Guanidinium Ylide

Mechanistic approaches to asymmetric aziridine synthesis have been carried out systematically using a variety of p-substituted benzaldehydes (Table 4.16). Two kinds of reaction mechanism, controlled by the nature of the p-substituent of aryl aldehydes, are proposed an S i-like mechanism, via cationic-like transition state for the fragmentation of intermediate adducts to aziridine products (step 2) by intramolecular nucleophihc substitution, when EDG-substituted benzaldehydes are used and an Si f2-like mechanism, where electron-withdrawing group (EWG) substituted benzaldehydes are used [99]. 

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A novel guanidinium ylide-mediated procedure has recently been reported by Ishi-kawa [62]. Though not an imine transformation, it does employ an imine precursor in the fonn of an aldehyde. Guanidinium ylides react with aldehydes to form aziridines (Scheme 1.35). The mechanism for the formation of the aziridine is believed to involve [3+2] cycloaddition between the guanidinium ylide 112 and the aldehyde, followed by stereospecific extrusion of the urea with concomitant aziridine formation. 

A spiro imidazolidine-oxazolidine intermediate (97) has been isolated in an azirid-ination of o -bromocinnamaldehyde mediated by a guanidinium ylide (98).275... 

The mechanisms of asymmetric synthesis of aziridines from guanidinium ylides and arylaldehydes have been probed by varying para-substituents in the aldehyde.339 Hammett plots show a mechanistic switchover going from electron-donating to electron-withdrawing groups. 

Guanidinium ylide 132 generated in situ reacts with aldehydes to give aziridine-2-carboxylic esters (cis/trans isomer mixtures). Chiral information located four bonds away is transmitted and it guides the alignment of the reactants. 

Scheme 4.40 Asymmetric synthesis of sphingsine from cis- and trans-aziridines obtained by guanidinium ylide participating aziridination...

Hada, K., Watanabe, T., Isobe, T. and Ishikawa, T. (2001) Guanidinium ylides as a new and recyclable source for aziridines and their roles as chiral auxiliaries. Journal of the American Chemical Society, 123, 7705-7706 [Chem. Eng. News, Aug. 13, 32 (2001)]. 

Haga, T. and Ishikawa, T. (2005) Mechanistic approaches to asynunetric synthesis of aziridines from guanidinium ylides and aryl aldehydes. Tetrahedron, 61, 2857-2869. 

Wannaporn, D. and Ishikawa, T. (2005) Chirality transfer from guanidinium ylides to 3-alkenyl (or 3-alkynyl) aziridine-2-carboxylates and application to the syntheses of (2/J,35)-3-hydro-xyleucinate and D-erythro-sphingosine. The Journal of Organic Chemistry, 70, 9399-9406. 

Wannaporn, D., Ishikawa, T., Kawahata, M. and Yamaguchi, K. (2006) Guanidinium ylide mediated aziridination identification of a spiro imidazolidine-oxazolidine intermediate. The Journal of Organic Chemistry, 71, 6600-6603. 

The mechanism and stereo-selectivity of aziridine synthesis from guanidinium ylides and a series of benzaldehydes have been studied using the mPWlK and... 

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