Reactions of Photochemically Formed Transients From 2-Nitrotoluene

Dedicated with best wishes to Prof. Edgar Heilbronner on the occasion of his 80th birthdayThe photochemistry of several 2-(2-nitrophenyl)ethyl-caged compounds including caged thymidine nucleosides was studied by nanosecond laser flash photolysis and stationary illumination experiments with quantitative HPLC analysis for quantum yields and product distribution. Effects of solvent basicity and acidity were investigated by varying the H 2 O content and HCl concentration, respectively, in MeCN/H 2 O mixtures. For all compounds 1 ± 7 investigated, intramolecular H abstraction by the nitro group from the exocyclic a-position with respect to the aryl moiety was found to be the primary process. The protolytic dissociation equilibrium of the resulting aci-nitro compound was kinetically characterized in the 0.1 ± 10 ms time region. In general, two reaction channels compete for the aci-nitro compound and its anion: b-elimination of the caged compound occurs from the anion, while from the undissociated aci-nitro compound, a nitrosobenzene derivative is formed with no release of the caged compound. The yield ratio of these two reaction channels can be controlled through shifts in the protolytic dissociation equilibrium of the aci-nitro compound. In solutions with either low basicity (H 2 O-free MeCN) or high acidity (higher concentration of HCl in H 2 O/MeCN), two as yet unidentified products are formed, each one specifically for one of the mentioned conditions. Introduction. ± Photolabile protecting groups play an important role in synthetic organic chemistry [1], for caging of biologically active molecules [2], and for lightdirected, combinatorial solid-phase syntheses of biopolymers [3]. A particularly attractive application is the generation of so-called high-density DNA chips [4] [5] which are needed in various types to perform the sequencing by hybridisation (SBH) method for oligonucleotide and gene sequencing [6]. The o-nitrobenzyl (oNB) type of protecting groups has been commonly used since 1901 when the photoreaction was discovered [7], and its reaction mechanism has been investigated in detail [8]. So far, the [(a-methyl-2-nitropiperonyl)oxy]carbonyl ([1-(6-nitro-1,3-benzodioxol-5-yl)-ethoxy]carbonyl; MeNPOC) group has been the preferred choice in DNA-chip production [4].The photochemical cleavage mechanism of the (o-nitrobenzyl)oxy function is triggered by the abstraction of a benzylic H-atom by the excited nitro group [9]. Recently, a new type of photolabile protecting group was developed on the basis of the [2-(2-nitrophenyl)ethoxy]carbonyl functionality [10] [11] which is cleaved by a lightinduced b-elimination process with formation of o-nitrostyrene, CO 2 , and the corresponding alcohol. It was suggested [10] that, in analogy to the o-nitrobenzyl groups, the cleavage of the [2-(2-nitrophenyl)ethoxy]carbonyl moiety, too, should start with the formation of an aci-nitro intermediate, through H abstraction by the nitro group at the exocyclic a-position with respect to the aryl moiety. Based on results from laser f...

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“…3 . These rate constants correspond to a pK a of 4 which is similar to what has been published for other o-quinonoid aci-nitro compounds [16].…”

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confidence: 84%

Photolabile Protecting Groups for Nucleosides: Mechanistic Studies of the 2-(2-Nitrophenyl)ethyl Group

Walbert

Pfleiderer

Steiner

2001

HCA

107

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“…Wettermark and coworkers [5-71 showed with flash photolysis studies that the phototropic activity of 2-nitro and 2,4-as well as 2,6-dinitrotoluenes was due to a similar type of rearrangement as the one shown in Mechanism 11: They further determined the pK, values of the equilibrium between the aciquinoid isomer and the conjugate base for 2-nitro and 2,4-and 2,6-dinitrotoluenes and found them to be equal to 3.7 [8], 1.1 [7], and 1.8 191, respectively.…”

Section: \ /mentioning

confidence: 84%

Flash Photolysis of 2,4,6-Trinitrotoluene Solutions

Suryanarayanan¹,

Capellos²

1972

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“…They further determined the pK, values of the equilibrium between the aciquinoid isomer and the conjugate base for 2-nitro and 2,4-and 2,6-dinitrotoluenes and found them to be equal to 3.7 [8], 1.1 [7], and 1.8 191, respectively.…”

Section: \ /mentioning

confidence: 99%

Flash Photolysis of 2,4,6‐Trinitrotoluene Solutions

Suryanarayanan

Capellos

1974

Int J of Chemical Kinetics

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Flash photolysis of 2,4,6‐trinitrotoluene solutions (TNT) indicates a photochemically induced isomerization leading to the aci‐quinoid isomer with an absorption maximum at 460 nm in nonpolar solvents, or the conjugate base of the aci‐quinoid isomer with absorption maxima at 500, 540, and 630 nm in polar solvents. Both species, the quinoid isomer and the conjugate base, have transient existence and their identification was based on detailed spectroscopic and kinetic studies presented here. The magnitude of the negative charge on the transient conjugate base was determined with salt effect experiments and found to be equal to unity.

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Reactions of Photochemically Formed Transients From 2-Nitrotoluene

Cited by 36 publications

References 15 publications

Photolabile Protecting Groups for Nucleosides: Mechanistic Studies of the 2-(2-Nitrophenyl)ethyl Group

Photolabile Protecting Groups for Nucleosides: Mechanistic Studies of the 2-(2-Nitrophenyl)ethyl Group

Flash Photolysis of 2,4,6-Trinitrotoluene Solutions

Flash Photolysis of 2,4,6‐Trinitrotoluene Solutions

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