11B and13C NMR studies on silica modified by borate esters

¹¹B and¹³C NMR has been applied along with programmed desorption and boron analysis to the surface of aerosil specimens modified by trimethoxyborane and dimethoxybutyl-borane. It is concluded that methanol is coordinated to the boron atoms in the dimethoxyborosilyl groups.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 118–121, January–February, 1988.     

Photochemistry of ommochromes and related compounds. Part II

The visible light irradiated solutions of the 1-methyl-1-(1′-[11-(β-aspartoyl-methyl ester-imino)]ethenyl]-ketal-1H,5H-pyrido[3,2-a]phenoxazin-5-one ( 1 ) and the 1,5-dimethoxy-11-(β-aspartoyl-N-acetyl-methyl ester)-pyrido[3,2-a]phenoxazine ( 2 ) [1], in methanol and acidic methanol, are examined. Both methanolic solutions undergo light induced transformation according to an opening of the phenoxazinone and phenoxazine systems, not reversible in darkness. On the contrary, 1 and 2 in methanol-acid solutions, under visible light irradiation, yield a solvent photoaddition, reversible in darkness. Some phototransformation products are examined and a plausible mechanism, for the reactions explanation, is suggested.     

Photochemistry of bertyadionol and related compounds

Irradiation of the macrocyclic diterpene bertyadionol resulted in a single photoisomer (2) the formation of which was rationalised in terms of a novel photorearrangement resulting in transannular bond formation. The structure of this photoproduct was elucidated by a combination of chemical and spectroscopic methods. Two related compounds, diterpene B and D, and their acetate derivatives when photolysed underwent E→Z isomerisation of the conjugated double bond and epimerization about one centre of the cyclopropyl moiety.     

Photochemistry of ommochromes and related compounds

The photochemistry of some synthetic ommochromes, the 1-methoxy-11-(β-aspartoyl-acetyl-methyl ester)-5H-pyrido[3,2-a]phenoxazin-5-one ( 2 ), the 1-methyl-1-(1′-[1′[11-(β-aspartoyl-methyl ester-imino)]ethenyl)-ketal-1 H,5H-pyrido[3,2-a]phenoxazin-5-one ( 3 ) and the 1,5-dimethoxy-11-(β-aspartoyl-N-acetyl-methyl ester)-pyrido[3,2-a]phenoxazine ( 4 ), obtained from the oxidation of the 3-hydroxykynurenine ( 1 ), has been examined. In an acidic methanol solution of 2 , a reversible solvent photoaddition was observed, associated with a yellow-red bathochromic shift. The mixture, stored in the dark, quantitatively records the starting absorption spectrum. In an acidic enviroment, 3 and 4 , affording 2 , show the same photochemical behaviour. A plausible mechanism, for explanation of the process is suggested.     

Boron Photochemistry : XII. The role of iodine in the photocyclization of anilinodimesitylboranes

The ratio of methyl group rearrangement to methyl group elimination during the photocyclization of anilinodimesitylborane is dependent upon the concentration of iodine present. At iodine concentrations of less than 10^?3M, the major product is a demethylated borazarophenanthrene, 7,9-dimethyl-6-mesityldibenz[ce][1,2]-azaborine (IV). At concentrations of iodine above 5 × 10^?3M, the major products is a methyl-rearranged borazarophenanthrene, 7,9,10-trimethyl-6mesityldibenz[ce][1,2]-azaborine (V). The role of iodine was at least threefold: assisting in the production of a reactive cation-radical at low iodine concentrations, quenching the formation of the cation-radical at higher iodine concentrations, and actively assisting at the higher concentrations in the formation of a reactive species.     

13C NMR studies of some hydroxycoumarins and related compounds

The ¹³C chemical shifts and one-bond carbon-hydrogen coupling constants have been obtained for some hydroxycoumarins and their corresponding acetoxy and methoxy derivatives. The changes in the one-bond carbon-hydrogen coupling constants resulting from the conversion of a hydroxy group to an acetoxy group represent a simple method of assignment of the ¹³C NMR signals in coumarins which contain one or more hydroxy groups in the benzenoid ring.     

