Photoprocesses of Molecules with 2‐Nitrobenzyl Protecting Groups and Caged Organic Acids

Abstract

The 308 nm photoinduced formation of the nitroso product and the intermediacy of the aci-nitro form(s) were studied for a series of 2-nitrobenzyl alkyl and aryl esters (1a-4e) and bis-(nitrophenyl)methyl acetates (5a-6b) by time-resolved UV-vis spectroscopy. A triplet state appears as major transient, when 2-nitrobenzyl derivatives 1 are substituted by 4,5-dimethoxy (2) and 4,5-methylenedioxy (3/4) groups. This triplet of charge transfer character is, however, not part of the route via the aci-nitro into the 2-nitroso form. The activation energy and preexponential factor of the longest lifetime component (tau(aci)), i.e. the major part of the aci-nitro decay, were determined. The carboxylic acids as leaving groups have rather small effects on tau(aci). An additional nitrated phenyl ring in alpha-position (5) leads generally to shorter tau(aci) value. Otherwise, the photogeneration of nitroso products is similar. The quantum yield (Phi(d)) varies only moderately with structure, the yield of the aci-nitro form and Phi(d) are correlated and little affected by solvent properties.

Photocleavage of the Polypeptide Backbone by 2-Nitrophenylalanine

“…We anticipated that incorporation of 2-NPA into proteins might allow photocleavage of the polypeptide backbone based on the photochemistry of (2-nitrophenyl)ethane derivatives[8-10] which like the more common 2-nitrobenzyl group [11, 12] are used as photochemical caging groups. Specifically, alkoxy substituents β to the nitrophenyl group undergo photochemical elimination reactions, suggesting that an amide group might behave similarly.…”

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

“…167 On the other hand, Görner showed that triplet states of CT character are formed by direct excitation of NV derivatives and related compounds that, however, do not participate in aci -nitro formation or the release of the substrate. 168 …”

Intramolecular Sensitization of Photocleavage of the Photolabile 2‐(2‐Nitrophenyl)propoxycarbonyl (NPPOC) Protecting Group: Photoproducts and Photokinetics of the Release of Nucleosides

“…[15,16] It is of the onitrobenzyl-type in which cleavage occurs at the benzylic carbon and the leaving moiety is an aromatic o-nitrosocarbonyl compound. [17] In photoremovable protecting groups involving the 2-(2-nitrophenyl)propoxycarbonyl (NPPOC) group, which was introduced by Pfleiderer and co-workers, [18][19][20] cleavage is due to b elimination, and formation of the reactive and problematic nitroso byproduct [21] can be suppressed by using suitable reaction conditions (Scheme 1). [22] This is probably the reason why chips of higher quality result when using the NPPOC group.…”