Unusual products from CO/ethene reactions catalysed by β-ketophosphine and related complexes of rhodium

Article abstract of DOI:10.1039/b007989h

Using rhodium complexes of tertiary phosphines with carbonyl groups β to the P atom, ethene and CO react in methanol to give products involving increased chain growth (octane-3,6-dione, methyl 4-oxohexanoate) compared with PEt₃ complexes and unsaturated products (methyl propenoate, penten-3-one and 1-methoxypentan-3-one from addition of methanol to penten-3-one); mechanistic studies suggest that the ligand carbonyl group prevents coordination of the keto group in the growing chain.

Full text of DOI:10.1039/b007989h

Unusual products from CO/ethene reactions catalysed by b-ketophosphine and
related complexes of rhodium
Ruth A. M. Robertson,a Andrew D. Poole,b Marc J. Payneb and David J. Cole-Hamilton*a
a School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK KY16 9ST.
E-mail: djc@st-and.ac.uk
b BP, Salt End, Hull, UK HU12 8DS
Received (in Cambridge, UK) 3rd October 2000, Accepted 20th November 2000
First published as an Advance Article on the web 15th December 2000
Using rhodium complexes of tertiary phosphines with
carbonyl groups b to the P atom, ethene and CO react in
methanol to give products involving increased chain growth
(octane-3,6-dione, methyl 4-oxohexanoate) compared with
PEt3 complexes and unsaturated products (methyl prope-
noate, penten-3-one and 1-methoxypentan-3-one from addi-
tion of methanol to penten-3-one); mechanistic studies
suggest that the ligand carbonyl group prevents coordina-
tion of the keto group in the growing chain.
2
Scheme 1 Proposed role of an h -3-oxopentyl intermediate in determining
the selectivity of ethene carbonylation to pentan-3-one catalysed by Rh/PEt3
complexes.3
Reactions between ethene and CO in methanol generally give
perfectly alternating polyketones or methyl propanoate (MP),
with the best catalysts being based on palladium complexes of
mono- or ditertiary phosphines,1,2 although we have recently
reported that highly electron rich rhodium phosphine complexes
can give high selectivities to pentan-3-one (DEK), with the two
extra H atoms required being derived from methanol, which
forms methyl formate (MF).3 We presented evidence that the
selectivity to pentan-3-one (DEK) arose because of binding of
the keto-oxygen atom in the growing chain to the rhodium atom
removal of HBr with base (Scheme 2). Catalytic reactions were
then carried out, synthesising the active catalyst in situ from the
ligand and [Rh(acac)(CO)2] (Hacac = pentane-2,4-dione). The
results of these reactions are shown in Table 1 and indicate that,
apart from the complex derived from But2PCH2C(O)Ph, which
does not give an active catalyst, catalysts based on these ligands
show quite different selectivities compared with those involving
PEt3. In particular, chain growth to octane-3,6-dione (OD) and
methyl 4-oxohexanoate (M4OH) has become significant and
the unsaturated products, penten-3-one (EVK) and methyl
propenoate (MA) are observed. A further product, 1-methoxy-
pentan-3-one (1M3P)† is a major product. We have shown in
separate experiments that this is formed by addition of methanol
to penten-3-one (EVK) in an uncatalysed reaction under the
experimental conditions employed. The selectivity to medium
chain products (!7 chain atoms) can be as high as 57.6%,
2
to give an h -3-oxopentyl intermediate. Since this complex has
18e, it more readily protonates and reductively eliminates
pentan-3-one than undergoing further insertion to give chain
growth (Scheme 1).3
One of our interests is in the production of CO/C2H4
oligomers for use as low-volatility solvents containing rela-
tively high oxygen content, so we were interested in the
possibility of encouraging chain growth and hence of prevent-
2
ing the formation of the h -3-oxopentyl intermediate. We,
therefore, synthesised a range of phosphines which themselves
contain carbonyl groups b to the P atom in the hope that these
carbonyl groups might compete with coordination of the keto
group in the growing chain and encourage chain growth.
The ligands shown in Table 1 were synthesised by the
reaction of R2PH (R = Et, But, Cy) with the appropriate bromo
compound, RACOCH2Br. (RA = Ph, Et, OEt), followed by
Scheme 2 Sythesis of b-ketophosphine ligands. RB = H, R = Et, RA = Ph,
OEt, Et; RB = H, R = But, RA = Ph, OEt; RB = H, R = Cy, RA = Ph; RB
= Me, R = Et, RA = Me. Reagent: i, NaOH.
Table 1 Products obtained from the carbonylation of ethene catalysed by rhodium complexes of b-ketophosphines and related ligandsa
Total
Ligand
MA
MP
EVK
DEK
1M3P
M4OH
OD
turnover MCP(%)
Et2PCH2C(O)Ph
Et2PCH2C(O)Et
Et2PCH2C(O)OEt
Et2PCH(Me)C(O)Me
Cy2PCH2C(O)Ph
But2PCH2C(O)OEt
But2PCH2C(O)Ph
Et2PC2H4OMe
4.4
6.7
12.2
7.2
—
—
—
4.1
—
—
4.4
7.8
6.8
6.9
4.0
2.1
—
15.0
26.9
10.7
5.1
2.0
3.1
3.7
1.4
—
1.7
—
1.5
trace
0.9
3.4
24.0
23.4
38.2
39.7
0.5
7.7
—
37.3
41.1
3.5
12.5
12.0
14.3
6.7
1.2
11.2
—
17.8
3.3
3.7
2.4
2.6
5.8
2.5
2.0
—
—
3.0
—
9.0
7.1
9.7
3.1
0.7
4.4
—
9.2
4.4
2.9
14.3
58.7
62.7
90.7
67.5
8.4
27.1
0
87.9
75.7
21.7
68.4
40.7
34.6
32.9
18.2
46.4
57.6
—
34.1
10.2
30.4
44.7
Et2PC2H4NEt2
Me2PCH2P(O)Me2
Me2PCH2P(O)Me2
—
2.9
b
4.4
24.9
13.4
a [Rh(acac)(CO)2] (0.1 mmol), phosphine (0.4 mmol), CO (35 bar), ethene (35 bar), methanol (10 cm3), 110 °C, 24 h. Amounts expressed as catalyst
turnovers. b 2 equivalents of ligand used, i.e. (0.2 mmol). MA (methyl propenoate, methyl acrylate), MP (methyl propanoate), EVK (penten-3-one, ethyl
vinyl ketone), DEK (pentan-3-one, diethyl ketone), 1M3P (1-methoxypentan-3-one), M4OH (methyl 4-oxohexanoate), OD (octane-3,6-dione), MCP
(medium chain products, 7+ atoms in backbone).
DOI: 10.1039/b007989h
Chem. Commun., 2001, 47–48
This journal is © The Royal Society of Chemistry 2001
47

