Enantioselective organocatalytic α-sulfenylation of substituted diketopiperazines

The asymmetric organocatalytic α-sulfenylation of substituted piperazine-2,5-diones is reported, with cinchona alkaloids as chiral Lewis bases and electrophilic sulfur transfer reagents. Catalyst loadings, the type of sulfur transfer reagent, temperature, and solvent were investigated in order to optimize the reaction conditions. The effects of ring substitution and the type of catalyst on the yield and enantioselectivity of the reaction are reported.     

Sequential One‐Pot Addition of Excess Aryl‐Grignard Reagents and Electrophiles to O‐Alkyl Thioformates

The sequential addition of aromatic Grignard reagents to O‐alkyl thioformates proceeded to completion within 30 s to give aryl benzylic sulfanes in good yields. This reaction may begin with the nucleophilic attack of the Grignard reagent onto the carbon atom of the O‐alkyl thioformates, followed by the elimination of ROMgBr to generate aromatic thioaldehydes, which then react with a second molecule of the Grignard reagent at the sulfur atom to form arylsulfanyl benzylic Grignard reagents. To confirm the generation of aromatic thioaldehydes, the reaction between O‐alkyl thioformates and phenyl Grignard reagent was carried out in the presence of cyclopentadiene. As a result, hetero‐Diels–Alder adducts of the thioaldehyde and the diene were formed. The treatment of a mixture of the thioformate and phenyl Grignard reagent with iodine gave 1,2‐bis(phenylsulfanyl)‐1,2‐diphenyl ethane as a product, which indicated the formation of arylsulfanyl benzylic Grignard reagents in the reaction mixture. When electrophiles were added to the Grignard reagents that were generated in situ, four‐component coupling products, that is, O‐alkyl thioformates, two molecules of Grignard reagents, and electrophiles, were obtained in moderate‐to‐good yields. The use of silyl chloride or allylic bromides gave the adducts within 5 min, whereas the reaction with benzylic halides required more than 30 min. The addition to carbonyl compounds was complete within 1 min and the use of lithium bromide as an additive enhanced the yields of the four‐component coupling products. Finally, oxiranes and imines also participated in the coupling reaction.     

A Sulfur Monoxide Surrogate Designed for the Synthesis of Sulfoxides and Sulfinamides**

Sulfur monoxide (SO) is a highly reactive species that cannot be isolated in bulk. However, SO can play a pivotal role as a fundamental building block in organic synthesis. Reported herein is the design and application of a sulfinylhydrazine reagent as an easily prepared sulfur monoxide surrogate. We show facile thermal SO transfer from this reagent to dienes where a reaction using a mechanistic probe suggests the generation of singlet SO. Combined with Grignard reagents and appropriate carbon or nitrogen electrophiles, the reagent serves as an effective “SO” donor to enable the one-pot, three-component synthesis of sulfoxides and sulfinamides.     

Synthesis of allyl sulfones from potassium allyltrifluoroborates

Potassium allyltrifluoroborates underwent a bora-ene reaction with sulfur dioxide in the absence of Lewis acid catalysts to give sulfinyloxy-trifluoroborates, which subsequently undergo alkylation with electrophiles to produce sulfones in up to 91% yield. Benzyl halides and haloacetic acid derivatives can be used as the alkylation reagents while the Sanger reagent undergoes a S_NAr reaction with sulfinyloxy-trifluoroborates to produce the corresponding 2,4-dinitrophenylsulfone. The developed method allows the transformation of potassium allyltrifluoroborates into allyl sulfones.     

Induced Alkene Reactions with Sulfur Dichloride. System Sulfur Dichloride-Dimethyl Sulfide as a Reagent for Polysulfonation and Conjugate Chlorination of Alkenes

In reaction of unsaturated compounds with sulfur dichloride in the presence of dimethyl sulfide radically new induced directions are realized: the reagent is involved as dimer or trimer (formation of di- and trisulfides). Alongside this process a conjugate chlorination occurred with participation of external nucleophiles, dimethyl sulfide and acetonitrile, furnishing sulfonium chlorides and N-substituted acetamides respectively. The relative importance of these alternative reactions depends on solvent, alkene structure, and order of reagents addition.     

Copper-Catalyzed Aminothiolation of 1,3-Dienes via a Dihydrothiazine Intermediate

Heteroatom-containing organic molecules are of particular interest to medicinal chemists and materials scientists. A strategy to reach these architectures via direct difunctionalization of abundant 1,3-dienes is especially attractive. Herein, we describe the development of a regio- and diastereoselective 1,4-aminothiolation of 1,3-dienes with a sulfur diimide reagent, a copper catalyst, and alkyl Grignard reagents. This unique protocol provides remote nitrogen and sulfur functionalities with high levels of stereocontrol. The reaction proceeds via a tandem hetero-Diels–Alder cycloaddition of N,N′-bis(benzenesulfonyl)sulfur diimide with 1,3-diene followed by copper-catalyzed Grignard substitution. Mechanistic studies support a copper catalyzed formation of an unprecedented [10-S-4] sulfurane that reductively eliminates to afford a 3,6-dihydrothiazine, which is selectively converted to 1,4-aminothiols.     

Unexpected reactivity of Imidazo[2,1-b]thiazolines with organometallic reagents

[reaction: see text] The reaction of imidazo[2,1-b]thiazolines with various organometallic reagents is described. Nucleophilic attack of organolithium reagents on sulfur occurs with extrusion of ethylene to produce 2-thioalkyl- or 2-thioarylimidazoles. The outcome with Grignard reagents, however, is less predictable, with some reagents adding at sulfur and others reacting at C-2 or not at all.     

