Direct synthesis of isothiocyanates from isonitriles by molybdenum-catalyzed sulfur transfer with elemental sulfur

The direct molybdenum-catalyzed sulfuration of a variety of isonitriles with elemental sulfur or propene sulfide as sulfur donors affords the corresponding isothiocyanates in good yields and under mild reaction conditions. A catalytic cycle is suggested, in which the molybdenum oxo disulfur complex operates as the active sulfur-transferring species. A novel adduct between the isonitrile and the molybdenum complex has been characterized by X-ray analysis and its association constant determined by UV-vis spectroscopy, but this adduct appears not to be involved in the sulfur-transfer process.     

Lewis acid catalyzed sulfur transfer from a sultene to strained cyclic alkenes

[reaction: see text] Sultene 1 reacts with cyclic alkenes under mild Lewis acid catalysis to form thiiranes diastereoselectively. With 1-methoxycyclooctene as sulfur acceptor, an unexpected insertion product is formed, which provides valuable mechanistic insight into the sulfur-transfer process.     

A computational study of chlorocarbene additions to cyclooctyne

Dichloro- and phenylchlorocarbene (CCl2 and PhCCl) add to cyclooctyne via a barrierless process (MP2/6-311+G*, B3LYP/6-311+G*, B3LYP/6-31G*) to yield the expected corresponding cyclopropene adducts. A three-dimensional potential energy surface (PES) for CCl2 addition to cyclooctyne (B3LYP/6-31G*) shows the formation of the cyclopropene product and also possible formation of a vinylcarbene. Residing in a shallow energy well, the vinylcarbene easily rearranges to the cyclopropene product, or to an exocyclic vinyl bicyclo[3.3.0]octane. Although the calculated three-dimensional PES indicates possible dynamic control of the cyclooctyne-chlorocarbene system through the putative formation of a vinylcarbene (in addition to the expected cyclopropene), additional calculations and preliminary experimental work show paths through the vinylcarbene to be unlikely. If the additions of chlorocarbenes to cyclooctyne are controlled by reaction dynamics, we predict that the vast majority of the reactions proceed via traditional carbene cycloaddition with only a very minor amount of products formed from the alternative pathway.     

N-terminal derivatization and fragmentation of neutral peptides via ion--molecule reactions with acylium ions: toward gas-phase Edman degradation?

The gas-phase ion-molecule reactions of neutral alanylglycine have been examined with various mass-selected acylium ions RCO(+) (R= CH(3), CD(3), C(6)H(5), C(6)F(5) and (CH(3))( 2)N), as well as the transacylation reagent O-benzoylbenzophenone in a Fourier transform ion cyclotron resonance mass spectrometer. Reactions of the gaseous dipeptide with acylium ions trapped in the ICR cell result in the formation of energized [M + RCO](+) adduct ions that fragment to yield N-terminal b-type and C-terminal y-type product ions, including a modified b(1) ion which is typically not observed in the fragmentation of protonated peptides. Judicious choice of the acylium ion employed allows some control over the product ion types that are observed (i.e., b versus y ions). The product ion distributions from these ion--molecule reactions are similar to those obtained by collision-activated dissociation in a triple quadrupole mass spectrometer of the authentic N-acylated alanylglycine derivatives. These data indicate that derivatization of the peptide in the gas-phase occurs at the N-terminal amine. Ab initio molecular orbital calculations, performed to estimate the thermochemistry of the steps associated with adduct formation as well as product ion formation, indicate that (i) the initially formed adduct is energized and hence likely to rapidly undergo fragmentation, and (ii) the likelihood for the formation of modified b(1) ions in preference to y(1) ions is dependent on the R substituent of the acylium ion. The reaction of the tetrapeptide valine--alanine--alanine--phenylalanine with the benzoyl cation was also found to yield a number of product ions, including a modified b(1) ion. This result suggests that the new experimental approach described here may provide a tool to address one of the major limitations associated with traditional mass spectrometric peptide sequencing approaches, that is, determination of the identity and order of the two N-terminal amino acids. Analogies are made between the reactions observed here and the derivatization and N-terminal cleavage reactions employed in the condensed-phase Edman degradation method.     

