Anodic Cleavage

C-C or C-X bonds (X = I, NR2, SR, OR) can be cleaved anodically by a) oxidation of the substrate to a radical cation that either dissociates (Eq. (215a) ) or is hydrolytically decomposed in a subsequent CgECjq-process (Eq. (215b)) and 

Alkyl iodides are cleaved in acetonitrile/lithium perchlorate to iodine and al-kyl.cations. These react prior to or after rearrangement in a Ritter reaction with acetonitrile to form N-alkylacetamides in 21% to 75%yield (Eq. (217) ) 4S8). 

In the analogous cleavage of l,l-diphenyl-2-iodoethylene a vinyl cation is postulated as an intermediate whose follow-up reactions yield benzophenone and benzoin 45 9K In contrast, aryl iodides are not decomposed anodically but form p-iododiaryliodonium salts via electrophilic substitution of the intermediate aryl iodide radical cation at iodobenzene (Eq. (218) ) 45S). 

2°- and 3°-amides are cleaved in aquous acetonitrile to protonated deal-kylated amides and aldehydes via immonium cations, which hydrolyze to product (Eq. (219), path a). In dry acetonitrile, however, the intermediate radical cation 158 abstracts hydrogen from acetonitrile to form the protonated amide and the O-cyanomethyl radical, the dimerization of which yields 83 - 95% succino-nitrile (path b), Eq. (219)) 460). 

The cleavage of primary amines in aprotic solvents, such as acetonitrile or DMF, was studied by cpe, controlled potential coulometry, and cyclic voltammetry 463 The results indicate that at low anode potential the primarily formed radical cation RCH2NH2 dissociates to an alkyl cation RCH2+ and an amino radical NH2 , which subsequently is oxidized to nitrogen. At higher anode potentials path a)ofEq. (219), leading to an aldehyde and an ammonium ion, preponderates. 

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The cleavage of 1,2-diols can be inexpensively achieved at a nickel hydroxide electrode [126], while chemically more expensive reagents such as PbjOAc) or 104 must be used. The latter can be used as mediator for the indirect anodic cleavage of starch [146]. Double bonds can be cleaved at the anode to carboxylic acids by applying the double mediator RuCh, 104 [147]. 

Anodic oxidation of alkyl substituted cyclopropanes and spiroalkanes in methanol/TEATos (tetraethyl ammonium tosylate) affords monomethoxy and dimethoxy products in yields ranging from 6 to 86% [30, 31]. The products result from the cleavage of the most highly substituted C,C bond. In contrast to the anodic cleavage the acid-catalyzed cleavage occurs selectively at the less substituted carbon. The cleavage of hetero-substituted cyclopropanes is reported in Ref [32-35]. 

Scheme 6 Indirect anodic cleavage ofthioethers with trisarylamines.

Oxidative ring cleavage of cyclohexanones is achieved in ethanol at a platinum anode. Cleavage of the carbon-carbonyl bond occurs and the carbonyl centee is trapped as the carboxylic acid. The radical centre formed after bond cleavage is oxidised to the carbocation. This rearranges to the most stable centre and is then trapped by the carboxyl group to form a lactone [4, 5], An identical process is... 

By indirect anodic cleavage of carbon-sulfur bonds it is also possible to initiate rearrangements (Eqs. (105)—(106)) (Table 11, No. 6). 

The indirect anodic cleavage of carbon-hydrogen bonds in the benzyl position using triarylamine mediators was also used for mild and selective deblocking of hydroxy, carboxyl, and amino groups. The primarily formed cation radical of the protective group is readily deprotonated in the benzyl position by an added base (Eq. (107)). This benzylic radical is easily further oxidized to the benzyl cation which subsequently is cleaved by attack of a nucleophile, such as water (Eq. (108)). 

Typical examples for type 1 are the anodic cleavages of two carbon-sulfur bonds in 1,3-dithianes [46] or dithiolanes [47]. This reaction is especially effective if performed under the conditions of indirect electrolysis using triarylamine cation radicals as regenerable oxidative mediators [47] ... 

The anodic cleavage of disulfides was used on the laboratory scale for the synthesis of other vulcanization enhancers 330 331), for the production of phenyl sulfinates 332), and for the synthesis of intermediates for penicillins and cephalosporins 333). 

The direct anodic cleavage of saturated aliphatic carbon-carbon bonds is only possible if an electron is removed from the highest occupied molecular orbital (HOMO) of die C— bond, e.g. due to die presence of strain in the bond. ... 

The initiation step of this anodic oxidation of glycols may be the electron transfer from the lone pah-electrons of the oxygen atom to the anode. This anodic cleavage of 1,2-glycols has beat utilized for a variety of organic syntheses. ... 

Torii and coworkers developed a convenient method for the introduction of an alkoxycarbonyl group at the a-positions of cycloalkanones. It involves a novel anodic cleavage at one of the external C-C bonds of the cyclopropane in the presence of Fe ions, leading to the formation of cyclic j -keto esters (43) in 39-79% yield (Table 14). The reaction temperature has a profound effect on the yield of 43 since at the higher temperature range the formation of 44 was favored. 

Anodic cleavage, where the cathodic reversal would be a C,C-bond-forming reaction, such as hydrodimerization. 

A novel O-deprotection protocol involving anodic cleavage of aromatic ether (LXXXV) was effectively used in the total synthesis of the natural products alkannin and shikonin [Eq. (42)]. The electrolysis was conducted at a carbon anode in MeCN/ H2O with LiC104 as electrolyte, giving an 80% yield at 50% conversion [97]. Although both naphthoquinone tautomers were initially formed after trapping of the radical cation by water and loss of CH2O, the tautomer with alkyl substitution at the quinone double bond was more thermodynamically stable. 

During the anodic cleavage of a-organothio ethers, performed in the presence of carbon nucleophiles, an inter- or intramolecular carbon-carbon bond formation was observed [120]. The electrolyses were run in CH2CI2-BU4NCIO4 or BU4NBF4 using a C rod anode. Two examples are summarized in Eqs. (65) and (66). 

Indirect anodic cleavage of 1,3-dithianes can be done using tris(/ -tolyl)-amine as electron transfer agent in acetonitrile-LiC104 containing NaHC03 the yields of recovered carbonyl compound were almost quantitative [139,140]... 

Cleavage reactions of radical ions may be conveniently applied in deprotection processes like the cathodic scission of sulfonamides (deprotection of amines) and the anodic cleavage of gemdithioacetals (deprotection of carbonyl groups). 

The French group also reported the anodic cleavage of REER or RER (where E = chalcogenide) in MeCN, in contrast to the previous electroreductive systems, and a number of products were prepared where RE+ had been trapped by appropriate nucleophiles [156]. 

Another anodic cleavage reaction which has been used is bis-decar-... 

Scheme 2. Functional group interconversions at the anode (left to right) and at the cathode (right to left). (2a) Anodic substitution and cathodic cleavage. (2b) Anodic dehydrogenation (X is almost always a heteroatom) and cathodic hydrogenation [X is a heteroatom or a carbon atom (CR2)]. (2c) Anodic addition and cathodic elimination (2d) Anodic cleavage...

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