Reactions of Diazomethanes with 5‐Benzylidene‐3‐phenylrhodanine – A Computational Study

It has been shown previously that the reaction of diazomethane with 5‐benzylidene‐3‐phenylrhodanine ( 1 ) in THF at ?20° occurs at the exocyclic C?C bond via cyclopropanation to give 3a and methylation to yield 4 , respectively, whereas the corresponding reaction with phenyldiazomethane in toluene at 0° leads to the cyclopropane derivative 3b exclusively. Surprisingly, under similar conditions, no reaction was observed between 1 and diphenyldiazomethane, but the 2‐diphenylmethylidene derivative 5 was formed in boiling toluene. In the present study, these results have been rationalized by calculations at the DFT B3LYP/6‐31G(d) level using PCM solvent model. In the case of diazomethane, the formation of 3a occurs via initial Michael addition, whereas 4 is formed via [3+2] cycloaddition followed by N₂ elimination and H‐migration. The preferred pathway of the reaction of 1 with phenyldiazomethane is a [3+2] cycloaddition, subsequent N₂ elimination and ring closure of an intermediate zwitterion to give 3b . Finally, the calculations show that the energetically most favorable reaction of 1 with diphenyldiazomethane is the initial formation of diphenylcarbene, which adds to the S‐atom to give a thiocarbonyl ylide, followed by 1,3‐dipolar electrocyclization and S‐elimination.     

Reaction Kinetics of Carbon Dioxide (CO2) Absorption in Sodium Salts of Taurine and Proline Using a Stopped‐Flow Technique

The pseudo–first‐order reaction rate constants (k₀, s^?1) for the reaction of carbon dioxide in aqueous solutions of sodium taurate (NaTau) and sodium prolinate (NaPr) were measured using a stopped‐flow technique at a temperature range of 298–313 K. The solutions concentration varied from 5 to 50 mol m^?3 and from 4 to 12 mol m^?3 for NaTau and NaPr, respectively. Comparing the k₀ values, aqueous NaPr was found to react very fast with CO₂ as compared with the industrial standard aqueous monoethanolamine (MEA) and aqueous sodium taurate (NaTau) was found to react slower than aqueous MEA at the concentration range considered in this work. For the studied amino acid salts, the order of the reactions was found to be unity with respect to the amino acid salt concentration. Proposed reaction mechanisms such as termolecular and zwitterion reaction mechanisms for the reaction of CO₂ with aqueous solutions were used for calculating the second‐order reaction rate constants (k₂, m³ mol^?1 s^?1). The formation of zwitterion during the reaction with CO₂ was found to be the rate‐determining step, and the deprotonation of zwitterion was instantaneous compared to the reverse reaction of zwitterion to form an amino acid salt. The contribution of water was established to be significant for the deprotonation of zwitterion. Comparing the pseudo–first‐order reaction rate constants (k₀, s^?1) of various amino acid salts with CO₂, NaPr was found to be the faster reacting amino acid salt. The activation energy for NaTau was found to be 48.1 kJ mol^?1 and that of the NaPr was found to be 12 kJ mol^?1. The Arrhenius expressions for the reaction between CO₂ and the studied amino acid salts are      

Kinetics of the reaction of carbon dioxide with blends of amines in aqueous media using the stopped‐flow technique

The effect of mixing 2‐amino‐2‐methyl‐1‐propanol (AMP) with a primary amine, monoethanolamine (MEA), and a secondary amine, diethanolamine (DEA), on the kinetics of the reaction with carbon dioxide in aqueous media has been studied at 298, 303, 308, and 313 K over a range of blend composition and concentration. The direct stopped‐flow conductimetric method has been used to measure the kinetics of these reactions. The proposed model representing the reaction of CO₂ with either of the blends studied is found to be satisfactory in determining the kinetics of the involved reactions. This model is based on the zwitterion mechanism for all the amines involved (AMP, MEA, and DEA). Blending AMP with either of the amines results in observed pseudo‐first‐order reaction rate constant values (k_o) that are greater than the sum of the k_o values of the respective pure amines. This is due to the role played by one amine in the deprotonation of the zwitterion of the other amine. Steric factor and basicity of the formed zwitterion and the deprotonating species have a great bearing in determining the rate of the reactions studied. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 391–405, 2005     

