial and axial-
equatorial coordination geometries, it was found that making
minor changes to ligand structure completely switched the
coordination mode from 100% bis-equatorial to 100% axial-
equatorial. Of the ligands synthesised and tested; it was
found that the least bulky ligand 2 leads to only an axial-
equatorial species. Increasing steric bulk leads to an increase
in the bis-equatorial species until only the bis-equatorial iso-
mer was detected by NMR with the more bulky ligand 3.
These catalysts were then tested in the hydroformylation of
propene and 1-octene and were found to be amongst the
most successful catalysts for maximising the branched prod-
uct at industrially applicable temperatures. The differences
in selectivity within the ligand family is very small, limiting
the scope of the conclusions that can be drawn. Differences
in selectivity between ligand 2, which forms less iso-aldehyde
and ligand 3 are more likely ascribed to steric interactions in
the transition states for Rh-alkene > Rh-alkyl or Rh-alkyl >
Rh-acyl species. Ligands 1 and 3 show very different catalyst
geometries but very similar selectivities in hydroformylation,
which means there is no strong correlation between coordi-
nation mode (alone) and iso-selectivity in propene hydro-
formylation. While this is not entirely unexpected,
confirming that there is no isomeric preference towards iso-
selectivity focuses attention on the design of a coordination
sphere that can promote the formation of branched interme-
diates. Highly iso-selective hydroformylation of propene is
clearly a formidable challenge and our studies continue. It is
hoped that the knowledge gained here will lead us to the
rational design of catalysts that form a single isomer and can
lead to even higher iso-selective catalysts in the future.
General procedure for ligand synthesis
Synthesis and characterisation of ferrocenyl-ligand precur-
sors, chlorophosphites and phosphine-phosphoramidite
ligands is available in ESI.† General synthesis of ligand 1 is
given from amine precursor 7 and chlorophosphite 8a.
Synthesis of phosphine-phosphoramidite ligand 1. Amine
7 (0.30 g, 0.70 mmol) was dissolved in ethyl acetate (1.5 mL)
and N-methylpyrrolidine (0.11 mL, 1.07 mmol) under Ar. The
solution was cooled to 0 °C and was purged with argon for 15
minutes then chlorophosphite 8a (0.340 g, 0.85 mmol) in
CH2Cl2 (2 mL) was added and stirred at 0 °C for 1 hour. The
solution was warmed to room temperature and stirred for 16
hours. The solution was concentrated in vacuo to afford a
crude solid. The solid was purified by flash column chroma-
tography (pre-treated with a solution of 95 : 5 toluene : Et3N)
using 30 : 1 hexane : ethyl acetate as eluent under N2 to give
phosphoramidite 1 as an orange solid (0.38 g, 0.48 mmol,
69%).
Acknowledgements
We thank the Eastman Chemical Company for funding, the
EPSRC for the use of the national mass spectrometry service,
and all the technical staff in the School of Chemistry for their
assistance.
Notes and references
1 (a) T. Besset, D. W. Norman and J. N. H. Reek, Adv. Synth.
Catal., 2013, 355, 348–352; (b) V. F. Slagt, J. N. H. Reek,
P. C. J. Kamer and P. W. N. M. van Leeuwen, Angew. Chem.,
Int. Ed., 2001, 40, 4271–4274; (c) V. F. Slagt, P. C. J. Kamer,
P. W. N. M. van Leeuwen and J. N. H. Reek, J. Am. Chem.
Soc., 2004, 126, 1526–1536; (d) M. Kuil, T. Solder,
P. W. N. M. van Leeuwen and J. N. H. Reek, J. Am. Chem.
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2 (a) G. M. Noonan, J. A. Fuentes, C. J. Cobley and M. L.
Clarke, Angew. Chem., Int. Ed., 2012, 51, 2477–2480; (b) G. M.
Noonan, C. J. Cobley, T. Mahoney and M. L. Clarke, Chem.
Commun., 2014, 50, 1475–1478.
3 (a) M. L. Clarke, Curr. Org. Chem., 2005, 9, 701–718; (b)
Rhodium Catalysed Hydroformylation, ed. P. W. N. van
Leeuwen and C. Claver, Kluwer Academic Publishers,
Netherlands, 2000.
4 T. J. Devon, G. W. Phillips, T. A. Puckette, J. L. Stavinoha
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Experimental
General information
Full experimental details are available in ESI.†
All manipulations were carried out under an inert atmo-
sphere of nitrogen or argon using standard Schlenk tech-
niques. Solvents were dried and degassed before use, with
the exception of octafluorotoluene which was degassed only.
General procedure for propene hydroformylation
Ligand (6.40 μmol (Rh : L 1 : 1.25)) was added to a schlenk
tube, which was then purged with N2. ijRhIJacac)IJCO)2] (5.12
μmol) was added in a toluene stock solution (2 mg mL−1).
Toluene was then added to make up to 20 mL total volume,
followed by the addition of internal standard 1-methylnaph-
thalene (0.2 mL). The solution was transferred via syringe to
the pressure vessel (which had been purged with CO/H2)
through the injection port. CO/H2 (1 : 1) (20 bar) was added
5 C. P. Casey, G. T. Whiteker, M. G. Melville, L. M. Petrovich,
J. A. Gavney and D. R. Powell, J. Am. Chem. Soc., 1992, 114,
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Catal. Sci. Technol.
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