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Photodesulfonation

General Aspects

The term photodesulfonation describes the elimination of a sulfonyloxy group (–SO3X–) by photochemical methods. Although there are few investigations on this reaction type, it offers an application in some interesting experimental fields:

  • the development of new synthesis routes,
  • as protection for or blocking of functional groups,
  • the removal of sulfur-containing groups in waste water treatment,
  • the evaluation of photoreactivity and phototoxicity of sulfonamides used as drugs and
  • the development of mechanistic models.

The thermal initiation of desulfonations usually needs harsh reaction conditions and only low yields and selectivities can be accomplished. By its directing effect, sulfonyloxy groups can direct potential substituents to the desired position of e.g. an aromatic ring. The most studied substance families are substituted anthraquinone sulfonic acids (Aq–SO3H), arenesulfonic acids (Ar–SO2OH) and their derived esters Ar–SO2OR, amides Ar–SO2NRR’ and aromatic sulfones Ar–SO2Ar.

The photochemical reactivity of these compounds is based on the sulfonyl group as the sulfur atom lacks a pair of free electrons. For this reason, it has almost no conjugation effect with the adjacent aromatic π-system, which significantly shifts the absorption spectrum of the aromatic system allowing photonic excitation. The subsequently occurring excited states show reaction characteristics similar to the non-sulfonated hydrocarbons. Thus, introducing the sulfonyloxy or sulfonyl group to a hydrocarbon offers the possibility of subsequent photochemical conversion at low temperatures as an alternative to the direct thermally conducted reaction of the hydrocarbons.

Arenesulfonic acids and their salts can be desulfonated by UV-light independent of the pH value of the solvent. Keeping the reaction solution at room temperature, thermal desulfonation can be avoided to allow an appropriate reaction control. The reaction can be of radical or ionic character and may involve several excited states. In principle, three different reaction types can be distinguished regarding desulfonation:

  • Desulfonation or substitution of a sulfonyloxy group by a hydrogen atom,
  • Substitution of a sulfonyloxy group by another functional group and
  • Dimerization under desulfonation.

 

Desulfonation or Substitution of a Sulfonyloxy Group by a Hydrogen Atom

 

photodesulfonationeq1

This reaction can be carried out in an aqueous solution under the absence of oxygen. A proton adds to the excited molecule forming a positively charged transition state, finally dehydration leads to the desulfonated aromate. Depending on the basicity of the aromatic ring caused by the excitation, the yield varies from 2% of the ortho-aminated compound to 50% of the para-aminated compound considering the example of anilinesulfonic acid.[1;2] The absorption spectra in strongly acidic solutions are similar to α-anthracene, α-anthraquinone and α-anthrahydrochinone sulfonic acids. This suggests that the aromatic ring is not protonated in the ground state but in the state of the lowest singlet excitation state S1.[3]

According to the relatively low yields, the quantum yields of desulfonation in the range of 3.5·10−4 to 1.4·10−2 are rather small due to the high tendency of the protonated excited state to simply deactivate again. Hereby, the order of the reactivity equals that of thermal initiation. The yield can be raised up to 90% for e.g. anthraquinone sulfonic acid by working in a basic alcohol solution, in the presence of amines and by using the whole absorption range of the sulfonated molecule.[4] In the first step, an electron transferred from the alcoholate or the amine gives a radical anion of the quinone system that is detectable via ESR spectroscopy.[5] The increased lewis acceptor properties of the aromatic system in the exited state promotes the electron transfer. If oxygen would be present, the additional electron would be transferred to the oxygen with high yields disabling photodesulfonation to occur. The second step is the actual photodesulfonation of at least one of the reduced forms under pH-dependent dismutation forming either the monoanion A–SH(pH above 10) or the dianion A–S2of the anthraquinone system. Good yields are accessible for all substitution patterns as long as the photochemical reduction to the anthrahydroquinone is possible. For example, nitro- or aminoanthraquinone alpha-sulfonic acids do not react but form nonreactive charge-transfer states in the lowest excitation state. Another possible hindrance of high yield desulfonation is the competing desulfonylation forming SO2 in a photolysis process.

