Electron transfer. 144. Reductions with germanium(II)

Solutions 0.2-0.4 M in Ge(II) and 6 M in HCl, generated by reaction of Ge(IV) with H3PO2, are stable for more than 3 weeks and can be diluted 200-fold with dilute HCl to give GeCl3- preparations to be used in redox studies. Kinetic profiles for the reduction of Fe(III) by Ge(II), as catalyzed by Cu(II), implicate the odd-electron intermediate, Ge(III), which is formed from Cu(II) and Ge(II) (k = 30 M-1 s-1 in 0.5 M HCl at 24 degrees C) and which is consumed by reaction with Fe(III) (k = 6 x 10(2) M-1 s-1). A slower direct reaction between Ge(II) and Fe(III) (k = 0.66 M-1 s-1) can be detected in 1.0 M HCl. The reaction of Ge(II) with I3- in 0.01-0.50 M iodide is zero order in oxidant and appears to proceed via a rate-determining heterolysis of a Ge(II)-OH2 species (k = 0.045 s-1) which is subject to H(+)-catalysis. Reductions of IrCl6(2-) and PtCl6(2-) by Ge(II) are strongly Cl(-)-catalyzed. The Ir(IV) reaction proceeds through a pair of 1e- changes, of which the initial conversion to Ge(III) is rate-determining, whereas the Pt(IV) oxidant probably utilizes (at least in part) an inner-sphere PtIV-Cl-GeII bridge in which chlorine is transferred (as Cl+) from oxidant to reductant. The 2e- reagent, Ge(II), like its 5s2 counterpart, In(I), can partake in 1e- transactions, but requires more severe constraints: the coreagent must be more powerfully oxidizing and the reaction medium more halide-rich.     

Theoretical study of the energetics of the reactions of triplet dioxygen with hydroquinone, semiquinone, and their protonated forms: relation to the mechanism of superoxide generation in the respiratory chain

One-electron reduction of the dioxygen molecule by the reduced form of mitochondrial ubiquinones (Q) of the NADH dehydrogenase (complex I) and mitochondrial cytochrome bc1 (complex III) is believed to be the main source of the superoxide anion radical O2*- and the hydroperoxide radical OOH*. In this work, we modeled the energetics of four possible reactions of the triplet ((3)Sigma(g)) dioxygen-molecule reduction by fully reduced and protonated ubiquinone (QH2; reaction 1), its deprotonated form (QH-; reaction 2), the semiquinone radical (QH*; reaction 3), and the semiquinone anion radical (Q*-; reaction 4), by means of ab initio calculations with the 6-31G(d) and 6-31+G(d) basis set in the restricted open-shell Hartree-Fock (ROHF), unrestricted Hartree-Fock (UHF), and complete active space self-consistent field (CASSCF) with dynamic correlation [at the second-order M?ller-Plesset (MP2) or multiple reference M?ller-Plesset (MRMP), respectively] schemes and the basis set superposition error (BSSE) correction included, as well as semiempirical AM1 and PM3 calculations in the UHF and ROHF schemes. 2-Butene-1,4-dione and p-benzoquinone were selected as model compounds. For the reduced forms of both compounds, reaction 1 turned out to be energetically unfavorable at all levels of theory, this agreeing with the experimentally observed diminished reductive properties of hydroquinone derivatives at low pH. For 2-butene-1,4-dione treated at the most advanced MRMP/CASSCF/6-31+G(d) level, the energies of reactions 1-4 are 4.7, -34.3, -15.0, and -4.1 kcal/mol, respectively. This finding suggests that reactions 2 and 3 are the most likely mechanisms of electron transfer to molecular oxygen in aprotic environments and that proton transfer is involved in this process. Nearly the same energies of reactions 2 and 3 were calculated at the MRMP/CASSCF/6-31+G(d) level for reduced forms of p-benzoquinone. Inclusion of diffuse functions in the basis set and dynamic correlation at the CASSCF level appears essential. Because deprotonated ubiquinol is unlikely to exist in physiological environments, reaction 3 appears to be the most likely mechanism of one-electron reduction of oxygen; however, if oxygen can penetrate cytochrome bc1 as far as the Q(o) center where ubiquinol can be deprotonated, reaction 2 can also come into play. The energies of reactions 2 and 3 calculated at the MRMP/CASSCF/6-31+G(d) level are most closely reproduced in the ab initio and semiempirical UHF PM3 calculations. Additional semiempirical calculations on more realistic models of ubiquinone, 2,3-dimethoxy-6-methyl-p-benzoquinone and 2,3-dimethoxy-5-isoprenyl-6-methyl-p-benzoquinone, gave qualitatively the same relations between the energies of reactions 2 and 3 as those carried out for p-benzoquinone species, thereby suggesting that this method could be used in studying electron-transfer reactions from reduced quinone derivatives to molecular oxygen in more complex systems, such as a model of the Q(o) site of cytochrome bc1, where applying ab initio methods is unfeasible.     

