Kinetic stabilizing effect of the 4-N-methylacetamido substituent on the phenylnitrenium ion

2001 ◽  
Vol 79 (12) ◽  
pp. 1875-1880 ◽  
Author(s):  
Patrick H Ruane ◽  
Robert A McClelland
Keyword(s):  
Positive Charge ◽  
Flash Photolysis ◽  
Ph Profile ◽  
Phenyl Azide ◽  
Acidic Solutions ◽  
Nitrenium Ion ◽  
Azide Ion ◽  
Methoxy Substituent ◽  
Long Lifetime ◽  
Arylnitrenium Ions

Photolysis of 4-(N-methylacetamido)phenyl azide in aqueous solution results in quantitative formation of the 4-(N-methylacetamido)phenylnitrenium ion, this cation arising from solvent protonation of an initially formed singlet arylnitrene. The cation is observed by flash photolysis, and is identified through characteristic quenching by azide ion and by 2'-deoxyguanosine, both excellent nucleophiles for arylnitrenium ions in water. The nitrenium ion is protonated in acidic solutions to form the 4-(N-methylacetamido)aniline dication, whose pKa is determined to be 1.5 based on the rate–pH profile. This means that the nitrenium ion is relatively basic, which suggests that there is significant positive charge on the N-methylacetamido group. Further evidence for this is seen in the remarkably long lifetime (5 ms) of the nitrenium ion in water. In fact, the 4-(N-methylacetamido)phenylnitrenium ion is 5000-fold longer-lived than the 4-methoxy-substituted analog. A 4-methoxy substituent on a phenyl ring is more electron donating according to σ+ values (–0.78 for MeO vs. –0.60 for NMeAc). The dramatic reversal in the arylnitrenium ions is another example of the failure of these to follow the carbocation scale.Key words: nitrenium ion, flash photolysis, amide substituent.

Carbocation-like reactivity patterns in X'-substituted-4-biphenylylnitrenium ions

1998 ◽  
Vol 76 (1) ◽  
pp. 78-84 ◽  
Author(s):  
Daniel Ren ◽  
Robert A McClelland
Keyword(s):  
Positive Charge ◽  
Flash Photolysis ◽  
Substituent Effects ◽  
Diffusion Limit ◽  
Poor Correlation ◽  
Carbenium Ions ◽  
Azide Ion ◽  
Arylnitrenium Ions ◽  
Reaction With Water ◽  
Two Parameter

4-Azido-X'-substituted biphenyls (X' = 4'-MeO, 4'-Me, 4'-F, 3'-Me, 4'-Cl, H, 3'-MeO, 3'-Cl, 4'-CF3) have been prepared and subjected to 248 nm flash photolysis irradiation in 20:80 acetonitrile:water. Transient X'-substituted 4-biphenylylnitrenium ions 10 (Ar C6H4-N + H) are observed, with lifetimes ranging from 0.6 ms (4'-MeO) to 26 ns (4'-CF3). These cations are quenched by azide ion, with values of kaz ranging from 6 to 10 x 109 M-1 s-1, with the majority in the range (9-10) x 109. This near constant kaz provides further evidence that arylnitrenium ions are quenched by azide ion at the diffusion limit. The solvent reactivities, plotted in a single-parameter Hammett plot against σ sup + (X), exhibit a poor correlation, with the points for the para π-electron donors deviating from the correlation line based on the other substituents in the direction of requiring a more negative substituent parameter. The data are more satisfactorily fit to the two-parameter Yukawa-Tsuno equation; the parameter r + obtained in this fit is 2.8. Thus the resonance interaction of the para π -donor X'-substituents with the positive charge of the cation is underestimated by σ sup + , a situation that has previously been observed with benzylic-type carbenium ions. The conclusion is made that, in their reaction with water, 4-biphenylylnitrenium ions behave like benzyl cations bearing two additional stabilizing vinyl groups, i.e., as if they had the structure Ar-C + (C = C)2. The inherent reactivity and the pattern of the aryl substituent effects are in fact similar to those in the carbocation series Ar-C + (Ph)2.Key words: nitrenium, nitrene, aryl azide, photochemistry, lifetime.

Reaction of arylnitrenium ions with guanine derivatives: N1-methylguanosine and N2,N2-dimethylguanosine

2001 ◽  
Vol 79 (12) ◽  
pp. 1881-1886 ◽  
Author(s):  
Bernice Cheng ◽  
Robert A McClelland
Keyword(s):  
Rate Constants ◽  
Flash Photolysis ◽  
Initial Step ◽  
Direct Reaction ◽  
Ph Profile ◽  
Nitrenium Ion ◽  
Base Form ◽  
Guanine Derivatives ◽  
Arylnitrenium Ions ◽  
Conjugate Base

A prior flash photolysis study of the direct reaction of arylnitrenium ions with 2'-deoxyguanosine identified a second intermediate that grew in as the transient nitrenium ion reacted with the nucleoside. This intermediate was identified as the the product of the addition of the nitrenium ion to the C-8 position of guanine prior to loss of the C-8 proton — the C-8 intermediate. A feature of the C-8 intermediate is that it exists in acid–base forms. This behavior was evident in both a spectroscopic analysis as well as in the rate–pH profile, which showed a break around pH 4 from a pH-independent reaction to a reaction that was first-order in H+. The present study was designed to identify the structure of the conjugate base form. This involved a kinetic study of the decay of the C-8 intermediate derived from the reaction of the 2-fluorenylnitrenium ion with N1-methylguanosine and N2,N2-dimethylguanosine. The rationale was that the former is unable to lose the N-1 proton, while the latter cannot deprotonate at the NH2 group. The rate–pH profiles clearly show that it is the N-1 proton that is acidic. The rate constants for the C-8 intermediate of N2,N2-dimethylguanosine show the same downward break observed with 2'-deoxyguanosine and guanosine associated with conversion to the conjugate base form. In contrast, the rate constants for the N1-methylguanosine intermediate are independent of pH. Rate constants for the reaction forming the C-8 intermediate are also reported. These show that the reaction of nitrenium ions with the N2,N2-dimethylguanine derivative is significantly faster (except where the reactions are diffusion controlled). This is consistent with the initial step of the reaction of an arylnitrenium ion and guanine occurring by direct addition at C-8. The developing positive charge in such a reaction can be delocalized to the C-2 position where π donors such as NH2 and NMe2 can exert a stabilizing effect.Key words: nitrenium, arylnitrenium, guanosine, DNA adduct.