Photochemistry of phenyl-substituted 1,2,4-thiadiazoles. 15N-labeling studies

Irradiation of 5-phenyl-1,2,4-thiadiazole (6) resulted in the formation of benzonitrile (5), 3-phenyl-1,2,4-thiadiazole (4), phenyl- and diphenyl-1,3,5-triazines (7 and 8), and a trace quantity of diphenyl-1,2,4-thiadiazole (9). The formation of 4,5, 7, and 8 can be explained in terms of photoinduced electrocyclic ring closure resulting in the formation of an intermediate 4-phenyl-1,3-diaza-5-thiabicyclo[2.1.0]pentene. 15N-labeling experiments revealed that sulfur can undergo sigmatropic shifts around all four sides of the diazetine ring. Thus, irradiation of 6-4-15N led to the formation of 6-2-15N and an equimolar mixture of 4-2-15N and 4-4-15N. The thiabicyclo[2.1.0]pentene intermediate is also suggested to undergo sulfur elimination resulting in the formation of phenyldiazacyclobutadiene, which can undergo complete fragmentation to benzonitrile or [4+2] cycloaddition leading to unstable tricyclic adducts, the suggested precursors of the 1,3,5-triazine products 7 and 8. The observed 15N distribution in 7 and 8 is consistent with this mechanism. Irradiation of 4 led only to the formation of 5. 15N-labeling experiments show that 4 does not undergo electrocyclic ring closure but reacts exclusively by photofragmentation of the thiadiazole ring.     

Photochemistry of methylenenitrones and related compounds. A study of oxaziridine and nitrene formation

A series of N-alkylmethylenenitrones has been prepared and can be irradiated cleanly to simple N-alkyloxaziridines. Further irradiations of these systems lead to N-alkylformamides via a singlet state rearrangement process. Triplet state fragmentation to give amines, presumably via alkyl nitrenes, has been observed but with very low efficiency. Only N-aryl systems appear to lead to significant nitrene formation.     

Refinement of borate structures from 11B MAS NMR spectroscopy and density functional theory calculations of 11B electric field gradients

The refinement of borate structures using DFT calculations combined with experimental (11)B quadrupole coupling parameters from solid-state NMR spectroscopy is presented. The (11)B electric field gradient (EFG) tensors, calculated using the WIEN2k software for trigonal and tetrahedral boron sites in a series of model compounds, exhibit a convincing linear correlation with the quadrupole coupling tensor elements, determined from (11)B MAS NMR spectra of the central or satellite transitions. The model compounds include Li(2)B(4)O(7), Mg(2)B(2)O(5), Mg(3)B(2)O(6), NH(4)B(C(6)H(5))(4), and colemanite (CaB(3)O(4)(OH)(3).H(2)O). The (11)B quadrupole moment, Q = 0.0409 +/- 0.0002 barn, derived from the linear correlation, is in excellent agreement with the accepted value for Q((11)B). This demonstrates that DFT (WIEN2k) calculations can provide precise (11)B quadrupole coupling parameters on an absolute scale. On the other hand, DFT calculations based on the reported crystal structures for datolite (CaBSiO(4)(OH)) and danburite (CaB(2)Si(2)O(8)) cannot reproduce the experimental (11)B quadrupole coupling parameters to the same high precision. However, optimization of these structures by minimization of the forces between the atoms (obtained by DFT) results in a significant improvement between the calculated and experimental (11)B quadrupole coupling parameters, which indicates that reliable refinements of the borate structures are obtained by this method. Finally, the DFT calculations also provide important structural information about the sign and orientation of the EFG tensor elements in the crystal frame, a kind of information that cannot be achieved from (11)B NMR experiments on powdered samples.     

11B solid-state NMR investigation of the rhamnogalacturonan II-borate complex in plant cell walls.

The boron in plant cell walls, which is water-insoluble and in the solid state, is solubilized by pectinase digestion to give a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex. To clarify the nondestructive structure of boron present in plant cell walls (as represented by sugar beet fiber), we performed 192- and 96-MHz 11B solid state NMR measurements. The use of a high field magnet frequency of 192-MHz enabled us to observe 11B isotropic chemical shifts at -9.7 and -9.6 ppm for dRG-II-B and sugar beet fiber in the solid state, respectively, demonstrating that the boron in isolated dRG-II-B and in plant cell walls is present as a borate-diol ester (1:2). The observation of the magnetic field dependence of the chemical shift and lineshape for the borate-diol ester (1:2) by quadrupolar interaction suggested that the borate complex had a distorted tetrahedral boron structure.     