Comparison of the reactivity of thiophene,thiophene 1-oxide,and thiophene 1,1-dioxide as diene for diels-alder reactions. An ami semiempirical study

A semiempirical AMI theoretical study was carried out to examine the very low reactivity of thiophene; for example, the high reactivity of thiophene 1-oxide as a diene in the Diels Alder reactions. The reactivities of cyclopentadiene, thiophene, thiophene 1-oxide, and thiophene 1,1-dioxide were studied as dienes in the reaction with ethylene, cyclopropene, and benzoquinone. Ethylene was chosen as a standard, while cyclopropene, with its high strain energy was released in the course of a reaction making it relatively reactive. The benzoquinone has a lower LUMO energy, making it a very reactive dienophile for the Diels-Alder reaction. Frontier molecular orbital energy gap between the reactants was considered, and the disadvantage of this approach in studying the reactivity was demonstrated. For all combinations, the corresponding transition structures are generated and the activation energies are estimated. The estimated activation barrier for sulfur dioxide elimination from the adduct was used to explain the failure to accumulate the cycloadduct in the reaction mixture. The obtained results are compared with experimental data when available. An excellent agreement of theory and experiment was obtained.     

Electron-impact induced fragmentation of thiirene and thiophene dioxides

The electron impact mass spectra of several substituted thiirene and thiophene oxides and dioxides have been investigated using both low and high resolution mass spectrometry. The predominant fragmentation process in the spectra of the thiirene compounds is the elimination of the hetero (SO or SO₂) function and formation of a substituted acetylene ion. The 5-membered ring thiophene dioxides exhibit mainly elimination of SO rather than SO₂. Plausible mechanisms leading to the formation of the principal ions are proposed on the basis of metastable transitions.     

Trapping unstable terminal M-O multiple bonds of monocyclopentadienyl niobium and tantalum complexes with Lewis acids

Hydrolysis of [NbCp'Cl(4)] (Cp' = η(5)-C(5)H(4)SiMe(3)) with the water adduct H(2)O·B(C(6)F(5))(3) afforded the oxo-borane compound [NbCp'Cl(2){O·B(C(6)F(5))(3)}] (2a). This compound reacted with [MgBz(2)(THF)(2)] giving [NbCp'Bz(2){O·B(C(6)F(5))(3)}] (2b), whereas [NbCp'Me(2){O·B(C(6)F(5))(3)}] (2c) was obtained from the reaction of [NbCp'Me(4)] with H(2)O·B(C(6)F(5))(3). Addition of Al(C(6)F(5))(3) to solutions containing the oxo-borane compounds [MCp(R)X(2){O·B(C(6)F(5))(3)}] (M = Ta, Cp(R) = η(5)-C(5)Me(5) (Cp*), X = Cl 1a, Bz 1b, Me 1c; M = Nb, Cp(R) = Cp', X = Cl 2a) afforded the oxo-alane complexes [MCp(R)X(2){O·Al(C(6)F(5))(3)}] (M = Ta, Cp(R) = Cp*, X = Cl 3a, Bz 3b, Me 3c; M = Nb, Cp(R) = Cp', X = Cl 4a), releasing B(C(6)F(5))(3). Compound 3a was also obtained by addition of Al(C(6)F(5))(3) to the dinuclear μ-oxo compound [TaCp*Cl(2)(μ-O)](2), meanwhile addition of the water adduct H(2)O·Al(C(6)F(5))(3) to [TaCp*Me(4)] gave complex 3c. The structure of 2a and 3a was obtained by X-ray diffraction studies. Density functional theory (DFT) calculations were carried out to further understand these types of oxo compounds.     

Theoretical Analysis of Reactions between Phosphanylnitrenes and Boranes: The Fate of the Adducts

Ab initio molecular orbital calculations have been carried out to determine the minimum-energy pathways and thereby to probe the mechanism of reactions between phosphanylnitrenes (R(1)R(2)P&tbd1;N, R(1), R(2) = H, F) and boranes (H(2)XB, X = H, F, CH(3), and C(2)H(5)). Geometries have been determined using the MP2/6-31G(d,p) model, while relative energies have been estimated using, depending on the size of the system, the quadratic configuration interaction model (QCISD and QCISD(T)) with various basis sets including 6-31G(d,p), 6-311G(d,p), and 6-311++G(d,p). The stability of the primary complex adduct is strongly dependent on the substituents of the boranes. When the borane bears a H atom, the primary adduct is not at all stable and readily collapses to an amine isomer via a H-shift from B to N. This shift becomes more difficult if the substituent is F or CH(3). In the F case, a phosphorane isomer, owing to the strength of the P-F bond, turns out to be favored. When non-hydrogen boranes (BF(3) and B(CH(3) )(3) for example) could be used, the primary adducts could be stabilized and even exist as discrete intermediates. F substituents on the nitrene show no significant qualitative effect. In the H(2)PN + H(2)BC(2)H(5) reaction, a retro-ene reaction of the adduct directly gives rise to an amine product via a five-membered transition structure. In the reverse reaction of a HX molecule plus an iminoborane (RB&tbd1;N-PR(1)R(2) ), both 1,2-addition to B and N and 1,3-addition to B and P reactions are possible.     

