The present invention relates in general to olefin hydroformylation and in particular to the production of an aldehyde by the regioselective hydroformylation of an olefin in the presence of homogeneous rhodium catalysts. The hydroformylation of olefins is one of the most thoroughly investigated reactions in homogeneous catalysis. Hydroformylation catalysts are generally based on either rhodium or cobalt and the product of the reaction is generally either an aldehyde or an alcohol depending largely on the particular catalyst employed. In olefin hydroformylation processes the possibility generally exists for the production of a linear and a branched product. It is not uncommon for one or other but not both of the linear or branched products to be commercially attractive and, as a consequence of this alone it would be highly desirable to achieve regioselective control over the reaction. Additionally, the ease of separation can be considerably enhanced by improvement in the regioselective control.

Efforts to exert regioselective control over the hydroformylation reaction in the past have not generally met with great success. Recently Neibecker and Reau (in New J Chem. , 1991, 15, 279) reported on the use of rhodium phosphole and phosphanobornadienes [from Rh(C0) ₂ Cl and added ligand] as catalysts for the hydroformylation of ethyl acrylate. Although the reaction is regiospecific, an investigation was not conducted on the scope and limitations using a variety of alpha, beta-unsaturated -esters. The work of Neibecker and Reau derived from the previous study by

Tanaka et al (Bull Chem, Soc. Japan, 1977, 5 , 2351) who used the same rhodium catalyst, but with added bidentate phosphine ligands.

It has also recently been reported by Amer and Alper in J Amer Chem Soc, 1990, 112, 3674 that the Zwitterionic rhodium complex having the formula (A) below:-

(COD) (wherein COD represents cyclooctadiene) is an excellent catalyst for the hydroformylation of.olefins, with high regioselectivity observed for the branched (equation 1 below) or linear (equation 2 below) aldehyde depending on the nature of the reactant.

(equation 1)

(equation 2) However, regiochemical control is not universally achievable using this catalyst system, for example poor control was found during attempts to apply it to the hydroformylation of alpha, beta-unsaturated esters, such as methyl acrylate and methyl methacrylate.

We have now found that aldehydes can be produced regioselectively from olefins in the presence of a novel catalyst system.

Accordingly the present invention provides a process for the production of an aldehyde from an olefin which process comprises reacting in the liquid phase at elevated temperature the olefin with gaseous carbon monoxide and either gaseous hydrogen or a reducing agent or a combination thereof in the presence of a soluble catalyst comprising (i) either a Zwitterionic rhodium complex or the

precursors thereof, and (ii) a bidentate phosphine ligand.

In one embodiment therefore the invention provides a process for the production of an aldehyde from an olefin which process comprises reacting in the liquid phase at elevated temperature an olefin in the absence of a reducing agent with gaseous carbon monoxide and gaseous hydrogen in the presence of a soluble catalyst comprising (i) either a Zwitterionic rhodium complex or the precursors thereof, and (ii) a bidentate phosphine ligand.

In another embodiment the invention provides a process for the production of an aldehyde from an olefin which process comprises reacting in the liquid phase at elevated temperature the olefin with gaseous carbon monoxide in the presence of a reducing agent and a catalyst comprising (i) either a Zwitterionic rhodium complex or the precursors thereof, and (ii) a bidentate phosphine ligand. In this emobodiment of the invention gaseous hydrogen may also be present in the reaction mixture if desired.

The olefin reactant may be any compound having an olefinic double bond. Thus, the olefin may be a simple alpha-olefin, for example ethylene, propylene, a butylene, a pentene, a hexene, or a heptene, or the like, or a functionalised olefin. Preferred olefins are those functionalised with at least one ester group. Thus, there may be employed alkyl esters of unsaturated acids, for example acrylic acid, methacrylic acid, crotonic acid and allylic acid. There may also be employed cyclic esters, for example lactones. Examples of suitable functionalised olefins for use in the process of the invention include methyl acrylate, ethyl acrylate, sec-butyl acrylate, methyl methacrylate, alpha-methylene-gamma-butyrolactone, ethyl crotonate (both cis- and trans-) and allyl acetate. The regioselectivity of the process depends upon the nature of the reactant olefin. It is believed and indeed it has been our experience that in the case of unsaturated esters as the reactant olefin, those esters having a carbonyl group adjacent to the olefinic double bond, eg ethyl acrylate, methyl methacrylate and cis- or trans- ethyl crotonate, provide the branched aldehydic esters, in good yields and high selectivities. On the other hand,

