Studies on the Stereochemistry of 2‐(Nitromethylidene)‐Heterocycles

Abstract

SummaryThe 'H-NMR spectra of 2-(nitromethy1idene)pyrrolidine (7), l-methy1-2-(nitromethy1idene)imidazolidine (10) and 3-(nitromethy1idene)tetrahydrothiazine (1 1) in CDC1, and (CDJ,SO indicate that these compounds have the intramolecularly H-bonded structures (2)-7, (E)-10 and (2)-11 while the N-methyl derivative 8 of 7 is (E)-configurated in both solvents. 1 -Benzylamino-l-(methylthio)-2-nitroethylene (13), an acylic model, has the H-bonded configuration (E)-13 in CDC1, and in (CD,),SO. 2-(Nitromethy1idene)thiazolidine (3) has the (E)-configuration in CDC1, but exists in (CD,),SO as a mixture of (2)-and (E)-isomers with the former predominating. Both species are detected to varying proportions in a mixture of the two solvents. "N-NMR spectroscopy of 3 ruled out unambiguously the nitronic acid structure 6 and the nitromethyleneimine structure 5. The N-methyl derivative 4 of 3 is (Z)-configurated in (CD,),SO. Comparison of the olefinic proton shifts of (2)-3 and (2)-4 with those of analogues and also of 1,1-bis(methylthio)-2-nitroethylene (12) shows decreased conjugation of the lone pair of electrons of the ring N-atom in (2)-3 and (2)-4. This is also supported by T -N M R studies. Plausible explanations for the phenomenon are offered by postulating that the ring N-atoms are pyramidal in (2)-3 and (2)-4 and planar in other cases or, alternatively, that the conjugated nitroenamine system gets twisted due to steric interaction between the NO,-group and the ring S-atom. Single-crystal X-ray studies of 3 and 8 show that the former exists in the (Z)-configuration and the latter in (E)-configuration; the ring N-atom in the former has slightly more pyramidal character than in the latter.

A DFT Study on the Mechanism of the Annulation Reaction of Trichloronitroethylene with Aniline in the Synthesis of Quinoxalinone-N-oxides

“…Interestingly, TS17 is 0.9 kcal/mol more stable than intermediate 21 . One can expect that structures 21 and 25 can be interconverted in a dynamic equilibrium (Scheme ) on the basis of the experimental observation of Rajappa et al…”

“…Activation barriers of the rate-determining steps (WE and CTI) of chosen paths (3 and 5) were predicted to be 31.9 and 29.0 kcal/mol, respectively, by B3LYP/6-31+G**. In order to see how other computational methods would affect these barriers, B3LYP/6-31+G** was compared to the PBE1PBE, BLYP, B3PW91, MPW1PW91, MPW1K, and MP2(SP) methods by using 6-31+G** and different basis sets (6-31G*, 6-31G**, and 6-31++G**) with B3LYP.…”

Regioselective Synthesis of New 5-Ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene Bromides and Rearrangement Reaction Thereof

“…in a geometrically favorable position for the formation of intramolecular H-bonding, as in 3c-Z isomer, was previously reported for similar compounds. 2 The 1 H NMR analysis revealed that 3c undergoes further chemical transformation with time (vide infra) in the reaction medium, which is acidic due to the evolution of HBr. However, on a preparative scale, rapid a-bromination (~10 min) of 2a-c-Z at r.t., in alcohol or in boiling CCl 4 as a solvent (Scheme 1), resulted due to the lower solubility, in isolation of the primary bromination compounds 3a-c in good yields of 60.5-66%.…”

“…[1][2][3][4] These compounds, bearing the electron-donor and acceptor groups at the termini of the double bond, are of interest not only for their mechanistic properties, [5][6][7] but also as versatile synthetic intermediates. 4,8 Synthetic routes to a number of such compounds have been described.…”

ChemInform Abstract: Regioselective Synthesis of New 5‐Ethoxycarbonylmethyl‐4‐oxothiazolidin‐2‐ylidene Bromides and Rearrangement Reaction Thereof.

“…In recent years, numerous studies of push-pull alkenes have been reported. [1][2][3][4] These compounds, bearing the electron-donor and acceptor groups at the termini of the double bond, are of interest not only for their mechanistic properties, [5][6][7] but also as versatile synthetic intermediates. 4,8 Synthetic routes to a number of such compounds have been described.…”