Methylene radical anions

Attempted cyclopropanation of cyclohexene with methylene halides using sodium naphthalene was largely unsuccessful, the initially formed methylene being much more efficiently trapped by electron transfer to give the methylene radical anion than by addition to cyclohexene. Less than 1 % of norcarane was formed, the major products being methane, ethane, propane, and ethylene. ... 

The unique chemical behavior of KO2 is a result of its dual character as a radical anion and a strong oxidizing agent (68). The reactivity and solubiHty of KO2 is gready enhanced by a crown ether (69). Its usefiilness in furnishing oxygen anions is demonstrated by its appHcations in SN2-type reactions to displace methanesulfonate and bromine groups (70,71), the oxidation of benzyHc methylene compounds to ketones (72), and the syntheses of a-hydroxyketones from ketones (73). 

The ESR spectrum of the thioxanthene S, S-dioxide radical anion itself shows that the two possible conformers coexist, since the two methylene protons are not equivalent. In the case of the 9-monoalkyl derivatives, the large coupling constant observed for the 9-proton leads to the conclusion that the 9-substituent is in the boat equatorial position as in II1 F Thus the radical anions and the neutral molecule display different conformations. The protons in the 9-position of the radical anions of cis-9-methylthioxanthene S-oxides (2, n — 1, R1 = H, R2 = CH3) have an appreciable coupling constant10 which suggests that these radical anions have the substituent in the pseudo-axial position. Furthermore, in the radical anions the S—O bond is pseudo-axial. These situations are exactly the opposite of that observed for the neutral compound. 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

Chlorides RMe2CCH2Cl [(a) R = Me, R = Ph and (b) R = CH2Ph] reacted with diphenylphosphide ions in liquid ammonia, via a proposed 5rn 1 mechanism and their reactivities were measured. The higher reactivity of (a) has been attributed to efficient intramolecular electron transfer from the phenyl ring to the C—Cl a bond (intra-ET catalysis). The lower reactivity of (b) is ascribed to a decrease in the rate of the intra-ET by elongation of the bridge by one methylene unit. The relative reactivity of (a) versus (b) is proposed to indicate the ratio of the intra-ET rates of the radical anions of both compounds. ... 

Methylated aromatic heterocycles (HetCHj) form cation-radicals that are typical n acids and expel a proton. Methylene radicals are formed. These radicals give rise to the corresponding carbocations if an oxidant was taken in excess. Nucleophiles attack the ions, completing the reaction. If water is the reaction medium (the hydroxyl anion is a nucleophile), an alcohol is formed. The alcohol rapidly transforms into an aldehyde on the action of the same oxidant. 

Further support for the rehybridization model was provided by analysis of the spin densities of annelated 1,4-naphthoquinones, n hthalenes, and cyclobutaben-zene radical anions by ESR. The hyperfine coupling constants at the methylene position decrease as ring strain increases. Calculation of spin densities using the rehybridization model leads to an excellent fit of experimental spin densities, while calculation of spin densities using either the Coulson-Crawford hyperconjugation model or INDO do not accurately correlate experimental values. Such results suggest that rehybridization can account for the observed changes in spin densities. 

As Stated in the previous section, reduction of thieno[3,2-2 ]thiophene (2) with Na-K alloy at —100 results in the formation of a radical-anion. With AlClj in nitromethane at —20°, or SbClj in methylene chloride at —60°, a radical-cation was obtained. The experimental hyperfine splitting constants (HFSC) data are shown in Eq. (73). 

The reduction of thienothiophene 2 with Na-K alloy produced a radical-anion (see Section III,D), and a radical-cation resulted from oxidation with AlCl, in nitromethane or SbCl, in methylene chloride (see Section IV,B 5). No such conversion was observed in the case of thienothiophene 1 this is explained by the extended conjugation throughout the thienothiophene 2 molecide, impossible in thienothiophene 1. 

