Ozonolysis of Allenes and Alkylidenecyclopropanes (Homoallenes)

Abstract

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.

Heterocycles from Alkylidenecyclopropanes

“…The same argument, however, does not apply to an explanation of the observed regioselectivity. HOMO and LUMO energies and atomic orbital coefficients for nitrone 178 and ACPs 2, 167, and 2,2-dimethylmethylenecyclopropane (102) were calculated by ab initio (up to the STO 6-311G level) 52b, 53 and DFT methods. 53 The calculated ∆E values between the frontier orbitals for different approaches of reactants cannot justify the observed regioselectivities.…”

“…10 The reaction of diphenylmethylenecyclopropane (393) with ozone was recently described by Beck to afford, in mixtures with cyclobutanone 441 and benzophenone (443), the peroxide 442 (Scheme 65). 102 The authors suggested a single electron-transfer mechanism, rather than a 1,3-dipolar cycloaddition, for this reaction.…”

“…The reaction of diphenylmethylenecyclopropane ( 393 ) with ozone was recently described by Beck to afford, in mixtures with cyclobutanone 441 and benzophenone ( 443 ) , the peroxide 442 (Scheme ) …”

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

“…In 2001, Beck proved the possibility of an MCP oxidation pathway via SET because MCP 13 is oxidized by ozone to afford cyclobutanone, peroxide, and ketone derivatives. 54 To extend the reactivity of MCPs, in 2021, our group reported the cascade cyclization of MCPs through the direct SET oxidation the MCPs 91 by merging photocatalysis and cobalt catalysis for the synthesis of 4-aryl-1,2-dihydronaphthalene derivatives 92 and 93 (Scheme 20). 55 In this report, two different oxidation pathways were proposed.…”

Catalytic Enantioselective Reaction of Allenylnitriles with Imines Using Chiral Bis(imidazoline)s Palladium(II) Pincer Complexes

“…Palladium acetate catalyzed the coupling‐cyclization of 3 d with allyl bromide, and afforded product 6 in moderate yield without racemization (Scheme b) . Ozonolysis of 3 d gave acyl cyanide, which was directly converted into amino acid methyl ester 7 using MeOH and Et 3 N . The reduction of 7 with LiBH 4 afforded amino alcohol 8 , which was cyclized to oxazolidinone 9 .…”