Activated hydrodimerization

Scheme 5.2-5 Formation of the active Pd-catalyst from [BMIM]2 PCICI4 for the hydrodimerization of 1,3-butadiene.

A wide variety of activated olefins (126) undergo reductive electrochemical dimerization to compounds of structure 127 (electrolytic hydrodimerization) 129 i. While the product 127 is capable of existing in either dl or meso modifications, relatively little attention has been paid to the stereochemistry of hydrodimers... 

Intramolecular hydrodimerization of activated olefins has been exploited for elegant one-step cyclizations and heterocouplings (Fig. 53) [263, 278, 279]. 

The linear telomerization reaction of dienes was one of the very first processes catalyzed by water soluble phosphine complexes in aqueous media [7,8]. The reaction itself is the dimerization of a diene coupled with a simultaneous nucleophilic addition of HX (water, alcohols, amines, carboxylic acids, active methylene compounds, etc.) (Scheme 7.3). It is catalyzed by nickel- and palladium complexes of which palladium catalysts are substantially more active. In organic solutions [Pd(OAc)2] + PPhs gives the simplest catalyst combination and Ni/IPPTS and Pd/TPPTS were suggested for mnning the telomerizations in aqueous/organic biphasic systems [7]. An aqueous solvent would seem a straightforward choice for telomerization of dienes with water (the so-called hydrodimerization). In fact, the possibility of separation of the products and the catalyst without a need for distillation is a more important reason in this case, too. 

Interestingly, various phosphonium salts have been applied [13] as constituents of palladium catalysts for hydrodimerization of butadiene and isoprene about the same time when the results of Kuraray were disclosed. These were obtained by quatemization of aminoalkylphosphines with methyl iodide or HQ (Ph2P-R-NH2 type compounds are known to yield phosphonium salts with these reagents). Although the catalysts prepared in situ from [PdCU] were reasonably active (TOF-s of 10-20 h ) the reactions always yielded complex product mixtures with insufficient selectivity towards the desired 1,7-octadienyl derivatives. 

The formation of dimers by reduction of a,p-unsaturated ketones in aqueous media is well documented in the early literature of electrochemistry. Reductants include sodium or aluminium amalgams [58], dissolving zinc and a lead cathode in both acid and alkaline conditions [59,60]. Mixtures of dimers and dihydro derivatives were isolated. As the concept of the hydrodimerization of activated alkenes... 

In order to improve the activity in the absence of co-solvent, the use of a surfactant was studied in the presence of TPPTS-based catalyst [55]. Monflier et al. reported the hydrodimerization of 1 in the presence of surfactants in order to improve butadiene mass transfer in pure water solution [56-58]. Such an additive used in very low amount avoided the presence of an organic co-solvent. It was shown in the case of hydrodimerization that neutral or cationic surfactants played a significant role in the process. Similar behaviors were reported for the telomerization of 1 with 21. While 30% conversion of 1 was achieved in pure water after 24 h reaction time at 50°C using 0.4 mol% of catalyst, the conversion reached 87% when polyether surfactant (POEA) was added to the reaction medium under similar reaction conditions (Table 12). It was found that the conversion is strongly affected by the nature of the surfactant (Table 13). 

Finally, a third means of ligand formation from an imidazolium cation, described by Dupont and co-workers, should be mentioned here [34]. They investigated the hydrodimerization/telomerization of 1,3-butadiene with palladium(II) compounds in [BMIM][BF4] and described the activation of the catalyst precursor complex [BMIM]2[PdCl4] by a palladium(lV) compound formed by oxidative addition of the imidazolium nitrogen atom and the alkyl group with cleavage of the C-N bond of the [BMIM] ion, resulting in bis(methyHmidazole) dichloropalladate (Scheme 5.2-5). However, this reaction was only observed in the presence of water. 

Activated methylene components like malonic esters and P-ketoesters can be coupled anodically using small amounts of potassium iodide as redox catalyst (Table 4, No. 4-7) i45-i5n -pj g cathodically formed metallic potassium is used to deprotonate the methylene component generating the oxidizable carbanion. The combination of this reaction with the cathodic hydrodimerization of acrylic esters has been studied several times (Table 4, No. 7) Thus both electrode reac-... 

Cathodic hydrodimerization of active olefins and carbonyl compounds. 