Quantum mechanical study of the potential energy surface of the ClO + NO2 reaction

In the study of the reaction pathways of the ClO + NO2 reaction including reliable structures of the reactants, products, intermediates, and transition states as well as energies the MP2/6-311G(d), B3LYP/6-311G(d), and G2(MP2) methods have been employed. Chlorine nitrate, ClONO2, is formed by N-O association without an entrance barrier and is stabilized by 29.8 kcal mol(-1). It can undergo either a direct 1,3 migration of Cl or OCl rotation to yield an indistinguishable isomer. The corresponding barriers are 45.8 and 7.1 kcal mol(-1), respectively. ClONO2 can further decompose into NO3 + Cl with an endothermicity of 46.4 kcal mol(-1). The overall endothermicity of the NO2 + ClO --> NO3 + Cl reaction is calculated to be 16.6 kcal mol(-1). The formation of cis-perp and trans-perp conformer of chlorine preoxynitrite, ClOONO(cp) and ClOONO(tp), are exothermic by 5.4 and 3.8 kcal mol(-1), respectively. Calculations on the possible reaction pathways for the isomerization of ClOONO to ClONO2 showed that the activation barriers are too high to account for appreciable nitrate formation from peroxynitrite isomerization. All quoted relative energies are related to G2(MP2) calculations.     

Asymmetric allylic substitution catalyzed by C1-symmetrical complexes of molybdenum: structural requirements of the ligand and the stereochemical course of the reaction

Application of new chiral ligands (R)-(-)-12 a and (S)-(+)-12 c (VALDY), derived from amino acids, to the title reaction, involving cinnamyl (linear) and isocinnamyl (branched) type substrates (4 and 5 --> 6), led to excellent regio- and enantioselectivities (>30:1, < or =98 % ee), showing that ligands with a single chiral center are capable of high asymmetric induction. The structural requirements of the ligand and the mechanism are discussed. The application of single enantiomers of deuterium-labeled substrates (both linear 38 c and branched 37 c) and analysis of the products (41-43) by (2)H{(1)H} NMR spectroscopy in a chiral liquid crystal matrix allowed the stereochemical pathways of the reaction to be distinguished. With ligand (S)-(+)-12 c, the matched enantiomer of branched substrate was found to be (S)-5, which was converted into (R)-6 with very high regio- and stereoselectivity via a process that involves net retention of stereochemistry. The mismatched enantiomer of the branched substrate was found to be (R)-5, which was also converted into (R)-6, that is, with apparent net inversion, but at a lower rate and with lower overall enantioselectivity. This latter feature, which may be termed a "memory effect", reduced the global enantioselectivity in the reaction of the racemic substrate (+/-)-5. The stereochemical pathway of the mismatched manifold has been shown also to be one of net retention, the apparent inversion occurring through equilibration via an Mo-allyl intermediate prior to nucleophilic attack. Incomplete equilibration leads to the memory effect and thus to lower enantioselectivity. Analysis of the mismatched manifold over the course of the reaction revealed that the memory effect is progressively attenuated with the nascent global selectivity increasing substantially as the reaction proceeds. The origin of this effect is suggested to be the depletion of CO sources in the reaction mixture, which attenuates turnover rate and thus facilitates greater equilibrium. The linear substrate was also converted into the branched product with net syn stereochemistry, as shown by isotopic labeling. An analogous process operates in the generation of small quantities of linear product from branched substrate.     