The kinetics of complex formation between Ti(IV) and hydrogen peroxide

The kinetics of the formation of the titanium‐peroxide [TiO²⁺₂] complex from the reaction of Ti(IV)OSO₄ with hydrogen peroxide and the hydrolysis of hydroxymethyl hydroperoxide (HMHP) were examined to determine whether Ti(IV)OSO₄ could be used to distinguish between hydrogen peroxide and HMHP in mixed solutions. Stopped‐flow analysis coupled to UV‐vis spectroscopy was used to examine the reaction kinetics at various temperatures. The molar absorptivity (ε) of the [TiO²⁺₂] complex was found to be 679.5 ± 20.8 L mol^?1 cm^?1 at 405 nm. The reaction between hydrogen peroxide and Ti(IV)OSO₄ was first order with respect to both Ti(IV)OSO₄ and H₂O₂ with a rate constant of 5.70 ± 0.18 × 10⁴ M^?1 s^?1 at 25°C, and an activation energy, E_a = 40.5 ± 1.9 kJ mol^?1. The rate constant for the hydrolysis of HMHP was 4.3 × 10^?3 s^?1 at pH 8.5. Since the rate of complex formation between Ti(IV)OSO₄ and hydrogen peroxide is much faster than the rate of hydrolysis of HMHP, the Ti(IV)OSO₄ reaction coupled to time‐dependent UV‐vis spectroscopic measurements can be used to distinguish between hydrogen peroxide and HMHP in solution. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 457–461, 2007     

Generation and cyclization of thiocarbonyl S‐ylides by reaction of diazocompounds with C‐sulfonyldithioformates

The unexpected 1,3‐benzodithiine derivatives 5b,c were obtained from the reactions of trimethylsilyldiazomethane 2 with C‐sulfonyldithioformates, bearing pentachlorophenylthio group, 1b,c via unprecedented cyclization of the transient thiocarbonyl ylides 4b,c . While the corresponding reaction with C‐sulfonyldithioformates, bearing phenylthio group, afforded 5a via [2 + 3]‐cycloadditive dimerization of a transient thiocarbonyl ylides 4a . Under the same reaction condition, C‐sulfonyldithioformates 1d–f react with diazomethane and/or phenyldiazomethane to afford the unsymmetrical 1,3‐dithiolane 7d,e and thiirane 8e,f derivatives, respectively. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:28–33, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20246     

DFT studies on the mechanism of the reaction of C2H5S with NO2

The mechanisms for the reaction of C₂H₅S with NO₂ are investigated at the QCISD(T)/6‐311++G(d, p)//B3LYP/6‐311++G(d, p) level on both single and triple potential energy surfaces. The geometries, vibrational frequencies and zero‐point energy (ZPE) corrections of all stationary points involved in the title reaction are calculated at the B3LYP/6‐311++G(d, p) level. The results show that the reaction is more predominant on the single potential energy surface, while it is negligible on the triple potential energy surface. Without barrier height in the whole process, the major channel is R → C₂H₅SONO (IM1 and IM2) → P1 (C₂H₅SO+NO). With much heat released in the formation of C₂H₅SNO₂ (IM3) and the transition state involved in the subsequent step more stable than reactants, P4 (CH₃CHS + t‐HONO) is subdominant product energetically. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Study of the oxidation of some catechols in the presence of 4‐amino‐3‐thio‐1,2,4‐triazole by digital simulation of cyclic voltammograms

