2-Nitrobenzyl ethers photochemical cleavage

Scheme 3.—Proposed Mechanism for the Photochemical Cleavage of 2-Nitrobenzyl Ethers and 2-Nitrobenzyl Glycosides.

Photolysis of o-Nitrobenzyl Derivatives - The cleavage of o-nitrobenzyl derivatives is one of the relatively few classes of photofragmentations in which the two fragments can both bear complex functionality. The protection of alcohols as o-nitrobenzyl ethers is well known, and provides a photochemical method of deprotection, but other applications include the design of photoclea-vable polymers and photochemically active links for molecules synthesized on polymer beads. 

The fj-nitrobenzyl and p-nitrobenzyl ethers can be prepared and cleaved by many of the methods described for benzyl ethers. In addition, the o-nitrobenzyl ether can be cleaved by irradiation (320 nm, 10 min, quant, yield of carbohydrate 280 nm, 95% yield of nucleotide ). This is one of the most important methods for cleavage of this ether. These ethers can also be cleaved oxidatively (DDQ or electrolysis) after reduction to the aniline derivative." Clean reduction to the aniline is accomplished with Zn(Cu) (acetylacetone, rt, >93% yield).Hydrogenolysis is also an effective means for cleavage. A polymeric version of the o-nitrobenzyl ether has been prepared for oligosaccharide synthesis that is also conveniently cleaved by photolysis. An unusual selective deprotection of a bis-o-nitrobenzyl ether has been observed. The photochemical reaction of o-nitrobenzyl derivatives has been reviewed. ... 

Shimizu et al report that while [2.2] paracyclophane (55) undergoes two-photon dissociation in low temperature matrices by way of the triplet state, in the gas phase, the efficient two-photon process proceeds via a hot molecule formed by internal conversion from the initially formed singlet excited state. The photocleavage of 2-nitrobenzyl ethers and ester has been widely reported and has now been evaluated as a deprotection methodology for indoles, benzimidazole, and 6-chlorouracil (Voelker et al). The mechanism of the cleavage of such compounds is considered to involve the o-quinonoid intermediate, but previously these had only been deduced from transient electronic spectra produced in flash photolysis experiments. Infrared spectral data from photochemical studies of 2-nitrobenzyl methyl ether in argon and nitrogen matrices have now been published which confirm that the intermediate does indeed have the o-quinonoid structure... 

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