Transition state stabilization and substrate strain in enzyme catalysis: ab initio QM/MM modelling of the chorismate mutase reaction

To investigate fundamental features of enzyme catalysis, there is a need for high-level calculations capable of modelling crucial, unstable species such as transition states as they are formed within enzymes. We have modelled an important model enzyme reaction, the Claisen rearrangement of chorismate to prephenate in chorismate mutase, by combined ab initio quantum mechanics/molecular mechanics (QM/MM) methods. The best estimates of the potential energy barrier in the enzyme are 7.4-11.0 kcal mol(-1)(MP2/6-31+G(d)//6-31G(d)/CHARMM22) and 12.7-16.1 kcal mol(-1)(B3LYP/6-311+G(2d,p)//6-31G(d)/CHARMM22), comparable to the experimental estimate of Delta H(++)= 12.7 +/- 0.4 kcal mol(-1). The results provide unequivocal evidence of transition state (TS) stabilization by the enzyme, with contributions from residues Arg90, Arg7, and Arg63. Glu78 stabilizes the prephenate product (relative to substrate), and can also stabilize the TS. Examination of the same pathway in solution (with a variety of continuum models), at the same ab initio levels, allows comparison of the catalyzed and uncatalyzed reactions. Calculated barriers in solution are 28.0 kcal mol(-1)(MP2/6-31+G(d)/PCM) and 24.6 kcal mol(-1)(B3LYP/6-311+G(2d,p)/PCM), comparable to the experimental finding of Delta G(++)= 25.4 kcal mol(-1) and consistent with the experimentally-deduced 10(6)-fold rate acceleration by the enzyme. The substrate is found to be significantly distorted in the enzyme, adopting a structure closer to the transition state, although the degree of compression is less than predicted by lower-level calculations. This apparent substrate strain, or compression, is potentially also catalytically relevant. Solution calculations, however, suggest that the catalytic contribution of this compression may be relatively small. Consideration of the same reaction pathway in solution and in the enzyme, involving reaction from a 'near-attack conformer' of the substrate, indicates that adoption of this conformation is not in itself a major contribution to catalysis. Transition state stabilization (by electrostatic interactions, including hydrogen bonds) is found to be central to catalysis by the enzyme. Several hydrogen bonds are observed to shorten at the TS. The active site is clearly complementary to the transition state for the reaction, stabilizing it more than the substrate, so reducing the barrier to reaction.     

Sensitized photoxygenation of piroxicam in neat solvents and solvent mixtures.

Detection of O(2)(1Delta(g)) phosphorescence emission, lambda(max)=1270 nm, following laser excitation and steady state methods were employed to determine the total rate constant, k(T), for the reaction between the non-steroidal anti-inflammatory drug piroxicam (PRX) and singlet oxygen in several solvents. Values of k(T) ranged from 0.048+/-0.003 x 10(6) M(-1) s(-1) in chloroform to 71.2+/-2.2 x 10(6) M(-1) s(-1) in N,N-dimethylformamide. The chemical reaction rate constant, k(R), was determined by using thermal decomposition of 1,4-dimethylnaphthalene endoperoxide as the singlet oxygen source. In acetonitrile, the k(R) value is equal to 5.0+/-0.4 x 10(6) M(-1) s(-1), very close to the k(T) value. This result indicates that, in this solvent, the chemical reaction corresponds to the main reaction path. Dependence of total rate constant on the solvent parameters pi* and beta can be explained in terms of a reaction mechanism that involves the formation of a perepoxide intermediate. Rearrangement of the perepoxide to dioxetane followed by ring cleavage and transacylation accounts for the formation of N-methylsaccharine and N-(2-pyridyl)oxamic acid, the main reaction products. Data obtained in dioxane-water (pH 4) mixtures with neutral enolic and zwitterionic tautomers of piroxicam in equilibrium show that the zwitterionic tautomer reacts with singlet oxygen faster than the enolic tautomer.     

