Electrolytic hydrodimerization

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... 

Monsanto (1) A process for making adiponitrile, an intermediate in the manufacture of Nylon 66, by the electrolytic hydrodimerization (EHD) of acrylonitrile ... 

Many salts are soluble in DMSO, so the choice of supporting electrolyte is less restricted than in most other nonaqueous solvents. In general, perchlorates, even KCIO4, nitrates, and halides, are soluble, whereas fluorides, cyanides, sulfates, and carbonates are not thus not only NaC104, LiCl, NaNO, and tetraalkylammonium salts can be used but also such salts as NH4PF6 and NH4SCN. The ability of DMSO to solvate ions is also of importance in the indirect electrolytic hydrodimerization of, for example, acrylonitrile using Na(Hg) [388]. 

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 unavoidable presence of sodium ions in the double layer during a Na(Hg) reduction introduces differences compared to the electrolytic hydrodimerization with respect to hydration and ion pairing. 

Scko, WL, Devdopxnent of electrolytic hydrodimerization of acryloiiitrik" Chem. Econ. and Engng Rev, 7 (10,89) 20-24 (1975). 

The second alternative route to adiponitrile involves the electrolytic hydrodimerization of acrylonitrile ... 

Reduction of stilbene [18] or dipheny-lacetylene [214] in DME yields 1,2,3,4-tetraphenylbutane, whereas phenanthrene [214] provides 9,9, 10,10 -tetra-hydro-9.9 -biphenanthrene. Hydrodimerization was also observed with benzalfluo-rene [225]. If DME is replaced by acetonitrile, protonation completely dominates hydrodimerization [18]. In carefully dried ethers, using alkali or alkaline earth metals salts as supporting electrolyte, 1,1-diphenylethylene can be reduced ca-thodicaUy to give stable solutions of 1,1,4,4-tetraphenylbutane dianions [226]. These dianions can be cleaved by flash... 

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. 

Lactones such as coumarin and its derivatives (14) undergo hydrodimerization even in acidic hydroxylic media. When the supporting electrolyte contains Li, coumarin (14a) gives the 4,4 -hydrodimer in 90% yield in a methanol/acetate buffer (pH 5) [79]. The mechanism of the reduction of 4-methylcoumarin (14b) in DMF, MeOH, and Me0H/H20 has been examined [73]. In all cases the mechanism of dimerization was found to be of the RR type, again with a considerable increase in the observed second-order rate constant in going from aprotic to protic solvents (Table 3), [73]. 

This method was applied to the asymmetric reduction of ketones [440 44] and imines [439,445]. Maximum asymmetric yields reported for the former and the latter are 20% [441] and 8.95% [445], respectively. A higher asymmetric yield (20.6%) was obtained in the hydrodimerization of a ketone [444]. It is a problem that lower asymmetric yields are obtained using much larger amounts of asymmetry inducers, which must play the role of supporting electrolyte. 

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). 

A very impressive illustration of the relative costs of the electrolysis step compared to workup and purification procedures is given in Ref. 14. The electrochemical hydrodimerization of acrylonitrile to adiponitrile is reported here. The electrolytic part makes up less than 20% of the whole process sheet, and the costs for the electrolytic part are less than 30%. The process is later discussed in more detail. 

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. 

Careful selection of the supporting electrolyte is a key point in the hydrodimerization process. Monsanto has chosen tetraethylammonium p-toluenesulfonate for their solution process. In concentrations of from 20% to 40%, it increases the solubility of acrylonitrile in the anolyte and retards the formation of byproduct. Asahi Chemical uses less hydrophobic supporting electrolyte, in concentrations of about 10%, for their emulsion process. Thus, adiponitrile formed in the emulsion process is separated from the reaction mixture by physically breaking down the emulsion and then separating the oil layer. The loss of supporting electrolyte and the electric power consumption are lower in the emulsion process than in the solution process. 

The history of the Monsanto adiponitrile process demonstrates well the development of a synthetic process[1,23-25]. The first electrolytic process for the hydrodimerization of acrylonitrile was introduced on a 14,000 tons/year scale in 1965. The late 1970 s saw a large expansion of this process both in the UK and USA and it now produces some 200,000 tons adiponitrile/year. At least the recent cell houses are based on quite different technology to that used in the first plant and this has allowed large reductions in both capital investment and energy consumption. 

These are the roles of additives for corrosion inhibition and the modification of electrodeposits. For electrode reactions where the overall sequence includes chemical steps, however, the role of the adsorbate layer may be quite different. Rather, it may be to create an environment which ts more favourable than the bulk solution for a particular reaction, e.g. the proton availability may be different it is not unusual for an adsorbate layer to be relatively aprotic compared with an aqueous electrolyte and such modifications of electrode processes have been used in the electrosynthesis of adiponitrilc (Chapter 6). The presence of tetraalkylammonium ions in the electrolyte leads to the desired hydrodimerization of acrylonitrile to adiponitrile. In their absence, only propionitrilc is formed. Tt is thought that the tetraalkylammonium ions adsorb on the cathode surface and create an environment where an intermediate is protected from protonation. 

By the mid 1970s it was clear that the hydrodimerization of acrylonitrile could be run very effectively with only a low concentration of tetraalkylammonium ion and that a saturated solution of acrylonitrile in an aqueous buffer was an appropriate medium. In such circumstances, it seemed likely that the electrolysis could be run in an undivided cell without the additional complication of an anode depolarizer. Such a system was first reported by Phillips Petroleum. They ran a pilot plant with an undivided cell consisting of a lead cathode and a steel anode a very simple electrolyte, 6% acrylonitrile and 0.03% tetrabutylammon-ium salt in aqueous dipotassium hydrogen phosphate, was employed and the anode reaction was oxygen evolution. The yield of adiponitrile remained above 90% and the chief by-products were propionitrile and trimer. No base-initiated chemistry was observed due to the use of an effective buffer. 

The reductive hydrodimerization of activated olefins was considered as a good test system for initial SECM studies, due to the fact that the dimerization rate constant could be tuned via the substituent activating the olefinic double bond." " For the relatively slow dimerization of the dimethyl fumarate (DF) radical anion, a large UME (25 jam diameter Pt electrode) was employed so that the value of K2 (Equation 7.19) was in the range where the SECM response would be sensitive to the kinetics. Experiments were carried out with solutions of 5.15 and 11.5mM DF in dimethylformamide (DMF), containing 0.1 M tetrabutylammoniumtetrafluoroborate as a supporting electrolyte. The oxidation of -tetramethyl-l,4-phenylenediamine (TMPD), included in solutions at a con-... 

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