Menahem Kaftory

Kinetics of Solid State Photodimerization of 1,4-Dimethyl-2-pyridinone in its Molecular Compound

The [4 + 4] photodimerization of 1,4-dimethyl-2-pyridinone (A) in its molecular compound with 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol (I) was investigated by irradiation of a single crystal of the compound. The conversion to the product in a single-crystal to single-crystal transformation enabled the determination of its crystal structure after each exposure cycle of 10-20 min with a pulsed 355 nm laser light. The unit cell changes as a function of the conversion were monitored and showed monotonic changes. The kinetics of the reaction was studied at two different temperatures (230 and 280 K) and revealed a sigmoidal behavior that could be explained by the JMAK model for crystal growth with a mechanism that is intermediate between random distribution of products in the crystal and the existence of growing nuclei. The Arrhenius plot provided calculated activation energy of 32.5 kJ/mol.

Irreversible single-crystal to polycrystal and ­reversible single-crystal to single-crystal phase transformations in cyanurates

4,6-Dimethoxy-3-methyldihydrotriazine-2-one (1) undergoes a single-crystal to single-crystal reversible phase transformation at 319 K. The low-temperature phase crystallizes in monoclinic space group P2(1)/n with two crystallographically independent molecules in the asymmetric unit. The high-temperature phase is obtained by heating a single crystal of the low-temperature phase. This phase is orthorhombic, space group Pnma, with the molecules occupying a crystallographic mirror plane. The enthalpy of the transformation is 1.34 kJ mol(-1). The small energy difference between the two phases and the minimal atomic movement facilitate the single-crystal to single-crystal reversible phase transformation with no destruction of the crystal lattice. On further heating, the high-temperature phase undergoes methyl rearrangement in the solid state. 2,4,6-Trimethoxy-1,3,5-triazine (3), on the other hand, undergoes an irreversible phase transformation from single-crystal to polycrystalline material at 340 K with an enthalpy of 3.9 kJ mol(-1); upon further heating it melts and methyl rearrangement takes place.

Methyl Rearrangement of Methoxy-Triazines in the Solid- and Liquid-State

Abstract 2,4,6-Trimethoxy-1,3,5-triazine 11, and 6-methoxy-3,5-dimethyl-tetrahydrotriazine-2,4-dione 13 undergo intermolecular O→N methyl rearrangement in the liquid-state to 1,3,5-trimethyl 2,4,6-trioxohexahydro-s-triazine 14. 4,6-Dimethoxy-3-methyl-dihydro-triazine-2-one 12 was found to exhibit a different thermal behavior, and the methyl rearrangement takes place in the solid-state. The thermal behavior of each was investigated by calorimetry and high-temperature X-ray diffraction. It was found that 11 undergo phase transition to 11′, and the methyl rearrangement takes place in the melt. The solid-state methyl rearrangement of 12 is topochemically controlled. Two courses of methyl migration in the solid-state of 12 are proposed. A quantitative analysis of samples of 12 heated to different temperatures proves the existence of the two courses. A computer simulation was used to rationalize the reaction routes in 12 and 13.

Photodimerization of anthracene derivatives in their neat solid state and in solid molecular compounds

The Cambridge Crystallographic Data Center provides valuable data for analysis and detection of compounds with potential to undergo photodimerization in the solid-state. Such analysis is presented for anthracene derivatives, and showed that there are many compounds that their packing in the crystal meets the requirement needed for [4 + 4] photodimerization in the solid-state. The crystal structures of various neat acyl anthracenes before and after exposure to UV light are described and the molecular structures of the dimers are compared with those obtained by exposing molecular compounds possessing a light-stable host (1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol) and light-sensitive acyl anthracenes. While exposing neat compounds to UV light causes disintegration of the single crystal, the reaction in the molecular compounds proceeds in a single-crystal to single-crystal mode until a certain rate of conversion, and then disintegrates.

The effective ‘size’ of the tris(trimethylsilyl)silyl group in several molecular environments

The effective size of the tris(trimethylsilyl)silyl group in several molecular environments has been estimated. 2,2-Dimesityl-1-tris(trimethylsilyl)silylethanol 1g has been prepared and its structure determined by X-ray crystallography. The Mes–CC torsional angles are 59.6 (φ2) and 63.3°(φ2) and the CC–Si bond angle α4 is 133.8°. The two-ring flip barrier for the correlated rotation of the two mesityl rings around the Mes–C bonds is ΔGc‡= 10.2 kcal mol–1. The structures of enols Mes2CC(OH)R, R = H, Me, Et, Pri, But(1a–1e), Me3Si (1f), (Me3Si)3Si (1g) and (Me3Si)3C (1h) and the two-ring flip barriers have been calculated by the MM2* force-field. The calculated and the experimental values are in good agreement, except for somewhat lower calculated α4 for 1b–1e and a shorter C–Si distance in 1g. From the linear correlations between the observed cos φ2 or ΔGc‡ values and Es values for the enols 1a–1e, and the values observed for 1g an average Es value of –1.46 has been calculated for (Me3Si)3Si. MM2* calculations gave an A value for (Me3Si)3Si of 4.89 kcal mol–1. These steric parameters resemble those for the But group (Es=–1.54; A= 4.9 kcal mol–1) indicating a similar effective size for the But and (Me3Si)3Si groups in these specific environments. (Me3Si)3C is significantly larger (A= 13.3 kcal mol–1; estimated Es=–3.7).

