Electron transfer with olefins

Cation-radicals are also known to undergo electron transfer with olefins The intermediate carbon centered cation-radical often dimerizes and polymerization may be initiated by the resulting dication. 

Photo-NOCAS reactions of p-dicyanobenzene with 2-methylpropene in acetonitrile afforded novel 3,4-dihydroisoquinoline derivatives, as shown in Scheme 132 [482], This photoreaction is initiated by a single electron transfer from olefin to p-dicyanobenzene. Acetonitrile as a nucleophile combined with the alkene radical cation and the resulting radical cation adds to the radical anion of 1,4-di-cyanobenzene. Cyclization to the ortho position of phenyl group followed by loss... 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

Upon oxidation, the subsequent radical cation can decompose in a number of different ways to generate reactive intermediates (Scheme 10.1). The first possibility involves direct H-atom abstraction of the a-C-H bond of the oxidized amine (I) to generate an iminium ion (II), which is susceptible to nucleophilic attack via polar reaction mechanisms (pathway a). Deprotonation of I may also form a carbon-centered radical species (III) that can react with typical radical traps, such as olefins or arenes (pathway b). Generation of the iminium ion may also occur indirectly through oxidation of III via SET to the photocatalyst or another oxidant (pathway c). Finally, radical cation I can undergo non-productive pathways such as back-electron transfer with the reduced photocatalyst (PC" ) to re-generate the neutral amine and PC" (pathway d). 

The photo-NOCAS reaction was first described by McMahon and Arnold and is a photonucleophilic Sfj2Ar aromatic substitution between dicyanobenzene and an olefin in the presence of electron donor photosensitizers (phenanthrene or biphenyl) in acetonitrile-methanol solutions. This reaction system has been researched extensively in recent times. As shown in Scheme 6, the single electron transfer from olefin to photo-excited electron-deficient dicyanobenzene forms the cation radical of the olefin, which initiates a quenching reaction with nucleophile solvent methanol molecules and forms the methoxyalkyl radical. Addition of an electron transfer photosensitizer (phenanthrene or biphenyl) to the reaction mixture increases the efficiency of the reaction simply by absorbing more Hght. The excited state of the photosensitizer donates an electron to dicyanobenzene to give the photosensitizer radical cation and dicyanobenzene radical anion. The photosensitizer radical cation then oxidizes the olefin. 

Apparently the alkoxy radical, R O , abstracts a hydrogen from the substrate, H, and the resulting radical, R" , is oxidized by Cu " (one-electron transfer) to form a carbonium ion that reacts with the carboxylate ion, RCO - The overall process is a chain reaction in which copper ion cycles between + 1 and +2 oxidation states. Suitable substrates include olefins, alcohols, mercaptans, ethers, dienes, sulfides, amines, amides, and various active methylene compounds (44). This reaction can also be used with tert-huty peroxycarbamates to introduce carbamoyloxy groups to these substrates (243). 

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

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... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

It has been reported that molecnlar oxygen plays an important role in the allylic oxidation of olefins with TBHP (25, 26). Rothenberg and coworkers (25) proposed the formation of an alcoxy radical via one-electron transfer to hydroperoxide, Equation 4, as the initiation step of the allylic oxidation of cyclohexene in the presence of molecnlar oxygen. Then, the alcoxy radical abstracts an allylic hydrogen from the cyclohexene molecnle. Equation 5. The allylic radical (8) formed reacts with molecular oxygen to yield 2-cyclohexenyl hydroperoxide... 

JOC3172, 1996J(P2)2085>. Chemical electron transfer (CET) oxidation of these bicyclic housanes with tris(4-bromophenyl)aminium hexachloroantimonate performed at 0°C in the absence or presence of base (2,6-di-/-butyl-pyridine) leads to the formation of two olefinic products 369 and 370, which were isolable after flash chromatography on silica (Scheme 56) <1996JOC3172>. 

Cu(II) EPR signal in nitriles as solvent as well as by polarographic measurements 144>. Similarly, the EPR signal disappeared when Cu(OTf)2 was used for catalytic cyclo-propanation of olefins with diazoesters 64). In these cases, no evidence for radical-chain reactions has been reported, however. The Cu(acac)2- or Cu(hfacac)2-eatalyzed decomposition of N2CHCOOEt, N2C(COOEt)2, MeCOC(N2)COOEt and N2CHCOCOOEt in the presence of cyclopropyl-substituted ethylenes did not furnish any products derived from a cyclopropylcarbinyl - butenyl rearrangement128. These results rule out the possible participation of electron-transfer processes and radical intermediates which would arise from interaction between the olefin and a radical species derived from the diazocarbonyl compound. 

Mattay et al.5i suggested from the photoreaction of biacetyl with highly electron-rich olefins that an initial electron transfer from an electron-rich olefin to photoexcited ketone is the key step in the oxetane formation via the ion-radical pair (equation 26). 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Indeed, the extent of disproportionation of NO according to equation (89) clearly depends on the donor strength of the aromatic hydrocarbon.240 For example, hexamethylbenzene which is a strong donor (IP = 7.85 V) promotes the ionization of NO to an extent of 80% whereas the weaker donor durene (IP = 8.05 V) affords less than 25% ion-pair formation. Furthermore, the resulting NO+ cation is a powerful electron acceptor (Erea = 1.48 V versus SCE) in contrast to NO (Ered = 0.25 V versus SCE) and thus readily forms donor/acceptor complexes with a variety of aromatic, olefinic and heteroatom-centered donors. Accordingly, the donor/acceptor complexation and electron-transfer activation are the critical steps in various transformations in Chart 8 as described below. 

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

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