Selective formation of pentan-3-one from carbon monoxide and ethene in methanol

Article abstract of DOI:10.1039/b002629h

In the presence of rhodium complexes of triethylphosphine, CO and ethene react in methanol to give pentan3-one with selectivities up to 85%; model studies and deuterium labelling suggest that the mechanism involves hydroxycarbene and Tj2-3-oxopentyl deriv

Full text of DOI:10.1039/b002629h

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Selective formation of pentan-3-one from carbon monoxide
and ethene in methanol
a
b
c
a
Ruth A. M. Robertson, Andrew D. Poole, Marc. J. Payne and David J. Cole-Hamilton*
a
School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST.
E-mail: djc@st-and.ac.uk
BP-Amoco Chemicals, Salt End, Hull, UK HU12 8DS
BP-Amoco Chemicals, Sunbury Research Centre, Chertsey Rd., Sunbury-on-Thames,
b
c
Middlesex, UK TW16 7LN
Received 3rd April 2000, Accepted 5th May 2000
Published on the Web 24th May 2000
In the presence of rhodium complexes of triethylphos-
phine, CO and ethene react in methanol to give pentan-
furan or propan-2-one reduces the yield without greatly affect-
ing the selectivity. In propan-2-one, 2,2-dimethoxypropane is
also formed by a direct ketalisation reaction.
3
-one with selectivities up to 85%; model studies and
deuterium labelling suggest that the mechanism involves
hydroxycarbene and ꢀ -3-oxopentyl derivatives.
At first sight, this high selectivity towards the formation of
pentan-3-one, without the formation of ethane is rather surpris-
ing, since the intermediates that would lead to these products
are very similar. Ethane would be formed from an ethyl
complex, whilst pentan-3-one would arise from a 3-oxopentyl
intermediate. Both should have very similar environments at the
metal (see Fig. 1). CO insertion would lead to propanoyl or
3-oxohexanoyl intermediates, whilst methanolysis would lead
to ethane and pentan-3-one respectively. The observed products
demonstrate that for the ethyl intermediate, CO insertion
occurs exclusively, whilst for the 3-oxopentyl intermediate,
methanolysis is much preferred. We have attempted to probe
the origin of this unusual selectivity by studying a closely
related model system and by deuterium labelling studies.
2
In methanol, in the presence of a variety of complexes of
palladium(), CO and ethene react to give either perfectly
alternating copolymers or methyl propanoate, which is formed
being dependent upon the other ligands present in the
1
complex. Pentan-3-one is sometimes observed as a side-
product, but high selectivities to this product, which is a useful
solvent of relatively low volatility, are only obtained if water or
2
hydrogen gas is also a component of the mixture. Using
triphenylphosphine complexes of rhodium, long chain
polymers are not formed, but rather a mixture of oligoesters
3
and oligoketones. Hydrogen is again required, but this can be
31
produced from added water via the water gas shift reaction; no
P NMR studies have shown that [RhH(PEt ) ] dissolves in
3 4
ϩ
3
reaction occurs in dry methanol in the absence of hydrogen.
methanol to give [RhH (PEt ) ] and that this reacts with CO to
2 3 4
4
We have previously shown that trialkylphosphine complexes
of rhodium show high activity for the direct formation of
give [RhH(CO)(PEt ) ] and then [Rh (CO) (PEt ) ]. This last
3 3 2 2 3 6
product is also formed from [Rh(acac)(CO) ] and excess PEt3
2
4,5
alcohols in hydrocarbonylation reactions of alkenes and that
similar products are also formed in the absence of hydrogen,
in methanol at room temperature. Assuming that the active
species in the catalytic reaction is [RhH(CO)(PEt ) ], we
3
2
6
with the alcohol solvent acting as the source of hydrogen. We
attempted to study the reactions of [RhMe(CO)(PEt ) ], a
3 2
now report that the same system can afford relatively high
selectivity to pentan-3-one from CO and ethene in methanol.
Heating a solution of [RhH(PEt ) ] (1) (Strem) or [Rh(acac)-
model of the necessary ethyl intermediate. We have already
4
reported that [RhMe(CO)(PEt ) ] reacts with CO in toluene to
3
2
give [Rh(MeCO)(CO) (PEt ) ] and that on addition of meth-
3
4
2
3 2
4
(
CO)2] (2)/4PEt3 (acacH = pentane-2,4-dione) in methanol
anol and passing argon, this complex is transformed into the
ϩ
under an atmosphere of CO and ethene at 110 ЊC produces
hydroxycarbene complex, [Rh(᎐C(OH)Me)(CO)(PEt ) ] ,
᎐
3 2
pentan-3-one as the major product, with selectivities up to 85%
characterised by the very low field chemical shift of the C atom
of the hydroxycarbene (δ 301).† In methanol, ethene also con-
(
see Table 1). The other main products are methyl propanoate
and methyl formate, whilst traces of products formed by further
chain growth, octane-3,6-dione and methyl 4-oxohexanoate
are also detected. Analysis of the gas phase at the end of the
reaction shows that no ethane is produced. Increasing the
temperature increases the rate but reduces the selectivity to
pentan-3-one, whilst diluting the methanol with tetrahydro-
verts [Rh(MeCO)(CO) (PEt ) ] into [Rh(᎐C(OH)Me)(CO)-
2 3 2
᎐
ϩ
(PEt ) ] , although we have not found direct evidence
3
2
for, the product of the reaction of this hydroxycarbene complex
with ethene.
Deuterium labelling studies obtained by carrying out the
reaction in CD OD (Table 2) shed further light on the mech-
3
Table 1 Products (expressed as catalyst turnovers) after 24 h from the reaction of CO (35 bar) and ethene (35 bar) in methanol
Catalyst
system
Methyl
propanoate
Octane-3,6-
dione
Selectivity to
pentan-3-one (%)
a
Solvent
Ratio
T/ЊC
Pentan-3-one
MeOH
MeOH
MeOH/H O
MeOH/propan-2-one
MeOH/propan-2-one
—
—
4:1
1:3
1:3
—
1
2
1
1
2
1
2
110
110
110
110
110
140
110
168
164
106
79
81
221
25
34
56
19
21
24
111
10
tr
7
tr
3
2
tr
—
83
72
85
77
76
66
71
2
b
MeOHc
MeOH
—
a
Ϫ3
1
[RhH(PEt ) ] (0.01 mol dm ) as catalyst precursor, 2 [Rh(acac)(CO) ] (0.01 mmol)/PEt (0.04 mmol) as catalyst precursor. Methyl formate (not
3 4 2 3
b c
quantified) is also a product in all cases. 6 h reaction time. CO was omitted from this reaction, but paraformaldehyde (2 g) was added instead.
DOI: 10.1039/b002629h
J. Chem. Soc., Dalton Trans., 2000, 1817–1819
This journal is © The Royal Society of Chemistry 2000
1817

