Deprotonation donor radical cations

However, there are some exceptions. One of them is the possibility of (photo)-protonation or -deprotonation. If a matrix is doped with sufficient amounts of a proton donor or acceptor, chances are that the substrate will give up or accept a proton already on cocondensation or on subsequent photoexcitation. In fact, the higher noble gases (Ar, Kr, Xe) are themselves good proton acceptors, forming (NG H)+ complexes that can be identified by their characteristic IR vibrations. This feature allows occasionally to observe radicals formed by deprotonation of radical cations formed in noble gas matrices, for example, benzyl radical from ionized toluene. However, we know of no examples where a carbanion was formed by deprotonation in matrices. 

Radical cations are strongly oxidizing intermediates, but also after deprotonation at a heteroatom (in the present systems at nitrogen) some of this oxidizing property remains. Thus a common feature of these intermediates is that they are readily reduced by good electron donors. Since the heteroatom-centered radicals and the radical cations are always in equilibrium, it is, at least in principle, possible that such intermediates react with water at another site (canonical mesomelic form), that is at carbon. This reaction leads to OH-adduct radicals. Although deprotonation at a heteroatom is usually faster (but also reversible) than deprotonation at carbon, the latter reaction is typically "irreversible". This also holds for a deprotonation at methyl (in Thy). 

If fcf > feBET- the overall transformation can occur rapidly despite unfavorable driving forces for the electron transfer itself. Only follow-up reactions with high kf can compete with back electron transfer. Different kinds of such unimolecular processes can drive the equilibria toward the final product. A representative example is the mesolytic cleavage of the C-Sn bond in the radical cation resulting from the oxidation of benzylstannane by photoexcited 9,10-dicyanoanthracene (DCA). This is followed by the addition of the benzyl radical and the tributyltin cation to the reduced acceptor DCA [59]. In the arene/nitrosonium system, [ArH, NO+] complexes can exist in solution in equilibrium with a low steady-state concentration of the ion-radical pair. However, the facile deprotonation or fragmentation of the arene cation radical in the case of bifunctional donors such as octamethyl(diphenyl)methane and bicumene can result in an effective (ET) transformation of the arene donor [28, 59]. Another pathway involves collapse of the contact ion pair [D+, A- ] by rapid formation of a bond between the cation radical and anion radical (which effectively competes with the back electron transfer), as illustrated by the examples in Chart 5 [59]. 

Benzylic deprotonation is often an inefficient process. It may be more important than it would appear from the end products, however, since radical cation deprotonation followed by reduction of the radical and reprotonation may regenerate the starting material. This mechanism has been proposed to explain the inefficiency of some PET alkylations [68]. In suitable models such a process has been revealed, e.g. deuterium incorporation at the bis-benzylic position in 2-(4-methoxyphenyl)-2-phenylethyl methyl ether and cis-trans isomerization in 2-methoxy-l-(4-methoxyphenyl)indane (but not in the corresponding 3-methoxyphenyl derivatives) [204], as well as deconjugation of 1-phenylalkenes to 3-phenylalkenes in the presence of 1,4-dieyanobenzene, biphenyl (as a secondary donor) and a hindered pyridine as the base [205]. Deprotonation of N,N-dimethylaniline has likewise been observed (Scheme 38) [206-207],... 

The studies related to the interactions of electronically excited arene molecules with tertiary amines have provided a basis for the present understanding of exciplexes and radical ion-pair phenomena [41,82], PET reactions of amines yield planar amine radical cations (Eq.20) which are deprotonated to give a-amino radicals (Eq.21) and usually cross-coupling (Eq.22) between radical pairs of donor-acceptor terminates the photoreaction [32a, 83]. Mechanistic studies revealed contact ion pair (CIP) intermediate for these reactions [84, 85]. 

Cyclopropyl-substituted ketones are suitable substrates for generating distonic radical anions from ketyl radical anions. A series of cycloalkanone substrates with unsaturated side-chains, to trap the primary radical formed after cyclopropylcar-binyl ring opening, has been investigated (Scheme 31) [118, 119]. For the first electron-transfer step triethylamine is used as electron donor. The reaction sequence is terminated by proton or hydrogen transfer from the solvent or the a-amino radical formed after deprotonation of the amine radical cation. 

