Hydrodimerization reactions

Conjugated, electron-deficient alkenes can be reductively coupled in the presence of Sml2, providing hydrodimerized products virtually instantaneously in excellent yields [93], Both inter- and intramolecular versions of the reaction have been established, the latter leading to the construction of 3- and 6-membered rings (Eq. 83). Alkynes also take part in the reaction, albeit in modest yields. 

Some success has been achieved in conferring enantioselectivity into the hydrodimerization process [94], but the method as it exists does not appear to be general and furthermore requires huge excesses of expensive reagents (Eq. 84). 

In an interesting adaptation of this chemistry, cyclopropanes serve as pseudo alkenes, resulting in homologous hydrodimerizations [95]. Yields in these cases can be quite high, but the process is rarely stereocontrolled (Eq. 85). 

Several sequential intermolecular hydrodimerization/intramolecular condensation reactions have been reported [30f, 96]. Yields and diastereoselectivities in these processes can be quite high, and the products possess an array of useful functionality (Eqs. 86, 87). 

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This hydrodimerization reaction forms adiponitrile, which is a precursor for nylon. However, the two-electron reduction of this reactant results in propio-nitrile, an undesired product ... 

Methacrylonitrile can be subjected to a similar cathodic hydrodimerization reaction (Asahi 348-350 ) ... 

The reduction of acetylene dicarboxylic acid (or its monoethyl ester) leads to an interesting hydrodimerization reaction (Scheme 6). 

An intramolecular version of the hydrodimerization reaction discussed in the preceding section has proved to be especially attractive and efficient for natural product synthesis. A total synthesis of the sesquiterpene sterpurene exploits just such a reaction, at a very early stage of the synthesis, for the closure of the five-membered ring (Scheme 72) [120]. 

The aromatic sulfonates have been employed mostly for reductions, but they would be expected to be reasonably stable toward oxidation. To obtain a reasonable hydrotropic effect, rather high concentrations are used in the original hydrodimerization reaction of acrylonitrile, the medium may be regarded as either a solution of tetraalkylammonium p-toluenesulfonate in water or the salt containing some water. 

Electrolytic formation of carbon bonds during formation of heterocyclic compounds occurs in the reduction of ketones to pinacols, in the hydrodimerization reaction, in some radical coupling reactions, and in the oxidative coupling of activated aromatic systems. 

Of more interest than the saturation of double bonds is the hydrodimerization reaction (Chapter 21) sodium amalgam hydrodimerizations of a, -unsaturated ketones were reported early [28], and the product distribution resembled that obtained from direct electrolytic reduction (Chap. 10). 

After the development of the electrolytic reductive coupling of acrylonitrile to adiponitrile (Chapter 21), many investigations have been directed toward developing an alternative, indirect electrolytic process. The electrolytic hydrodimerization reaction in neutral solution [32,33] is critically dependent on the proton activity at the electrode. If the proton activity is too high, the product is predominantly propionitrile, whereas oligo-or polymerization occurs at too low a proton activity. The establishment of a reaction layer with a suitable proton activity in which conditions are favorable for a hydrodimerization of acrylonitrile to adiponitrile is, thus, of paramount importance. This can be accomplished in different ways. 

The hydrodimerization reaction to 1-octanol was developed commerdally in 1991 by Kuraray using an aqueous homogeneous catalyst [1, 2], i.e., a hgand-modified palladium catalyst (Figure 1). 

First, we should consider the role of the tetraalkylammonium ion in the hydrodimerization reaction. Certainly the ions are essential to the process in their absence (but with for example a lithium or sodium salt as the electrolyte) the reduction of acrylonitrile leads only to propionitrile and a critical concentration of R4N is necessary to obtain a good yield of adiponitrile. This critical concentration decreases along the series (CH3)4N > (C2H5)4N > (C4H9)4N and with the latter can be as low at 0.01%. It may also be noted that the presence of these ions suppresses hydrogen evolution and there is evidence that they adsorb on the cathode surface. This led to the proposal that the adsorption of the tetraalkylammonium ion produces a layer at the electrode surface which is relatively aprotic. In this layer anionic coupling can occur because protonation is slow compared with that in the bulk solution. 

These reactions are catalytic with respect to hydroxide (the OH is always regenerated). They probably represent the major diflkulty with the process since the hydrodimerization reaction (6.1) itself dqwnds on protonation steps and leads to the foimation of hydroxide ion if a neutral solution is employed. In addition, any water reduction at the rather negative pwtential necessary for the reduction of acrylonitrile (about —1.8 V) will also lead to hydroxide ion. 