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Fig. 1 Possible products of reactions of ethyl and 3-oxohexanoyl complexes of rhodium with (i) methanol or (ii) CO. The reactions marked with a
cross are disfavoured. P = PEt3.
Fig. 2 Proposed mechanism for multiple incorporation of deuterium into the methyl group of pentan-3-one, during CO, C H reactions in CD OD.
2
4
3
(
i) CD OD, P = PEt .
3
3
Table 2 Labelling pattern (% of each isotopomer) for ethyl groups of products obtained from ethene and CO in CD OD
3
2
Compound
CH CH
CH CHD
CH CD
CH DCH
CH DCHD
CH DCD
CHD CH
CHD CHD
CHD CD
3
2
3
3
2
2
2
2
2
2
2
2
2
2
2
Et CO
16.8
39.0
35.0
23.4
21.2
15.6
4.4
11.0
9.1
6.6
5.5
4.4
1.8
0
3.8
0
2.3
0
2
EtCO Me
2
anism of formation of the products. Control experiments show
that exchange of D with the CH (but none of the CH ) H
atoms in both methyl propanoate and pentan-3-one occurs
exchange with the solvent cannot, however, be responsible for
the multiple deuteriation of the methyl groups in pentan-3-one,
since this would also lead to multiple deuteriation in the methyl
groups of methyl propanoate, which is not observed.
2
3
under the reaction conditions. The CH resonance of the
3
13
1
2
pentan-3-one in the C{ H, H} NMR spectrum is remarkable
since it shows α, β and δ shifts, so we can be sure that the two
methylene groups are equally exchanged. The important result
is that the CH groups in pentan-3-one are d (73%), d (19%)
This multiple deuteriation must, therefore, arise via an
enolate mechanism, as shown in Fig. 2 and provides evidence
2
for a η -3-oxopentyl intermediate. Without the binding of the
oxo group, penten-3-one would be released and should be
observed as a product and/or react with methanol to produce
1-methoxypentan-3-one, since we have observed these products
when we have deliberately hindered oxo binding in the
3
0
1
and d (8%) and that <1% of the pentan-3-one molecules have
2
d2 at each end. In contrast, none of the methyl propanoate
molecules contains d in the methyl group (d (78%), d (22%)).
2
0
1
7
The presence of a large amount of d0 methyl groups in
pentan-3-one and in methyl propanoate confirms that the
hydride mechanism is operating since these isotopomers must
form by β-H abstraction in Rh–CH CH D, initially formed by
3-oxopentyl intermediate. For the PEt3 complexes, these
2
products are not observed, confirming that η binding of the
oxopentyl intermediate occurs. Chelating oxoalkyl intermedi-
ates have been proposed before to explain the perfectly altern-
ating nature of the polymers obtained from palladium catalysed
2
2
migration of Rh–D to coordinated ethene, to give RhH(CH2-
CHD). H migration onto the undeuteriated end of the coordin-
1
copolymerisation of CO and ethene, but they have seldom
8,9
ated CH ᎐CHD or exchange with CH ᎐CH followed by H
been directly observed for ethene. Furthermore, a similar eno-
late mechanism has been shown by D labelling studies to
2
᎐
2
᎐
2
migration would then give the observed isotopomer. The latter
clearly occurs since, for both pentan-3-one (17%) and for
methyl propanoate (39%) a significant proportion of C H is
8
operate in CO/C H copolymerisation using Pd complexes and
2
4
10
in the hydrocarbonylation of ethene to pentan-3-one.
2
5
2
present. Multiple reversible insertions of this kind with H/D
The formation of the η -3-oxopentyl complex then explains
1
818 J. Chem. Soc., Dalton Trans., 2000, 1817–1819