Remarkable kinetic stability of α-thiocarbamoyl substituted 4-methoxybenzylcations

1998 ◽  
Vol 76 (12) ◽  
pp. 1910-1915 ◽  
Author(s):  
Robert A McClelland ◽  
Victoria E Licence ◽  
John P Richard ◽  
Kathleen B Williams ◽  
Shrong-Shi Lin
Keyword(s):  
Key Words ◽  
Strong Interaction ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Visible Absorption ◽  
Methyl Group ◽  
Thioamide Group ◽  
Azide Ion ◽  
Uv Visible ◽  
Leaving Groups

4-Methoxybenzyl cations bearing α-(N,N-dimethylcarbamoyl) and α-(N,N-dimethylthiocarbamoyl) substituents have been generated photochemically upon irradiation of precursors with pentafluorobenzoate or 4-methoxybenzoate leaving groups. The ions have been observed with flash photolysis in 40:60 acetonitrile:water and in 50:50 methanol:water, and rate constants were measured for their decay in solvent alone and for their capture by azide ion. The cations so studied and their lifetimes in 40% acetonitrile are 6, ArC+H-CONMe2, 0.6 μs; 2, ArC+H-CSNMe2, 7 ms; and 4, ArC+(CH3)-CSMe2, 6 ms, where Ar = 4-MeOC6H4. The cation 4 reacts with solvent by elimination of a proton from the α-methyl group, and the rate constant for solvent addition must be less than 1 s-1. The CSNMe2 substituted cations are 105-107-fold longer lived than analogs where the thioamide group has been replaced with an α-methyl. The UV-visible absorption spectra of these two cations also show significant differences from those of typical 4-methoxybenzyl cations. Thus, both the lifetimes and spectra point to a strong interaction of the benzylic centre with the thioamide group. Key words: flash photolysis, thiocarbamoyl stabilized carbocation, photosolvolysis.

Intermediates in the oxidation of azide ion in acidic solutions

1986 ◽  
Vol 90 (6) ◽  
pp. 1198-1203 ◽  
Author(s):  
Zeev B. Alfassi ◽  
W. A. Pruetz ◽  
Robert H. Schuler
Keyword(s):  
Acidic Solutions ◽  
Azide Ion

Structures of Reactive Nitrenium Ions:  Time-Resolved Infrared Laser Flash Photolysis and Computational Studies of SubstitutedN-Methyl-N-arylnitrenium Ions

2000 ◽  
Vol 122 (34) ◽  
pp. 8271-8278 ◽  
Author(s):  
Sanjay Srivastava ◽  
Patrick H. Ruane ◽  
John P. Toscano ◽  
Michael B. Sullivan ◽  
Christopher J. Cramer ◽  
...  
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Infrared Laser ◽  
Computational Studies ◽  
Time Resolved ◽  
Nitrenium Ions ◽  
Arylnitrenium Ions

Formation and nucleophilic capture of N-nitrosiminium ions

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1148-1161 ◽  
Author(s):  
Latifa Chahoua ◽  
Alain Vigroux ◽  
Yvonne Chiang ◽  
James C Fishbein
Keyword(s):  
Ionic Strength ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Azide Ion ◽  
Iminium Ions ◽  
Nucleophilic Trapping

A study of the solvolysis of a series of (N-nitrosomethylamino)arylmethyl esters and azides and the products of nucleophilic trapping of the corresponding N-nitrosiminium ion intermediates in aqueous media, 25°C, ionic strength 1 M is reported. Structure-reactivity data for the forward and reverse reactions have been obtained. In three cases, the rate constants for reactions of the cations with nucleophiles have been measured directly by laser flash photolysis. The data allow a comparison of the degree to which the N-methyl-N-nitroso functionality enhances cation stability from a thermodynamic and kinetic perspective. It has been possible to deduce that the carbon basicity of azide ion is less than 1 kcal/mol greater than that of acetate ion.Key words: nitrosiminium ions, α-acetoxynitrosamines, carbocations, iminium ions, nucleophilicity.

scholarly journals Generation of N,N-Di(4-bromophenyl)nitrenium Ion Under Acidic Conditions: Search for a Nitrenium Dication

2020 ◽  
Author(s):  
Andrea Zeppuhar ◽  
Daniel Falvey
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Cation Radical ◽  
Laser Flash ◽  
Strong Electron ◽  
Nitrenium Ion ◽  
Mechanistic Analysis ◽  
Separate Pathway ◽  
Acidic Conditions ◽  
Aqueous Conditions

The behavior of the N,N-di(4-bromophenyl)nitrenium ion under acidic aqueous conditions was examined via laser flash photolysis experiments. A long-lived species forms and can be assigned as the cation radical or the dication. This species is unreactive towards nucleophiles and reactive towards strong electron donors, consistent with a cation radical. Mechanistic analysis indicates its formation is through a separate pathway than that of the nitrenium ion, suggestive of a triplet mechanism.

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