Kolbe radicals

Kolbe radicals can be added to olefins that are present in the electrolyte. The primary adduct, a new radical, can further react by coupling with the Kolbe radical to an additive monomer I (Eq. 9, path a), it can dimerize to an additive dimer II (path b), it can be further oxidized to a cation, that reacts with a nucleophile to III (path c), or it can disproportionate (path d). 

To some degree the ratio of additive monomer to additive dimer can be infiuenced by the current density. High current densities favor the formation of additive monomers, low ones these of additive dimers (Table 8, Nos. 4, 5). This result can be rationalized according to Eq. 9 At high current densities, which corresponds to a high radical concentration in front of the electrode, the olefin can trap only part of the Kolbe radicals formed. This leads to a preferred coupling to the Kolbe dimer and a combination of the Kolbe radical with the primary adduct to the additive monomer. At low current densities the majority of the Kolbe radicals are scavenged by the olefin, which leads to a preferential formation of the additive dimer. 

Additions of Kolbe radicals to dienes are reported in refs. [45, 215, 228] and in the reviews named in chap. 1. 

The addition of various Kolbe radicals generated from acetic acid, monochloro-acetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate to acceptors such as ethylene, propylene, fluoroolefins and dimethyl maleate is reported in ref. [213]. Also the influence of reaction conditions (current density, olefin-type, olefin concentration) on the product yield and product ratios is individually discussed therein. The mechanism of the addition to ethylene is deduced from the results of adsorption and rotating ring disc studies. The findings demonstrate that the Kolbe radicals react in the surface layer with adsorbed ethylene [229]. In the oxidation of acetate in the presence of 1-octene at platinum and graphite anodes, products that originate from intermediate radicals and cations are observed [230]. 

Kolbe radicals can also be trapped by oxygen to yield dialkylperoxides, aldehydes, and ketones [97]. Furthermore methyl and trifluoromethyl radicals from acetic acid and trifluoroacetic acid are trapped, although inefficiently, by pyridine (3-20%) [234], benzotrifluoride and benzonitrile[ 235]. 

If carboxylates are subjected to Kolbe electrolysis in the presence of olefins, the generated Kolbe radicals add to the double bonds to afford mainly additive dimers (Table 8, entries 10-17). 

Scheme 2 Intramolecular addition of a Kolbe radical to a double bond.

It is therefore intriguing to understand what is the particular role of the platinum/electrolyte interface in the Kolbe synthesis favoring that reaction path—Eqs. (39a)-(39c)—which is thermodynamically disfavored and unlikely to occur. A closely related reaction whose kinetics are easier to investigate with conventional electrode kinetic methods is the anodically initiated addition of N3 radicals to olefins, discovered by Schafer and Alazrak (275). The consecutive reactions, which follow the initial generation of the reactive intermediate, an Na radical, are somewhat slower than that of the Kolbe radicals, so that their rate influences the shape and potential of the current voltage curves which can be evaluated in terms of reaction rates and rate laws. 

The consecutive reactions of the addition radical (R-CH-CH2R)- follow the same pattern as the reaction of the Kolbe radicals. At Pt anodes they undergo radical-radical reactions forming either dimers (4Id) or bisubstituted monomers (41e),... 

Thus contrary to the naive understanding, Pt does not catalyze but prohibits the thermodynamically favored oxidation, allowing for homogenous dimerization of the Kolbe radicals and similar radicals due to blocking the anode by a layer of adsorption-stabilized species, whereas C anodes allow for physisorption of radicals with subsequent immediate anodic formation of carbenium cations, which constitutes a completely different reaction path. 

The heterocoupling of carboxylic acids bearing chiral auxiliaries has been used to study the diastereoselectivity of the coupling of Kolbe radicals [204b]. For that purpose, 2-substituted malonic acid amides bound to seven different chiral auxiliaries were coelectrolyzed with different coacids [Eq. (200]- The yields in heterodimer ranged between 13 to 69%, and the diastereoselectivities between 20 and 86% de. 

In the addition of Kolbe radicals to 1,3-dienes allyl radicals are intermediates, that can then couple to form 1,1 -, 1,3 - and 3,3 -dimers (Scheme 9). In general, the reactivity of the allyl radical is two to three times higher at C-1 than at C-3, so that the 1,1 -dimer predominates. Besides the examples given in Table 8, further additions of Kolbe radicals to dienes can be found in refs. 33,179 and 191. 

The addition of the 5-pentanoate radical from methyl adipate to butadiene has been intensively investigated, because in this way long chain l,n-diacids are easily accessible a total yield of 96% has been claimed for this reaction (Table 8, entry 7). Different Kolbe radicals from acetic acid, monochloroacetic acid, trichloroacetic acid, oxalic acid, methyl adipate and methyl glutarate have been added to ethylene, propylene, fluoroalkenes and dimethyl maleate. In this detailed study the influence of current density, alkene type and alkene concentration on the product yields and product ratios have been discussed. 

In some cases reactive alkenes can be polymerized with Kolbe radicals as initiators, e.g. styrene, acrylonitrile, vinyl acetate, methyl acrylate, acrylic acid, acrylamide or vinyl chloride. - The addition of Kolbe radicals to pyridine, benzotrifluoride or benzonitrile affords only low yields. 

Table 8 Addition of Kolbe Radicals to Alkenes (continued) ...

Compared to chemical radical cyclizations, the ring closures via Kolbe radicals have the advantage that here two carbon-carbon bonds can be joined in one step, whilst in the majority of chemical alternatives only one carbon-carbon bond is formed. Furthermore electrolysis avoids the toxic tributyltin hydride, that is usually needed as initiator or reagent in chemical radical cyclizations. 

If carboxylates are subjected to Kolbe electrolysis in the presence of olefins, the generated radicals add to the double bonds to afford mainly additive dimers (Table 8, entries 12-20). In vicinal disubstituted styrenes, upon addition of the Kolbe radical Me02CCH2, the yields of adducts decrease with increasing size of the /f-substituent H = 42%, Me = 27%, Et = 11%, /Pr = 5%, tBu = 2% [125]. The ratio of additive dimer 87 (Eq. 11) to monomer 89 can be changed to some extent by the current density i. Upon electrolysis of trifluoroacetate in MeCN-H20-(Pt) in an undivided cell in the presence of electron-deficient olefins, additive dimers and additive monomers are obtained. The selectivity can be controlled by current density, temperature and the substitution pattern of the olefin [126]. Trifluoromethylation of various aromatic compounds with -M substituents has been achieved in satisfactory yield via electrolysis of pyridinium trifluoroacetate in acetonitrile [127]. 

---