Hydroesterification of Olefins

The intramolecular hydroesterification of olefins provides a route to lactones. Alihough this reaction has not been applied extensively in the synthesis of complex molecules, several examples demonstrating the scope and potential utility of this reaction have been reported, As shown in Equations 17.42-17.44 this reaction can be used to prepare optically active lactones,benzo-fused lactones, and lactams. The reaction in Equation 17.43 illustrates how the product can result from a combination of isomerization and carbonyla-tion, and the reaction in Equation 17.44 shows how the ring size can be controlled by the composition of the catalyst. 

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Very recently, a new strategy for the hydroesterification and hydroamidation of olefins was reported by Chang and coworkers [83]. They used a chelation-assisted protocol for the hydroesterification of olefins. The reaction of 2-pyridylmethyl formate with 1-hexene in the presence of a Ru3(CO)12 catalyst gave the hydroesterification product in 98% yield as a mixture of linear and branched isomers (Eq. 54). The chain length of the methylene tether is important for a successful reaction. Thus, the reaction of 2-pyridyl formate (n=0) afforded 2-hydroxypyridine, a decarbonylation product, and the reaction of 2-pyridiylethyl formate (n=2) resulted in a low conversion (7% conversion) of the starting formate. From these results, the formation of a six-membered ruthenacycle intermediate is crucial for this chelation-assisted hydroesterification. 

Hydrocarboxylation and hydroesterification of olefins by Rh and Ir catalysts are considered here together since the catalysis schemes are similar and the two metals re treated as one unit in many patents. 

Copper(I) carbonyl catalyzes the hydroesterification of olefins and alcohols under very mild conditions (25 °C, 1 bar) in strong acids [12-15]. 

Hydro carbonylation of olefins, hydroformylation, hydroesterification and hy-droxycarbonylation are reactions which appear to be of particular interest. Indeed, they allow the simultaneous creation of a new C - C bond as well as the introduction of a functional group (aldehyde, ester and acids). One or two new stereogenic centres can thus be formed at the same time (Scheme 26). Despite the difficulty of using high carbon monoxide pressure, the aheady existing industrial processes prove that such reactions can be performed on a very large scale [107]. 

Many other addition reactions of olefins, dienes, and acetylenes are known, which are catalyzed by metal carbonyls including Ni(CO)4, Fe(CO)5, and Co2(CO)8 and by carbonyl derivatives such as hydrocarbonyls or phosphine-substituted carbonyls. Among these are the hydro-carboxylation, hydroesterification, and hydrocyanation of olefins the synthesis of hydroquiniones from acetylenes, carbon monoxide, and water ... 

As mentioned earlier, palladium, rhodium, and platinum catalysts lead to superior regioselectivities because they work under milder reaction conditions (20-80 °C, 0.1-1 MPa CO) [11], e.g., bimetallic catalysts based on tin(II) chloride and either platinum or palladium complexes afford linear esters in up to 98 % selectivity [12]. In addition, catalyst systems with preference for branched isomers are known. A recent example employed palladium acetate immobilized on montmorillonite in the presence of triphenylphosphine and an acid promoter for the hydroesterification of aryl olefins (eq. (3)). The reaction is totally regiospecific for the branched isomer of aromatic olefins, while aliphatic olefins afford branched chain esters only regioselectively with n/i = 1 3 [13]. 

The regioselectivity of hydroesterification of alkyl acrylates or aromatic olefins catalyzed by [PdCl2L2] complexes (L = phosphine ligand) could be largely controlled by variation of the ligands. Triphenylphosphine promotes preferential carboxylation to the branched isomer, whereas with bidentate bisphosphines the linear product is produced overwhelmingly [14]. 

Carbonylation of olefins in the presence of alcohols to give esters is called hydroesterification. Similarly, olefin carbonylation in the presence of carboxylic acids yields acid anhydrides. Both hydroesterification and acid anhydride formation by olefin carbonylation are covered in section 14.6.4. Other carbonylation variations, including the use of acetylenic substrates, thiols and amines as hydrogen sources and the carbonylation of allylic halides are not discussed. Several excellent reviews of hydrocarboxyiation and carbonylation of olefinshave appeared. 

Hydroesterification of butadiene with CH3OH employing a Co catalyst can proceed in a stepwise fashion, initially to methyl 3-penteneoate and subsequently, under more rigorous conditions, to dimethyl adipate. Thus hydroesterification of an olefin stream containing 44% butadiene (remainder mostly butenes) with CH3OH, employing Co2(CO)g catalyst and an isoquinoline promoter, gave 98% methyl pent-3-enoate ° after... 

