2-Nitrobenzyl, photochemical cleavage

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

The substitutions by o-nitrobenzyl groups are used for dioxolane and 1,3-dioxane derivatives, but the photochemical cleavage is only partially regioselective. However,... 

Holmes, C.P. Model Studies for New o-Nitrobenzyl Phololabile Linkers Substituent Effects on the Rates of Photochemical Cleavage, J. Org. Chem. 62, 2370-2380 (1997). 

Other variations of the nitrobenzyl linker are the nitroveratryl (4) and hydroxynitrobenzyl (5) linkers, which are characterized by one or two alkoxy substituents on the nitrobenzene ring. The increased electron density at the benzylic position in these linkers improves photochemical cleavage efficiency through stabilization of radical intermediates in photolysis and also retards diketopiperazine formation during the coupling of the third amino acid in peptide synthesis. 

The photochemical cleavage of the phosphorus-carbon bond in many benzylic phosphonic acids occurs through intramolecular electron transfer and gives monomeric metaphosphate together with substituted toluenes and, in many cases, 1,2-diarylethanes. The ultraviolet irradiation of (4-nitrobenzyl)phosphonic acid dianion in solution in aqueous ethanol yields l,2-bis(4-nitrophenyl)ethane, 4-nitrotoluene, H3PO4 and monoethyl phosphate (Scheme Irradiation of the same acid in the presence of dbu (other tertiary... 

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. ... 

Benzobenzvalene (100) has been shown to add sulphur dioxide to give the sulphone (101) and the 7-sultine (102) both adducts extrude sulphur dioxide on direct photolysis regenerating benzobenzvalene accompanied by naphthalene. Competing homolytic and heterolytic bond cleavages have been observed in singlet excited triarylsulphonium salts, and intramolecular electron transfer is involved in the photodissociation of p-nitrobenzyl 9,10-dimethoxy-anthracene-2-sulphonate.A photochemical method for the detosyl-ation of sulphonamides has been described,and the thermolysis and photolysis of some thiourea derivatives have been compared. 

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]. 

In addition to Woodward s scavenger treatment [18], the use of an a-substituted nitrobenzyl Hnker was used to solve the problem of aldehyde condensation, since the ketone photolysis byproduct is substantially less reactive than an aldehyde. On the basis of the accepted cleavage mechanism, though, photochemical deprotection should still be feasible with an a-substituted nitrobenzyl linker. 

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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