Photochemistry diazo compounds

Photo-de-diazoniation has found relatively little application in organic synthesis, as is clearly evident from the annual Specialist Periodical Reports on Photochemistry published by the Royal Society of Chemistry. Since the beginning of these reports (1970) they have contained a section on the elimination of nitrogen from diazo compounds, written since 1973 by Reid (1990). In the 1980s (including 1990), at least 90% of each report is concerned with dediazoniations of diazoalkanes and non-quinon-oid diazo ketones, the rest being mainly related to quinone diazides and only occasionally to arenediazonium salts. 

The photoelimination of nitrogen from azo compounds, diazo compounds, and azides is a topic that has been thoroughly reviewed elsewhere. These transformations are found in oxygen and sulfur heterocycles, but as they are not unique to these systems, they will not be discussed in this article. Examples can be found in a review on the photochemistry of nitrogen-containing heterocycles.1... 

SCHEME 2. Photochemistry of (silyl)diazo compounds in argon matrix... 

The photochemistry of diazo compounds has been discussed in detail in Houben-Weyl, Vol. 4/5 b, pp 1158 1257. Photochemical cleavage of an a-diazocarbonyl compound into an acylcarbene and nitrogen is typically carried out by UV irradiation at 1 = 254-300 nm. 

The areas covered by this chapter deal with the photochemistry and radiation chemistry of triply bonded systems. The definition of a system with a triple bond is fairly loose and the review will deal with compounds which contain the following functionalities alkynes, nitriles, diazo compounds, diazonium salts and azides. The photochemical reactivity of these compounds has been reviewed extensively This review is not intended to be encyclopaedic but will highlight some of the areas currently of interest. 

Several reviews have dealt with the photochemical reactions of the types of compounds included in this chapter. Alkynes, J. D. Coyle m The Chemistry of the Carbon-Carbon Triple Bond (Ed. S. Patai), Wiley, Chichester, 1978, p. 523 Nitriles, in The Chemistry of the Cyano Group, diazonium salts and diazo compounds by W. Ando, m The Chemistry of Diazonium and Diazo Groups (Ed. S. Patai), Wiley, Chichester, 1978, p. 341 and radiation chemistry by Z. B. Alfassi, m The Chemistry of Functional Groups, Supplement C(Eds. S. Patai and Z. Rappoport), Wiley, Chichester, 1983, p. 187. Azides have been the subject of another extensive review [E. F. Scriven and K. Turnbull, Chem. Rev., 88,298 (1988)]. In addition to these references, extensive information on the photochemical reactivity of such groups is to be found in the appropriate chapters in Volumes 1-24 of Photochemistry published by the Royal Society of Chemistry, London, from 1968-1993. 

Generation of Alkyl and Aryl Carbenes. - A review of recent experimental and theoretical progress in the photochemistry of diazirines has been published. These compounds, unlike their valence isomers, diazo compounds, are relatively stable to most organic reagents and are reasonably stable in dilute solutions, and are therefore useful photochemical and thermal precursors to carbenes. 

The following additional examples offer a hint of the utility of organic photochemistry in s)mthesis. A [2 + 2] photochemical cycloaddition was a key step in the synthesis of cubane. S)mthesis of 2-bromocyclopenta-2,4-dienone resulted in a spontaneous Diels-Alder reaction, which produced 132. The monoketal of the dimer (133) underwent intramolecular photocycloaddition to produce 134 (equation 12.85). The Favorskii reaction of 134 gave 135, which was decarboxylated to 136 (equation 12.86). Hydrolysis of the ketal gave 137. Subsequent Favorskii reaction to 138 and then decarboxylation produced cubane, 139. Similarly, de Meijere and co-workers reported that the intramolecular [2 + 2] photochemical cycloaddition of 140 produced octacyclopropylcubane (141, equation 12.87) Meder and coworkers utilized the reaction sequence in equation 12.88 to convert the diazo compound 142 into tetra-f-butylcyclobutadiene (143) and tetra-f-butyltetra-hedrane (144). " ... 

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. 

As discussed by Bally [33], matrix photochemistry has obtained excellent results, provided that back reaction is not overwhelming. This is a problem with radicals, much less for carbenes and nitrenes when formed by elimination reactions from diazo compounds and respectively azides. In the latter case, N2 is eliminated with no significant alteration of the matrix. There are exceptions, however, as is the case of parent carbene, the identification of which has long been hindered by the efficient recombination occurring, and finally demonstrated by exchange [42]. 

Chapman, O.L., Photochemistry of diazo compounds and azides in argon, Pure Appl. Chem., 51, 331, 1979. 

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. 

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

In the photochemistry of a-diazo carbonyl compounds, singlet carbenes undergo a Wolff rearrangement while triplet carbenes react with alkenes to afford cydopropanes. 

Photfrfysis of a-Diazo Carbonyl Compounds - Some recent advances in the matrix photochemistry of diazoketones, including some heterocyclic species, have been reviewed. Flash photolysis of 10-diazo-9(10//)-phenanthrenone (35) in aqueous solution led to the detection of two transient species on the pathway to the final product, fluorene-9-carboxylic acid. These were identified, from solvent isotope effects and the nature of the observed acid-base catalysis, as fluorenylideneketene (36, X = CO) and the enol of fluorene-9-carboxylic acid (36, X = C(0H)2), formed by hydration of the ketene. In related studies, fluorenylideneketene was found to react with amines to give ylides as intermediates on the route to the amide final products. The product distribution from the photochemical reactions of 2-diazo-3-oxo-5,10,15,20-tetraphenylchlorins with alcohols strongly depends on the central metal ion of the irradiated diazoketones. ... 

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