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

Abstract

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.

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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 to give an η²-3-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

Scheme 1 Proposed role of an η²-3-oxopentyl intermediate in determining the selectivity of ethene carbonylation to pentan-3-one catalysed by Rh/PEt₃ complexes.³

One of our interests is in the production of CO/C₂H₄ oligomers for use as low-volatility solvents containing relatively high oxygen content, so we were interested in the possibility of encouraging chain growth and hence of preventing the formation of the η²-3-oxopentyl intermediate. We, therefore, synthesised a range of phosphines which themselves contain carbonyl groups β 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, Bu^t, Cy) with the appropriate bromo compound, R′COCH₂Br. (R′ = Ph, Et, OEt), followed by 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 Bu^t₂PCH₂C(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-methoxypentan-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%, 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 β-ketophosphines are obtained (Table 1) using other ligands with O γ 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.

Table 1 Products obtained from the carbonylation of ethene catalysed by rhodium complexes of β-ketophosphines and related ligands^a

Ligand	MA	MP	EVK	DEK	1M3P	M4OH	OD	Total turnover	MCP(%)
a [Rh(acac)(CO)2] (0.1 mmol), phosphine (0.4 mmol), CO (35 bar), ethene (35 bar), methanol (10 cm^3), 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).
Et2PCH2C(O)Ph	4.4	4.4	2.0	24.0	12.5	2.4	9.0	58.7	40.7
Et2PCH2C(O)Et	6.7	7.8	3.1	23.4	12.0	2.6	7.1	62.7	34.6
Et2PCH2C(O)OEt	12.2	6.8	3.7	38.2	14.3	5.8	9.7	90.7	32.9
Et2PCH(Me)C(O)Me	7.2	6.9	1.4	39.7	6.7	2.5	3.1	67.5	18.2
Cy2PCH2C(O)Ph	—	4.0	—	0.5	1.2	2.0	0.7	8.4	46.4
Bu^t2PCH2C(O)OEt	—	2.1	1.7	7.7	11.2	—	4.4	27.1	57.6
Bu^t2PCH2C(O)Ph	—	—	—	—	—	—	—	0	—
Et2PC2H4OMe	4.1	15.0	1.5	37.3	17.8	3.0	9.2	87.9	34.1
Et2PC2H4NEt2	—	26.9	trace	41.1	3.3	—	4.4	75.7	10.2
Me2PCH2P(O)Me2	—	10.7	0.9	3.5	3.7	—	2.9	21.7	30.4
Me2PCH2P(O)Me2^b	4.4	5.1	3.4	24.9	13.4	2.9	14.3	68.4	44.7

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Scheme 2 Sythesis of β-ketophosphine ligands. R″ = H, R = Et, R′ = Ph, OEt, Et; R″ = H, R = Bu^t, R′ = Ph, OEt; R″ = H, R = Cy, R′ = Ph; R″ = Me, R = Et, R′ = Me. Reagent: i, NaOH.

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.

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 β 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 occurs via β-H abstraction in the η²-3-oxopentyl intermediate to give an enolate. A similar β-H abstraction in the η¹-3-oxopentyl 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.

Scheme 3 Proposed mechanism of ethene carbonylation catalysed by Rh/β-ketophosphine (PO) complexes. The products shown in boxes have been observed (Table 1), but the assignments of the metal containing intermediates are tentative.

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 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 β-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 β to the P atom can be explained as in Scheme 3 if one carbonyl group in the phosphine is coordinated to the rhodium.

Acknowledgements

We thank BP and the EPSRC for funding (R. A. M. R.) and Dr E. Ditzel and Mr P. Howard for helpful discussions.

Notes and references

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..

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Footnotes

† 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

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