Synthesis of Novel Heteroquaterphenoquinones and Their Electrochemical, Structural, and Spectroscopic Characterization

Novel heteroquaterphenoquinones, 5,5'-bis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-5,5'-dihydro-2,2'-bithienyl (3) and its 2,2'-biselenienyl (4), 2,2'-bifuryl (5), and 2,2'-bi-N-methylpyrrolyl (6) analogues, were synthesized by a stepwise cross-coupling reaction or by a more convenient one-pot oxidative homocoupling reaction of heterocycle-substituted phenols. Despite a highly conjugation-extended system, the quinones 3-6 are very stable in the solid state and in solution in common organic solvents. These quinones undergo a reversible one-stage, two-electron reduction up to dianions and a two-stage, one-electron oxidation reaction up to dications. The E(1)(red) of these quinones increases with the increase in the aromaticity of the incorporated heterocycles. The E(1)(ox) of these quinones appears to be specified by the ionization potential of the incorporated heterocycles. Thus, the N-methylquinone 6 exhibiting the lowest E(1)(ox) value exhibits the smallest E(1)(sum) among the quinones 3-6. The quinone 5 was revealed to exist in an unusual O-cis conformation in the solid state by X-ray crystallography, whereas the quinone 3 exists in an S-trans conformation in the solid state. The cis and trans isomers are interconvertible in solution in 3-5, whereas only N-trans isomer was detected for 6 in (1)H NMR spectroscopy. The quinones 3-6 exhibit a very intense absorption maximum in the near-infrared region of 662-827 nm. Of these, the maximum absorption wave length of 6 shifts to a more bathochromic region by 149-165 nm than those of 3-5. The quinones 3-6 can be used as dyestuffs in various fields for laser-driven high-density optical storage media.     

C_(70)与臭氧反应机理的理论研究

用半经验AM1方法研究了C_(70)与臭氧环加成反应的反应机理。采用Berny梯度 法优化得到反应的过滤度，并进行了振动分析确诊。计算结果表明：臭氧在C_(70) 6－6单、双键上的环加成反应均为复杂反应，由三步组成：第一步是O_3分子与C_ (70)的6-6单、双键发生1,3-偶极环加成反应，生成分子臭氧化物（即中间体I）， 6－6双键加成为放热反应，6－6单键加成为吸热反应，活化势垒分别为84.7和181. 2 kJ·mol~(-1)；第二步是中间体I的加成，C－C键断裂，生成两性离子中间体II 的放热反应，其势垒分别为61.3和13.3 kJ·mol~(-1)；第三步是中间体II脱去一 个Q_2分子生成具有环氧结构的C_(70)O，均为放热反应，活化势垒分别为169.3和 101.2 kJ·mol~(-1)；第三步是中间体I脱去一个O_2分子生成具有环氧结构的C_ (70)O，均为放热反应，活化势垒分别为169.3和101.2 kJ·mol~(-1)，从反应机理 和动力学角度解释了6－6双键加成优于6-6双键加成优于6-6双键加成优于6-6单键 加成的原因。O_3分子与C_(70)6-6双键的加成反应是协同且同步进行的，与6－6单 键的加成反应是协同但不同步的过程。     

o-Quinone methide as alkylating agent of nitrogen, oxygen, and sulfur nucleophiles. The role of H-bonding and solvent effects on the reactivity through a DFT computational study

The reactivity of the alkylating agent o-quinone methide (o-QM) toward NH(3), H(2)O, and H(2)S, prototypes of nitrogen-, oxygen-, and sulfur-centered nucleophiles, has been studied by quantum chemical methods in the frame of DF theory (B3LYP) in reactions modeling its reactivity in water with biological nucleophiles. The computational analysis explores the reaction of NH(3), H(2)O, and H(2)S with o-QM, both free and H-bonded to a discrete water molecule, with the aim to rationalize the specific and general effect of the solvent on o-QM reactivity. Optimizations of stationary points were done at the B3LYP level using several basis sets [6-31G(d), 6-311+G(d,p), adding d and f functions to the S atom, 6-311+G(d,p),S(2df), and AUG-cc-pVTZ]. The activation energies calculated for the addition reactions were found to be reduced by the assistance of a water molecule, which makes easier the proton-transfer process in these alkylation reactions by at least 12.9, 10.5, and 6.0 kcal mol(-1) [at the B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p) level], for ammonia, water, and hydrogen sulfide, respectively. A proper comparison of an uncatalyzed with a water-catalyzed reaction mechanism has been made on the basis of activation Gibbs free energies. In gas-phase alkylation of ammonia and water by o-QM, reactions assisted by an additional water molecule H-bonded to o-QM (water-catalyzed mechanism) are favored over their uncatalyzed counterparts by 5.6 and 4.0 kcal mol(-1) [at the B3LYP/6-311+G(d,p) level], respectively. In contrast, the hydrogen sulfide alkylation reaction in the gas phase shows a slight preference for a direct alkylation without water assistance, even though the free energy difference (DeltaDeltaG(#)) between the two reaction mechanisms is very small (by 1.0 kcal mol(-1) at the B3LYP/6-311+G(d,p),S(2df) level of theory). The bulk solvent effect, evaluated by the C-PCM model, significantly modifies the relative importance of the uncatalyzed and water-assisted alkylation mechanism by o-QM in comparison to the case in the gas phase. Unexpectedly, the uncatalyzed mechanism becomes highly favored over the catalyzed one in the alkylation reaction of ammonia (by 7.0 kcal mol(-1)) and hydrogen sulfide (by 4.0 kcal mol(-1)). In contrast, activation induced by water complexation still plays an important role in the o-QM hydration reaction in water as solvent.     

