Staudinger reaction, intermediates detection

More recently, Wilson et have used ESI-MS to detect transient intermediates in the Wittig, Mitsunobu and Staudinger reactions directly from solution. Ionic intermediates were directly detected while zwitterionic intermediates were indirectly detected following acid quenching. The systems studied were those which had been previously investigated via P NMR. This work demonstrated that the progress of these reactions can be... 

Detection of transient ionic intermediates in the [241] Wittig, Mitsunobu, and Staudinger reactions directly from solution... 

The general nature of these reactions was first recognized by Staudinger and his coworkers in the reaction of ketenes with olefins. Huisgen and his coworkers demonstrated that the cycloaddition reaction of diphenylketene with vinyl ethers is stereospecific, indicating a concerted one-step mechanism. However, more otfen the [2+2] cycloaddition reactions proceed in a stepwise fashion. In recent examples it was demonstrated that the initial reaction of ketenes with several substrates produces adducts which are different from the isolated ones (see Chapter 4, Section 4.1.4.2) . Also switter ionic intermediates are detected by low temperature spectroscopy. For example, Machiguchi and coworkers have detected the formation of 1,4-switter ionic species as intermediates in the reaction of bis(trifluoromethyl)ketene with ethyl vinyl ether. 

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