Reaction of 2,4,6-substituted pyrylium salts with compounds containing a C = N bond

2,4,6-Substituted pyrylium salts add azomethines to give pyridinium salts and aromatic aldehydes. The latter can be condensed with the methyl groups of the pyridinium salts. Benzaldoxime, benzalazine, benzalphenylhydrazine, urea, thiourea, and phenyl isothiocyanate react with 2,4,6-triphenylpyrylium perchlorate similarly to give, respectively, 2,4,6-triphenylpyridine N-oxide, 2,4,6-triphenylpyridine, or N-substituted 2,4,6-triphenylpyridinium perchlorates.     

Dye-sensitized photooxygenation of the C=N bond. 5. substituent effects on the cleavage of the C=N bond of C-aryl-N-aryl-N-methylhydrazones

The title compounds are cleaved cleanly at the C=N bond by singlet oxygen ((1)O(2), (1)Delta(g)) yielding arylaldehydes and N-aryl-N-methylnitrosamines. These reactions take place more rapidly at -78 degrees C than at room temperature. The effects of substituent variation at both the C-aryl and N-aryl groups were studied using a competitive method. Good correlations of the resulting rate ratios with substituent constants (sigma(-) or sigma(+)) were obtained yielding small to very small rho values indicative of small to very small changes in charge distribution between the reactant and the rate determining transition state. Electron withdrawing groups on the C-aryl moiety retard reaction somewhat by preferential stabilization of the hydrazone. Electron donors on the other hand slightly stabilize the rate determining transition state. Substituents on the N-aryl group have almost no effect. Inhibition by 3,5-di-tert-butylphenol was not observed showing that free (uncaged) radical intermediates are not involved in the mechanism. We postulate a mechanism in which the initial event is exothermic electron transfer from the hydrazone to (1)O(2) leading to an ion-radical caged pair. Subsequent covalent bond formation between the hydrazone carbon and an oxygen atom is rate controlling. The transition state for this step also has a lower enthalpy than the starting reactants, but the free energy of activation is dominated by a large negative TDeltaS++term leading to the negative temperature dependence. Direct formation of a C-O bond in the initial step is not unambiguously ruled out. Subsequent steps lead to C-N cleavage.     

Luminescent complexes with ligands containing C=N bond

Data on luminescent complexes with azomethine ligands are generalized and systematized. The synthesis and luminescent properties of complexes with acyclic and cyclic azomethines are considered.     

Trichlorosilane mediated asymmetric reductions of the C=N bond

Chiral amines are key components in numerous bioactive molecules. The development of efficient and economical ways to access molecules containing this functional group still remains a challenge at the forefront of synthetic chemistry. Of the methods that do exist, the trichlorosilane mediated organocatalytic reduction of ketimines offers significant potential as an alternative strategy. In this perspective, we wish to highlight the progress made in the past decade in this field and offer a direct quantitative comparison to transition-metal mediated process.     

The reaction of lead tetraacetate with hydrocarbons

The free radical reaction of lead tetraacetate with hydrocarbons has been investigated. The products of these reactions are the acetate esters. The hydrogen abstracting species from lead tetraacetate is found to have a primary to secondary to tertiary selectivity of 1:27:123 based upon relative reactivities.     

Displacement of Activated Amino Groups. C,N- vs. N,S-Bond Cleavage in the Reaction of N-Alkyl-disulfonamides with Nucleophiles

Sodiumthiophenoxide and sodiumphenylselenide react with N-benzyl- and N-hexyl-di-p-toluenesulfonamides ( 1 and 2 ) via displacement at the C-atom to afford the corresponding organosulfides and selenides in yields of 68–96%. In contrast, sodium cyanide converts disulfonamides to monosulfonamides by attack on the S-atom. The different selectivities of phenylsulfide and selenide as compared to cyanide anions with respect to attack on the C- and S-atoms are rationalized on the grounds of the HSAB (hard and soft acids and bases) principle of Pearson.     

Addition of heterocyclic CH acids to the C=N bond of azomethines

The aminomethylation of oxindole, 1-phenyl-3-methyl-5-pyrazolone, and N-phenyl-rhodanine was studied. Derivatives of these CH acids were obtained as a result of aminomethylation. The addition products were subjected to acid and base hydrolysis; the corresponding arylidene derivatives are formed in the case of the products of aminomethylation of oxindole and 1-phenyl-3-methyl-5-pyrazolone, while thioglycolic acids are formed in the case of N-phenylrhodanine derivatives.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1088–1093, August, 1981.     