those esters having a carbonyl group remote from the olefinic double bond, e.g. allyl acetate, provide the linear ester in good yields and high selectivities.

The preferred olefin for use in the process of the invention is allyl acetate, which provides, as the major product,

4-acetoxybutanal (ie the linear product), from which 1,4-butanediol, a desirable commercial product, is readily obtainable.

The olefin is reacted in the liquid phase with either carbon monoxide and hydrogen or carbon monoxide alone as gaseous reactants. Mixtures of carbon monoxide and hydrogen, commonly referred to as synthesis gas, are readily available commercially and may be used in the process of the invention with or without further purification. Alternatively, both carbon monoxide and hydrogen are individually available and may be used as such with or without further purification.

Instead of gaseous hydrogen, or in addition thereto, there may be used a reducing agent. An example of a suitable reducing agent is an alkalimetal .borohydride, eg sodium borohydride. Suitably the molar ratio of olefin reactant to reducing agent employed may be in the range from 1:0.1 to 4, preferably from 1:0.2 to 1.2.

Component (i) of the catalyst comprises either a Zwitterionic rhodium complex or the precursors of a Zwitterionic rhodium complex. Thus, component (i) of the catalyst may be added to the reactants as a pre-formed Zwitterionic rhodium complex or as compounds which if reacted together would form the Zwitterionic rhodium complex. An example of a pre-formed Zwitterionic rhodium complex is the compound of structure (A) hereinbefore referred to. Instead of using the aforesaid pre-formed complex, a mixture of RhCl2- (cyclooctadiene) complex and a source of tetraphenylborate may be employed. Other pre-formed Zwitterionic rhodium complexes which may be used as component (i) of the catalyst are:

+

(Ph ₃ P) ₂ Rh(COD)

(I ) PhBPh ₃

Other useful precursors of a Zwitterionic rhodium complex which may be used as component (i) of the catalyst are the RhCl2- cyclooctadiene complex and the compound of formula (III).

As an alternative to cyclooctadiene in the aforesaid complexes these may be employed, for example, either isoprene, 2, 3-dimethyl-l, 3-butadiene or 1 ,5-hexadiene.

As component (ii) of the catalyst there is used a bidentate phosphine ligand. The bidentate phosphine ligand is suitably a bisphosphine, for example, either 1,4-bis (diphenylphosphino)butane, hereinafter referred to as dppb, 1 , 2-bis(diphenylphosphino)ethane or 1 , 3-bis(diphenylphosphino) propane, of which dppb is preferred.

The liquid phase may be provided by the reactants in the event that the olefin reactant is a liquid under the reaction conditions employed. Preferably, however, a suitable inert solvent is employed. Suitable solvents include, but are by no means limited to, chlorinated paraffins and their mixtures with, for example, monohydric alcohols. Examples of suitable solvents include dichloromethane and a mixture of dichloromethane and isopropanol.

The elevated temperature employed may suitably be in the range from 50 to 200°C, preferably from 70 to 120"C. The pressure may suitably be in the range from 50 to 1000 psi, preferably from 300 to 700 psi.

Certain of the products of the process are believed to be novel. Thus, the aldehydic lactone product formed by reacting alpha-methylene-gamma butyrolactone in dichloromethane with synthesis gas in the presence as catalyst of the Zwitterionic

rhodium complex (A) and dppb is believed to be a novel compound. In another aspect therefore the present invention provides

3-methyltetrahydrofuran-2-one-3-carboxaldehyde.

The invention will now be illustrated by reference to the following Examples and Comparison Tests.