Recently great attention has been paid to the electrochemical hydrodimerization of nitriles, by means of which it is possible to obtain at the cathode a large number of bifunctional compounds with high, and in some cases practically quantitative, current yields [7-13]. Such compounds are of great interest in certain branches of polymer chemistry. In general form the electrochemical hydrodimerization reaction can be represented by the following equation ... 

Dir, whereas for small distances d < r), /r Did. The large effective obtainable enables fast heterogeneous reaction rates to be measured under steady-state conditions. Zhou and Bard measured a rate constant of 6 x 10 Ms for the electro-hydrodimerization of acrylonitrile (AN) and observed the short-lived intennediate AN for this process [65]. 

Perhaps the best known example of adsorption effects in electrosynthetic reactions is the beneficial role of tetraalkylammonium ions in the hydrodimerization of acrylonitrile... 

Reactions of this type are called electrochemical hydrodimerization. They are of great value for the synthesis of various bifunctional compounds. A reaction that has found wide commercial nse is the hydrodimerization of acrylonitrile to adiponitrile (the dinitrile of adipic acid) ... 

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

Scheme 4. Possible reaction pathways for the hydrodimerization of acrylonitrile to adiponitrile. The asterisk indicates that electron transfer can be from the cathode or from [CH2CHCN] in homogeneous solution...

At the cathode, olefins with electron deficient double bonds can be hydrodimer-ized (Eq. 1). This reaction has been developed for acrylonitrile [22] in a technical adipodinitrile synthesis [23] with a scale of more than 300.000 tons per year. The scope of this hydrodimerization has been substantiated with many examples [24-33]. 

Formally related reactions are observed when anthracene [210] or arylole-fines [211-213] are reduced in the presence of carboxylic acid derivatives such as anhydrides, esters, amides, or nitriles. Under these conditions, mono- or diacylated compounds are obtained. It is interesting to note that the yield of acylated products largely depends on the counterion of the reduced hydrocarbon species. It is especially high when lithium is used, which is supposed to prevent hydrodimerization of the carboxylic acid by ion-pair formation. In contrast to alkylation, acylation is assumed to prefer an Sn2 mechanism. However, it is not clear if the radical anion or the dianion are the reactive species. The addition of nitriles is usually followed by hydrolysis of the resulting ketimines [211-213]. 

CathodicaUy reduced hydrocarbons not only undergo homogeneous ET and nucleophilic attack but also coupling reactions resulting in hydrodimerization and polymerization. 

Intermolecular coupling Many papers on hydrodimerization of aromatic carbonyl compounds have appeared indicating the importance of this reaction. The rac/meso ratio for the pinacolization of acetophenone in aqueous ethanol ranges between 0.9 and 1.4 in acidic medium and between 2.5 and 3.2 in basic medium. The diastereoselectivity is independent of the cathode material mercury, tin, or copper. Electrolysis conditions such as current density, potential, or current-controlled electrolysis also do not influence the diastereoselectivity. The same holds for propiophenone. For benzaldehyde, the rac/meso ratio is 1.1 to 1.2 in acidic as well as in basic media [283]. In the presence... 

Typical acceptors in Michael additions are reducible and their radical anions often undergo dimerization (hydrodimerizations). Either the radical anions or more likely the dimer dianions can act as EGBs toward the donor in the Michael addition. Since the reaction is catalytic in base when the product anion is more basic than the donor anion, the Michael addition can take place by reduction of a small fraction (2-10%) of the acceptor [129]. The reaction takes place in 20 to 77% yield... 

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 first formed enolate products from hydrodimerization of esters are trapped by reaction with added chlorotrimethylsilane. By starting with allyl esters, this hy-... 

It is commercially advantageous to operate cells with no diaphragm since the cell diaphiagm is the weakest point in the system. Achievement of this aim rests upon finding an anode reaction that destroys neither the substrate nor the product. Russian workers [63] showed that up to 90 % yields of adiponitrile can be obtained at a graphite cathode in an undivided cell with an iron oxide anode, provided that phosphate and tetraalkylammomum ions are present. Further research contributions from Monsanto, BASF and Japanese companies led to the present system for hydrodimerization of acrylonitrile using an undivided cell [64,65]. 

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