A facile [4+1] type synthetic route to thiophenes from dienol silyl ethers and elemental sulfur

A facile method for the synthesis of thiophene derivatives via the reaction of readily accessible dienol silyl ethers with elemental sulfur is described. Dienol silyl ethers and elemental sulfur, when heated at 180 °C in the presence of MS4A, provided 3-siloxythiophene derivatives in yields up to 98%. In this reaction, thiophene derivatives might be formed through 1,2-dithiines and eight-membered cyclic tetrasulfides.     

A study of the reaction of sulfur with organic compounds

The reactions of sulfur with alip-halogen derivatives of 1,2-diphenylethane, 1,2-diphenylethylene, and tetraphenylethane have been studied. A new method for the production of tetraphenylthiophene (from C₆H₅CHClCHClC₆H₅), benzothieno[3,2-b]benzothiophene (from C₆H₅CHClCCl₂C₆H₅), and of 2-phenylbenzo[b] thiophene (from C₆H₅CHBrCHBrC₆H₅) has been developed. On being treated with sulfur, tetraphenyl-1,2-dichloroethane is converted into tetraphenylethylene. 1,2-Diphenylethylene reacts with sulfur in the presence of hydrogen bromide forming 2-phenylbenzo[b]thiophene and tetraphenylthiophene.For part XVII, see [1].     

Photoelectron spectra of planar sulfur heterocycles

The photoelectron spectra of benzo[b]thiophene (2), benzo[c]thiophene (3), thieno[3,2-b] thiophene (4) and thieno[2,3-b]thiophene (5) together with 2,1,3-benzothiadiazol (6) and benzofurazan (7) have been recorded and the first three bands have been assigned to π-orbitals on the basis of band shapes and semiempirical calculations. The good agreement obtained between measured and calculated ionization potentials suggests that sulfur 3d orbital participation must be very small.     

Kinetics and mechanism of the photoinitiated autoxidation of sulfur(IV) in the presence of iodide ion

The kinetics and mechanism of the photoinitiated and iodide ion-catalyzed aqueous autoxidation of sulfur(IV) has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Light absorption of both the iodide ion and sulfur(IV) contribute to the initiation of a highly efficient radical chain reaction, the overall rate of which depends on the reactant and catalyst concentrations, the pH, and the light intensity in a complex manner. To interpret all the experimental findings, an elaborate scheme is proposed, in which the chain carriers are SO3-*, SO4-*, SO5-*, I*, and I2-*. There are three termination steps, each of them is second-order with respect to the chain carriers. Model calculations and nonlinear fitting have been used to show that the proposed scheme gives an excellent quantitative interpretation of the experimental results.     

Synthesis of a novel fused thiirene sulfoxide with endoperoxide and its diels-alder adduct

Photooxidation of thiiranoradialene sulfoxide afforded a new thiirene sulfoxide fused with an endoperoxide ring, which gave the [2+4]cycloadduct with furan.     

AM1 semiempirical searching for suitable thiophene derivatives that will enable thiophenes to act as a diene in the diels-alder reactions

The AM1 semiempirical method was used for theoretical searching of activation of thiophene as a diene for the Diels-Alder reaction. The reactivity of thiophene, electron-withdrawing and electron-donating substituted thiophenes, as well as the S-methylthiophenium ion were studied as the diene for Diels-Alder reactions by evaluating their frontier orbital energies and by calculating reaction barriers with activated and deactivated dienophiles. It was demonstrated that slight activation of the thiophene ring can be obtained with both electron-donating and electron-withdrawing groups attached to the thiophene ring. It was predicted that the actual transformation of thiophenes into the corresponding S-methylthiophenium anions is the best means of activating the thiophenes. The calculated activation energies for normal (non-activated) dienophiles are moderate so mild reaction conditions are predicted. If dienophiles are activated with electron-donating substituents, AM1 calculations predict a two step cycloaddition reaction with a very small activation barrier.     

