Conformational Transformations in Aromatic Nitroso Oxides

Ortho-Cyclization in Asymmetrically Substituted Arylnitroso Oxides

The mechanism of the photooxidation of a number of asymmetrically substituted phenyl azides in acetonitrile was studied. The key intermediates of this reaction are the corresponding nitroso oxides, the unimolecular consumption of which occurs via the cis form when the terminal oxygen atom of the NOO moiety reacts with the ortho position of the aromatic ring. As a result, it is opened to form a nitrile oxide. In the case of 3-methylphenyl azide, the reaction proceeds via the cis/syn form of nitroso oxide with a regioselectivity of 91%. The methoxy substituent at the para position changes the direction of the ortho-cyclization so that it occurs via the cis/anti form of nitroso oxide independently on the nature of a meta substituent. Nitrile oxides, which are formed as a result of these transformations of nitroso oxides, are stabilized by [3 + 2] cycloaddition with acetonitrile to give 1,2,4-oxadiazoles. The observed regioselectivity of the ortho-cyclization of nitroso oxides was explained using theoretical methods. Its cause consists in the extra-stabilization of the transition state of the reaction of the cis/anti form due to a stereoelectronic effect of the para-methoxy substituent.

Experimental and theoretical substantiation of differences of geometric isomers of copper(II) α-amino acid chelates in ATR-FTIR spectra

Stereo and structural isomerism of the copper(II) chelate complexes define their biological activity. At the same time, the identification of the geometric isomers of such complexes is a nontrivial task of modern coordination chemistry. In the presented work we have studied the trans- and cis-isomers of chelates bis(S-valinato)copper(II), (R,S-valinato)copper(II) and other mixed ligand copper(II) amino acid complexes with the joint use of experimental by ATR-FTIR spectroscopy and DFT simulations. Using DFT simulations (method M06/6 311+G(d)) the optimum conformers of the geometric isomers of copper(II) a-amino acid chelate complexes were found and their characteristic stretching vibrations were established in the mid-wave region of the IR spectra. The experimental ATR-FTIR bands of the compounds well agree with the theoretical estimates. Such a joint use allows to determine of cis- and trans-isomers of copper(II) N,O-amino acid chelates in the mid-wave region of the ATR-FTIR spectrum.

The substituent effects on the [3+2] cycloaddition of nitrile oxides generated by photooxidation of arylazides to acetonitrile

The DFT approach in M06L/6-311 + G(d,p) approximation was used to study the transformation of unsaturated nitrile oxides (RCNO), which were generated by photooxidation of the corresponding aromatic azide, to oxadiazoles via [3 + 2]cyclization with acetonitrile. It was found that the cycloaddition activation enthalpy was within 60-93 kJ/mol, depending on the structure of the nitrile oxide. A significant mesomeric effect of the substituent and its position in the conjugated molecular system on the activation barrier of the reaction studied was identified. The relationship between the thermodynamic characteristics of [3 + 2] cycloaddition and electron density distribution in RCNO was demonstrated by a representative set of compounds including mono- and disubstituted nitrile oxides.

Interplay of Conformational and Chemical Transformations of Ortho-Substituted Aromatic Nitroso Oxides: Experimental and Theoretical Study

The mechanism of the photooxidation of aromatic azides containing a substituent at one of the ortho positions (2,4-dimethoxyphenyl azide (1a) and 2-methyl-4-[(2E)-1-methylbut-2-en-1-yl]phenyl azide (1b)) was studied in acetonitrile. The electronic spectra and the kinetic regularities of the consumption of corresponding nitroso oxides, which are the reaction intermediates, were investigated by flash photolysis. Owing to the one-and-a-half order of the C-N and N-O bonds and asymmetric molecule structure these nitroso oxides exist as four conformers (cis/syn, cis/anti, trans/syn, and trans/anti). The conformers differ in the spectral properties and in the reactivity in various irreversible transformations. The only product, (2Z,4E)-4-methoxy-6-oxohepta-2,4-dienenitrile oxide (7a), was observed during photooxidation of 1a, whereas transformations of the nitroso oxide isomers derived from 1b led to a set of stable products: the cis/anti isomer was transformed into (3,4,7-trimethyl-3a,4-dihydro-2,1-benzisoxazol-5(3H)-ylidene)ethanal (10), the trans isomers recombined forming the corresponding nitro and nitroso compounds, and the most reactive cis/syn isomer was transformed into ortho-nitrosobenzyl alcohol 11. The last was oxidized slowly to the corresponding benzaldehyde 12. Interaction of 11 and 12 led to the formation of (Z)-1,2-bis(2-formyl-4-((2E)-1-methylbut-2-en-1-yl)phenyl)diazene-1-oxide (13). The DFT simulation and kinetic modeling of the nitroso oxide transformations as well as the product analysis allowed revealing the fine details of the mechanism of decay for these species.