Phosphonium-enamine zwitterion

Most recently, Tong et al. isolated the stable phosphonium-enamine zwitterion 23, which has long been postulated as one of the key intermediates in the aza-MBH reaction, from the PPhs-catalyzed reaction between propiolate and N-tosylimine (Scheme 31.8), providing experimental evidence to support the postulated reaction mechanism of the phosphine-catalyzed MBH reaction [22]. 

Zimmerman-Traxler transition state 57 zinc(ll) ions 99 zwitterions 632, 905 - phosphonium-enamine 905... 

The adduct 65 of aziridine to the acetylenic phosphonium salt 64 readily rearranges to the enamine 66, which in refluxing acetonitrile is converted into the pyrroline derivative 68 by opening of the aziridine ring and recyclization of the resulting zwitterion 67 (equation 33)54. 

---