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Fig. 3 Proposed mechanism for the formation of pentan-3-one, methyl propanoate and methyl formate (all shown in bold) from CO and ethene in
methanol catalysed by rhodium triethylphosphine complexes. An alternative is that pentan-3-one is released by protonation of the enolate, as shown
in Fig. 2. P = PEt3.
the selectivity shown in Fig. 1. Since this complex has 18 e, it
will need to decomplex a ligand to allow CO coordination and
chain growth. Protonation on the other hand does not require
the creation of a vacant site and produces pentan-3-one.
The alternative of direct CO insertion would require an
unfavourable ring expansion from a 5- to a 6-membered ring.
The analogous ethyl complex has only 16 e so that CO coordin-
ation and chain growth are both facile.
Notes and references
3
1
†
In ref. 4, a typographical error gives the wrong P NMR data for
ϩ
[Rh(᎐C(OH)Me)(CO)(PEt ) ] , which had not been measured at that
3 2
time. The spectrum consists of a doublet at δ 19.0, JPRh = 147 Hz.
1
E. Drent, J. A. M. van Broekhoven and P. H. M. Budzelaar, Applied
Homogeneous Catalysis with Organometallic Compounds, ed. B.
Cornils and W. A. Herrmann, VCH, Weinheim, 1996, vol. 1, p. 333
and refs. therein.
Methanol is the source of the two H atoms that are required
for the formation of pentan-3-one and the product derived
2
G. N. Il’inich, V. N. Zudin, A. N. Nasov, V. A. Rogov and
V. A. Likholobov, J. Mol. Catal. A, 1995, 101, 221 and refs.
therein.
1
3
from it is methyl formate. A reaction carried out in CH OH
3
13
13
13
produced (C H ) CO, C H CO CH and H CO CH . This
2
5
2
2
5
2
3
2
3
last product shows that in the presence of CO, the formalde-
hyde formed by dehydrogenation of methanol provides one
extra proton and the metal formyl complex undergoes nucleo-
philic attack by methanol rather than α-hydride abstraction.
Paraformaldehyde can also be used as a source of CO if it is
used to replace CO in the catalytic system (Table 1). It has been
reported that formaldehyde can be used as a source of CO and
3 A. Sen, J. S. Brumbaugh and M. Lin, J. Mol. Catal., 1992, 73,
2
97.
4
5
6
J. K. MacDougall, M. C. Simpson, M. J. Green and D. J. Cole-
Hamilton, J. Chem. Soc., Dalton Trans., 1996, 1161.
M. C. Simpson, A. W. S. Currie, J. M. Andersen, D. J. Cole-
Hamilton and M. J. Green, J. Chem. Soc., Dalton Trans., 1996, 1793.
J. K. MacDougall and D. J. Cole-Hamilton, Polyhedron, 1990, 9,
1
235.
11
H with a related catalytic system.
7 R. A. M. Robertson and D. J. Cole-Hamilton, unpublished
observations.
8 M. J. Green, G. J. P. Britovsek, K. J. Cavell, B. W. Skelton and A. H.
White, Chem. Commun., 1996, 1563.
2
We conclude that rhodium trialkylphosphine complexes
show good selectivity for the formation of pentan-3-one from
CO and ethene, with the extra H atoms being derived from
methanol. The mechanism of formation of this product, methyl
propanoate and methyl formate is shown in Fig. 3.
9
M. A. Zdideveld, P. C. J. Kramer, P. W. N. M. van Leeuwen, P. A. A.
Klusener, H. A. Stil and C. F. Roobek, J. Am. Chem. Soc., 1998, 120,
7
977.
1
0 V. N. Zudin, V. D. Chinakov, V. M. Nekipelov, V. A. Rogov,
Acknowledgements
V. A. Likholobov and Yu. I. Yermakov, J. Mol. Catal., 1989, 52,
2
7.
We thank BP-Amoco Chemicals and the EPSRC for a student-
ship (R. A. M. R.) and Dr Evert Ditzel for helpful discussions.
11 T. Okama and T. Toshida, Yuki Gosei Kagaku Kyokaishi, 1983, 41,
359; Chem. Abstr., 1983, 99, 53950.
J. Chem. Soc., Dalton Trans., 2000, 1817–1819
1819