Compounds and Diazirines

Diazo compounds have long been exploited as excellent sources of carbenes (see Section 10.11.2). A valuable feature of this route to carbenes is that the spin multiplicity of the carbene can be controlled. Experiments such as those shown in Eqs. 16.71 and 16.72 were among the first to provide definitive evidence of the differing reactivity patterns of singlet vs. triplet carbenes. 

Photolysis of a-diazoketones produces acyl carbenes, which efficiently rearrange to ke-tenes that are subsequently trapped (Eq. 16.73). This photochemical version of the Wolff re- 

Diazirines are simply cyclic isomers of diazo compounds (Eq. 16.74). In fact, the two forms can sometimes be interconverted photochemically. The photochemistry of a diazirine mirrors that of the analogous diazo compound—that is, loss of N2 to produce a carbene. In some circumstances the diazirine form is more desirable, especially in photoaffinity labeling applications. 

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Finally, in some cases diradical negative ions can even be generated directly npon ionization of appropriate precnrsors. For example, nitrene and carbene anions can be formed by El of organic azides, diazo-compounds, and diazirines, whereas Branman and co-workers have reported the formation of oxyallyl anions by El of flnorinated acetyl componnds (Eq. 5.12). ... 

Similar conclusions concerning the photochemistry of diazo carbonyl compounds and diazirines have been reached using similar experiments and logic. 

Carbenes, which may be formed photochemically from precursors such as diazo compounds and diazirines, are highly reactive entities containing divalent carbon. The first photogenerated reagent was a carbene, formed by irradiation of diazoacetyl chymotrypsin (Singh et al., 1962). 

In Chapter 2 an account was given of the various classes of molecules that have been used as photoactivatable reagents. This chapter concentrates on the three classes, aryl azides, diazo compounds and diazirines, which can be considered as proven and established. 

In Section 3.5, only the most generally useful methods for making azides, diazo compounds and diazirines can be covered. In the unlikely event that these methods fail the reader may wish to try less commonly used alternatives. These may be found by searching for the syntheses of analogous compounds in Chemical Abstracts, or by consulting the appropriate volumes in Houben-Weyl Methoden der Organischen Chemie, Beilstein , and The Chemistry of. .. series, e.g. The Chemistry of the Azido Group, S. Patai, ed., Interscience (1971). 

It is known that diazo compounds and diazirines are photochemical precursors to carbenes [54]. 

Recently, new photochemical carbene precursors which differed from diazo compounds and diazirines have been reported [62, 63], Platz et al. synthesized a photosensitive precursor 86 to phenylsulfanylcarbene 87 (Scheme 4.39) [62], The photolysis of 86 in the presence of 2,3-dimethyl-2-butene yielded the [2 +1] cycloadduct 88 in 80% yield. In contrast, Jenks et al. reported that S,C-ylides of malonates were photochemical precursors of carbene 90 [63]. Thus, the S,C-ylide 89 was irradiated at 254 nm in acetonitrile with 10% cyclohexene to give 91 in 44% yield (Scheme 4.40). 

Elimination of Nitrogen from Diazo Compounds and Diazirines... 

There is ample evidence in the literature that excited states of diazo compounds and diazirines often display carbene-like reactivity. For review, see M. S. Platz, In Advances in Carbene Chemistry, Vol. 2, U. H. Brinker, Ed., JAI Press, Greenwich, CT, 1998, p. 133. 

Thermal or photochemical decomposition of diazo compounds and diazirines gives carbenes. The formation of very stable gaseous nitrogen compensates for the formation of the unstable carbene. Photolysis or thermolysis of a ketene-Hke diazo compound eliminates a stable molecule carbon monoxide (CO) to yield a carbene. The reactions are not widely used since ketenes are not readily available precursors, and tend to polymerize under the reaction conditions (Scheme 5.5). 

Contents of Vol. 5 Electron Spin Resonance of Irradiated Organic Fluorine Compounds (M. Iwasaki) The Preparation and Chemistry of Fluorinated Diazo Compounds and Diazirines (C. G. Krespan and W. J. Middleton) Fluorine-containing Epoxides (P. Tarrant, C. G. Allison, K. P. Barthold, and E. C. Stump) The Chemistry of Fluoroalkyl Radicals (A. P. Stefani). 

Due to the complication of alternate pathways involved in the photochemistry of diazo compounds or diazirines, new strategies for carbene generation from non-nitrogenous precursors have been studied. Phenyliodonium ylides" (27) andmethyl-8-chloro-3a,7a-methanoindan-8-carboxylates" (28) are alternative precursors to dicarboethoxycarbene and carbomethoxychlorocarbene, respectively. Photoreactions from these precursors presumably will not be plagued by the excited-state chemistry observed for the diazo compounds and diazirines. 

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