Nonaqueous dimerization

As anode material, smooth platinum in the form of a foil or net seems to be most universally applicable [32, 33]. In nonaqueous media, platinized titanium, gold, and nonporous graphite can also be used [56]. PbO -, MnOj- or FejO -anodes do not lead to Kolbe-dimers [57], except for PbO in acetic acid [58]. 

An oxoiron(V) species such as 6 derived from 1 and peroxides is accessible in nonaqueous media (51). The reaction of the tetraphenylphosphonium salt of la with 2 to 5 equivalents of m-chloroperbenzoic acid (mCPBA) at — 60°C in n-butyronitrile produces within about 10 s a bis-iron(IV)- i-oxo dimer followed by an as - yet uncharacterized EPR - silent iron(IV) intermediate. After 15 min, the deep green oxoiron(V) species 6 forms with distinct absorption maxima at 445 nm (e — 5400 Mr1 cm-1) and 630 nm (s — 4200 M-1 cm-1). At —60°C, 6 decays by 10% in 90 min, but it is stable for at least one month at 77 K. Selected spectral data for the oxoiron(V) species are shown in Fig. 13. DFT calculations favor the low-spin (S = 1/2) configuration of the ground state. The calculated Fe-0 bond length of 1.60 A is in excellent agreement with the EXAFS results. The Fe atom is displaced out of the 4-N plane by 0.5 A. 

NAD+, NMN+, NADP+) in nonaqueous media (acetonitrile, DM SO) involve 1 F mol to form neutral free radicals that dimerize at the 6-position (Scheme 153) [270],... 

On one-electron rednction, aldehydes and ketones give anion-radicals. It is the carbonyl group that serves as a reservoir for the unpaired electron Ketones yield pinacols exclusively. Thus, acetophenone forms 2,3-diphenylbutan-2,3-diol as a result of electrolysis at the potential of the first one-electron transfer wave (nonaqueous acetonitrile as a solvent with tetraalkylammonium perchlorate as a supporting electrolyte) (van Tilborg and Smit 1977). In contrast, calculations have shown that the spin densities on the carbonyl group and in the para position of the benzene ring are equal (Mendkovich et al. 1991). This means that one should wait for the formation of three types of dimer products head-to-head, tail-to-tail, and head-to-tail (cf. Section 3.2.1). For the anion-radical of acetophenone, all of the three possible dimers are depicted in Scheme 5.21. 

In addition, lactams can be prepared by the present technique under heterogeneous conditions although most amino acids are barely soluble in nonaqueous solvents (Table II). Interestingly, (S)-(-)-proline selectively gives the cyclic dimer with no measurable loss of enantiomeric purity. 

Planar tetradentate Schiff-base ligands are extremely common for VO(IV) complexes. The prototypal example, VO(salen), has been studied under a variety of conditions. Under nonaqueous acidic conditions the chemistry is complicated by dimerization driven by the release of water (Eq.4) [84]. 

M (CO)6 complexes all undergo irreversible electrochemical reduction in nonaqueous electrolytes at peak potentials close to —2.7 V versus SCE in tetrahydrofu-ran (THF) containing [NBu4][Bp4]. The product of the reductions are the din-uclear dianions [M2(CO)io] although under some conditions polynuclear products can also been obtained, Sch. 3 [2]. It was initially proposed that the primary step involved a single-electron transfer with fast CO loss and subsequent dimerization of the 17-electron radical anion [M(C0)5] [34]. A subsequent study showed that a common intermediate detected on the voltammetric timescale was the 18-electron species [M(CO)5] and that the overall one-electron process observed in preparative electrolysis arises by attack of the dianion on the parent material in the bulk solution, Sch. 2 [35]. 

Molybdenum(V) chemistry is dominated by oxo complexes many of these exist as dimers, but monomeric species can be isolated from strongly addic solutions or under nonaqueous conditions. Non-oxo compounds are also known, both as monomers and as dimers or polymers with halide or sulfur bridges. ESR spectroscopy has been used extensively to investigate the properties of monomeric Mov systems, and has shown the participation of this oxidation state in the reactions of the oxomolybdoenzymes (see Section 36.6.7). 

