Clean catalysis with ionic solvents—phosphonium tosylates for
hydroformylation
Nazira Karodia, Steven Guise, Craig Newlands and Jo-Ann Andersen*
School of Chemistry, University of St Andrews, St Andrews, Fife, UK KY16 9ST. E-mail: jma2@st-andrews.ac.uk
Received (in Basel, Switzerland) 10th July 1998, Accepted 7th September 1998
High-melting phosphonium tosylates are synthesised and
applied for the first time as solvents in catalytic hydro-
formylation reactions; variation in the substituents attached
to phosphorus can lead to markedly different results; the
catalyst systems are non-corrosive, easily manipulated and
can readily be recovered and reused.
One of the major problems associated with homogeneous
catalysts is that they are difficult to recover at the end of the
reaction. Frequently distillation is used as a separation tech-
nique, but this is a very energy-intensive process. High-boiling
products can remain with the catalyst and lead to catalyst
deactivation or product contamination. This problem, especially
for expensive and/or toxic precious metal catalyst systems, has
precluded their more widespread application in the catalysis
industry.
In today’s environmentally conscious world, another prob-
lem with homogeneous catalysts has emerged, viz. many of the
All four salts were fully characterised; selected data are
shown in Table 1. They were then applied as solvents in the
hydroformylation of hex-1-ene to heptanal (A) and 2-me-
thylhexanal (B) [eqn. (3)] in the presence of [Rh2(OAc)4], both
solvents traditionally used in transition metal catalysis, such as
chlorinated hydrocarbons, acetonitrile, DMF to name but a few,
are currently on the ‘environmental blacklist’. It is rapidly
becoming apparent that the way in which solvents are used in
organic synthesis needs rethinking. The way forward may thus
be to choose the solvent on environmental grounds and then
optimise the reaction in that solvent.1 In this respect, the use of
high-melting ionic solvents (molten salts/ionic liquids) will be
highly advantageous if the product(s) can readily be decanted
off the stable catalyst system. The advantages of these systems
will be manifold: in addition to facilitated catalyst recovery,
they may exhibit low viscosity, high thermal and air stability,
good electrical conductivity, low vapour pressure and they will
readily solubilise the reagents and catalyst. They also exhibit a
large ‘liquid range’, allowing for extensive kinetic control.2
Room temperature ionic liquids such as alkylimidazolium
chloride/AlCl3 have been developed by Seddon and others2,3
and have been found to function as highly efficient catalytic
systems for reactions such as dimerisation and alkylation.
However, much less attention has been paid to higher melting
ionic solvents such as tetraalkyl ammonium and phosphonium
salts. These offer advantages over the room temperature
systems in that (i) they are not corrosive and (ii) being solid at
room temperature, they are more easily manipulated and
product separation is simple, being accomplished by decanta-
tion rather than by biphasic extraction. They are also stable to
much higher temperatures, thereby enabling more forcing
reaction conditions to be applied. We now report our results on
the synthesis of tetraalkyl/aryl phosphonium tosylates and their
application as solvents in hydroformylation reactions.
with and without added phosphine ligand. Selected results are
shown in Table 2.
In some instances, trace amounts (ca. 1–2%) of additional
reaction products were observed. The isomerisation products
2-methylhexanal (B) and 2-ethylpentanal (C) (derived from the
isomerisation of hex-1-ene to hex-2-ene and subsequent
hydroformylation) were typically present in small amounts. In
the case of Ph3PBu+OTs2 and Ph3POc+OTs2, the correspond-
ing C5 aldehyde and C9 aldehyde were observed, respectively.
These are most likely to derive from elimination of the bulky
alkyl moiety (butyl or octyl) to form the corresponding alkene,
analogous to Hoffmann eliminations in ammonium salts. The
alkene is then hydroformylated to the corresponding aldehydes
(F–J; see Scheme 1). This elimination process does not occur
with Bu3PEt+OTs2 or with Ph3PEt+OTs2; presumably, in these
cases, the ethyl group is small enough to remain bound to the P
atom and does not eliminate.
Salts 1–4 were synthesised by reaction of the tosylate esters
[eqn. (1)] with the appropriate tertiary phosphine [eqn. (2)].
Table 1 Characterisation of phosphonium salts
Salt
Melting point/°C [literature value]
Yield (%)
d(31P)
n(P–C)/cm21
Ph3PEt+OTs2; 1
Ph3PBu+OTs2; 2
Ph3POc+OTs2; 3
Bu3PEt+OTs2; 4
94–95 [93–94]4
116–117 [139–140]4
70–71
98
98
95
94
26.6
24.8
26.7
24.6
1460 (P–Caryl); 1380 (P–Calkyl
1470 (P–Caryl); 1380 (P–Calkyl
1450 (P–Caryl); 1390 (P–Calkyl
)
)
)
81–83 [70–78]4
1380 (P–Calkyl)
Chem. Commun., 1998, 2341–2342
2341
Karodia, Nazira
