Electrochemical cleavage

Electrochemical cleavage reactions are not easily accommodated within a general scheme. In many cases they might well be classified as substitution processes however, the preparative outcome normally would seem to make the reaction conform more closely with a cleavage process. 

Akiyama s group developed an anodic oxidative decarboxylation of oxabi-cyclo [2.2.1] substrates that subsequently undergo skeletal rearrangement to yield 1,2,3-trisubstituted cyclopentanols [146, 147]. An example of this reaction which generates the carbocyclic framework of hydrindanes is shown in Eq. 99. 

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While electrochemical cleavage of several other kinds of single bonds has been reported, stereochemical information is sparse. Erickson and Fischer examined the electrochemical reduction of O-benzoylatrolactic acid (76, R = H) and its methyl ester (76, R = CH3) 69>. The products (77, R = H and CH3) of the... 

Horner and Singer examined the electrochemical cleavage of alkyl p-tol-uenesulfonates ( 79) 70). The reduction can be carried out under very mild... 

Hoemer and Neumann found that electrochemical cleavage of the sulfur-nitrogen bond of alkyl p-toluenesulfonamides proceeds in high yield, under mild conditions, and with little or no racemization 71 ) ... 

Scheme 5.2. Demonstration of retention of configuration during electrochemical cleavage of the carbon-phosphorus bond, Ref. [49. ...

In the following example, the electrochemical cleavage of the Mo—Cl bond of the seven-coordinate complex [MoCl(dtc)2(dppe)]+ (dtc = 5 2CNR2 R = Me or Ft dppe = Ph2PCH2CH2PPh2)... 

Electrochemical cleavage of metal-ligand bond(s) is a convenient way of generating vacant coordination sites at a metal center (see Sch. 5). The eliminated ligand can be neutral (L) or anionic (X). 

The indirect electrochemical cleavage of halides has been studied in detail by Lund and Simonet et al. -272) course of the reaction is strongly influenced by the structure of the substrates. Thus, aryl and benzyl halides do not form alkylation products of the mediators. The products Of the perturbed redox catalysis are, however, favored in the case of aliphatic halides. Primary halides give predominantly monoalkylation products, while tertiary halides favor the formation of dialkylation products. 

The indirect electrochemical cleavage of sulfones demonstrates the increased selectivity of this method as compared with direct electrolysis (Eq. (98))... 

The electrochemical cleavage of C-halogen bonds allows to remove one halogen atom from polyhalogenated compounds selectively, to introduce C=C double bonds, and to produce organometallic compounds. Some examples from the patent literature are summarized below ... 

The electrosynthetic method has played a principal role in obtaining coordination compounds with N,0,S- and N,0,Se-ligand environment 810 [202,512,513], The reaction of electrochemical cleavage of the S — S and Se — Se bonds, discovered by Tuck and coworkers [554a,603,604], was used for this goal. In particular, the complexes 811 (X — S [605,606] and Se [607]) were obtained electrochemically (3.254) on the basis of the formally tetradentate ligand 810 ... 

The composition of the complexes above depends on the metal nature and its oxidation state. Thus, divalent copper, zinc, and cadmium form complexes of the type 811a (L = py, bipy, 1,10-phen m= 1, 2) [605], tetravalent tin forms chelates 811b (X — S [606], Se [607]). The electrochemical cleavage of the S — S bond is observed not only in ligands with aromatic bridge 810, but also in those with aliphatic one, for example (CH2) in 812 [608] ... 

Zollinger D, Griesbach U, Putter H, Comninellis C (2004) Electrochemical cleavage of 1, 2-diphenylethanes at boron-doped diamond electrodes. Electrochem Commun 6 605-608... 

Alternatively, an electrochemical procedure was employed in the synthesis of amino acids from nitridomolybdenum complex 4 via nitrogen-carbon and carbon-carbon bond formations involving imide 5 and nitrogen ylides 6. trans-[MoCl(N)(Ph2PCH2CH2PPh2)] 4 reacted with methyl iodoacetate to give the cationic complex 5. Complex 6 was obtained by the deprotonation of 5 at the a-carbon followed by treatment with Mel to afford the cationic methyl derivative 7. Electrochemical cleavage of the Mo-N bond of the complex 5 or 7 proceeded in the presence of acetic acid to release amino acid esters in 70-80% yield (Scheme 2) [12]. 

The synthesis of chiral phosphines from resolved phosphonium salts or phosphine oxides is an intrinsically limited approach. The groups attached to phosphorus must be present prior to resolution and, furthermore, the preparation of phosphine oxides and phosphines from phosphonium salts by chemical or electrochemical cleavage reactions requires that one of the groups bonded to phosphorus be substantially easier to cleave than the other three. 

Mackor, A., Koster, T.P.M., Spee, C.I.M.A. and De Kreuk, C.W. 1987a. Stabilisation of electrodes in the photo-electrochemical cleavage of water. Condensed annual report 1986 (15 pp in Dutch). Available upon request. Mackor, A., Koster, T.P.M., Spee, C.I.M.A. and De Kreuk, C.W. 1987b. Stabilisation of Si photoanodes by SiOxNy coating for their use in photo-electrochemical water cleavage. Final report, in preparation. Available in 1988 upon request. 

Some of the previously discussed processes, especially cathodic substitution reactions according to eqn (14) with E+ = H+, might also be considered as cleavage reactions. In fact, it is difficult to accommodate electrochemical cleavage reactions within a unique formal scheme (for a review within a rather broad definition, see Horner and Lund, 1973), and we are therefore content merely to show a few examples which conform reasonably well to the commonly accepted notion of cleavage. 

The mechanism of the electrochemical cleavage of the C-X (X = F, Cl, Br and I) bond has been extensively studied (see for example, Miller and Riekena 1969 Pause et al. 2000 Costentin et al. 2003 Wang et al. 2004 Sanecki and Skital 2007) and theoretical models (Battistuzzi et al. 1993 Kuznetsov et al. 2004 Zhang et al. 2005 Golinske and Voss 2005), mostly based on the Marcus theory (Marcus 1964) for the homogeneous and heterogeneous (electrochemical) electron transfer (ET), have been formulated (Saveant 1987, 2000 Maran et al. 2001 Costentin et al. 2006a, b) and tested with the aim to predict the experimental outcomes. 

A synthesis of substituted bicyclic azo compounds utilizes the electrochemical cleavage (Chap. 23) of a bis-(2,2,2-trichloroethyl)dicarbamate in DMF followed by ferri-cyanide oxidation. In this way acid-, base-, and heat-sensitive azo compounds may be prepared [82] ... 

The electrochemical cleavage of nitriles can also be performed in anhydrous amine media [146], making possible C-CN bond cleavage in aliphatic nitriles, such as cyclohep-tanecarbonitrile and heptanecarbonitrile. This reaction was interpreted as due to reductive cleavage by solvated electrons. This reaction has also been accomplished with electrogenerated solvated electrons [147]. 

Folic acid [LXVII R = NHC6H4CONHCH(COOH)(CH2)2COOH] is in the first step reduced as LXVII (R = H) in acidic solution 7,8-dihydrofolic acid is reduced with cleavage of the C-9/N-10 bond to LXIX (R = H) [365]. Besides the electrochemical cleavage, a nonelectrochemical cleavage with loss of the 4-aminobenzoylglutamate side chain from the 5,8-dihydrofolic acid has been observed [368]. In neutral medium with low buffer capacity, it is possible to reduce 7,8-dihydrofolic acid to the 5,6,7,8-tetrahydrofolic acid [369]. 

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