MO Study on the photochemical isomerization of isoxazole

The reaction mechanism of photoisomerization of isoxazoles to oxazoles through azirine intermediates is investigated theoretically by means of ab initio MO CI calculations. Azirine intermediates in S₁ state [(n →*) state of the carbonyl chromophore] cause the C? N bond rupture of azirine ring and transform to isoxazoles via intersystem crossing to T₁ state. On the other hand, azirine intermediates in S₂ state [(n →*) state of the ketimine chromophore] lead to the C? C bond rupture of azirine ring and convert to oxazoles via intersystem crossing to T₁ state.     

Template-directed synthesis employing reversible imine bond formation

The imine bond--formed by the reversible condensation of an amine and an aldehyde--and its applications as a dynamic covalent bond in the template-directed synthesis of molecular compounds, will be the focus of this tutorial review. Template-directed synthesis--or expressed another way, supramolecular assistance to covalent synthesis--relies on the use of reversible noncovalent bonding interactions between molecular building blocks in order to preorganise them into a certain relative geometry as a prelude to covalent bond formation to afford the thermodynamically preferred product. The use of this so-called dynamic covalent chemistry (DCC) in templated reactions allows for an additional amount of reversibility, further eliminating potential kinetic products by allowing the covalent bonds that are formed during the template-directed reaction to be 'proofread for errors', thus making it possible for the reaction to search out its thermodynamic minimum. The marriage of template-directed synthesis with DCC has allowed chemists to construct an increasingly complex collection of compounds from relatively simple precursors. This new paradigm in organic synthesis requires that each individual piece in the molecular self-assembly process is preprogrammed so that the multiple recognition events expressed between the pieces are optimised in a highly cooperative manner in the desired product. It offers an extremely simple way of making complex mechanically interlocked compounds--e.g., catenanes, rotaxanes, suitanes, Borromean rings and Solomon knots--from relatively simple precursors.     

Recyclable polybutadiene elastomer based on dynamic imine bond

Catalyst‐free recyclable polybutadiene (PB) elastomer cross‐linked by dynamic imine bonds is prepared by the reaction between amine functionalized PB and aldehyde cross‐linkers. The dynamic nature of imine bond is investigated by rheometry and creep‐recovery experiments. The cross‐linking degrees are regulated by incorporating different amount of aldehyde, and their influence on the cross‐linked elastomers is investigated in detail. The temperature‐induced imine exchange reactions enable recycling of the cross‐linked PB elastomers and their mechanical properties are almost unchanged after several cycles. It is important to note that the elastomers also show excellent solvent resistance even at high temperature. The good mechanical properties, solvent resistance and recycling ability of the resultant PB elastomer demonstrate the superiority of the imine bonds in the design of recyclable polymers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2011–2018     

Conical intersection control of heterocyclic photochemical bond scission

The photochemistry of the heterocycle 5,6-dihydro-1-methyl-5,5-diphenylpyridin-2(1H)-one (compound 1 in the text) leads to two competitive reactions (the reactions are depicted in the Introduction to the article). These arise from fission of bond a, between the nitrogen (N-6) and C-1, and bond b, between C-4, with the two phenyl substituents, and C-5, adjacent to the nitrogen. Scission of bond a alone leads to a zwitterionic intermediate which can be trapped by nucleophiles, while cleavage of bonds a and b together affords two fragments--a ketene and an imine. The ketene could be intercepted with nucleophiles and the imine trimerized. Computation reveals little weakening of bonds a and b. But as stretching begins, conical intersections are encountered, leading to ground-state products.     

Carbon-carbon bond formation from azaallyl and imine couplings about metal-metal bonds

Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation.     

Graphical linking of a MO multicenter bond index to VB structures

The multicenter bond index proposed within the MO framework depends on the order of the centers for which it is calculated, outside from the 3-c case. For the 6-c case, the eventually different values are 60. A graphical approach links the MO values to VB structures. Benzene, chosen as our paradigm for the 6-c case, illustrates our proposition.     

Determination of approximate bond dissociation energies by a semi-empirical MO technique

It is suggested that the Extended Hückel Method by Hoffmann may be used for the determination of approximate bond dissociation energies.The author would like to express his sincere gratitude to Miss Etta Rosenzweig for her assistance in this project.     

On the mechanism of the copper-mediated C-S bond formation in the intramolecular disproportionation of imine disulfides

The mechanism of the copper-mediated disproportionation of aromatic imine disulfides to benzothiazoles in the gas phase is investigated by experimental and theoretical methods. Application of infrared multiphoton dissociation and hydrogen/deuterium exchange experiments combined with density functional theory (DFT) calculations of the relevant molecular structures and the associated infrared spectra allows the identification of the observed ionic intermediates. The theoretical investigation of the possible reaction pathways supported by collision-induced dissociation experiments provides a consistent mechanistic picture of the reaction catalyzed by a single copper(I) ion. Activation of the substrate proceeds via homolytic sulfur-sulfur bond cleavage, yielding metal complexes in the formal +3 oxidation state; carbon-sulfur coupling and hydrogen-atom transfer complete the transformation to the products. Exploratory studies demonstrate that in the gas phase, the disproportionation of the imine disulfide can also be mediated by other metal ions via different either homo- or heterolytic mechanisms without involving high-valent intermediates.     

