Aziridine cationic ring-opening

The cationic ring opening polymerization of oxolane (THF) or of N-substituted aziridines can be initiated by oxocarbenium salts [42]. The methacrylic ester unsaturation is insensitive to cationic sites, and polyoxolanes (poly-THF) macromonomers are obtained in good yields. 

Polyamines can also be synthesized by cationic ring-opening polymerization of ethyleneimines (aziridines), trimethyleneimines (azetidines), and 2-oxazolines. 

In cationic ring-opening polymerization, there are not too many examples of the systems in which ratios of kplk, are known. In the polymerization of substituted aziridines and substituted thietanes the ratios of rate constants of chain transfer to polymer to the rate constants of propagation have been measured and at least the value obtained for polymerization of N-/-butylaziridine (1.2-104) [260], indeed indicates the living character... 

The earliest reported PHAI was hyperbranched PEI (B-PEI), synthesized by the cationic ring-opening polymerization (CROP) of aziridine in water. The formation of a hyperbranched structure is due to the high nucleophilicity of the amino groups in both the monomer and the polymer causing significant chain transfer of cationically growing chains to already formed polymer chains, followed by reinitiation of a new chain by so-called proton transfer (Scheme 2.2). 

The research groups of Mariano and West developed a photoinduced electrocydi-zation/nucleophilic addition sequence. Thus, irradiation of N-alkylpyridinium perchlorates as 5-19 in an aqueous solution led to the aziridine cations 5-20, which react in a nucleophilic addition with OH to give the isolable azabicyclo[3.1.0]hex-2-enols 5-21. These can be further transformed by a nucleophilic ring-opening of the aziridine moiety under acidic conditions to lead to useful unsymmetrically trans,trans-trisubstituted cyclopentenes 5-22 (Scheme 5.5) [10]. 

Some of the more important monomers whose ring opening polymerisations have been induced by stable cation salts include, 1,4-epoxides, notably tetra-hydrofuran (20,112,113), 1,2-epoxides (114), 1,3-episulphides (thietans) (33,53), 1,2-episulphides (thiiranes) (53), azetidines (115,116), aziridines (117), the cyclic formals, 1,3-dioxolan (23,54, 118-120), and 1,3-dioxepan (118,119), trioxane (121,122) and more recently lactones (123). Aldehydes (124) may also be included since these molecules can be regarded as the smallest possible oxygen hetero-... 

The direct synthesis of the morphinan skeleton (107) from (105) involves the intramolecular ring opening by the enolate anion of the in situ generated aziridine cation (106) (Scheme 41) <88J0C2144>. Compound (108) was found to rearrange thermally to the thermodynamically favored azabicyclo-[3.3.1]octane (110) via the aziridinium ion (109) (Scheme 42) <93CC758>. 

The ability of non-racemic C2-symmetric aziridines 848 and 849 to undergo rapid nucleophilic ring-opening to provide synthetic equivalents for the j5-cation of aspartic acid illustrates the utility of the azide 822. Treatment of 822 with triphenylphosphine in refluxing benzene results in reduction of the azide to an amine that readily undergoes ring closure to aziridine 845. Subsequent tosylation of the secondary nitrogen affords 848 in 73% yield for the two steps. 

Though very stable thermally, aziridine 31 slowly decomposes at 220°C, producing imidoyl fluoride 195 and amine 196 in a 1 1 ratio in 95% yield. A likely course of events entails reversible ring opening to azomethine ylide 197, which abstracts a fluoride ion from the starting aziridine to form anion 198 and cation 199 (akin to 179 above). Ring closure of 199 yields 200, and then transfer of a proton from 200 to 198 gives the final products. 

Similar ring-opening reactions are known for aziridines. Hydrolysis constitutes an important process for the synthesis of aminoalcohols (Scheme 10.2). This is usually performed with aqueous acid an aziri-dinium cation is first formed and then is opened by reaction with water. 

The more crown-like tetra-aza-molecule (447), which had previously been prepared by the action of acid on N benzyl aziridine, is now reported as the product of an anodic oxidation. The mechanism postulated proceeds via a ring-opened radical cation (446), which polymerizes, cyclizes, and then initiates another sequence. 

On the basis of ring opening reactions of 2 substituted aziridines described in the literature [17,18,19], the polymerization initiated with dimethylsulfate or boron trifluoride etherate was expected to propagate mainly by nucleophilic attack of the free aziridine on the methylene carbon of the aziridinium cation following the scheme ... 

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