Interest in CdS mediated surface photochemistry has continued. The irradiation (wavelenyth > 420 nm) of the alkene (3) in an aerated suspension of CdS results in the formation of the products shown in Scheme 1. Under deaerated conditions all the products shown apart from the ketone (4) are formed. The results obtained are interpreted as involving electron transfer from the alkene to the CdS affording a radical cation (5). Subsequent cyclization and back electron transfer or disproportionation yields (6) and (7). Deprotonation followed by back electron transfer or disproportionation yields (8) and (9). The reactions described were all quenched when irradiation was carried out in the presence of the electron donor, 1,2,4,5-tetramethoxybenzene. A comparison of various CdS samples was carried out. 

Owing to their relatively low ionization energies (IE) of ca 8.0-8.5 eV, phenols are also good electron donor solutes. Recent experimental studies of phenols in non-protic solvents showed that ionized solvent molecules react with phenol to yield not only phenol radical cations by electron transfer, but also phenoxy radicals by hydrogen transfer. An obvious question is whether, under these conditions, the latter radicals were formed from ionized phenols rather than by direct hydrogen abstraction, because proton transfer reactions could be facilitated upon ionization. This also raises a question about the influence of solvent properties, both by specific and non-specific interactions, on the mechanism and kinetics of deprotonation processes ... 

In polar solvents the excited state of sufficiently electron deficient arenes will accept an electron from donors. The fates of the radical ion pairs produced include formation of products of addition to the arene ring. A new example of this mode of reactivity is the photochemical reaction of 1,4-dicyanonaphthalene with benzyl methyl ether in acetonitrile. This yields stereoisomers of the addition product (120). The reaction most likely involves electron transfer from the ether to the naphthalene excited state and subsequent ionisation of a proton from the benzyl ether radical cation. This produces a benzyl ether radical which adds to the naphthalene derivative. An analogous sequence is proposed to explain the photochemical formation of (121)-(124) from ultra-violet light irradiated solutions of naphthalene-1,2-dicarboxylic acid anhydride in methanolic benzene or acetonitrile containing isobutene, 2-butene or 2-methyl-2-butene. Here it is suggested that the alkene radical cation, formed by electron transfer to the excited state of the naphthalene, is attacked by methanol deprotonation... 

At pH above pKu2, where the amino group is deprotonated, these substrates contain two possible donor sites, sulfur and nitrogen. From the polarization patterns, it could be concluded that 48 quenches the sensitizer triplet by electron transfer from sulfur only. The CIDNP effects arise in two successive radical pairs because the sulfur-centered radical cations 50 +... 

Recently, substituted amides and lactams, such as dimethylformamide (DMF) or 2-pyrrolidones, were used as electron donors in the photoalkylation of 1,2,4,5-tetracyanobenzene (TCB) [45], The success of the reaction was ascribed to the high reduction potential of TCB in the excited state that made the initial step-that is, the oxidation of amides [Ei/2 DMF = 2.29 V versus standard calomel electrode (SCE)]-possible. It should be noted here that the ensuing deprotonation step was found to be chemoselective when using N-methylpyrrolidone (NMP), as illustrated in Scheme 14.8b. Accordingly, deprotonation of the lactam radical cation intermediate occurred exclusively at the methylene, and not from the methyl group. Tricyano benzene 12 was thus isolated in 41% yield. 

Ammonia or amines were likewise used as nucleophiles in place of alcohols [52]. Here, PET between an arene S (e.g. triphenylbenzenes) and 1,4-dicyanoben-zene (DCB) led to the arene radical cation S and the radical anion DCB. Secondary ET from a donor (D) to S produced the radical cation of the former (D ) that added ammonia. After deprotonation, coupling of the resulting neutral radical with DCB and cyanide loss led to the corresponding NOCAS product Suitable donors were arylcyclopropanes, quadricyclane and dienes that gave 4-(l-aryl-... 

Ionic disproportionation of NO can be promoted in non-polar media by the addition of Bronsted acids and Lewis acids [10]. The photochemical activation of the nitrosonium donor-acceptor complex via irradiation of the charge-transfer absorption band produces the aromatic radical cation. The most direct pathway to aromatic nitration proceeds via homolytic coupling of the aromatic radical cation with NO [16] because the intermediate subsequently undergoes very rapid deprotonation ... 

AUcenes are reasonably good donors. Allylic deprotonation from the radical cation is usually a slow process, but it does occur to some extent as indicated by the allylation of aromatic nitriles used as electron transfer sensitizers, which takes place to a variable extent (see Equation 4.19). ... 

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