Carbenes matrix photochemistry

The matrix photochemistry of 2n24 and 2o92 is completely analogous to that of 2a. The primary irreversible loss of nitrogen from 2 produces carbenes 1 in photostationary equilibria with cyclopropenes 3 (Scheme 15). The relative amounts of 1 and 3 formed in the matrix depends very much on the wavelength used for the irradiation. Both carbenes In and lo were chemically identified by oxygen trapping. UV irradiation (248 nm) of In produces a mixture of indeneketene 21, CO, and indenylidene 22 (Scheme 14). 

The matrix photochemistry of 2v proved to be fairly complicated.108 The primary product of the photolysis of 2v is carbene lv, which was identified by ESR spectroscopy. Under the conditions of matrix isolation the carbene showed the expected reactivity towards molecular oxygen (formation of carbonyl oxide 7v) and carbon monoxide (formation of ketene lOv) (Scheme 22). In contrast to the oxocyclohexadienylidenes (la and derivatives) carbene lv slowly reacted with CO2 to give an a-lactone with the characteristic C=0 stretching vibration at 1896 cm-1. The latter reaction indicates that lv is — as expected — more nucleophilic than la. 

Platz, M.S., Leyva. E., Haider, K. (1991), Selected Topics in the Matrix Photochemistry of Nitrenes, Carbenes, and Excited Triplet States, Org. Photochem. (Padwa, A., Ed.) 11, 367. 

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]. 

Komnick, P. and Sander, W., Intramolecular carbene addition to a triple bond matrix photochemistry of a-diazo-2-ethynylacetophenone, Liebigs Ann. Chem., 7, 1996. 

Photochemistry of (2-nitrophenyl)diazomethane 70 has been studied by excitation at 350 nm in argon matrix isolation system54. That shows that at 10 K, 2-nitroso-benzaldehyde is formed by intramolecular oxygen migration from (2-nitrophenyl) carbene... 

Decarboxylation of (175) occurs on its irradiation in an argon matrix at 10 K using 254 nm light. Spectroscopic analysis of the resulting matrix indicates the presence of a complex between carbon dioxide and the carbene (176). Tiaprofenic acid (177) undergoes facile photochemical decarboxylation, and this is reported to take place from an upper triplet excited state." A study of the transient photochemistry of 5-(p-toluyl)-l-methyl-2-pyrrolylacetic acid has been reported. Decarboxylation results in the formation of a carbanion in its triplet state. A laser-flash study using irradiation at 266 nm of the xanthene-9-carboxylate (178) has shown that the radical (179) is formed. This study used NaY zeolites and studied the oxidation of the radical within the cage structure. Calculations have indicated that decarboxylation of (180) and (181) and deprotonation of cycloheptatriene and cyclopentadiene affords the same anions (182) and (183), respectively. ... 

An interesting intramolecular oxidation was found by Tomioka et al. (1992b) when studying the photochemistry of (2-nitrophenyl)diazomethane (8.56) in an argon matrix at 10 K. Irradiation at A > 350 nm provided 2-nitrosobenzaldehyde (8.57), presumably as a result of intramolecular reduction of the nitro group with the neighboring carbene center (8-32). ... 

A prominent strategy in matrix isolation photochemistry has been the expulsion of N2 from stable precursors. Both diazo/diazirine precursors and 1,2-diazene precursors have been useful. The former generate carbenes and related species, while the latter are useful precursors to biradicals. 

Matrix IR Spectra of Radicals Photochemistry of Matrix-Isolated Radicals Carbenes and Their Reactions Cyclopentadienylidene and Related CarbenesAryl Carbenes ... 

Tomioka, H., Ichikawa, N., and Komatsu, K., Photochemistry of (2-nitrophenyl)diazomethane studied by a matrix isolation technique (nitrophenyl)carbene to (carboxylphenyl)nitrene rearrangement by successive reduction of nitro group with carbenic center, /. Am. Chem. Soc., 114, 8045, 1992. 

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