Electron transfer dimerization reactions

Second-order irreversible chemical reaction following a reversible electron transfer dimerization. It is quite common in chemical reactions that newly formed radicals couple to each other. This also often happens in the electrochemical generation of radicals according to a dimerization process that can be written as ... 

Both stereoisomers were formed, implying a loss of stereochemical integrity during the formation of the second carbon-carbon bond. When the reaction was conducted on ZnO, surface-related processes affected both the rate and stereochemistry. The effect of various quenchers could be explained as competitive adsorption at active sites, with or without interference with electron transfer. A reaction scheme involving formation of dimer, both in the adsorbed state and in solution, was proposed, the former route being the more important On CdS, the reaction could sometimes be induced in the dark as well because of the presence of acceptor-iike surface states. Neither particle size, surface area, nor crystal structure appeared to significantly influence the dimerization observations parallel to those found in the CdS photoinduced dimerization of N-vinylcarbazole... 

The electron transfer induced reaction of this diene system results in rapid [4 + 2]dimerization conversely, the dimer rapidly undergoes cycloreversion upon electron transfer. Both reactions result in strong CIDNP effects. The monomer polarization supports a radical cation with a spin density distribution like those of the butadiene or fulvene radical cations. The dimer polarization identifies a dimer radical cation with appreciable spin density only on two carbons of the dienophile fragment this species can only be the doubly linked radical cation D [135, 136], Significantly, a second dimer radical cation is implicated in a pulsed... 

Figure 12.1.1 Schematic representation of possible reaction paths following reduction and oxidation of species RX. a) Reduction paths leading to (1) a stable reduced species, such as a radical anion (2) uptake of a second electron (EE) (3) rearrangement (EC) (4) dimerization (EC2) (5) reaction with an electrophile, E , to produce a radical followed by an additional electron transfer and further reaction (ECEC) (6) loss of X followed by dimerization (ECC2) (7) loss of X followed by a second electron transfer and protonation (ECEC) (8) reaction with an oxidized species. Ox, in solution (EC ), b) Oxidation paths leading to (1) a stable oxidized species, such as a radical cation (2) loss of a second electron (EE) (3) rearrangement (EC) (4) dimerization (EC2) (5) reaction with a nucleophile, Nu , followed by an additional electron transfer and further reaction (ECEC) (6) loss of X followed by dimerization (ECC2) (7) loss of X" followed by a second electron transfer and reaction with OH (ECEC) (8) reaction with a reduced species. Red, in solution (EC ). Note that charges shown on products, reactants, and intermediates are arbitrary. For example, the initial species could be RX, the attacking electrophile could be uncharged, etc.

Studies of ionic reactions were extended to processes other than propagation of polymerization. A variety of protonation processes, electron transfers, dimerizations, cis-trans isomerizations, etc., were investigated with emphasis on the role of different types of ionic species on the course of these reactions. The results of these studies were reviewed in two volumes entitled "Ions and Ion-Pairs in Organic Reactions" edited by Professor Szwarc, who contributed four chapters to this extensive compilation. These books were published by Wiley in 1970 (volume I) and 1972 (volume II). 

In this review, we will first describe the photochemical dimerization of ACN in solution, summarizing studies that have been carried out over the past 30 years, focusing on the mechanistic aspects and the reactivity of the S, and the T, states and of radical ions generated by photochemical electron transfer. Thereafter, reactions in the solid state and constrained media, including the crystalline state, silica gel surfaces, micelles, Uquid crystals, zeolite, and other heterogeneous media will be described. 

The acetylide anion 3 is likely to form an alkynyl-copper complex by reaction with the cupric salt. By electron transfer the copper-II ion is reduced, while the acetylenic ligands dimerize to yield the -acetylene 2 ... 

At high copper(II) chloride indole ratios the pyrrole ring of 2-methylindole was chlorinated in yields approaching 92%. This reaction is believed to involve radical cations of indoles formed in an electron-transfer process. At low copper(ll) chloride indole ratios dimers were formed [86JCS(P 1)2305]. 

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Kattenberg and coworkers54 studied the chlorination of a-lithiated sulfones with hexachloroethane. These compounds may react as nucleophiles in a nucleophilic substitution on halogen (path a, Scheme 5) or in an electron transfer reaction (path b, Scheme 5) leading to the radical anions. The absence of proof for radical intermediates (in particular, no sulfone dimers detected) is interpreted by these authors in favour of a SN substitution on X. 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

These reactions are postulated to proceed by electro-transfer to give the radical cation of alkoxynaphtalene, which either undergoes reaction with copper(II) bromide or dimerizes (ref. 15). That is, one-electron transfer from the electron-rich alkoxynaphtalene to Cu(II) results in generation of the corresponding radical cation. The radical cation reacts with bromide anion leading to the brominated compound, whereas the radical cation undergoes reaction with another alkoxynaphtalene leading to the binaphtyl (eqns. 2-4). 

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

These present an interesting dichotomy in their reductions by tm(l,10-phen-anthroline)iron(ri) (ferroin) °. That of CIO2 to CIOJ is rapid, is first-order in each component ki = 1.86 0.13 l.mole sec at 35 °C) and is independent of acidity. Ferriin is the immediate product and an outer sphere electron-transfer is proposed. The reduction of CIO2 is much slower, proceeding at the same rate as dissociation of ferroin at high chlorite concentrations and a major product is feriin dimer, possibly [(phen)2Fe-0-Fe(phen)2] . Clearly the reaction depends on ligand-displacement followed by an inner-sphere electron transfer. 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

Whatever the reason may be behind the strict necessity to deprotonate the flavin donor, the reduced and deprotonated flavin was established in these model studies to be an efficient electron donor, able to reduce nucle-obases and oxetanes. In the model compounds 1 and 2 the pyrimidine dimer translates the electron transfer step into a rapidly detectable chemical cycloreversion reaction [47, 48], Incorporation of a flavin and of a cyclobutane pyrimidine dimer into DNA double strands was consequently performed in order to analyse the reductive electron transfer properties of DNA. 

The cycloreversion experiments showed a clean Tf=T-DNA to T/T-DNA transformation. No by-products were detected, which supports the idea that DNA may be more stable towards reduction compared to oxidation. Even heating the irradiated DNA with piperidine furnished no other DNA strands other then the repaired strands, showing that base labile sites - indicative for DNA damage - are not formed in the reductive regime. The quantum yield of the intra-DNA repair reaction was therefore calculated based on the assumption that the irradiation of the flavin-Tf=T-DNA strands induces a clean intramolecular excess electron transfer driven cycloreversion. The quantum yield was found to be around 0=0.005, which is high for a photoreaction in DNA. A first insight into how DNA is able to mediate the excess electron transfer was gained with the double strands 11 and 12 in which an additional A T base pair compared to 7 and 8 separates the dimer and the flavin unit. 

Flavin-cyclobutane pyrimidine dimer and flavin-oxetane model compounds like 1-3 showed for the first time that a reduced and deprotonated flavin is a strong photo-reductant even outside a protein environment, able to transfer an extra electron to cyclobutane pyrimidine dimers and oxetanes. There then spontaneously perform either a [2n+2n cycloreversion or a retro-Paternd-Buchi reaction. In this sense, the model compounds mimic the electron transfer driven DNA repair process of CPD- and (6-4)-photolyases. 

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