Electrochemical reduction substituents

However, the hydride reduction of FeCp(arene)+ salts [124, 125] gives [FeCp(r 5-cyclohexadienyl)] complexes [125, 126] (via an ET mechanism [127] for the directing effect of substituents see Refs. [126, 128-130]. The electrochemical reduction of the carboxylic substituents at an Hg cathode in water leads to the primary alcohol [131-133] Eq. (39) ... 

The electrochemical reduction of 4//-thiopyrans bearing four electron-withdrawing substituents leads to 5,6-dihydro-2//-thiopyrans. Four diastereoisomers are produced, their relative proportions depending on the electrolytic conditions. Their conformations have been established using the vinylic proton as an nmr probe and confirmed in some instances by X-ray analysis <96JCS(P2)2623>. 

Electrochemical reduction of aryl sulphones in methanol leads to cleavage of one carbon-sulphur bond in a two-electron process. Alkyl aryl sulphones with an electron donating substituent in the aryl ring give the arenesulphinic acid and an alkane [68, 69]. Methyl phenyl sulphone and alkyl aryl sulphones with an electron withdrawing substituent in the aryl ring are cleaved to the substituted arene and the alkylsulphinic acid [70], see Scheme 5.2. In addition, a bulky orfAo-substituent such as tert.-butyl favours cleavage to the arene and alkanesulphinic acid [71]. 

Benzene mono- di- and tri-sulphonic acids are stable towards electrochemical reduction. More highly sulphonated compounds undergo a two-electron cleavage of a carbon-sulphur bond with elimination of sulphurous acid [82]. Thus benzene-hexa-sulphonic acid 14, in aqueous solution at pH 8, shows three two-electron po-larographic waves with half-wave potentials -1.00, -1.45 and 1.65 V vs. see, due to the sequential loss of substituents. The half-wave potentials are independent of pH. 

Cleavage of a Mo L bond. The electrochemical reduction of the acetonitrile complex ds-19 + is an overall two-electron process leading to the cleavage of the Mo—NCMe bond. The CV of cis-[Mo2(cp )2(At-SR)2(CO)3(MeCN)]2+ appeared to be strongly dependent upon the nature of the sulfur substituents and of the... 

Boujlel et al. <2003SC1675> have prepared oxazolooxazoles such as 90 by the rather more unusual method of electrochemical reduction of 91 which then undergoes dimerisation giving good yields of the symmetrical heterocycles. Various substituents have been included. Their proposed mechanism involves capture of an electron to give radical anion 92 followed by dimerisation and cyclization as outlined in Scheme 9. 

Electron-attracting substituents facilitate electrochemical reduction. The reduction potentials for the polarographic reduction of quinoline and isoquinoline derivatives are much less negative than those for the pyridine analogues. Diazines are reduced electrochemically stepwise, usually as far as tetrahydro derivatives (70AHC(12)262). 

Electron spin resonance (ESR) spectroscopy reveals that an increase in the electron-donating ability of substituents in the 2-position of the furan ring produces an increase in the spin density at the 3-position and a decrease at the 4-position the coupling constants in the ESR spectra of anion radicals generated by electrochemical reduction of 2-substituted 5-nitrofurans equate to o values.295... 

Electrochemical reductions of Nin(HCor) and CuM(HCor) are both ligand-centred processes, and the first step accompanies hydrogen elimination (Scheme 81).247 The JV-alkyl analogues are reduced at 30-50 mV less negative potentials without loss of the substituent. 

More recently, Rudolph et al. was able to reduce C02 to oxalate with faradaic efficiencies approaching 100% with their most active and stable complex [102]. These authors examined a variety of macrocyclic nickel chelate complexes with various substituent groups on the ring in acetonitrile solution. Whilst it is interesting that the group was able to produce oxalate catalyzed by a metal complex, the potentials required for reduction were —1.9 to —2.2 V (versus SCE), similar to the potential required for the direct reduction of C02 in aprotic solvent (—2.21 V versus SCE). The very negative potentials in this reaction highlight the overall theme of the electrochemical reduction of C02. 

The influence of carbon group substituents ER3 (E = C to Sn R = H, Me, f-Bu) on the energy barrier of bond shift and electrochemical reduction of substituted cyclooc-tatetraenes (COT-ER3) has been studied with experimental and theoretical methods by Staley et al.164. The ring inversion transition state (Figure 45) was taken as a model for the steric interactions in the bond shift transition state which could not be calculated... 

Perylenediimides represent another class of photoactive dyes which are characterized by their strong fluorescence emission and facile electrochemical reduction. Recently, a supramolecular bis(phthalocyanine)-perylenediimide hetero-triad (compound 42) has been assembled through axial coordination [47]. Treatment of perylenediimide 43, which has two 4-pyridyl substituents at the imido positions, with 2.5 equiv. of ruthenium(II) phthalocyanine 44 in chloroform affords 42 in 68% yield (Scheme 3). This array shows remarkable stability in solution due to the robustness of the ruthenium-pyridyl bond. Its electronic absorption spectrum is essentially the sum of the spectra of its molecular components 43 and 44 in... 

Electrochemical reduction of the salts (4) provides radicals (18) which dimerize or undergo further reduction to anions (32) or dianions (80MI43100). The reduction potentials are not much affected by substituents. Reduction with zinc in aprotic conditions gives bi(l,2-dithiol-3-yls) (59), and 3-chloro-l,2-dithiolylium salts (35a X = Cl) are converted into bi(l,2-dithiol-3-ylidenes) (20) (75TL3473). Divalent chromium converts the 3,5-dimethyl-l,2-dithiolylium cation into a dithioacetylacetonate ligand (72AJC2547). The reaction of 3,5-diamino-l,2-dithiolylium salts (8) or alkyl derivatives with thiols provides dithiomalonamides (60) by electron transfer (63ACS163). 

However, based on UV-vis data " and other crystallographic studies, the description of the reduced species as porphyriu tt-anion radicals cannot be excluded iu all cases. Electrochemical reduction of (Br)4TPPFeCl, (CN)3TPPFeCl, and (CN)4TPPFeCl (the Br and CN substituents are on the -pyrrole positions in... 

---