Alkenes cathodic reduction

Radical anions resulting from cathodic reductions of molecules react with electrophilic centers. As an example (Scheme 8), the reduction of compounds in which a double bond is not conjugated with a carbonyl group, involves an intramolecular coupling reaction of radical anion with alkene [12]. 

A powerful method to convert non-conjugated keto olefins into cyclic alcohols [46,47] utilizes the cathodic reduction of a ketone to afford a ketyl, which subsequently undergoes cyclization onto the pendant alkene. As illustrated in Eqs. (27H30), the process provides access to mono-, bi-, and heterocyclic systems from very simple starting materials. It is especially well-suited to the construction of five-membered rings, less so for six-, and is not effective in producing seven-membered rings. 

Intramolecular cyclization of bis(activate alkenes) by reduction at a mercury cathode in aqueous acetonitrile containing tetraethylamraonium toluene-4-sulphonate. Data from ref [120]. 

The electrochemical formation of ylides described in reactions 1 and 2 has been utilized for producing alkenes by reaction with aldehydes through reaction 3. The potential of the cathode is controlled at or near the cathodic reduction peak potential of the probase. The yields of the alkenes are given in Table 3. 

Indeed only alkynes could be reduced in (CH3)4N+ solutions while alkenes were inactive. Reduction of 1-hexyne, propargyl alcohol and 1,4-butyne diol were performed 13 at a mercury cathode with (CH3)4NC1 as the electrolyte. The corresponding olefins were formed and the respective yields were 45 %, 62 % and 82 %. The diacetate of 15 behaved similarly. However only the trans isomer 16 was formed from 15 while a mixture of trans and cis (6 4) isomers resulted from the reduction of the diacetate. Polarography of several alkynes in methanol with (C4H9)4N+ electrolytes showed, 3) that they react close to background decomposition. It was therefore proposed 14) that (CH3)4N+-mercury may be involved in the cathodic reduction of alkynes when (CH3)4N+ salts serve as the electrolytes. 

From the limited data available, it seems that terminal alkynes can be efficiently reduced to the corresponding alkenes at mercury cathodes in (C4H9)4N+ electrolyte solutions. The cathodic reduction can be carried out in an organic-aqueous medium in which base related complications, associated with other electron-transfer reductions, can be avoided. Efficient reduction of alkenes has not proven possible. In competition, both benzenoid and alkyne functionalities are reduced. Selectivity can be improved by controlling the water content of the medium so that a terminal alkyne can be converted to an alkene in the presence of a benzenoid aromatic functionality. 

Looking back at the data, we find A/fr = 9 less favourable for addition of H+ and probably < 20 kcal/mol more favourable for addition of H to QH as compared with C2H4. However, equilibrium figures are deceptive. We have seen that significant substituent and solvation effects can reduce the energy gap. In respect to electrophilic rates, this occurs in A (CsQ>A (C=C), although this order is admittedly unusual. As for nucleophilic attacks, cathodic reductions may occasionally turn out to be exceptional otherwise, the order, k C=C)>k C=C), seems to be followed. A revised statement of alkyne-alkene reactivity now reads nucleophiles react faster with alkynes radicals react faster with alkenes polar electrophiles usually react faster with alkenes. 

Cathodic reduction mechanisms follow several patterns, depending on the alkyne, the medium, the supporting electrolyte, the properties of the cathode and the applied potential - On a spongy nickel electrode in 95% ethanol and sulphuric acid, a number of alkyl- and arylalkynes are reduced in good yields in a process which resembles, but is not identical with, catalytic hydrogenation—alkenes are not reduced under these conditions . In no sense does the presumed mechanism (equation 72) appear to involve nucleophilic attacks. 

The reductive elimination of the cyclic sulfate of 1,2-butanediol led to tmns-2-butene selectively. Aromatic vicinal dioxalates underwent fragmentation and elimination on cathodic reduction to give alkenes. Mexo-(Et02COCH(QH5))2 gave 80% trans-stilbene [365]. 

