Reactive intermediate generation carbenes, nitrenes

Note 1 Chemical reactions that are induced by a reactive intermediate (e.g., radical, carbene, nitrene, or ionic species) generated from a photo-excited state are sometimes dealt with as a part of photochemistry. 

The great majority of matrix isolation studies of carbenes and nitrenes have employed their formal adducts with molecular nitrogen, that is, diazo compounds or diazirines in the case of carbenes, azides in the case of nitrenes, as precursors for their in situ generation. Usually, these compounds will readily release N2 on irradiation with a low-pressure mercury lamp (254 nm), and this fragment has the advantage that it will usually not react with or perturb the targeted reactive intermediate (see Scheme 17.2). 

Phosphinidenes (R-P), short-lived, reactive intermediates that are isoelec-tronic with nitrenes and carbenes, are accessible by fragmentation of phos-phiranes, phosphol-3-enes, triazaphospholenes, and oxazaphospholenes and can be characterized by trapping reactions. The metal-complexed species [R-P-M, e.g., M = W(CO)5] are more stable (but also not isolable) and react more selectively. As a model example we describe the transformation of (3,4-dimethyl-l-phenylphosphole)(pentacarbonyl)tungsten 4 with dimethyl acetylenedicarboxylate to the 7-phosphanorbomadiene derivative 5.11 The (phosphinidene)(pentacarbonyl)tungsten complex 6 is generated from the latter by a [4+ 1]-cycloreversion and trapped with diphenylacetylene via a [2+1]-cycloaddition to furnish the triphenyl-2-phosphirene complex 7 (Scheme l).12... 

These authors have emphasized that the best way to distinguish between these two possible mechanisms of labeling is by the addition of scavengers of carbenes, nitrenes or whatever the chemical nature of the activated form of the photoaffinity label is. A true photoaffinity label (i.e. one which reacts irreversibly with the same protein molecule to which it was bound at the instant of photolysis) will not be inhibited by added scavengers, whereas a pseudo photoaffinity label (i.e. one which accomplishes derivatization via a reactive intermediate which has been generated in solution) will be. In their studies with nitrenes, Ruoho et al. (1973) found that 10 M p-aminobenzoic acid was an effective scavenger. It should be emphasized that the two possible mechan-... 

Structure and Reactivity of Carbenes Generation of Carbenes Addition Reactions Insertion Reactions Rearrangement Reactions Related Reactions Nitrenes and Related Intermediates Rearrangements to Electron-Deficient Nitrogen... 

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