This qualitative interpretation of structural and electronic similarity has also been employed to rationalize the fact that the quantum yield for the dioxetane derivative 6, in which the phenoxy substituent is directly linked to the peroxidic ring, is two orders of magnitude higher than for the dioxetane 7, in which the trigger function is separated by a methylene bridge. Furthermore, the different quantum yields were rationalized in terms of a competition between the intramolecular (pathway A) and intermolecular back-electron transfer (pathway B) in the decomposition of 7, whereas the intramolecular back-electron transfer was believed to occur exclusively in the decomposition of 6, due to the higher stability of the radical anion of the benzaldehyde derivative, as compared with the radical anion of acetone (Scheme 14). 

The intramolecular photoinduced electron transfer reaction of N-(o-chlorobenzyl)aniline 440 in the presence of sodium hydroxide in aqueous acetonitrile afforded, 9,10-dihydrophenanthridine and its dimer, which is reasonably explained by dechlorination from the radical anion of chlorobenzene chro-mophore followed by the cyclization (Scheme 130) [481], Similar photocyclization 9-(io-anilinoalkyl)-10-bromophenantherens 441 takes place to give spiro compounds, cyclized products, and reduction products dependent on the methylene chain length. The efficient intramolecular photocyclization occurs when the methylene tether is n = 3 [476] (Scheme 131). 

Cathodic reduction of 1,1,2,2-tetracyanocyclopropane or 1,1,2,2,-tetracyano-ethane yields the radical anion of tetracyanoethylene via a formal reductive de-methylenation and dehydrogenation, respectively (Eq. (256) ) 592K... 

It is known that acetone enolate anion does not react with primary alkyl radicals, and that nitromethane anion is not capable of initiating the SRN1 reactions even under irradiation [99]. Thus, the photo stimulated reactions of 25 with nitromethane anion as nucleophile and acetone enolate anion as entrainment reagent (which enables SRN1 initiation but cannot compete with the coupling of the methylene radical with nitromethane anion after cyclization) render the cyclized products 26 (Sch. 25) [98]. 

The photochemical dechlorination of 4-chlorobiphenyl in the presence of anthracene and triethylamine is supposed to proceed via two electron transfer steps, first from triethylamine to excited anthracene and then from the anthracene radical anion to the aryl halide410. This type of mechanism is also operative in the dechlorination of 4-chlorobiphenyl, 4,4 -dichlorobiphenyl, 1,3,5-trichlorobenzene, 1,2,3,5-tetrachloroben-zene and pentachlorobenzene photosensitized by visible dyes (protoporphyrin IX, acriflavin, rose bengal, zinc protoporphyrin, methylene green) in the presence of triethylamine411. The various steps of the mechanism are summarized in equations 112-115. A mechanistic alternative is that the excited photosensitizer transfers an electron directly to the aryl chloride and becomes regenerated by electron transfer from the amine. 

The transient absorption of the radical anions observed in pulsed styrene and a-methylstyrene were extremely sensitive to water they were greatly diminished or sometimes not observed at all if a small amount of water, even moisture in the atmosphere, was introduced into the sample. Similar phenomena have been observed in the pulse-irradiated monomers in cyclohexane solutions [16, 17]. Addition of ethanol, methylene chloride, chloroform, carbon tetrachloride, and n-butyl amine also reduced the yield of the anions [18]. 

Nearly at the same time, the same group reported a study dealing with the electron-transfer initiated oxidation of trans-stilbene (TS) 10a sensitized by the singlet excited states of both DCA and methylene blue (MB+) [124]. The authors proposed that, although the initial electron-transfer step was identical for the two systems, the subsequent steps leading to products (predominantly benzaldehyde) must be different. In fact, dicyanoanthracene radical anion DCAT reduces molecular oxygen to superoxide, whereas reduced methylene blue MB°, owing to its lower redox potential (Ered = —0.25 V vs SCE), doesn t. 

The mechanism of this reaction has been investigated in some detail. RX must be an activated halide, such as a benzyl halide, a-halo ether, or a 1,1-dihalocarbon where at least one of the halides is Br or L Simple halocarbons do not react, nor does methylene chloride. Reaction with chloroform is slow, while reaction with carbon tetrachloride is instantaneous at room temperature. The (pseudo) first-order rate constant is linearly correlated to the polarographic reduction potential of the halide thus electron transfer from (38) to RX (equation 57) is the rate-determining step, followed by rapid cleavage of the RX radical anion into R and X . 

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