Example 1

A mixture of ethyl acrylate (4.0 mmol), the Zwitterionic rhodium complex of the structure (A) (0.040 mmol), dppb (0.090 mraol) in dichloromethane (10 ml) was and pressurised with a lrl mixture of carbon monoxide and hydrogen to 600 psi and heated in an autoclave to 80°C. After reaction at this temperature for 12 hours, the mixture was passed through a silica gel column and pure products were obtained by thin-layer chromatography using 7:3 hexane-ethyl acetate as the developer. Identification of the products was accomplished by comparison of spectral data with authentic materials and with literature data.

Comparison Test A

Example 1 was repeated except that dppb was omitted.

Example 2 The procedure of Example 1 was repeated except that instead of ethyl acrylate there was used sec-butyl acrylate.

Example 3

The procedure of Example 1 was repeated except that instead of ethyl acrylate there was used methyl methacrylate. Example 4

The procedure of Example 3 was repeated except that instead of

80*C the temperature was 130*C and instead of 12 hours the reaction duration was 24 hours.

Example 5 The procedure of Example 4 was repeated except that instead of methyl methacrylate there was used alpha-methylene-gamma-butyrolactone.

The aldehyde product was characterised by the following data:-

¹ H-NMR(CDC1 ₃ ) delta 1.52 (S,3H,CH ₃ ), 2.17 (m.lH.CH of CH ₂ ), 2.83 (m.lH.CH of CH ₂ ), 4.33 (m,2H,CH ₂ 0), 9.56, (S.IH.CHO).

The product was also characterised by reaction of the lactonic aldehyde with benzylamine to give the corresponding Schiff base [RCH=NCH ₂ Ph]: ¹ H-NMR(CDC1 ₃ ) delta 1.50 (s,3H,CH ₃ ), 2.15 (m.lH.CH of ring CH ₂ ), 2.91 (m.lH.CH of ring CH ₂ ), 4.45 (t,2H,CH ₂ 0), 4.69 (S,2H,PhCH ₂ ), 7.35 (m,5H,Ph), 7.82 (S, lH,CH=N)ppm; ¹³ C-NMR(CDC1 ₃ ) delta 20.87 (CH ₃ ), 32.63 (CH ₂ CH ₂ 0), 48.50 (saturated quaternary C), 64.13, 65.87 (CH ₂ Ph,CH ₂ 0) , 127.08, 127.61, 128.50 (aromatic CH) , 138.59 (aromatic C), 163.25 (CN) , 178.37 (CO)ppm; MS(m/e) 217 [M ⁺ ]. The product was alloted the structure (IV):- Example 6 The procedure of Example 4 was repeated except that instead of methyl methacrylate there was used cis-ethyl crotonate and the reaction duration was 12 hours instead of 24 hours. Example 7

The procedure of Example 4 was repeated except that instead of methyl methacrylate there was used trans-ethyl crotonate and the reaction duration was 12 hours instead of 24 hours.

The results of Examples 1 to 7 and Comparison Test A are given in the following Table. Example 8 Allyl acetate (0.400g; 4.0 mmol), sodium borohydride (0.160g; 4.3 mmol), the Zwitterionic rhodium complex of structure (A) (0.024g; 0.04 mmol) and dppb (0.040g; 0.94 mmol) were dissolved in a mixture of iso-propanol (0.5 ml) and dichloromethane (10ml) in an autoclave. The mixture was pressurised to 500 psi with carbon monoxide and the temperature was increased to lOO'C. The autoclave was maintained at the temperature for 12 hours.

At the end of the reaction water was added to the mixture to consume unreacted sodium borohydride and the phases were separated. The dichloromethane phase was subjected to rotary evaporation to give a crude product, which was passed through a silica gel column.

Pure products were obtained by thin-layer chromatography using 7:3 hexane/ethyl acetate affording aldehydes In 45% yield. 96% of the product was identified as OHC(CH ) ₃ OAc and 4% as CH ₃ CHCH0

CH ₂ 0Ac.