1,3-dipolar cycloaddition of fluorinated azomethine ylides at the C=N bond

Azomethine ylides generated by reaction of difluorocarbene with N-alkyl- and N-arylimines derived from benzaldehyde and benzophenone react with N-benzylidenebenzenesulfonamide in a regioselective fashion, yielding the corresponding imidazolidin-4-ones via 1,3-dipolar cycloaddition at the C=N bond. Ylides generated from benzaldehyde imines give rise to mixtures of stereoisomeric 2,5-diphenyl-1-(phenylsulfonyl)-imidazolidin-4-ones, the cis isomer prevailing.Translated from Zhurnal Organicheskoi Khimii, Vol. 40, No. 10, 2004, pp. 1542–1548.Original Russian Text Copyright © 2004 by Novikov, Khlebnikov, Egarmin, Kopf, Kostikov.     

Theoretical Study of the Reaction Formalhydrazone with Singlet Oxygen. Fragmentation of the C=N Bond,Ene Reaction and Other Processes

Photobiologic and synthetic versatility of hydrazones has not yet been established with ¹O₂ as a route to commonly encountered nitrosamines. Thus, to determine whether the “parent” reaction of formalhydrazone and ¹O₂ leads to facile C=N bond cleavage and resulting nitrosamine formation, we have carried out CCSD(T)//DFT calculations and analyzed the energetics of the oxidation pathways. A [2 + 2] pathway occurs via diradicals and formation of 3‐amino‐1,2,3‐dioxazetidine in a 16 kcal/mol^?1 process. Reversible addition or physical quenching of ¹O₂ occurs either on the formalhydrazone carbon for triplet diradicals at 2–3 kcal mol^?1, or on the nitrogen (N(3)) atom forming zwitterions at ~15 kcal/mol^?1, although the quenching channel by charge‐transfer interaction was not computed. The computations also predict a facile conversion of formalhydrazone and ¹O₂ to hydroperoxymethyl diazene in a low‐barrier ‘ene’ process, but no 2‐amino‐oxaziridine‐O‐oxide (perepoxide‐like) intermediate was found. A Benson‐like analysis (group increment calculations) on the closed‐shell species are in accord with the quantum chemical results.     

Quantum-chemical study on the reaction of phenyl isocyanate with linear methanol associates. Addition at the C=N bond

Quantum-chemical calculations at the B3LYP/6-311++G(df,p) level of theory showed that reactions of phenyl isocyanate with methanol associates involve formation of pre-and post-reaction complexes. The reactions proceed through late asymmetric cyclic transition states. The height of the energy barrier decreases as the degree of association of the alcohol increases. The relative change in the Gibbs energy in the reaction of phenyl isocyanate with methanol also becomes smaller as the degree of alcohol association increases.     

Anti-configuration of the N = N bond in azobilirubin pigments

The conformational and internal energy barriers for rotation of the phenylazo group in azobilirubin pigments were determined by the PCILO method. The calculations showed a restricted rotation of the azo group at 180–300° dihedral angle.     

Activated sterically strained C=N bond in N-substituted p-quinone mono- and diimines: XIV. Reaction of some 3,5-dimethyl-1,4-benzoquinone monoimines with alcohols

Steric strain in the C=N-C fragment in 3,5-disubstituted N-acyl-1,4-benzoquinone monoimines, unlike their N-arylsulfonyl analogs, leads to increase of the C=N-C angle above 130° or twisting of the double C=N bond and loss of planarity of the quinoid ring. This structural transformation enhances the reactivity of the C=N bond so that 1,2-addition of alcohols becomes possible with formation of sterically unstrained cyclohexadienone structure with sp ³-hybridized C⁴ carbon atom.     

Propellanes-LXXV A [2+2]photocycloaddition of an N = n bond to a C=c bond

Irradiation of an azo-propellane derivative in which an olefinic bond is proximate to the azo group forms a cage compound containing a 1,2-diazacyclobutane ring.     

Oxidation of arylacetylhydrazones of carbonyl compounds with lead tetraacetate

Oxidation of the title compounds 8, 9 with lead tetraacetate at room temperature gives a variety of products depending on the substituents on the carbonyl carbon atom. Thus, on oxidation of the aldehyde derivatives 8 1,3,4-oxadiazolo derivatives 10 are obtained in good yields. However in some cases formation of N-acetyl-N-arylacetyl-N′-benzoylhydrazines 11 is also observed, whereas oxidation of the ketone hydrazones 9 gives in good yields the 2H,5H-1,3,4-oxadiazoles 15 or substituted monoacetoxy- 17 and diacetoxyalkanes 18 . The reaction mechanisms are also discussed.