功能化酸性离子液体催化柴油氧化脱硫的研究

以功能化酸性离子液体为催化剂,30%双氧水为氧化剂,将加氢柴油中的含硫化合物氧化为相应的砜类物质,并用N-甲基吡咯烷酮（NMP）萃取一次。同时考察了反应温度、反应时间和催化剂用量等因素对氧化脱硫反应的影响,得出最佳反应条件为,3mL油样（硫的质量分数为200×10-6）、1.5g酸性离子液体、 0.3mL H2O2、 25℃,3.5h、VNMP/Vdiesel=1∶1,脱硫率可达到86.7%,柴油硫的质量分数仅为25×10-6左右,反应结束后,可通过简单的倾倒将油样和催化剂分离,重复使用5次,其催化活性变化不大。     

Importance of interfacial adsorption in the biphasic hydroformylation of higher olefins promoted by cyclodextrins: a molecular dynamics study at the decene/water interface

We report herein a molecular dynamics study of the main species involved in the hydroformylation of higher olefins promoted by cyclodextrins in 1-decene/water biphasic systems at a temperature of 350 K. The two liquids form a well-defined sharp interface of approximately 7 A width in the absence of solute; the decene molecules are generally oriented "parallel" to the interface where they display transient contacts with water. We first focused on rhodium complexes bearing water-soluble TPPTS(3-) ligands (where TPPTS(3-) represents tris(m-sulfonatophenyl)phosphine) involved in the early steps of the reaction. The most important finding concerned the surface activity of the "active" form of the catalyst [RhH(CO)(TPPTS)(2)](6-), the [RhH(CO)(2)(TPPTS)(2)](6-) complex, and the key reaction intermediate [RhH(CO)(TPPTS)(2)(decene)](6-) (with the olefin pi-coordinated to the metal center) which are adsorbed at the water side of the interface in spite of their -6 charge. The free TPPTS(3-) ligands themselves are also surface-active, whereas the -9 charged catalyst precursor [RhH(CO)(TPPTS)(3)](9-) prefers to be solubilized in water. The role of cyclodextrins was then investigated by performing simulations on 2,6-dimethyl-beta-cyclodextrin ("CD") and its inclusion complexes with the reactant (1-decene), a reaction product (undecanal), and the corresponding key reaction intermediate [RhH(CO)(TPPTS)(2)(decene)](6-) as guests; they were all shown to be surface-active and prefer the interface over the bulk aqueous phase. These results suggest that the biphasic hydroformylation of higher olefins takes place "right" at the interface and that the CDs promote the "meeting" of the olefin and the catalyst in this peculiar region of the solution by forming inclusion complexes "preorganized" for the reaction. Our results thus point to the importance of adsorption at the liquid/liquid interface in this important phase-transfer-catalyzed reaction.     

General pathway of sulfur-chain breakage of polythionates by iodine confirmed by the kinetics and mechanism of the pentathionate-iodine reaction

The pentathionate-iodine reaction has been studied spectrophotometrically at T = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95-5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S(5)O(6)I(-) along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S(5)O(6)I(-), including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S(5)O(6)(2-) + 10I(2) + 14H(2)O → 5SO(4)(2-) + 20I(-) + 28H(+). A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included.     