Example 9

A mixture of allyl acetate (0.400gj 4.0 mmol), RhCl . cyclooctadiene (COD) complex (O.OlOg; 0.02 mmol), dppb (0.040g;

0.094 mmol) and the compound of structure (III) was dissolved in dichloromethane (10ml) and placed in an autoclave containing a glass line. The autoclave was pressurised to 700 psi using synthesis gas

(1:1 CO/H mixture) and the mixture was stirred at 80"C for 12 hours. The crude product obtained by rotary evaporation of dichloromethane was chromatographed on silica gel using 7:3 hexane/ethyl acetate, affording 0.356g of aldehydes (67% yield) consisting of OHC(CH ₂ ) ₃ OAc (93%) and CH ₃ CH(CH ₂ OAc)CHO (7%).

Example 10

The procedure of Example 9 was repeated except that instead of the reaction mixture described there was used the following : - allyl acetate (0.400g; 4.0 mmol), the rhodium compound of formula

(I) (0.020g; 0.04 mmol), dppb (0.040gj 0.094 mmol) and dichloromethane (10ml).

There was obtained a 59% yield of aldehydes consisting of

OHC(CH ₂ ) ₃ OAc (95%) and CH ₃ CH(CH ₂ OAc)CHO (5%). Example 11

The procedure of Example 9 was repeated except that instead of the reaction mixture described therein there was used the following:- allyl acetate (0.400g; 4.0 mmol), [RhCl ₂ C0D] (O.OlOg;

0.02 mmol), dppb (0.040g; 0.094 mmol), sodium borohydride (0.035g;

0.101 mmol) and dichloromethane (10ml-) . Any excess sodium borohydride remaining after reaction was destroyed by addition of water.

There was obtained a 70% yield of aldehydes consisting .of 0HC(CH ₂ ) ₃ 0Ac (97%) and CH ₃ CH(CH ₂ 0Ac)CH0 (3%).

Example 12

The procedure of Example 9 was repeated except that instead of the reaction mixture described there was used the following mixture:- allyl acetate (0.400g; 4.0 mmol), the rhodium compound having the formula (II) and dppb (0.030g; 0.04 mmol), sodium borohydride (0.035g; 0.1 mmol) and dichloromethane (10ml).

There was obtained a 67% yield of aldehydes consisting of OHC(CH ₂ ) ₃ OAc (80%) and CH ₃ CH(CH ₂ OAc) (20%). Comparison Test B Example 1 was repeated using the well-known hydroformylation catalyst HRh(CO)(PPh ₃ ) ₃ . There was obtained 34% branched and 13% linear aldehydes, together with 53% ethyl propionate. Example 13

Allyl butyrate, the Zwitterionic complex of structure (A) and dppb were dissolved in dichloromethane in an autoclave. The mixture was pressurised to 500-600 psi with synthesis gas (1:1 C0/H ₂ mixture) and the temperature was increased. Some hours later the autoclave was cooled to room temperature and opened. Rotary evaporation of dichloromethane gave a crude product which was chromatographed on silica gel using 7:3 hexane/ethyl acetate affording a mixture of aldehydes consisting of:-

0 II 0CH(CH ₂ )30CC ₃ H ₇ (63%) and

CH ₃ CHCH ₂ OCC ₃ H ₇ (6%) CHO 0

Example 14

The procedure of Example 13 was repeated except that instead of allyl butyrate there was used the acetate of l-buten-3-ol. There was obtained a mixture of aldehydes consisting of:- OAc

I CH ₃ CHCH ₂ CH ₂ CHO (69%) and

OAc I CH ₃ CHCCH ₃ (2%)

CHO Comparison Test C

Example 14 was repeated except that dppb was omitted. There was obtained a mixture of aldehydes consisting of:-

OAc i CH ₃ CHCH ₂ CH ₂ CHO (65%), and

OAc

CH ₃ CHCHCH ₃ (15%) CHO

10

TABLE

m π

* Enol content is 40% in CDCI3