Synthesis, structural, and magnetic characterization of substituted benzoimidazole-l-yl N,N′-dioxides

The crystal structures, EPR spectra and magnetic properties of the novel halogen- and cyano-substituted nitronyl nitroxide radicals 2-(2,6-dichlorophenyl)benzimidazolyl N,N′-dioxide, 6, 2-(2,6-difluorophenyl)benzimidazolyl N,N′-dioxide, 7, 2-(2-chloro-6-fluorophenyl)benzimidazolyl N,N′-dioxide, 8, 2-(2,3,6-trichlorophenyl)benzimidazolyl N,N′-dioxide, 9, 2-(2,3,4,5,6-pentafluorophenyl)benzimidazolyl N,N′-dioxide, 10, and 2-(3-cyanophenyl)benzimidazolyl N,N′-dioxide, 11, are reported. Compound 6 crystallizes in the triclinic crystal system in space group P. The molecules of 6 are arranged in pairs with short intermolecular distances between the NO groups. 7 crystallizes in two different modifications: polymorph α is orthorhombic, space group Pbca; polymorph β is monoclinic, space group P21/c. 8 crystallizes in two modifications: the α polymorph is monoclinic, space group P21/c; and the β polymorph is monoclinic, space group P21/n. 9 crystallizes in the monoclinic system, space group P21/c. 10 crystallizes in the monoclinic system, space group C2/c. The molecules of 10 are packed in pairs of two types that form a chain perpendicular to the c-axis. 11 crystallizes in the monoclinic crystal system in space group P21/c. The rotation angle between the two rings in compounds 6–10 is 54.2–76.7°. The rotation angle between the two rings is only 21.0° in 11 and it strongly affects the packing of the molecules that adopt the stacking mode. The magnetic measurements show that 6, 7, 10 and 11 exhibit large magnetic coupling. The best fitting with the experimental data for 6 and 11 was obtained using the Bleaney–Bowers singlet–triplet model plus the Curie–Weiss spin impurity (S = 1/2; H = −2JS1·S2) J/kB = −84.2 K and θimp = 0.3 K and J/kB = −95.3 K, θimp = 1.8 K, respectively. A Pade expression for 7 revealed Jintra/kB = 66.0 K and zJinter/kB = −14.0 K. Compound 10 shows evidence for large antiferromagnetic spin coupling (θ = −37.0 K Curie–Weiss model).

Channels formation through photodimerization of guest molecules within solid inclusion compounds

Photodimerizations of pyridone or methyl pyridone which serve as guest molecules in crystalline inclusion compounds are associated with a decrease in the volume of the guest molecules; at the end of this single crystal to single crystal transformation channels are formed through which water molecules diffuse.

Thermal rearrangement of cyanurates in the solid state

2,4,6-trimethoxy-1,3,5-triazine undergoes methyl transfer in few stages, some in the melt and some in the solid-state, to the end product, 2,4,6-trioxo-1,3,5-trimethylazine. The three possible intermediates have been prepared and their methyl transfer reaction was investigated. The mechanism and the dependence of the reactivity on the packing of the molecules in the crystal lattice have been studied by high temperature X-ray diffraction (HTXRD), by thermal analysis (DSC) and by crystal structure determination

Controlled photochemical reaction of 4-oxo(phenylacetyl)morpholine and 1-(phenylglyoxylyl)piperidine in solid supramolecular systems

Six inclusion compounds containing a photoreactive guest molecule, 4-oxo(phenylacetyl)morpholine or 1-(phenylglyoxylyl)piperidine, with different host molecules have been crystallized. The guest molecules underwent photochemical reaction upon irradiation. Examining their structures suggests that γ-hydrogen abstraction by an oxygen, the first step in cyclization of α-oxoamides, should be possible in all cases. In four cases crystallinity was maintained during and at the end of the conversion process i.e. the process was a single-crystal to single-crystal transformation. The crystal structure of the product crystals revealed that in two of the inclusion compounds the product obtained is a result of enclosure to a four-membered ring while in the other two the product obtained results from enclosure to a five-membered ring. The morpholine molecules adopt two different conformations. The photochemical reaction may take different courses either as a result of the different conformation or as a result of the different shapes of the cavities provided by the host molecules.

Reactions in the solid state. III: Structural aspects of photochemical reactions in crystalline inclusion compounds

Abstract Stereoselective photochemical dimerization to products adopting the syn head-to-tail configuration is obtained by irradiation of crystalline inclusion compounds. The structural aspects of these reactions regarding the relations between the arrangement of the host molecules in the crystalline lattice forming the matrix and the packing of the potentially reactive molecules included in the matrix are discussed. The possibilities of using non-symetrical host molecules as a probe for stereoselective and enantioselective intramolecular reactions are also discussed.