Substitution of a Sulfonyloxy Group by Another Functional Group

 

photodesulfonationeq2

Alternatively to the substitution with a simple H-atom, some halogens or even whole functional groups (e.g. NH2 or NO2) can substitute the sulfonyloxy group. Photosubstitution with halogens can reach almost quantitative yields whereas several other substitutions are limited to about 45% to 60% because of secondary product formation by hydroxylation and amination of the aromatic ring.[6] The reaction rate can vary depending on the other substituents and their position on the aromatic system influencing the basicity of the molecule. The quantum yields and excitation wavelengths are comparable to the ones of substitution by a hydrogen atom. The reaction mechanism for the photosubstitution by e.g. a bromide ion starts with the transfer of an electron from the bromide to an excited reactant molecule. Subsequently, the radical substitution of another molecule containing a sulfonyloxy group promotes the formation of the brominated product. Besides SO2, the hydroxyl radicals formed by decomposition of the dissociated HSO3radical are causal for the secondary hydroxylation reactions.

Dimerization Under Desulfonation

photodesulfonationeq3

Photodesulfonation of benzenesulfonic acids and most of their salts leads to either the corresponding hydrocarbons Ar–H or the dimerization product Ar–Ar. The yield mainly dependents on the reaction conditions and usually varies from 5% to 34%. The initial key step is the homoleptic scission of the Ar–S-bond after irradiative excitation. Dimerization with another non-excited starting molecule as well as recombination gives the biarylic compound. Alternatively, the corresponding hydrocarbon is the product of an aryl radical with a suitable hydrogen donor, e.g. a simple hydrocarbon. As already mentioned in section 0.0.1.3, the HSO3radicals decompose preferably in acidic solution into SO2 and OHto form in sum quantitative amounts of sulfur dioxide, sulfuric acid and hydrogen peroxide. In some cases, a hydroxylated arylic species can start a polymerization ending with polyphenolic products. In addition, a potentially interesting option in synthesis is the photolysis of α-substituted cyclic sulfones to hydrocarbonic ring systems (cyclopentenes and cyclobutenes) by reducing the ring size from six to five or even from five to four carbon atoms. This path is also possible for the transformation of episulfones into alkenes, 3-sulfolenes into 1,3-dienes or diketosulfones into diones (figure 1). All these reactions can only be carried out photochemically because if thermally initiated, the reactants would decompose to a significant amount.

 cyclic_desulfonations

Figure 1: Transformation of an episulfone into an alkene, a 3-sulfolene into an 1,3-dienes and a diketosulfone into a dione.[7]

 

  1. O.P. STUDZINSKII; A. V. EL’TSOV; N.I. RTISHCHEV; G.V. FOMIN: Russian Chem. Rev., 1974, 43, 155.
  2. EL’TSOV, A. V.; STUDZINSKII, O.; KUL’BITSKAYA, O.; OGOL’TSOVA, N.; EFROS, L.: J. Org. Chem. USSR, 1970, (6), 641.
  3. P. STUDZINSKII; N.I. RTISHCHEV; N.N. KRAVSHENKO; A. V. EL’TSOV: J. Org. Chem. USSR, 1967, (11), 377.
  4. P. STUDZINSKII; A. V. EL’TSOV; N.I. RTISHCHEV: J. Org. Chem. USSR, 1971, (7), 1312.
  5. P. STUDZINSKII; A. V. EL’TSOV; N.I. RTISHCHEV; G.V. FOMIN; A.V. DEVEKKI; L. GURDZHIYANM.: J. Org. Chem. USSR, 1973, (9), 1949.
  6. A. V. EL’TSOV; O.P. STUDZINSKII; A.V. DEVEKKI: J. Org. Chem. USSR, 1973, (9), 762.
  7. BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 978–0–471–92652–8, DOI: 1002/ange.19921041147.


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