Electrochemical oxidation of catechol and its derivatives ( 1a–d ) has been studied in the presence of 4‐amino‐3‐thio‐1,2,4‐triazole ( 3 ) at various pHs. Some electrochemical techniques such as cyclic voltammetry using the diagnostic criteria derived by Nicholson and Shain for various electrode mechanisms and controlled‐potential coulometry were used. Results indicate the participation of catechols ( 1a–d ) with 3 in an intramolecular cyclization reaction to form the corresponding 1,2,4‐triazino[5,4‐b]‐1,3,4‐thiadiazine derivatives. In various scan rates, based on an electron transfer–chemical reaction–electron transfer–chemical reaction mechanism, the observed homogeneous rate constants (k_obs) for Michael addition reaction were estimated by comparing the experimental cyclic voltammetric responses with the digital simulated results. The oxidation reaction mechanism of catechols ( 1a–d ) in the presence of 4‐amino‐3‐thio‐1,2,4‐triazole ( 3 ) was also studied. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 340–345, 2007     

Metal‐induced rearrangement of benzothiazoline ring: Synthesis and characterization of some new organoantimony(V) derivatives of N,O, and S atom containing schiff base ligands

Some new triphenylantimony(V) derivatives of Schiff bases having the composition have been synthesized by the equimolar reactions of Ph₃SbBr₂ with newly synthesized benzothiazoline ligands, (where R = C₆H₅(H₂L¹), 4‐BrC₆H₄(H₂L²), 4‐ClC₆H₄(H₂L³), 4‐CH₃‐OC₆H₄(H₂L⁴), 4‐CH₃C₆H₄(H₂L⁵)). The reaction proceeds with the rearrangement of benzothiazoline ring. All the derivatives have been characterized by elemental analyses, molecular weight measurements and their plausible structures have been proposed on the basis of IR and NMR (¹H and ¹³C) spectral studies. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:70–75, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20260     

Correlation analysis of reactivity in the addition of substituted benzylamines to benzylidenemalononitrile

The kinetics of addition of a number of ortho‐, meta‐, and para‐substituted benzylamines to benzylidenemalononitrile (BMN) in acetonitrile have been studied. The reaction is first‐order with respect to BMN. The order with respect to the amine is more than one. It has been shown that the reaction followed two mechanistic pathways, uncatalyzed and catalyzed by the amine. The enthalpy of activation for the catalyzed path is negative indicating the presence of a preequilibrium (k₁, k₋₁) leading to the formation of a zwitterion. The values of rate constant, k₁, for the nucleophilic attack have been determined for twenty‐eight benzylamines. The rate constant, k₁ was subjected to correlation analyses using various single‐ and multi‐parametric equations. The best correlation is obtained in terms of Charton's LDR and LDRS equations. The polar regression coefficients are negative indicating the formation of a cationic species in the transition state. The reaction is subject to steric hindrance by ortho‐substituents. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 245–252, 1999     

Reactions of 9,9′‐bibenzonorbornenylidene sulfoxides with TMSOTf: Anomalous pinacol‐type rearrangement of thiirane 1‐Oxides

syn‐9,9′‐Bibenzonorbornenylidene sulfoxide 8b underwent pinacol‐type rearrangement to form 9 , together with a mixture of thiiranes 4a and 4b by reaction with TMSOTf in CH₂Cl₂ at room temperature. The rearrangement of anti‐sulfoxide 8a proceeded more slowly giving a mixture of 9, 4a , and 4b . © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:29–34, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20507     

Synthesis and anti‐HIV activity of new chiral 1,2,4‐triazoles and 1,3,4‐thiadiazoles

5‐substituted 4‐(4‐chlorophenyl)‐4H‐1,2,4‐triazol‐3‐thiones 3 and 2‐substituted 5‐(4‐chlorophenylamino)‐1,3,4‐thiadiazoles 4 were prepared from the intermediate thiosemicarbazides 2 under basic and acidic conditions, respectively. The thiosemicarbazides, in turn, were prepared by the reaction of hydrazides 1 with 4‐chlorophenylisothiocyanate in MeOH. Some of the new synthesized compounds were assayed against HIV‐1 and HIV‐2 in MT‐4 cells. All the compounds were inactive except 3f , which showed an EC₅₀ value of 23.9 μg/mL and 9.9 μg/mL against HIV‐1 and HIV‐2 with a therapeutic index of 3 and 7, respectively. It means that compound 3f was cytotoxic to MT‐4 cells at CC₅₀ of 72.7 μg/mL in both strains. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:316–322, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20282     