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

The first reported electroorganic synthesis of a sizeable amount of material at a modified electrode, in 1982, was the reduction of 1,2-dihaloalkanes at p-nitrostyrene coated platinum electrodes to give alkenes. The preparation of stilbene was conducted on a 20 pmol scale with reported turnover numbers approaching 1 x 10. The idea of mediated electrochemistry has more frequently been pursued for inorganic electrode reactions, notably the reduction of oxygen which is of eminent importance for fuel cell cathodes Almost 20 contributions on oxygen reduction at modified... 

Given the large number of tandem cyclization processes that have been explored [63], it is disappointing to note that so few have been promoted electrochemi-cally. There appears to be a significant opportunity for additional exploration. Two tyiws of tandem cathodic cyclizations are discussed below. The first involves generation of a ketyl, and its subsequent cyclization onto a pendant alkene to afford a new radical that closes onto a second alkene [64,65]. The second focuses on chemistry not yet discussed involving the reductive cyclization of enol phosphates of 1,3-dicarbonyl compounds [66]. 

The formation of dimers by reduction of a,p-unsaturated ketones in aqueous media is well documented in the early literature of electrochemistry. Reductants include sodium or aluminium amalgams [58], dissolving zinc and a lead cathode in both acid and alkaline conditions [59,60]. Mixtures of dimers and dihydro derivatives were isolated. As the concept of the hydrodimerization of activated alkenes... 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

Reductive elimination from 1,2-dibromides generates the alkene in excellent yields. Conformationally rigid, periplanar tro 5-diaxial, also staggered trans-diequatorial, cyclohexane dibromides all afford the alkene at a mercury cathode [110]. In the bicyclo[2,2,2]octane series, the rra s-2,3-dibromide forms the alkene on dissolving metal reduction [111]. The rigid cis-periplanar 1,2 dibromobicy-clo[2,2,l]heptane, at a mercury cathode, also gives the strained alkene which can be trapped as a furan adduct [112]. 

Alkyl alkanoates are reduced only at very negative potentials so that preparative scale experiments at mercury or lead cathodes are not successful. Phenyl alkanoates afford 30-36% yields of the alkan-l-ol under acid conditions [148]. Preparative scale reduction of methyl alkanoates is best achieved at a magnesium cathode in tetrahydrofuran containing tm-butanol as proton donor. The reaction is carried out in an undivided cell with a sacrificial magnesium anode and affords the alkan-l-ol in good yields [151]. In the absence of a proton donor and in the presence of chlorotrimethylsilane, acyloin derivatives 30 arc formed in a process related to the acyloin condensation of esters using sodium in xylene [152], Radical-anions formed initially can be trapped by intramolecular addition to an alkene function in substrates such as 31 to give aiicyclic products [151]. 

It is easier to oxidize an alkene electrochemically than to reduce it, because it is easier to reach powerfully oxidizing anode potentials12 (at which one can remove an electron from the HOMO of the alkene) without interference from the solvent or electrolyte than it is with reductions, where one can normally not achieve sufficiently negative cathode potentials to be able to add an electron to the LUMO. Even so, anodic oxidation of alkenes is relatively rarely observed almost always the alkene is part of a conjugated system or it bears an electron-supplying substituent, which raises the HOMO energy. 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

Ether solutions based on TAA salts are not reduced on noble metal electrodes. The major cathodic reaction of these solutions involves the cation reduction to trialkyl amine, alkane, and alkene (which are the stable disproportion products of the alkyl radical formed by the electron transfer to the cation) [3], Electrolysis of ethers such as THF or DME containing TBAP, formed in the catholyte tributyl amine, butane and butene, were unambiguously identified by NMR and GCMS analysis [3], In the presence of water (several hundred ppm and more), the electrolysis products were found to be tributyl amine and butene (butane was not detected) [3], The potential of this reduction reaction is higher than that of the dry solution, and it is clear that the initial electroactive species in this case is the... 

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