A pyridal formate group is used for hydroesterification of an olefin (Equation (42)). Use of a benzyl ester in place of the pyridyl group gave 0% yield. Pyridyl formamides were also found to give hydroamidation products with high linear/branched ratios. "" ... 

The insertion of alkynes into metal-hydride bonds occurs during a number of catalytic processes, including alkyne hydrogenation, hydrosilylation, silylformylation, hydroesterification and dimerization. This insertion chemistry is more complex mechanistically than the insertions of olefins into metal hydrides. In some cases, ds addition products have... 

The copolymerization of carbon monoxide and olefins forms the polyketone in Equation 17.65, and this polymerization is closely related to the hydroesterification and hydrocarboxylation of olefins. The rate of reaction of the acyl intermediate that was generated in the hydroesterification process with olefin or alcohol differentiates the formation of copolymer from the formation of monomeric esters. This difference in relative rates for reaction of the acyl intermediate with olefin versus alcohol results from a change in the ancillary ligand on the palladium, as described in this section. 

Up to now, palladium complexes do not play a significant role in the hydroformylation of olefins [1]. However, because of their widespread use in the related hydrocarboxylation, hydroesterification, and olefin copolymerization with CO [2], occasionally their utility for hydroformylation was elucidated [3]. Moreover, palladium catalysts have been used for the hydroformylation of aryl and enol triflates to produce the corresponding unsaturated aldehydes [4]. 

In the absence of H2, the Rh(I)-catalyzed reaction of olefins with CO and alcohols may lead to esters or ethers [32]. The hydroesterification tendency is enhanced by amine ligands [3]. Strong basic amines, such as 2,6-lutidine, give higher yields of esters than basic amines, such as 2-picoline [3b]. 

Subsequently, palladium-based complexes were used for car-bonylation of aryl halides,hydroesterification and hydroamidation of olefins," hydroamidation of alkynes, Heck reaction, and oxidation and hydrogenation of olefins. i° ... 

The cycle is started with the formation of a Pd-alkoxy complex that reacts with CO to an alkoxycarbonyl intermediate. In the next step, the approach of the olefin and insertion into the carbonyl palladium bond is predicted. In the last step, the starting complex is rebuilt by the addition of an alcohol and the cleavage of the hydroesterification product [59]. 

Support for the involvement of HPdCl(PPh3)2 as the active species in palladium-catalyzed hydroesterifications comes from the isolation of frans-Pd-(COPr)Cl(PPh3)2 from propene hydroformylation [5], while Pd(CO)(PPh3)3 is inactive as a catalyst in the absence of HCI [6]. In the case of PdX2L2/SnX2 catalyst systems olefins seem to be the hydrogen source for the formation of the active Pd-H species [7],... 

Strictly related to catalytic reactions involving CO and H20 are reactions in which CO and alcohols, ROH, or CO and amines, R2NH, are used as building blocks. The catalytic addition of carbon monoxide and an alcohol to an olefin yields carboxylic esters (hydroesterification). Thus, the synthesis of methyl propionate from ethylene, CO, and methanol using a catalytic system composed of Ru3(CO)u and [PPh4]I (190°C, 20 bar C2H4, 45 bar CO, 2.5 hr, yield 74%, CT 1000) has been reported (323) ... 

Hydrogen donors for hydroesterification may be primary or secondary alcohols, cyclohexanol, phenol or polyols. Linear or branched primary alcohols react similarly secondary alcohols are less active and tertiary alcohols are not suitable. The reactivity of various olefins has been compared. ... 

The use of CO containing 3% H2 in the hydroesterification reaction is standard, suggesting that a cobalt hydrocarbonyl is the active catalyst species. The reaction sequence involves olefin insertion into the Co—H bond, (carbonyl insertion) to give an acyl complex and cleavage with alcohol assisted by the pyridine promoter ... 

As with Co, Rh and Ir catalyze the olefin carbonylation reactions of hydrocarboxylation, hydroesterification and acid anhydride formation. Rhodium or Ir complexes and iodide promoters with HjO as the hydrogen source yields a mixture of linear and branched carbocylic acids the branched isomer predominates. Many soluble complexes, such as Irlj, Ir2(CO)4Br2, Rh(PPh3)2(CO)Cl or Ir[(C4H9)3P](CO)I can be utilized as a solution in a carboxylic acid solvent. The iodide source can be HI or any material which... 

Hydrocarboxyiation and hydroesterification reactions are catalyzed effectively by a variety of Pd and Pt complexes. Palladium and Pt are considered together in patents, since the conditions for reaction are similar and many principles of ligand modification apply to both. Considerable progress in the application of solvent, ligand and promoter effects, have led to milder reaction conditions. Functional olefins have been successfully carbonylated and process regioselectivity has been controlled. 

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