Scheme 3 Proposed mechanism of ethene carbonylation catalysed by Rh/b-ketophosphine (PEO) complexes. The products shown in boxes have been
observed (Table 1), but the assignments of the metal containing intermediates are tentative.
whereas with PEt3 the selectivity to pentan-3-one (DEK) can be
> 80% with only traces of medium chain products being
formed. Similar results to those for the b-ketophosphines are
obtained (Table 1) using other ligands with O g to the P atom
such as Me2PCH2P(O)Me2, or Et2PCH2CH2OMe, but not with
N in this position; Et2PCH2CH2NEt2 behaves more like PEt3,
although the rate is lower.
We have also carried out the reaction using PhCOCH2PEt2 in
CD3OD and find that the methyl groups of pentan-3-one (DEK)
and of methyl propanoate (MP) contain 0 or 1 D atoms.‡ This
contrasts with reactions involving PEt3,3 where multiple
deuteriation of the methyl groups of pentan-3-one (DEK) is
observed.
mechanism1 is operating in addition to the hydride mechanism
which is responsible for the other products. This suggests that
CO insertion into the Rh–OMe bond competes with b-H
abstraction and CO can insert in the 18e complex (C in
Scheme 3). Acrylates can be products of CO/alkene reactions in
the presence of oxygen.5,6
In conclusion, all of the products obtained from the reaction
of CO with ethene in methanol in the presence of rhodium
complexes containing phosphine ligands with a carbonyl b to
the P atom can be explained as in Scheme 3 if one carbonyl
group in the phosphine is coordinated to the rhodium.
We thank BP and the EPSRC for funding (R. A. M. R.) and
Dr E. Ditzel and Mr P. Howard for helpful discussions.
These results point to the conclusion that, in these systems,
the carbonyl group in the growing chain does not coordinate,
presumably because the coordination site is blocked by the
carbonyl group b to the phosphine (Scheme 3). Intermediate A
in Scheme 3 is an 18e complex, so may protonate and
reductively eliminate pentan-3-one (DEK). Alternatively, CO
may insert leading to chain growth. We have shown that
multiple D incorporation into the methyl group of pentan-3-one
(DEK) in the Rh/PEt3 catalysed reactions carried out in CD3OD
Notes and references
† Since this product has seven atoms in the chain and contains two O atoms
(bp 62 °C at 24 Torr),4 it may be a suitable component of a low volatility
solvent mixture, so is included when calculating the percentage of medium-
chain products produced in the reaction.
‡ The methylene groups contain from 0 to 2 D atoms on account of post-
reaction exchange with the solvent.3
2
occurs via b-H abstraction in the h -3-oxopentyl intermediate to
1
give an enolate. A similar b-H abstraction in the h -3-oxopentyl
1 E. Drent, J. A. M. van Broekhoven and P. H. M. Budzelaar, in Applied
Homogeneous Catalysis with Organometallic Compounds, ed. B. Cornils
and W. A. Herrmann, VCH, Weinheim, 1996, vol. 1, p. 333 and
references therein.
2 E. Drent and P. H. M. Budzelaar, J. Organomet. Chem., 2000, 593–594,
211.
3 R. A. M. Robertson, A. D. Poole, M. Payne and D. J. Cole-Hamilton,
J. Chem. Soc., Dalton Trans., 2000, 1817.
4 I. N. Nazarov and I. V. Torgov, Chem. Abstr., 1948, 42, 7736a.
5 D. M. Fenton and K. L. Olivier, US Pat., 3 349 119, 1967.
6 W. Gaenzler and K. Kabs, US Pat., 3 907 882, 1975.
intermediate, B in Scheme 3, would lead to penten-3-one (EVK)
bound only through the double bond and it appears that this
decoordinates to give free penten-3-one (DEK), rather than
undergoing reversible C–H bond breakage and multiple D
incorporation.
The formation of methyl propenoate (MA) is also of
considerable interest, not only because it indicates that acrylates
can be products from CO/C2H4 reactions under non-oxidative
conditions, but also because it indicates that a carbomethoxy
48
Chem. Commun., 2001, 47–48