Extraction of the rare earths with acetylacetone has been investigated [418, 419] and is found to be enhanced by the decreasing basicity of the rare earth ions. The gas chromatographic separation of rare earth complexes with 2,2,6,6-tetramethyl-3,5-heptanedione has already been mentioned. The acetylacetonate complexes of the rare earths are reported to exist as either anhydrous [420, 421], mono- [422], di- [422] or trihy-drates [422, 423], Stites et al. [424] have studied the pH of the precipitation of several rare earth acetylacetonates and reported the melting points of the complexes. The europium acetylacetonate precipitated at pH 6.5, and melted at 144—45° C. The existence of monomers and dimers for these complexes in nonaqueous solvents has been proposed [421, 425-427],... 

Acetic acid, the principal nonaqueous constituent of vinegar, exists as a dimer in the liquid phase, with two acetic acid molecules joined together by two hydrogen bonds. Sketch the structure you would expect this dimer to have. 

Increasing junction potential also increased carbinol yield as was seen with aldehyde reductions. In basic electrolytes or in nonaqueous media, the dimer 50 was the major product. 

Eggins and McNeill compared the solvents of water, dimethylsulfoxide (DMSO), acetonitrile, propylene carbonate, and DMF electrolytes for C02 reduction at glassy carbon, Hg, Pt, Au, and Pb electrodes [78], The main products were CO and oxalate in the organic solvents, while metal electrodes (such as Pt) which absorb C02 showed a higher production for CO. In DMF, containing 0.1 M tetrabutyl ammonium perchlorate and 0.02 M C02 at a Hg electrode, Isse et al. produced oxalate and CO with faradaic efficiencies of 84% and 1.7%, respectively [79], Similarly, Ito et al. examined a survey of metals for C02 reduction in nonaqueous solution, and found that Hg, Tl, and Pb yielded primarily oxalate, while Cu, Zn, In, Sn, and Au gave CO [80, 81]. Kaiser and Heitz examined Hg and steel (Cr/Ni/Mo, 18 10 2%) electrodes to produce oxalate with 61% faradaic efficiency at 6 mA cm-2 [82]. For this, they examined the reduction of C02 at electrodes where C02 and reduction products do not readily adsorb. The production of oxalate was therefore explained by a high concentration of C02 radical anions, COi, close to the surface. Dimerization resulted in oxalate production rather than CO formation. 

In an aprotic medium, such as DM SO, a C2a-hydroxybenzylthiazolium salt is formed by nucleophilic addition55. There have appeared suggestions that in nonaqueous media, the double enamine or syn-anti symmetrical dimer formed between two thiazolium salts on addition of base is the template for benzoin condensations (Scheme 4)56. The... 

Cyclic immonium salts such as 205 are reduced in a nonaqueous solvent such as acetonitrile through a radical to a dimer (206).207 The reaction is analogous to pinacol formation from carbonyl compounds. 

As mentioned in Section 1.5.2, the /r-oxo dimers PFeOFeP are readily cleaved by dry HCl in nonaqueous solvents (equation 22) ... 

In nonaqueous solvents, such treatment with dry hydrohahc acids is the only way to cleave the /r-oxo dimer nonoxidatively. However, the /x-oxo dimers of water-soluble porphyrins are readily cleaved by Tewis bases such as hydroxide, imidazole, histidine, and pyridine. Both the equilibria and kinetics of such reactions have been reported. In addition, /x-oxo dimers of water-insoluble Fe porphyrins in dichloromethane can be oxidatively cleaved to yield PFe p2 and (probably) an Fe species. Studies of the picosecond decay of the excited state of (TPPFe)20 in benzene following a 532- or 355-nm 25-ps pulse suggest that the intermediate state is a photodissociated pair, (TPP -)Fe -l-TPPFe — (0 ), and a small amount of disproportionation reaction products, TPPFe -f TPPFe = O. ... 

Quantitative information about amide hydrogen bonding is unfortunately very meager in nonaqueous liquids which are protein solvents. In later sections, however, much stress will be placed on the one result (Table II) that amide hydrogen bonds in dioxane solution are considerably stronger than in water the difference in AF° for the dimerization of iV-methylacetam-... 

Under conditions that are not strictly nonaqueous, the oxidized dimer may be trapped by water, as was observed during the oxidation of 1,1-diphenylethylene catalyzed by the radical cation of dibenzo-1,4-dioxin [91]. The dimer dication upon reaction with water undergoes a 1,2-phenyl shift, resulting finally in 1,2,4,4-tetraphenyl-3-buten-l-one [Eq. (42)], reminiscent of the 1,2-shifts observed during anodic oxidation of 1-phenyl- and 1,4-diphenylnaphthalene in acidic dime thy Iformamide (DMF) [92]. 

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