Benjamin Taljaard

Synthesis of 4-(6H-dibenzo[b, e]thiepin-11-ylidene)cyclohexa-2,5-dienone and 4-(6H-dibenzo[b, e]oxepin-11-ylidene)cyclohexa-2,5-dienone

The synthesis of two novel quinone methides (4a and 4b) containing tricyclic heterocyclic systems with 7-membered central rings has been achieved in good yields under relatively mild reaction conditions.

Synthesis of 5,5′‐Diformyl‐2,2′‐difuran from 2‐Furfural by Photochemical Aryl Coupling

The synthesis of 5,5′-diformyl-2,2′-difuran (IUPAC name: [2,2′-bifuryl]-5,5′-dicarbaldehyde) in good yields by the intermolecular coupling of 2-furfural and 5-bromo-2-furfural has been achieved. Optimum yields were obtained when mixtures of the substrates in acetonitrile were treated with polyvinylpyridine powder (Reillex 402), and irradiated with UV light through a quartz filter. Low yields of coupling product were obtained in the absence of this base or if hydrocarbon solvents were used. A mechanistic pathway involving a transient exciplex intermediate has been proposed.

Synthesis of N,N′-bis(9-phenylxanthen-9-yl)ethylenediamine and an investigation of its host–guest inclusion potential

The novel amine, N,N′-bis(9-phenylxanthen-9-yl)ethylenediamine 1, was synthesized by treating 9-phenylxanthen-9-ylium perchlorate with ethylenediamine. Its host potential was assessed by allowing it to crystallise from a range of organic solvents (single and binary mixtures), of which several were found to be included. The stoichiometries of these host–guest complexes were determined through 1H-NMR analysis and their stabilities assessed through thermal analysis. Single crystal X-ray crystallography was used to elucidate the crystal structure of the 1∶1 1·THF inclusion complex. It was found that only one of the two amine moieties of the host functions as a donor, forming a nearly linear hydrogen bond to the oxygen atom of the guest molecule. Each THF molecule is effectively surrounded by host molecules so that the THF molecules are found to occupy discrete “cavities” within the host lattice. The activation energy associated with desolvation of the 1·THF complex was determined through thermal analysis and was found to decrease as desolvation progressed.

The mechanism of benzophenone-sensitised oxidation of 9-phenylxanthene with oxygen

From an investigation of the effect of inhibitors, quenchers, and the failure of a singlet sensitiser to effect the oxidation, it is proposed that the benzophenone-sensitised oxidation of 9-phenylxanthene with oxygen is a type I process. Further support for this conclusion is afforded by spectroscopic considerations. Evidence is also presented that the oxidation is a non-chain process, and the effect of acid on the product distribution is discussed.

Desorption kinetics of a xanthenol-dioxane clathrate

The host xanthenol compound forms a 1:1 clathrate with dioxane, namely 9-(1-naphthyl)-9H-xanthen-9-ol-1,4-dioxane, C23H16O2.C4H8O2. The structure of this clathrate is reported, along with a study of the kinetics of desolvation and the determination of an activation energy. The guest molecules are stabilized by O(host)-H...O(guest) hydrogen bonds [O-H = 0.968 (2) A, O...O = 2.7532 (13) A and O-H...O = 151.9 (4) degrees].

14-Hydroxy-14-phenyldibenzo[a,j]xanthene as a lattice host: crystal and molecular structure of its (1:1) complex with diethyl ether

The title compound 14-hydroxy-14-phenyldibenzo[a,j]xanthene 1 formed a (1:1) complex with diethyl ether. 1,4-Dioxane was also enclathrated, but with a variable stoichiometric ratio. Single crystal X-ray crystallography was used to elucidate the crystal structure of the 1·diethyl ether complex. Crystals are orthorhombic with space group P212121, a = 8.532(3), b = 15.040(4), c = 18.491(5) Å, V = 2373(1) Å3, dc = 1.256 g/cm3, and Z = 4. Host and guest molecules were found to associate via hydrogen bonds, with the guest molecules residing in undulating channels lined by host molecules.

Synthesis of 11-Phenyl-6H-dibenzo[b,f][1,4]oxazocine and 11-Phenyl-6H-dibenzo[b,f][1,4]thiazocine†

A convenient synthesis of 11-phenyl-6H-dibenzo[b,f][1,4]oxazocine 1 and the corresponding thiazocine 2 is described.

Inclusion Compounds from a Host Mixture: A Cautionary Tale

The inclusion properties of a dibenzocycloheptenol host with a variety of volatile guests have been analysed. The packing and intermolecular interactions of the inclusion compounds have been examined. Unexpected structures arising from impurities in the host, termed the ‘host mixture’, have been elucidated.

Addition of acyl and sulfonyl hypohalites generated from N-halogeno amides to alkenes : synthesis of trans-vic-halogeno esters and their conversion to cis-1,2-diols

N-lodo- and N-bromo-p-nitrobenzamide have been shown to react with various organic acids to form the respective acyl or sulfonyl hypoiodites or hypobromites. These readily add to double bonds under mild conditions to yield trans-vic-halogeno esters. The stereochemistry, as well as the regioselectivity, of these additions have been rationalised in terms of an intermediate iodonium or bromonium ion. Alkaline hydrolysis of adducts from cyclohexene gave good yields of ciscyclohexane-1, 2-diols, suggesting this methodology to be a viable alternative to the Woodward–Prevost synthesis of cis-1, 2-diols.

Thermal and photochemical decomposition of 9-arylxanthen-9-yl hydroperoxides and peroxides

The thermolysis and photolysis of a series of 9-arylxanthenyl hydroperoxides and peroxides have been investigated by means of differential scanning calorimetry, electron paramagnetic spectroscopy and product studies. These relatively stable peroxidic compounds were found to rearrange smoothly on being heated at temperatures > 120°C and on photolysis, respectively, through concerted reaction pathways. Photolysis of 9-hydroperoxy-9-phenylxanthene and di(9-phenylxanthen-9-yl) peroxide, respectively, have been shown by EPR spectroscopy to generate 9-phenylxanthenyl radicals which dimerise at low temperatures and reform the free radicals at ambient temperature.