An attempt to explore aromatic sulfonate esters as agents for the condensation of alcohols with mercaptans revealed an unusual process for sulfonate esters: CS bond rupture. Two mechanistic possibilities for CS bond rupture are explored: (i) radical anion intermediacy via single electron transfer and (ii) nucleophilic aromatic substitution. Both experiments and molecular orbital computations are presented to support the conclusion that nucleophilic aromatic substitutions are occurring.Key words: sulfonyl esters, nitrobenzenesulfonates, CS bond rupture.
Allenyl methyl ether (1) and (diphenylmethylene)cyclopropane (5) were ozonized with an ozone/oxygen mixture. Formation of the observed products cannot be satisfactorily rationalized via the familiar modified Criegee mechanism. However, a single-electron transfer (SET) mechanism provides a simple rationale.1. Introduction. ± Bailey [1] has included known allene ozonolyses in a chapter on Special Liquid Phase Ozonolyses because they do not follow the classical scheme exactly. Whereas the so-called Kolsaker route [2] with simple allenes leads to two carbonyl compounds and CO (Scheme 1,a), sterically hindered allenes form different nonperoxidic products. Similar unexpected results have been observed in the ozonolyses of alkylidenecyclopropanes, i.e. Feists ester [3a], and cycloalkylidenecyclopropanes [3b,c], even though they are not sterically hindered (Scheme 1,b). Moreover, both nonperoxidic and peroxidic products have been found in these cases.A good case has been made for the position that ground-state ozone is best viewed as a singlet biradical rather than as a 1,3 dipole or a resonance-stabilized zwitterion [4]. Ozone reactions, particularly with electron-rich species, can be understood as redox reactions because the role of ozone as a strong oxidant is incontestable. A comparison of the classical and an alternative (single-electron transfer SET) mechanism for the ozonolysis of alkenes is outlined in Scheme 2.Hitherto, rationalizing the results of allene or alkylidenecyclopropane ozonolyses (cf. Scheme 1 [1 ± 3]) required that the Criegee mechanism be supplemented with additional hypotheses. An example that includes both an allene and an alkylidenecyclopropane system was first investigated by Hartzler [5] (Scheme 3). This reaction has been reinvestigated by Crandall and Schuster [6] [7] in the course of a general study of the mechanism for allene ozonolyses. The results in Scheme 3 show that the absence of steric hindrance does not guarantee that ozonolyses will follow Kolsakers route. Crandall and Schuster concluded that this reaction is found to be a general transformation that proceeds by a nonobvious pathway. Consequently, this example should be a challenge for our alternative (SET) mechanism (cf. Sect. 3). The alternative (SET) mechanism (Scheme 2) demands the formation of an intimate pair of radical ions resulting from initial electron transfer by the unsaturated hydrocarbon to the ozone. Suitably substituted allenes and alkylidenecyclopropanes should be interesting species in terms of the regioselective formation of radical cations. Thus, in this paper, the results of ozonations of allenyl methyl ether (1) and (diphenylmethylene)cyclopropane (5) in aprotic solvents are described.
Allenyl methyl ether (1) and (diphenylmethylene)cyclopropane (5) were ozonized with an ozone/oxygen mixture. Formation of the observed products cannot be satisfactorily rationalized via the familiar modified Criegee mechanism. However, a single‐electron transfer (SET) mechanism provides a simple rationale.
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