Synthesis and Antimicrobial Evaluation of Some New Thiophene Derivatives

2-(2-Cyano-acetylamino)-4,5,6,7-tetrahydro-benzo[b]thiophene-3-carboxamide(3) was used as starting material for synthesis of 4-thiazolidinone, thiazolidine, and thiophene derivatives 6, 7a, b, and 8a, b, respectively. Thiocarbomyl derivative 5, 4-thizolidinone 9, and thioxothiazolidine 10 were obtained from reaction of 3 with thioglycolic acid and phenyl isothiocyanate/sulfur, respectively. Condensation of 3 with selected cyclic ketones and aromatic aldehydes yielded the arylidine derivatives 11a, b and 13, respectively. Refluxing of 11a, b with sulfur and morpholine yielded the thiophene derivatives 12a, b, respectively. Diazocoupling of compound 3 withp-tolyl diazonium chloride yielded the hydrazone derivative 14. The newly synthesized compounds were characterized by infrared, ¹H NMR, and mass spectral studies. Representative compounds of the synthesized product were tested and evaluated as antimicrobial agents. Compound 12b gives very high antimicrobial activity against Ampicillin.     

Acidity effects of Hβ zeolite on olefin alkylation of thiophenic sulfur in gasoline

Olefin alkylation of thiophenic sulfur process was carried out in model gasoline, using Hβ zeolites with different Si/Al2 ratios as catalysts. In particular, the influence of acid properties of Hβ zeolites on its catalytic ability for the thiophene alkylation, xylene alkylation and hexene oligomerization was investigated. The results showed that the acidity of the Hβ zeolite was increased with the decrease of Si/Al2 ratio, but its catalytic ability was not always increased. In fact, it reached the maximal catalytic ability at Si/Al2 ratio of 66, and under the reaction conditions of 60 ℃, 1.5 MPa, WHSV 3.0 h^-1 and time on stream 2 h. At the ratio, the conversion of thiophene, xylene, and oligomerized hexene were 96.6%, 2.7% and 2.8%, respectively. An optimal Si/Al2 ratio exists for the catalytic performance of Hβ zeolite. By investigating the coke deposition of the used Hβ zeolite catalysts, it has been found that the optimal Si/Al2 ratio is attributed to the combined effect of the carbocation activation capability and the hydrogen transformation capability of the Hβ zeolite catalyst.     

微波辅助脱除煤中有机硫的机理研究

为阐明微波场联合过氧乙酸(PAA)脱除煤中有机硫的机理,选取山西临汾(LF)、宁夏宁东(ND)、山西灵石(LS)和河南洛阳(LY)脱矿物质煤及三种含硫模型化合物苄硫醇(Benzyl mercaptan, BM)、苯并(b)噻吩(Benzo (b) thiophene, BT)和二苯基亚砜(Diphenyl sulfoxide, DS)作为研究对象。微波功率为100 W,并联合PAA,辐照1-5 min,通过X射线光电子能谱仪(X-ray photoelectron spectroscopy, XPS)分析固相中硫形态的变化;通过离子色谱仪(Ion chromatography, IC)分析脱硫后液相中SO_4~(2-)的浓度;通过气相色谱-质谱联用仪(Gas chromatography/mass spectrometry, GC/MS)分析萃取物中硫形态的变化。结果表明,有机硫化物硫的含量高,脱硫率大,LY和LS最大脱硫率分别高达55.06%和45.78%,ND和LF最大脱硫率分别为31.24%和28.21%,煤中的硫醇比噻吩和亚砜更易脱除,且脱硫过程中硫形态逐渐向高价态转化,含硫键在微波场中断裂,最终可被PAA氧化为SO_4~(2-)。     

Isolation and characterization of a monomeric cationic titanium hydride

Methyl cations 1-Cp and 1-Cp*, stabilized by the tri-tert-butylphophinimine ligand and either C5H5 or C5Me5, were generated from the neutral dimethyl precursors and [Ph3C]+[B(C6F5)4]-. Reaction of these compounds with H2 resulted in contrasting reactions. For 1-Cp, hydrogenolysis of the Ti-CH3 group led to rapid reduction to Ti(III) and production of a cationic Ti(III) dimer, 2, presumably formed upon loss of H2 from a transiently generated Ti(IV) hydride. Compound 2 was characterized crystallographically and via its cleavage with donor solvents such as THF to form the monomeric [Cp(L)Ti(THF)2]+[B(C6F5)4]-, 3. In contrast, 1-Cp* reacted rapidly with H2 to form a cationic Ti(IV) hydride species, 4, which was resistant to reduction. While only moderately stable in solution under H2, a stable, isolable THF adduct preciptitated upon addition of THF, giving 4.THF, which was fully characterized, including via X-ray crystallography. Naked hydride 4 was very reactive toward haloarene solvents such as bromobenzene, giving the cationic bromide [Cp*(L)TiBr]+[B(C6F5)4]-, 5, which was fully characterized as its THF adduct 5.THF. The contrasting behavior of 1-Cp and 1-Cp* is a result of the greater steric protection and electron donation provided by the Cp* ligand relative to the Cp donor.     