Efficient organotin catalysts for urethanes: kinetic and mechanistic investigations

Dibenzyltin bis(2‐ethylhexanoate) 1 (4‐Y C₆H₄CH₂)₂Sn(OC(O)R¹)₂ [Y = H, 1a; MeO, 1b; Cl, 1c; Me, 1d; and R¹ = MeCH₂CH₂CH₂CH(Et) ] were synthesized either from the reaction of corresponding dibenzyltin dichlorides with silver 2‐ethylhexanoate or from the reaction of dibenzyltin oxides with 2‐ethylhexanoic acid. Compound 1a was further utilized as a catalyst for the reaction of mono‐ and di‐isocyanates [PhNCO, CH₃C₆H₃‐2,4‐(NCO)₂ and OCN(CH₂)₆NCO] with alcohols (primary, secondary, tertiary, cyclohexcyl, alkyl, allyl, benzyl and aryl) leading to the formation of the corresponding urethanes. The catalytic efficiencies of 1 vis‐à‐vis industrially known organotin catalysts have been determined through kinetic studies for the reaction of PhNCO and n‐BuOH at various temperatures. Compounds 1a, 1c and 1d show higher efficiency than dibutyltin bis(2‐ethylhexanoate). FTIR studies further provide mechanistic insights into the catalytic cycle, which comprises pre‐coordination of isocyanate to tin(IV), formation of stannyl carbamate and generation of dibenzyl(alkoxy)carboxylate as the active catalyst. Copyright © 2000 John Wiley & Sons, Ltd.     

Reactions of Di(tert‐butyl)diazomethane with Acceptor‐Substituted Ethylenes

Di(tert‐butyl)diazomethane ( 4 ) is a nucleophilic 1,3‐dipole with strong steric hindrance at one terminus. In its reaction with 2,3‐bis(trifluoromethyl)fumaronitrile ((E)‐ BTE ), a highly electrophilic tetra‐acceptor‐substituted ethene, an imino‐substituted cyclopentene 9 is formed as a 1 : 2 product. The open‐chain zwitterion 10 , assumed as intermediate, adds the second molecule of (E)‐ BTE . The ¹⁹F‐ and ¹³C‐NMR spectra allow the structural assignment of two diastereoisomers, 9A and 9B . The zwitterion 10 can also be intercepted by dimethyl 2,3‐dicyanofumarate ( 11 ) and furnishes diastereoisomeric cyclopentenes 12A and 12B ; an X‐ray‐analysis of 12B confirms the ‘mixed’ 1 : 1 : 1 product. Competing is an (E)‐ BTE ‐catalyzed decomposition of 4 to give 2,3,4,4‐tetramethylpent‐1‐ene ( 7 )+N₂; the reaction of (E)‐ BTE with a trace of water appears to be responsible for the chain initiation. The H₂SO₄‐catalyzed decomposition of diazoalkane 4 , indeed, produced the alkene 7 in high yield. The attack on the hindered diazoalkane 4 by 11 is slower than that by (E)‐ BTE ; the zwitterionic intermediate 21 undergoes cyclization and furnishes the tetrasubstituted furan 22 . In fumaronitrile, electrophilicity and steric demand are diminished, and a 1,3‐cycloaddition produces the 4,5‐dihydro‐1H‐pyrazole derivative 25 . The reaction of 4 with dimethyl acetylenedicarboxylate leads to pyrazole 29 +isobutene.     