Tunneling chemical reactions in solid parahydrogen: direct measurement of the rate constants of R + H2 --> RH + H (R = CD3,CD2H,CDH2,CH3) at 5 K

Tunneling chemical reactions between deuterated methyl radicals and the hydrogen molecule in a parahydrogen crystal have been studied by Fourier transform infrared spectroscopy. The tunneling rates of the reactions R + H2 --> RH + H (R = CD3,CD2H,CDH2) in the vibrational ground state were determined directly from the temporal change in the intensity of the rovibrational absorption bands of the reactants and products in each reaction in solid parahydrogen observed at 5 K. The tunneling rate of each reaction was found to differ definitely depending upon the degree of deuteration in the methyl radicals. The tunneling rates were determined to be 3.3 x 10(-6) s(-1), 2.0 x 10(-6) s(-1), and 1.0 x 10(-6) s(-1) for the systems of CD3, CD2H, and CDH2, respectively. Conversely, the tunneling reaction between a CH3 radical and the hydrogen molecule did not proceed within a week's time. The upper limit of the tunneling rate of the reaction of the CH3 radical was estimated to be 8 x 10(-8) s(-1).     

Photodegradation of folate sensitized by riboflavin

Folate is shown to react with singlet-excited state of riboflavin in a diffusion controlled reaction and with triplet-excited state of riboflavin in a somewhat slower reaction with (3)k(q) = 4.8 × 10(8) L mol(-1) s(-1) in aqueous phosphate buffer at pH 7.4, ionic strength of 0.2 mol L(-1), and 25°C. Singlet quenching is assigned as photo-induced reductive electron transfer from ground state folate to singlet-excited riboflavin, while triplet quenching is assigned as one-electron transfer rather than hydrogen atom transfer from folate to triplet-excited riboflavin, as the reaction quantum yield, φ = 0.32, is hardly influenced by solvent change from water to deuterium oxide, φ = 0.37. Cyclic voltammetry showed an irreversible two-electron anodic process for folate, E = 1.14 V versus NHE at a scan-rate of 50 mV s(-1), which appears to be kinetically controlled by the heterogeneous electron transfer from the substrates to the electrode. Main products of folate photooxidation sensitized by riboflavin were pterin-6-carboxylic acid and p-aminobenzoyl-L-glutamic acid as shown by liquid chromatographic ion-trap mass spectrometry (LC-IT-MS).     

Reaktionen von 3-(Dimethylamino)-2H-azirinen mit 1,3-Benzoxazol-2(3H)-thion

Reaction of 3-(Dimethylamino)-2H-azirines with 1,3-Benzoxazole-2(3H)-thione The reaction of 3-(dimethylamino)-2H-azirines 2 with 1,3-benzoxazole-2(3H)-thione ( 5 ), which can be considered as NH-acidic heterocycle (pK_aca. 7.3), in MeCN at room temperature, leads to 3-(2-hydroxyphenyl)-2-thiohydantoins 6 and thiourea derivatives of type 7 (Scheme 2). A reaction mechanism for the formation of the products via the crucial zwitterionic intermediate A ′ is suggested. This intermediate was trapped by methylation with Mel and hydrolysis to give 9 (Scheme 4). Under normal reaction conditions, A ′ undergoes a ring opening to B which is hydrolyzed during workup to yield 6 or rearranges to give the thiourea 7. A reasonable intermediate of the latter transformation is the isothiocyanate E (Scheme 3) which also could be trapped by morpholine. In i-PrOH at 55–65° 2a and 5 react to yield a mixture of 6a , 2-(isopropylthio)-1,3-benzoxazole ( 12 ), and the thioamide 13 (Scheme 5). A mechanism for the surprising alkylation of 5 via the intermediate 2-amino-2-alkoxyaziridine F is proposed. Again via an aziridine, e.g. H ( Scheme 6 ), the formation of 13 can be explained.     

Reaction of thioketones with propiolic acids

The reaction of adamantane-2-thione with propiolic acid afforded a novel type of cycloadduct, spiro[adamantane-2,2'-6'H-[1,3]-oxathiin]-6'-one (3a), in quantitative yield. The reaction of thiobenzophenone with propiolic acid gave 2,2-diphenyl-6'H-[1,3]-oxathiin]-6'-one and 4-phenyl-3-thia-3,4-dihydronaphthoic acid in 34% and 35% yields, respectively. The reaction might proceed through a concerted process, as confirmed by kinetics. The reaction of adamantane-2-thione with 2-butynoic acid or phenylpropiolic acid gave the corresponding adducts regioselectively. Interestingly, only one isomer was obtained by the reaction of thiofenchone with propiolic acid, suggesting that the reaction proceeded diastereospecifically. Oxidation of adducts by dimethyldioxirane or m-chloroperoxybenzoic acid gave the corresponding sulfoxides and sulfones. The sulfoxides were thermally decomposed to give disulfide or another type of 1,3-oxathiin-6-one.     

