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.     

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.     

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.     

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.     

Photochemistry and photophysics of stilbene dendrimers and related compounds

The syntheses and reactions of photoresponsive dendrimers are described. Dendrimers with photoreversible stilbene cores undergo mutual cis–trans isomerization in organic solvents to give photostationary state mixtures of cis- and trans-isomers. Even stilbene dendrimers with molecular weights as high as 6500 underwent mutual cis–trans isomerization within the lifetime of the excited singlet state. The large dendron group surrounding the photoreactive core may affect the excited state properties of the core to induce the efficiency of photoisomerization and/or reduce the fluorescence efficiency. The photochemistry of stilbene dendrimers, with various types of dendron groups, azobenzene dendrimers and other photoresposive dendrimers is discussed.     

High-resolution solid-state 11B and 13C MAS NMR of icosahedral carboranes

The globular-shaped icosahedral carboranes, o-, m-, and p-C₂B₁₀H₁₂, yield high-resolution ¹¹B and ¹³C MAS solid-state NMR spectra. Chemical shifts and linewidths are comparable to those of solution spectra. Spin—lattice relaxation times indicate the quadrupolar mechanism to be dominant for the B-11 relaxation. Linewidths of spectra recorded at 96 MHz and 64 MHz were similar. At 298 K, the ¹³C T₁ of p-carborane was found to be an order of magnitude smaller than those of the other two isomers. This can be attributed to the higher phase transition temperature of p-carborane indicative of the stronger intermolecular dipolar CH interactions made possible by the para-carbon vertices. Substitution on o-carborane by a methyl group yields spectra of comparable linewidths to those of the parent compound. Substitution by a bulkier phenyl group, however, results in extensive broadening of both ¹¹B and 13C signals of the derivative 1-Ph-o-C₂B₁₀H₁₁.     

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 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.     

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.     

Studies on aromatic amine boranes by 11B and 1H NMR

¹H and ¹¹B NMR spectroscopy was applied to mono- and bisborane adducts derived from aryl-, benzyl-, phenethyl- and phenylenediamines, but no simple relationship was established between the spectroscopic data and the nature of the NB bond. Comparative studies of the affinity of aromatic amines to BH₃ by equilibria reactions may be of great value in establishing a scale of relative basicity.