In addition to the cathodic hydrodimerization of activated olefins [see 3.2.1.1], the electrosyntheses of substituted benzaldehydes are among the few electroorganic reactions which are carried out on a large scale industrially. 

In addition to the cathodic hydrodimerization of activated olefins and the Kolbe reaction, the anodic dimerization of CH-acidic compounds is another possibility for the electrochemical C—C coupling. Monsanto 281 > has used the anodic dimerization of malonates in a laboratory synthesis of intermediates for useful sequestrants and detergency builders. 

Anodic dimerization of electron-rich olefins, the mirror image process to cathodic hydrodimerization of activated olefins (Sect. 12.2), affords a one-step synthesis for substituted butanes(Eq. (175) ) 268)( dienes (Eq. (176) ) 26S precursors of polyenes (Eq. (177)) 36,385 and 1,4-dicarbonyl compounds (Eq. (178)) 35>36). 

Recently, a large variety of ion-exchanged zeolites of type X and Y were examined (163) for propylene oligomerization activity. These included LaY, LaX, CeX, MgX, NiY, CoY, A1Y, MgX, MnY, NiX, CoX, and CaX zeolites which were tested in a fixed bed reactor at 190°C. With the exception of NiX, all the zeolites tested showed rather unselective hydrodimerization activity leading to a wide variety of paraffinic products. The appearance of saturated C2, C4, C5, and C7 products was indicative of cracking reactions was well as hydrogen transfer. 

Electrolytic formation of carbon-carbon bonds occurs in the reduction of ketones to pinacols, in the Kolbe synthesis, and in the hydrodimerization of activated double bonds. Of these only the last reaction has been used in the preparation of heterocyclic compounds. 

Nonactivated esters are reduced with difficulty unless strongly acidic solutions are used. Unsaturated esters, such as a, y6-unsaturated and aromatic esters, are reduced relatively easily because of the activating effect of the strongly electron-withdrawing ester group for reduction of the attached unsaturated function. In these cases, the products of reduction may include those of hydrodimerization and the hydrogenation of the unsaturated function (e.g., aromatic ring) as well as those of reduction of the ester function. The possibilities are summarized in Eqs. (8)-(12), and subsequent discussion focuses on reduction of the ester function. 

Hydrodimerization of activated alkenes is a well-established process. 7r-Electron-deficient heteroaromatic compounds activate a double bond similarly to a cyano or car-bethoxy group, and in accordance with that analogy vinylpyridines can be hydrodimer-ized. 4-Vinylpyridine [380] forms l,4-bis-(4-pyridyl)butane in 82% yield on electrolysis in a mildly alkaline solution containing methyltriethylammonium / -toluenesulfonate and some DMF. The mechanism is discussed in Chapter 21. 

This section concerns the classical hydrodimerization of alkenes activated by electron-withdrawing substituents, as in Eq. (1). The literature in this area is extensive and this chapter cannot be exhaustive. The focus will be on typical reactions and general conclusions, which may serve as guidelines for further work. Special emphasis will be put on the effect of reaction conditions on the mechanisms, product selectivity, and stereochemistry. Section II.A deals with the monoactivated alkenes, that is, structures of the type 1 where R and R" are H, alkyl, or aryl Sec. II.B deals with intramolecular coupling reactions where two identically activated alkenes are linked together within the same molecule. The reactions of alkenes activated by two electron-withdrawing groups either in a, a- or in a,yS-positions, are treated in Sec. II.C. 

Baizer and coworkers established the most brilliant industrial electroorganic synthesis of the hydrodimerization of acrylonitrile to adiponitrile. They extended this hydrodimerization to a variety of activated olefins and in some cases [41 3] paid attention to the stereochemistry of products. However, their stereochemical data were not enough to discuss the stereochemical course of the reaction. Afterward, an attempt was made to provide a working hypothesis in the hydrodimerization of cinnamates by considering an orientated adsorption of radical anion intermediates on a cathode surface, but this was not persuasive because of a lack of experimental data on the stereochemistry of both the starting olefins and products. Recently Utley and coworkers [44-46] have reported stereochemical data of hydrodimers derived from a variety of cinnamic acid esters with chiral alcohol components. 

The hydrodimerization of non-activated olefins at a platinum electrode affords the corresponding d, /-isomeric products in excess ( > 80%) [57]. 

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