Reactions of alkenes

The following contribution will describe the basics of aqueous two-phase hydro-formylation as they apply to C3 and C4 alkenes. The focus will be on TPPTS (cf. Section 3.2.1) as a ligand and rhodium as the active metal center, e.g., the complex HRh(CO)(TPPTS)3 [1], Emphasis will be put on the commercial applications and the basic description of the processes. 

The mechanism of the oxo reaction has been extensively studied in the past. A comparative study of both commercially applied oxo catalysts HRh(CO)(TPP)3 (TPP = triphenylphosphine) and HRh(CO)(TPPTS)3 was performed by Horvath [2]. The latter, water-soluble catalyst is considered to react according to the dissociative mechanism. However, remarkable differences exist in the catalytic activity and the selectivity of the organic- and water-soluble catalysts. The latter shows much lower specific activity but an increased selectivity to linear products in the hydroformylation of propene. From an Arrhenius plot it is concluded than the dissociation energy of TPPTS from HRh(CO)(TPPTS)3 is about 30 1 kcal/mol 

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Limited data are available for the kinetics of the oxo synthesis with HRh(CO)(TPPTS)3. The hydroformylation of 1-octene was studied in a two-phase system in the presence of ethanol as a co-solvent to enhance the solubility of the olefin in the aqueous phase [4]. A rate expression was developed which was nearly identical to that of the homogeneous system, the exception being a slight correction for low hydrogen partial pressures (Eq. 1). 

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Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

Oxidation of ethylene in alcohol with PdCl2 in the presence of a base gives an acetal and vinyl ether[106,107], The reaction of alkenes with alcohols mediated by PdCl2 affords acetals 64 as major products and vinyl ethers 65 as minor products. No deuterium incorporation was observed in the acetal formed from ethylene and MeOD, indicating that hydride shift takes place and the acetal is not formed by the addition of methanol to methyl vinyl etherjlOS], The reaction can be carried out catalytically using CuClj under oxygen[28]. 

Although it is not a reaction of alkenes, oxidation of some alkanes with Pd(ll) is cited here. 1-Adamantyl Irilluoroacetate (155) was obtained in above 50% yield by the reaction of adamantane with Pd(OAc)2 in trifluoroa-cetic acid at 80 C[171]. 

Alkenes coordinated by Pd(II) are attacked by carbon nucleophiles, and carbon-carbon bond formation takes place. The reaction of alkenes with carbon nucleophiles via 7r-allylpalladium complexes is treated in Section 3.1. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

The first report of oxidative carbonylation is the reaction of alkenes with CO in benzene in the presence of PdCh to afford the /3-chloroacyl chloride 224[12,206]. The oxidative carbonylation of alkene in alcohol gives the q, f3-unsaturated ester 225 and /3-alkoxy ester 226 by monocarbonylation, and succinate 111 by dicarbonylation depending on the reaction conditions[207-209]. The scope of the reaction has been studied[210]. Succinate formation takes... 

Unsaturated nitriles are formed by the reaction of ethylene or propylene with Pd(CN)2[252]. The synthesis of unsaturated nitriles by a gas-phase reaction of alkenes. HCN, and oxygen was carried out by use of a Pd catalyst supported on active carbon. Acrylonitrile is formed from ethylene. Methacrylonitrile and crotononitrile are obtained from propylene[253]. Vinyl chloride is obtained in a high yield from ethylene and PdCl2 using highly polar solvents such as DMF. The reaction can be made catalytic by the use of chloranil[254]. 

Palladation products formed from arylmercurials, carboalkoxymercurials, and alkylmercurials, which have no /3-hydrogen, are used in situ for the reaction of alkenes[367]. Particularly, the arylation of alkenes is synthetically useful. Styrene derivatives 402 and 403 are formed by the reaction of a... 

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

Under CO pressure in alcohol, the reaction of alkenes and CCI4 proceeds to give branched esters. No carbonylation of CCI4 itself to give triichloroacetate under similar conditions is observed. The ester formation is e.xplained by a free radical mechanism. The carbonylation of l-octene and CCI4 in ethanol affords ethyl 2-(2,2,2-trichloroethyl)decanoate (924) as a main product and the simple addition product 925(774]. ... 

Oxidative reactions of alkenes with Pd(II) are treated in Chapter 3,. Section 2, catalytic reactions of alkenes with organic halides are discussed in this chapter. Section 1.1, and other catalytic reactions of alkenes are discussed in this section. 

Although the yields are not high, PdCU or Pd black catalyzes oxo reactions of alkenes with CO and H2 to give aldehydes[32]. 

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