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Photoinduzierte Reaktionen von 3-Phenyl-2H-azirinen mit Carbonsäureestern


Gilgen, Paul; Hansen, Hans-Jürgen; Heimgartner, Heinz; Sieber, Willi; Uebelhart, Peter; Schmid, Hans; Schönholzer, Peter; Oberhänsli, Willi E (1975). Photoinduzierte Reaktionen von 3-Phenyl-2H-azirinen mit Carbonsäureestern. Helvetica Chimica Acta, 58(6):1739-1768.

Abstract

Irradiation (280-350 nm light) of a benzene solution of 3-phenyl-2H-azirines 1a-e in the presence of carboxylate esters, whose carbony1 groups are activated by electron withdrawing groups situated in the acyl or alkyl moiety, produces 5-alkoxy-3-oxazolines (Tab. 1 and 4, Scheme 2) isolated in 18-82% yield. These heterocycles undoubtedly originate by regiospecific addition of the ester carbonyl group to the azirine-derived benzonitrile-methylide ‘dipole’ (Scheme 1). The 5-(2’,2’,2‘-trifluoroethoxy)-3-oxazolines, derived from 2‘,2‘,2’-trifluoroethyl carboxylic esters, on treatment with methanolic hydrogen chloride at low concentration, are smoothly transformed into the corresponding 5-methoxy-3-oxazolines (e.g. 16 to 17, Tab. 5). Utilizing this process, various hitherto relatively unknown [9] 5-alkoxy-3-oxazolines become accessible. The constitution of the adducts is based essentially on spectral data. The structuro of trans-5-methoxy-2,4-diphenyl-5-trifluoromethyl-3-oxazoline (trans-14), the addition product of methyl trifluoroacetate and the benzonitrile-benzylide from 2,3-diphenyl-2H-azirine (1d), was determined by X-ray crystallography (Section 5). Benzonitrile-isopropylide (22), resulting from the photochemical transformation of 2,2-dimethyl-3-phenyl-2H-azirine (1a), also reacts with S-methyl thiobenzoate to give 2,2-dimethyl-5-methylthio-4,5-diphenyl-3-oxazolie (26). Ethyl cyanoacetate protonates predominantly the dipolar species derived from 1a at the nitrile C-atom and yields after work-up ethyl alpha-cyanocinnamate (29) and ethyl isopropylidene-cyanoacetate (30)(Scheme 4). Thc relative rate of addition (k rel) of benzonitrile-isopropylide (22) to methyl alpha-haloacetates and dimethyl oxalate was determined by competition experiments (Section 6). Log k rel correlated satisfactorily (r = 0.97) with the pKa of the acide derived from the ester reactant: log k rel = -1.72 pKa + 2.58 or with Taft's substituent constants sigma*: log k del = 2.06 sigma* - 4.11 [k rel(ethyl dichloroacetate) = 1; Section 7.1|. On the basis of the results obtained, the mode of reaction of the so-called benzonitrile-methylide ‘dipole’ is discussed and a model for the transition state of addition of ester-carbonyl groups is proposed that accounts for the observed regiospecifity and stereoselectivity.

Abstract

Irradiation (280-350 nm light) of a benzene solution of 3-phenyl-2H-azirines 1a-e in the presence of carboxylate esters, whose carbony1 groups are activated by electron withdrawing groups situated in the acyl or alkyl moiety, produces 5-alkoxy-3-oxazolines (Tab. 1 and 4, Scheme 2) isolated in 18-82% yield. These heterocycles undoubtedly originate by regiospecific addition of the ester carbonyl group to the azirine-derived benzonitrile-methylide ‘dipole’ (Scheme 1). The 5-(2’,2’,2‘-trifluoroethoxy)-3-oxazolines, derived from 2‘,2‘,2’-trifluoroethyl carboxylic esters, on treatment with methanolic hydrogen chloride at low concentration, are smoothly transformed into the corresponding 5-methoxy-3-oxazolines (e.g. 16 to 17, Tab. 5). Utilizing this process, various hitherto relatively unknown [9] 5-alkoxy-3-oxazolines become accessible. The constitution of the adducts is based essentially on spectral data. The structuro of trans-5-methoxy-2,4-diphenyl-5-trifluoromethyl-3-oxazoline (trans-14), the addition product of methyl trifluoroacetate and the benzonitrile-benzylide from 2,3-diphenyl-2H-azirine (1d), was determined by X-ray crystallography (Section 5). Benzonitrile-isopropylide (22), resulting from the photochemical transformation of 2,2-dimethyl-3-phenyl-2H-azirine (1a), also reacts with S-methyl thiobenzoate to give 2,2-dimethyl-5-methylthio-4,5-diphenyl-3-oxazolie (26). Ethyl cyanoacetate protonates predominantly the dipolar species derived from 1a at the nitrile C-atom and yields after work-up ethyl alpha-cyanocinnamate (29) and ethyl isopropylidene-cyanoacetate (30)(Scheme 4). Thc relative rate of addition (k rel) of benzonitrile-isopropylide (22) to methyl alpha-haloacetates and dimethyl oxalate was determined by competition experiments (Section 6). Log k rel correlated satisfactorily (r = 0.97) with the pKa of the acide derived from the ester reactant: log k rel = -1.72 pKa + 2.58 or with Taft's substituent constants sigma*: log k del = 2.06 sigma* - 4.11 [k rel(ethyl dichloroacetate) = 1; Section 7.1|. On the basis of the results obtained, the mode of reaction of the so-called benzonitrile-methylide ‘dipole’ is discussed and a model for the transition state of addition of ester-carbonyl groups is proposed that accounts for the observed regiospecifity and stereoselectivity.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:German
Date:1975
Deposited On:18 Nov 2014 15:01
Last Modified:05 Apr 2016 18:31
Publisher:Wiley-Blackwell Publishing, Inc.
ISSN:0018-019X
Funders:Schweizerischer Nationalfonds zur Förderung der wissenschaftlichen Forschung
Publisher DOI:https://doi.org/10.1002/hlca.19750580628

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