Ring opening of boriranes vis‐à‐vis aziridines: An ab initio and DFT probe on the mechanisms

Borirane undergoes ring opening reaction with NOCl and HNF₂ yielding the corresponding alkenes. Ab initio and density functional investigations of this reaction with cis‐ and trans‐2,3‐dimethylboriranes reveal that these reactions take place in a single step through the formation of a prereactive complex and a transition state giving the alkene with the same stereochemistry. Calculations clearly show that the concerted cleavage of C? B bonds leads to retention of stereochemistry. Further, it shows that HNF₂ cleaves boriranes more efficiently than does NOCl. Intrinsic reaction coordinate analyses and bond order analysis describe the nature of the transition state very well and fix the reaction mechanism. Solvent effect calculations through PCM model, with acetonitrile and CCl₄ as solvents, do not alter the gas phase results significantly. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Phosphaalkenes palladium(II) complexes in the suzuki and sonogashira cross‐coupling reactions

The 1‐methoxy‐2‐(supermesitylphosphanylidenemethyl)‐benzene ligand ( 1 ) was prepared by reacting the phospha‐Wittig reagent [Mes*PPMe₃] with o‐methoxybenzaldehyde. Reaction of 1 with one equivalent of the [Pd(allyl)Cl]₂ dimer in the presence of Ag(OTf) affords a neutral complex ( 4 ) in which the triflate ligand is coordinated to the palladium atom. DFT calculations show that the formation of complex 4 is favored by 22.4 kcal/mol with respect to that of a chelate species involving coordination of the ligand through the phosphorus atom of one lone pair at the oxygen of the pendant methoxy group. Reaction of two equivalents of ligand 1 with the [Pd(allyl)Cl]₂ dimer affords complex 5 , in which the two ligands are coordinated through their phosphorus atom. The catalytic activity of complex 5 was compared to that of the 1,3‐bis[2‐(supermesityl)phosphanediylmethyl]benzene palladium chloride complex (6). Performances of the two catalysts were found to be similar in the Suzuki cross‐coupling reaction between phenylboronic acid and some arylbromides (TON between 55.10⁵ and 99.10⁵) as well as in the Sonogashira coupling between phenylacetylene and arylbromides (TON between 400 and 950). © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:363–371, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20307     

Substituted pyridopyrimidinones, 1: Convenient PTC alkylation and halogenation of 2‐hydroxy‐4H‐pyrido[1,2‐a]pyrimidin‐4‐one

Alkylation of 2‐hydroxy‐4H‐pyrido[1,2‐a]pyrimidin‐4‐one ( 1 ) was investigated under solid–liquid phase transfer catalysis conditions (PTC), using tetrabutylammonium bromide and potassium carbonate. The reaction with alkyl halides led to the formation of various 2‐alkoxy products, in fair yields. Reaction of compound 1 with epichlorohydrin and chloroacetonitrile, under the same PTC conditions, afforded novel O1,O3‐disubstituted glycerol and oxazolopyridopyrimidone betaine derivatives, respectively. Some 3‐halo‐, 3,3‐dihalo, and/or 2,3‐dihalopyrido[1,2‐a]pyrimidines were also prepared using different halogenating agents at different reaction conditions. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:19–27, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20245     

Unusual Tritylation Reactions of Tricyclic Analogues of Acyclovir and an Attempt to Elucidate Their Mechanism

In reference to our earlier observation that the 3,9‐dihydro‐3‐[(2‐hydroxyethoxy)methyl]‐6‐methyl‐9‐oxo‐5H‐imidazo[1,2‐a]purine (6‐Me‐TACV) tricyclic antiviral agent derived from acyclovir undergoes unusual C‐tritylation to 7‐trityl and 7‐[4‐(benzhydryl)phenyl] derivatives enforced by a 6‐Me substituent, we studied tritylation of 6‐Ph ( 1a ) and 6‐(4‐MeOPh) ( 1b ) TACV derivatives. The treatment of 1a and 1b with TrCl in K₂CO₃/DMF resulted exclusively in the formation of 7‐[4‐(benzhydryl)phenyl] derivatives 2a , 2b , 3a , 3b , and 4a . Inhibition experiments with radical scavengers DNB and DBNO indicated a single‐electron‐transfer (SET) mechanism for this reaction. Analogous experiments with unsubstituted TACV and 6‐Me‐TACV suggest that the nature of the substituent at C(6) determines the reaction mechanism. The presence of a 6‐aryl substituent results in the exclusive formation of 4‐(benzhydryl)phenyl derivatives via a SET mechanism. On the contrary, when C(6) is unsubstituted, trityl derivatives are the only products of the S_n reaction. In the case of 6‐Me‐TACV, concomitant SET and S_n mechanisms direct the reaction towards 4‐(benzhydryl)phenyl and trityl products.     