11B NMR study of p-carboxybenzeneboronic acid ions for complex formation with some monosaccharides.

The 11B NMR spectra of p-carboxybenzeneboronic acid (PCBA) ions demonstrated that their chemical shifts depend on the pH. At a lower pH, PCBA exists as a mixture of neutral PCBA and the monoanion with a trigonal structure, and at higher pH as the dianion with a tetrahedral structure. In the intermediate pH region, both the mono- and dianion coexist. The pKa of the monoanion of PCBA has been estimated to be 8.53. The complex formation constants of PCBA with several monosaccharides suggest that PCBA-fructose complex is most predominant.     

1H and 13C NMR studies of 7-azaindole and related compounds

The ¹H and ¹³C chemical shifts, proton-proton coupling constants, and one-bond carbon-hydrogen coupling constants have been obtained for 7-azaindole, 1-methyl-7-azaindole, their corresponding methyl iodide salts, and the related compound 7-methyl-7H-pyrrolo [2,3-b]pyridine. are different from those of either 7-azaindole or 1-methyl-7-azaindole.     

Optimization of11B NMR line form of dynamic system B6H 7 −

Kuban State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 3, pp. 65–68, May–June, 1992.     

Photochemistry of poly(vinyl cinnamylideneacetate) and related compounds

The photochemistry of the cinnamylideneacetyl group was investigated with respect to a photosensitive polymer, poly(vinyl cinnamylideneacetate). The photochemical reaction of 1,4-butanediol dicinnamylideneacetate was intramolecular cyclobutane formation. The photosensitive polymer underwent dimerization of the cinnamylideneacetyl moiety to form a cyclobutane ring. The reactivity of the double bond adjacent to the carbonyl group was larger than that of the double bond adjacent to the phenyl group in the chromophore. The quantum yield of the reaction was larger in the solid state than in solution: ? > 1.2 in crystalline state, ? = 0.5 in polymer film, ? = 0.1 in solution. The reaction was sensitized by triplet sensitizers (E_T > 42 kcal/mole). The thermal reaction of the polymer was completely different from the photochemical reaction. A radical initiator was very ineffective for reaction of the polymer.     

Thermodynamics of metal-ligand bond formation: XVI. Base adducts of some organotin compounds

Thermodynamic data have been obtained by calorimetric titration in benzene solution at 30° for reaction of organotin compounds with Lewis bases; data are reported for forty acid/base systems.Ph₃SnCl forms $\text{1}\text{1}$ adducts of low stability with pyridine (py) or 4-methyl-pyridine (4-mepy). Ph₂SnCl₂, Me₂SnCl₂, Bu₂SnCl₂ and Bu₂Sn(NCS)₂ form simultaneously $\text{1}\text{1}$ and $\text{1}\text{2}$ adducts with py or 4-mepy and $\text{1}\text{1}$ adducts with 2,2′-bipyridine or 1,10-phenanthroline (phen); the enthalpies of formation of the phen adducts are similar to those of $\text{1}\text{2}$ adducts with 4-mepy. With BuSnCl₃ and PhSnCl₃ it was not possible to obtain data for each step in addition of pyridine or 4-mepy. Adduct stabilities increase with increasing chloride substitution and in the order Bu < Me < Ph; adducts of Bu₂Sn(NCS)₂ are more stable than those of Bu₂SnCl₂.Tributylphosphine does not react with Ph₃SnCl but gives $\text{1}\text{1}$ adducts with the other tin compounds; only PhSnCl₃ adds a second molecule of this base. The $\text{1}\text{1}$ adducts are more stable than those with heterocyclic bases. Tributylamine brings about disproportionation of the compounds R₂SnX₂ to R₄Sn and SnX₄NBu₃.     

Infrared and NMR study of some 2-styrylindolium dyes

A series of 18 molecules of the 2-styrylindolium dyes group is examined by means of infrared and ¹H NMR spectroscopy. The effect of electron-withdrawing and electron-releasing substituents on the spectral behavior of the compounds is assessed and the most appropriate structure for these molecules is suggested.