Subtle photochemical behavior in ferroin-bromate-benzoquinone reaction

This study investigated the photochemical behavior of the ferroin-bromate-benzoquinone reaction in a batch system, in which the reduction of ferriin was dominated by bromide ions rather than by organic substrates as known in most of bromate oscillators. The chemical oscillations exhibited ultrasensitive response to illumination, where both the oscillation frequency and lifetime were significantly increased by ceiling light with an intensity of less than 20 μW/cm(2). As the light intensity was increased, the influence of light evolved from constructive to inhibitory. At a constant illumination, the influence of light also underwent a transition from constructive to inhibitory when the acid concentration was increased from 0.03 to 0.09 M. NMR measurements and absorption spectroscopy indicated photoenhanced bromination of benzoquinone and photoenhanced reduction of ferriin by bromide ions.     

Ab initio study on the photochemical behavior of styrene

Ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations have been performed to examine the photochemical behavior of styrene upon the strong S₀‐S₂ electronic excitation in the low‐lying excited states. The optimized structure at the S₂/S₁ conical intersection (CIX) is characterized by a quinoid structure. The transition state (TS) in S₁ is in the vicinity of the S₂/S₁‐CIX. At the S₁‐TS, two reaction paths branch. One is the relaxation into the stable structure in S₁ and then emission into S₀. The other is the radiationless decay through the S₁/S₀‐CIX. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 950–956, 2002     

Evidence of the facile hydride and enolate addition to the imine bond of an aluminum-salophen complex

The isolation of complexes 2 and 3, unambiguously characterized by single-crystal X-ray diffraction, demonstrates that nucleophilic additions to the aluminum-coordinated imino bond of salophen complex 1 can be achieved under very mild conditions.     

The photochemical behavior of colchicone and thiocolchicone

The irradiation of colchicone 5 led to the formation of lumicolchicone 7. The same reaction cannot be obtained by using thiocolchicone 6 as substrate. Transient absorption spectroscopy of colchicone and beta-lumicolchicone showed that probably the photoisomerization occurred on colchicone in its first excited singlet state. The spectroscopic data are in agreement with the hypothesis that lumicolchicone was generated in the ground state from the S1 state of colchicone without the presence of any intermediate. Semiempirical calculations on colchicone and thiocolchicone showed that the highest single occupied molecular orbital and the lowest unoccupied molecular orbital of the singlet excited colchicone can give a disrotatory ring closure to 7, while thiocolchicone cannot give the same type of process.     

Semiempirical MO study on the abnormal bond orders of large networks of highly branched polyenes

The π-electronic structures of the ground state of linear and highly branched polyenes with up to 80 π-electrons are calculated with particular reference to the alternation of the bond orders. The MO methods adopted are HMO, PPP , variable-β, γ, and its improved version. The effect of the electron correlation through singly and doubly excited configurations is estimated with a second-order perturbation calculation. The calculated bond orders systematically vary with the degree of approximation used. Most of the bond order values can be grouped into either a single or double bond region. In certain series of highly branched polyenes the bond orders of double and single bonds at the root of branching, respectively, get smaller and larger as the size of the molecule increases and sometimes their difference gets diminishingly small. The origin of these abnormal bond orders is discussed in terms of the π-electron flow networks.     

Factors controlling photochemical cleavage of the energetically unfavorable Ph-Se bond of alkyl phenyl selenides

Primary photochemical paths of alkyl phenyl selenides (1) were investigated, and an origin of large deviations in the chemical yields of products obtained by carbon radical reactions induced by photolysis of phenyl selenides was clarified. KrF excimer laser photolyses of n-pentyl phenyl selenide (1a) yielded 1-pentene (2a), n-pentane (3a), n-decane (4a), dipentyl selenide (5a), benzene (6), dipentyl diselenide (7a), and diphenyl diselenide (7) as major photoproducts, with compounds 2a, 3a, 4a, 5a, and 7 formed by pentyl-Se bond cleavage, and 5a, 6, and 7a by Ph-Se bond cleavage. The selectivity of the photoproducts revealed the occurrence of an unexpected amount of Ph-Se bond cleavage (35% in n-hexane at 248 nm) during photolysis. Solvent viscosity, wavelength of light, and the structure of alkyl substituents were the major factors that controlled Ph-Se bond cleavage. The ratio of Ph-Se bond cleavage decreased with increasing solvent viscosity and laser wavelength. The effect of alkyl substituents on the ratio of bond cleavages, Ph-Se/total C-Se, was investigated for five alkyl phenyl selenides; the ratio decreased in the order pentyl > 2-methylallyl > allyl > 1-ethylpropyl > tert-butyl groups. The contribution of Ph-Se bond cleavage is most probably the origin of the large deviations in the yields of radical reactions induced by photolyses of 1, which can be minimized by selecting appropriate solvents and wavelength of light.     

The photochemical step of vision: The hydrogen bond and other associations

It is shown on the basis of quantum chemical and spectroscopic arguments that through a consideration of the hydrogen bond and other intermolecular associations affecting the Schiff-base of retinal in the ground and excited states a new assessment can be made of the conditions related to the photochemical primary step of vision.