Polymerization of carboxylic ester functionalized norbornenes catalyzed by (η3‐allyl)palladium complexes bearing N‐heterocyclic carbene ligands

An in situ generated cationic allylpalladium complex bearing N‐heterocyclic carbene (NHC) ligands, derived from the reaction of [(η³‐C₃H₅)Pd(NHC)Cl] with AgX (X = BF₄ or SbF₆), is an active catalyst for the addition polymerization of norbornene and norbornene derivatives [5‐norbornene‐2‐carboxylic acid methyl ester ( b ) and 5‐norbornene‐2‐carboxylic acid n‐butyl ester ( c )] with an ester group containing a large portion of endo‐isomers. The catalytic activities, polymer yields, molecular weights, and molecular weight distributions of polynorbornenes were investigated under various reaction conditions: the catalytic activity was highly dependent on the counteranion, the reaction solvent, and the reaction temperature. For b , as the portion of an endo‐isomer increased, the activity of 13 (SbF) was much higher than those of 14 and 15 , and for c (exo/endo = 95:5), the maximum turn over number (TON) was observed with 15 (SbF). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3042–3052, 2007     

Kinetics for the hydrogen‐abstraction of CH4 with NO2

The detailed mechanism of the NO₂+CH₄ reaction has been computationally investigated at the M06‐2X/MG3S, B3LYP/6‐311G(2d,d,p), and MP2/6‐311+G(2df,p) levels. The direct dynamics calculations were preformed using canonical transition state theory with tunneling correction and scaled generalized normal‐mode frequencies including anharmonic torsion. The calculated results indicate that the NO₂+CH₄ reaction proceeds by three distinct channels simultaneously, leading to the formation of trans‐HONO (1a), cis‐HONO (1b), and HNO₂ (1c), and each channel involves the formation of intermediate having lower energy than the final product. The anti‐Hammond behavior observed in channel 1a is well analyzed. Proper treatment of anharmonic torsions about the C···H···O (or N) axis in the transition structures greatly improves the accuracy of kinetics predictions. The activation energy for each channel increases substantially with temperature, but is not strictly a linear function of temperature. Therefore, the thermal rate constants are fitted to the four‐parameter expression recommended for this case over the wide temperature range 400–4000 K. © 2012 Wiley Periodicals, Inc.     

The reaction of triphenylmethylhalophosphines with tetrachloro‐orthobenzoquinone: Oxidation—isomerization—dehydrohalogenation

Treatment of solutions of the halophosphines TrtP(H)F (Trt = trityl, Ph₃C) 1a and TrtP(H)Cl 1b with equimolar amounts of TOB (tetrachloro‐orthobenzoquinone) led to the formation of mixtures of products. They contained the phosphoranes 2a and 2b, which were formed by oxidation with TOB and are in equilibrium with the phosphines 3a and 3b. Moreover, the trityl phosphonite 4, which was formed by dehydrohalogenation of 2a, 2b and 3a, 3b, was observed in both mixtures. The dehydrohalogenation was found to be reversible in the case of HF. The pure compounds 4 and 5 were obtained from the reaction of TrtPCl₂ with tetrachlorocatechol 4 and by the oxidation of 1a and 1b with two equivalents of TOB. Because of its importance in this reaction sequence, an X‐ray crystal structure determination was carried out on 4. The P–O bond lengths of 168.4(2) and 167.7(2) pm are probably to be attributed to a bond‐lengthening effect of the chlorine atoms of the quinone. As a comparison with analogous systems reveals, the phosphorane 5 is an example of a σ⁵λ⁵(P) species in which the phosphorus atom exhibits square‐pyramidal coordination. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 277–280, 1999