Syntheses,crystallographic/computational characterizations,and reactions of the first 10-vertex arachno- and nido-phosphamonocarbaboranes

A synthetic sequence involving the initial reaction of a substituted phosphorus dihalide (RPCl(2), R = CH(3), C(6)H(5)) with the arachno-CB(8)H(13)(-) (1-) monoanion followed by an in situ dehydrohalogenation reaction initiated by Proton Sponge, resulted in phosphorus cage insertion to yield the first 10-vertex arachno- and nido-phosphamonocarbaboranes, exo-6-R-arachno-6,7-PCB(8)H(12) (2a, 2b) and PSH(+)6-R-nido-6,9-PCB(8)H(9)(-) (PSH+3a-, PSH+3b-) (R = C(6)H(5) (a), CH(3) (b)). Alternatively, 2a and 2b were synthesized in high yield as the sole product of the reaction of the arachno-4-CB(8)H(12)(2-) (1(2-)) dianion with RPCl(2). Crystallographic determinations of PSH+3a- and PSH+3b- in conjunction with DFT/GIAO computational studies of the anions have confirmed the expected nido cage framework based on an octadecahedron missing the six-coordinate vertex. DFT/GIAO computational studies have also shown that while the gross cage geometries of the exo-6-R-arachno-6,7-PCB(8)H(12) compounds 2a and 2b resemble the known isoelectronic arachno-6,9-SCB(8)H(12), the phosphorus and carbon atoms are in thermodynamically unfavorable adjacent positions on the six-membered puckered face. They also each have an endo-hydrogen at the P6-position arising from proton transfer to the basic phosphorus during the cage-insertion reaction. Possible stepwise reaction pathways that can account for the formation of both the arachno and nido products are discussed. Deprotonation of 2a and 2b resulted in the formation of their corresponding conjugate monoanions, 6-R-arachno-6,7-PCB(8)H(11)(-) (2a-, 2b-), in which the proton that had been attached to the P6 atom was removed. Reactions of 2a- with O(2), S(8), BH(3).THF, or Br(2) further demonstrated the basicity of the P6-phosphorus yielding the new arachno-substituted compounds, endo-6-O-exo-6-(C(6)H(5))-arachno-6,7-PCB(8)H(11)(-) (4a-), endo-6-S-exo-6-(C(6)H(5))-arachno-6,7-PCB(8)H(11)(-) (5a-), endo-6-BH(3)-exo-6-(C(6)H(5))-arachno-6,7-PCB(8)H(11)(-) (6a-), and endo-6-Br-exo-6-(C(6)H(5))-arachno-6,7-PCB(8)H(11) (7a), respectively, in which the O, S, BH(3), and Br substituents are bound to the phosphorus at the endo position.     

Synthesis and structural characterization of a series of lanthanide(II) amides: steric effect of amido ligand

The synthesis and structures of a series of lanthanide(II) and lanthanide(III) complexes supported by the amido ligand N(SiMe3)Ar were described. Several lanthanide(III) amide chlorides were synthesized by a metathesis reaction of LnCl3 with lithium amide, including {[(C6H5)(Me3Si)N]2YbCl(THF)}2.PhCH3 (1), [(C6H3-iPr2-2,6)(SiMe3)N]2YbCl(mu-Cl)Li(THF)3.PhCH3 (4), [(C6H3-iPr2-2,6)(SiMe3)N]YbCl2(THF)3 (6), and [(C6H3-iPr2-2,6)(SiMe3)N]2SmCl3Li2(THF)4 (7). The reduction reaction of 1 with Na-K alloy afforded bisamide ytterbium(II) complex [(C6H5)(Me3Si)N]2Yb(DME)2 (2). The same reaction for Sm gave an insoluble black powder. An analogous samarium(II) complex [(C6H5)(Me3Si)N]2Sm(DME)2 (3) was prepared by the metathesis reaction of SmI2 with NaN(C6H5)(SiMe3). The reduction reaction of ytterbium chloride 4 with Na-K alloy afforded monoamide chloride {[(C6H3-iPr2-2,6)(SiMe3)N]Yb(mu-Cl)(THF)2}2 (5), which is the first example of ytterbium(II) amide chloride, formed via the cleavage of the Yb-N bond. The same reduction reaction of 7 gave a normal bisamide complex [(C6H3-iPr2-2,6)(SiMe3)N]2Sm(THF)2 (8) via Sm-Cl bond cleavage. This is the first example for the steric effect on the outcome of the reduction reaction in lanthanide(II) chemistry. 5 can also be synthesized by the Na/K alloy reduction reaction of 6. All of the complexes were fully characterized including X-ray diffraction for 1-7.     

