R H Duncan Lyngdoh
Articles written in Journal of Chemical Sciences
Volume 105 Issue 4-5 August 1993 pp 253-263 Organic
All O-alkylated DNA bases and nucleosides possess alkyl groups considerably more labile than those in N-alkylated bases and nucleosides, being prone to degradation through loss of the alkyl group at strongly acidic
Volume 107 Issue 3 June 1995 pp 221-231 Physical and Theoretical
The possibilities open for tautomerism, for protonation and for deprotonation in the five nucleic acid bases are examined theoretically using the semiempirical AM1 SCF-MO methodology. The tautomers predicted to be the most stable, other than the usual forms, all involve proton shifts to adjacent sites. The sites predicted to be the most favourable for protonation are the N7-G, N3-A, N3-C, O4-T and O4-U positions of guanine, adenine, cytosine, thymine and uracil respectively. The protons predicted to be the most acidic for each base are the N1-G, N9-A, N1-C, N3-T and N3-U protons. These predictions accord well with the conclusions drawn from experimental work so far as assignments of acidic protons and basic sites for the particular bases are concerned. However, the relative feasibility of these reactions for the different bases is not well predicted by these gas-phase calculations.
Volume 128 Issue 5 May 2016 pp 681-693 Regular Articles
Alkene aziridination by azides through uncatalyzed thermal gas phase routes has been studiedusing the DFT B3LYP/6-31G(d,p) method, where the possible role of discrete nitrene intermediates is emphasized.The thermal decomposition of azides is studied using the MP2/aug-cc-pVDZ strategy as well. The MP2(but not the B3LYP) results discount the existence of singlet alkylnitrenes where the alkyl group has an α-hydrogen. Addition of the lowest lying singlet and triplet nitrenes R-N (R = H, Me, Ac) to four different alkenesubstrates leading to aziridine formation was studied by the B3LYP method. Singlet nitrenes with alkenes canyield aziridines via a concerted mechanism, where H-N insertion takes place without a barrier, whereas Me-Nshows larger barriers than Ac-N. Methyl substitution in the alkene favors this reaction. Triplet nitrene additionto alkenes is studied as a two-step process, where the initially formed diradical intermediates cyclize to formaziridines by ISC (intersystem crossing) and collapse. Scope for C-C bond rotation in the diradical leads to lossof stereochemical integrity for triplet nitrene addition to cis- and trans-2-butenes. Geometries of the transitionstates in the various reaction steps studied here are described as “early” or “late” in good accordance with theHammond postulate.
Volume 131 Issue 1 January 2019 Article ID 0006
The B3LYP/6-31G(d,p) DFT method was used to study alkene aziridination by azides through uncatalyzed thermal gas phase routes which involve the whole azide reactant molecule without dissociation.Two mechanisms were studied – Route I involving concerted azide addition to alkene with the elimination of N2, and the multi-step Route II involving 1,3-dipolar cycloaddition between azide and alkene. Three azides RN3 (R = H, Me, Ac) are reacted with alkene substrates forming aziridine products. The concerted addition–elimination step of Route I is exothermic with an appreciable barrier, where the facility order Ac > Me > Hpoints to electrophilicity of the azide reactant. The initial 1,3-dipolar cycloaddition step of Route II involves smaller barriers than Route I, while thermal decomposition of the triazoline intermediate to aziridine and N2 involves two more steps with an N-alkylimine intermediate. The very high barrier for N-alkylimine cyclization to aziridine could be offset by the high exothermicity of the previous step. Geometries of the transition states for various reaction steps studied here are described as ‘early’ or ‘late’ in good accordance with the Hammondpostulate. Two other mechanisms (Routes A and B) studied earlier (involving discrete nitrene intermediates) are compared with Routes I and II, where Route II involving 1,3-dipolar cycloaddition is predicted to be energeticallythe most favored of all the four mechanisms for thermal gas-phase aziridination of alkenes by azides.
Volume 134, 2022
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