Nitrobenzyl photochemistry

Greenberg MM, Gilmore JL. Cleavage of oligonucleotides from solid-phase supports using o-nitrobenzyl photochemistry. J Org Chem 59 746-753, 1994,... 

Recently, nonionic acid precursors based on nitrobenzyl ester photochemistry have been developed for chemically amplified resist processes (78-80). These ester based materials (Figure 8) exhibit a number of advantages over the onium salt systems. Specifically, the esters are easily synthesized, are soluble in a variety organic solvents, are nonionic in character, and contain no potential device contaminants such as arsenic or antimony. In addition, their absorption characteristics are well suited for deep-UV exposure. 

Figure 8 Schematic representation of o-nitrobenzyl ester photochemistry.

For a detailed review of the photochemistry of 2-nitrobenzaldehyde and its derivatives as well as of o-nitrobenzyl compounds in general the reader is referred to Ref. 8). Photochromic s5rstems of the o-nitrobenzyl type will also not be discussed here. 

Bley, F., Schaper, K. and Gorner, H. (2008) Photoprocesses of molecules with 2-nitrobenzyl protecting groups and caged organic adds. Photochemistry and Photobiology, 84, 162—171. 

As noted in the introduction to this chapter, the photochemistry of the ortho-nitrobenzyl group parallels that of ortho-alkyl aromatic ketones in that in the excited state a hydrogen atom can migrate from the benzylic position to an oxygen atom of the nitro group. This reaction has now been studied by time resolved... 

The orffto-nitrobenzyl chromophore has also been employed for phosphate deprotection but, as in Eq. (12) for amides, this photochemistry again does not result from excited state carbon-oxygen bond cleavage [126,128]. Similarly, de-syl phosphates like 89 release phosphate photochemically but, as discussed in Sec. IV.D.3, by a pathway beginning with n,n excited state of the carbonyl group [13,126,129]. 

The photochemistry of nitrophenylacetates has been investigated by UV-pump/IR-probe spectroscopy, combined with quantum chemical calculations. For the meta and para isomers, decarboxylation proceeds via a triplet state with subnanosecond lifetime. In the case of the ortho derivative, photodecarboxylation is nearly suppressed by excited state proton or hydrogen atom transfer, but it can still be investigated due to the isolated spectral position of the CO2 band. Data analysis reveals that a weak ultrafast release channel represents the main photodecarboxylation pathway for this compound. Understanding CO2 uncaging mechanisms can be useful for the design of improved nitrobenzyl cages. 

Walker, J. W., McCray, J. A. Hess, G. P. (1986). Photolabile protecting groups for an acetylcholine receptor ligand. Synthesis and photochemistry of a new class of o-nitrobenzyl derivatives and their effects on receptor function. Biochemistry, 25, 1799-805. 

Direct heterolytic fission of the substrate-photoprotecting group bond is the required course for photorelease of most biologically important substrates. This process avoids the generation of destructive radicals that could result in reactions such as decarboxylation, radical dimerization, or redox processes. Thus, the effect of m-substitution on the photochemistry of benzyl, naphthyl, and other aromatic chromophores has become the object of many studies in search of alternatives to the o-nitrobenzyl class of protecting group. 

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