Reaction of stabilized Criegee intermediates from ozonolysis of limonene with sulfur dioxide: ab initio and DFT study

The mechanism of the reaction of the sulfur dioxide (SO(2)) with four stabilized Criegee intermediates (stabCI-CH(3)-OO, stabCI-OO, stabCIx-OO, and stabCH(2)OO) produced via the ozonolysis of limonene have been investigated using ab initio and DFT (density functional theory) methods. It has been shown that the intermediate adduct formed by the initiation of these reactions may be followed by two different reaction pathways such as H migration reaction to form carboxylic acids and rearrangement of oxygen to produce the sulfur trioxide (SO(3)) from the terminal oxygen of the COO group and SO(2). We found that the reaction of stabCI-OO and stabCH(2)OO with SO(2) can occur via both the aforementioned scenarios, whereas that of stabCI-CH(3)-OO and stabCIx-OO with SO(2) is limited to the second pathway only due to the absence of migrating H atoms. It has been shown that at the CCSD(T)/6-31G(d) + CF level of theory the activation energies of six reaction pathways are in the range of 14.18-22.59 kcal mol(-1), with the reaction between stabCIx-OO and SO(2) as the most favorable pathway of 14.18 kcal mol(-1) activation energy and that the reaction of stabCI-OO and stabCH(2)OO with SO(2) occurs mainly via the second reaction path. The thermochemical analysis of the reaction between SO(2) and stabilized Criegee intermediates indicates that the reaction of SO(2) and stabilized Criegee intermediates formed from the exocyclic primary ozonide decomposition is the main pathway of the SO(3) formation. This is likely to explain the large (~100%) difference in the production rate in the favor of the exocyclic compounds observed in recent experiments on the formation of H(2)SO(4) from exocyclic and endocyclic compounds.     

Reaction of 2,2-bis(trifluoromethyl)-1,1-dicyanoethylene with 6,6-dimethylfulvene: cycloadditions and a rearrangement

Reaction of 2,2-bis(trifluoromethyl)-1,1-dicyanoethylene (BTF; 1) with 6,6-dimethylfulvene (2) affords the expected Diels-Alder cycloadduct, 7-(1-methylethylidene)-3,3-bis(trifluoromethyl)bicyclo[2.2.1]hept-5-ene-2,2-dicarbonitrile (3), in good yield. The cycloadduct 3 is unstable and exists in equilibrium with the starting materials in less polar solvents. In more polar environment, the [4 + 2] adduct 3 either reverts to starting materials, or, in a competing process, is converted to the formal [2 + 2] adduct, 2-(1-methylethylidene)-7,7-bis(trifluoromethyl)bicyclo[3.2.0]hept-3-ene-6,6-dicarbonitrile (6). In the presence of acid, 3 is converted to a third isomeric form, 1,3a,5,6-tetrahydro-7-methyl-5,5-bis(trifluoromethyl)-4H-indene-4,4-dicarbonitrile (8). Both 6 and 8 are formed with complete regiospecificity. Quantum mechanical calculations and X-ray crystallographic studies of this ensemble of reactions by BTF, and the analogous set of reactions by its progenitor, tetracyanoethylene (TCNE), reveal several interesting facets. The conversion of 3 to 6 occurs in certain polar solvents, in the presence of silica gel and alumina, as well as in the solid state. Single crystals of 3 were observed by X-ray crystallography to undergo crystal-to-crystal rearrangement to 6. The conversion of 3 to 8 proceeds by the initial retro-Diels-Alder reaction followed by isomerization of the fulvene to 1-isopropenyl-1,3-cyclopentadiene that then reacts with BTF to give the alternative Diels-Alder product as a single regioisomer. A hybrid density functional theory (DFT) method at the B3LYP/6-31G(d) level of theory gave calculated relative energies of 0.0, -9.0, and -18.8 kcal/mol for 3, 6, and 8, respectively. The same method was also used to correctly predict the regiochemical outcome of the cycloaddition of BTF with 1-isopropenyl-1,3-cyclopentadiene. Finally, an explanation is offered for the preference of the persubstituted cyanoolefins BTF and TCNE to add to the exocyclic diene portion of 1-isopropenyl-1,3-cyclopentadiene and the contrasting preference of 2-acetyloxy-2-propenenitrile to add to the endocyclic diene.