Reaction of ketenyl radical with acetylene: a promising route for cyclopropenyl radical

The reaction of the ketenyl radical (HCCO) with acetylene (C(2)H(2)) is very relevant to the oxygen-acetylene flames and fuel-rich combustion process for nitrogen-containing compounds. Unfortunately, except for several rate constant measurements, the mechanism is completely unknown for this reaction. In this paper, detailed theoretical investigations are performed for the HCCO + C(2)H(2) reaction at the G3B3 level using the B3LYP/6-31G(d), B3LYP/6-311++G(d,p), and QCISD/6-31G(d) geometries. The exclusive fragmentation channel is the formation of the cyclopropenyl radical (c-C(3)H(3)) and carbon monoxide (CO) via the chainlike OCCHCHCH and three-membered ring OC-cCHCHCH intermediates. Thus, the mass spectroscopic peak of C(3)H(3)(+) in a previous experiment can be explained. The calculated overall reaction barrier is 4.4, 4.4, and 5.3 kcal/mol at the G3B3//B3LYP/6-31G(d), G3B3//B3LYP/6-311++G(d,p), and G3B3//QCISD/6-31G(d) levels, respectively. The title reaction may provide an effective route for generating the long-sought cyclopropenyl radical in the laboratory, which has been the long-standing subject of numerous theoretical studies as the simplest cyclic conjugate radical, and its bulky derivatives were already known. Future experimental investigations for the HCCO + C(2)H(2) reaction are greatly desired to test the predicted fragmentation channel. The implication of the present study in combustion and interstellar processes is discussed.     

Photochemical synthesis of intensely luminescent isocyano rhenium(I) complexes with readily tunable structural features

A new class of readily tunable isocyano rhenium(I) diimine luminophores, cis,cis-[Re(CO)(2)(CNR)(2)(N-N)](+) (R=2,4,6-Cl(3)C(6)H(2), 4-ClC(6)H(4), 4-Br-2,6-(CH(3))(2)C(6)H(2), 2,6-(CH(3))(2)C(6)H(3), 4-[(CH(3))(2)N]C(6)H(4), 4-(C(6)H(5))C(6)H(4), 4-nBuC(6)H(4), tBu), has been synthesized in high yield by a highly selective photochemical substitution reaction. These complexes were characterized by (1)H NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The X-ray crystal structures of one of the complexes and one of the precursor complexes for the photosubstitution reaction were also determined. As the isocyanide ligands are readily tunable, complexes with excellent solubility in benzene or other nonpolar solvents could be designed through slight modification of the isocyanide ligands with a short nBu substituent. With the characteristic strong infrared absorptions of the carbonyl (C≡O) and isocyanide (C≡N) stretches as well as the high solubility of the reactant and product in benzene, which is the solvent for the photoreaction, the photosubstitution reaction of [Re(CO)(3)(nBuC(6)H(4)NC)(2)Br] with 4,4'-di-tert-butyl-2,2'-bipyridine was also studied by in situ IR spectroscopy. The photophysical and electrochemical properties of these complexes were also investigated. Except for the complex with [(CH(3))(2)N]C(6)H(4)NC ligands, all complexes displayed intense luminescence with quantum yields of up to 0.37 in degassed CH(2)Cl(2) solution at room temperature. These emissions were assigned as the phosphorescence derived from the metal-to-ligand charge transfer [dπ(Re)→π*(N-N)] excited state. The emissive excited states of these complexes have also been characterized by transient absorption spectroscopic studies. The capability of tuning the emissive excited-state energy through the modification of the isocyanide ligands could be reflected by the significant shifting of the phosphorescence from 530 to 620 nm with the same phenanthroline ligand.