Efficient organocatalytic α-sulfenylation of substituted piperazine-2,5-diones

Organocatalytic α-sulfenylation of substituted piperazine-2,5-diones is reported through the use of cinchona alkaloids as Lewis bases and electrophilic sulfur transfer reagents. 1-Phenylsulfanyl[1,2,4]triazole, a novel sulfur transfer reagent, gave excellent product yields with a number of substituted piperazine-2,5-diones under mild conditions. Catalyst loading, stoichiometry of sulfur electrophile, temperature, and solvent were optimized to achieve high product yields.     

三氮烯试剂的结构与性能关系的研究

为建立三氮烯试剂结构与性能的关系, 我们研究了在非离子型表面活性剂TritonX-100存在下, 上述试剂亚氨基氢的酸性离解常数及镉(II)与上述试剂的显色反应。研究结果表明: (1)三氮烯试剂亚氨基氢的酸性离解常数遵循Hammett方程式, 其反应常数ρ=2.23; (2)试剂的亚氨基氢的酸性离解常数(pKa)与试剂和镉(II)显色反应的最佳酸度(pH最佳)之间的经验关系式为pH最佳=pKa-(1.70±0.20)。     

Copper‐Catalyzed Aminothiolation of 1,3‐Dienes via a Dihydrothiazine Intermediate

Heteroatom‐containing organic molecules are of particular interest to medicinal chemists and materials scientists. A strategy to reach these architectures via direct difunctionalization of abundant 1,3‐dienes is especially attractive. Herein, we describe the development of a regio‐ and diastereoselective 1,4‐aminothiolation of 1,3‐dienes with a sulfur diimide reagent, a copper catalyst, and alkyl Grignard reagents. This unique protocol provides remote nitrogen and sulfur functionalities with high levels of stereocontrol. The reaction proceeds via a tandem hetero‐Diels–Alder cycloaddition of N,N′‐bis(benzenesulfonyl)sulfur diimide with 1,3‐diene followed by copper‐catalyzed Grignard substitution. Mechanistic studies support a copper catalyzed formation of an unprecedented [10‐S‐4] sulfurane that reductively eliminates to afford a 3,6‐dihydrothiazine, which is selectively converted to 1,4‐aminothiols.     

Polymer-supported thioanisole: a versatile platform for organic synthesis reagents

A new cross-linked polystyrene-supported thioanisole reagent is reported. This reagent incorporates the flexible JandaJel™ cross-linker and can be treated with methyl trifluoromethanesulfonate to form the corresponding sulfonium salt. This salt can in turn be deprotonated to form a polymer-supported sulfur ylide that is able to react with aldehydes and ketones to form epoxides. The thioanisole reagent can also be oxidized to form an insoluble sulfoxide reagent that is useful in Swern oxidation reactions. In these reactions, the polymer-supported thioanisole-based reagents can be recovered, regenerated and reused.     

The quest for chiral Grignard reagents

The involvement of single electron transfer, i.e. of free radicals in the reactions of organomagnesium reagents could be detected with the aid of a chiral secondary Grignard reagent, in which the magnesium-bearing carbon atom is the sole stereogenic centre. So far, however, such reagents have not been accessible, because the standard preparation of Grignard reagents proceeds via free radicals. We review and summarize here our efforts to generate such a Grignard reagent 36 by asymmetric synthesis starting from an enantiomerically pure alpha-chloroalkyl-sulfoxide 30b using a sulfoxide/magnesium exchange reaction to give 33 followed by a carbenoid homologation reaction. Grignard reagent 36 turned out to be an ideal probe to learn about the extent to which SET is involved in reactions of organomagnesium reagents.     

Selective Synthesis of Monofluorinated Compounds Applying Amine/HF Reagents

For nucleophilic monofluorination, amine/HF reagents such as Et₃N⋅3HF, Pyr⋅9HF (Olah's reagent) and similar combinations belong to the most frequently used fluoride sources, whereupon the selectivity of these reagents can be very different depending of its acidity, the nucleophilicity of the fluoride equivalent, and the structure of the particular substrate. These reagents can be used safely in ordinary chemistry laboratories for nucleophilic substitution reactions by fluoride at sp³-hybridized carbon centers. For ring opening reactions of epoxides, the regio- and stereoselectivity is very much depending of the nature of the epoxide and the acidity of the HF reagent favoring either S_N1 or S_N2 type reactions. Similarly, the outcome of halofluorination and similar reactions with sulfur or seleno electrophiles can be controlled by the particular combination of the electrophile and the fluoride source. Examples for the application of these reaction types for the synthesis of fluorine-containing analogues of natural products or biologically relevant compounds are in the focus of this personal account.     

pH oscillations in the BrO3--SO3(2-)/HSO3- reaction in a CSTR

Large-amplitude pH oscillations have been measured during the oxidation of sulfur (IV) species by the bromate ion in aqueous solution in a continuous-flow stirred tank reactor in the absence of any additional oxidizing or reducing reagent. The source of the oscillation in this simple chemical reaction is a two-way oxidation of sulfur (IV) by the bromate ion: (1) the hydrogen-ion-producing self-accelerating oxidation to sulfur (VI) (SO4(2-)), and (2) a hydrogen-ion-consuming oxidation to sulfur (V) (S2O6(2-)). In such a way, both the H+-producing and H+-consuming composite processes required for a pH oscillator take place in parallel in a reaction between two reagents in this system. A simple reaction scheme, consisting of the protonation equilibria of SO3(2-) and HSO3-, the oxidation of HSO3- and H2SO3 by BrO3- to SO4(2-), and the oxidation of H2SO3 to S2O6(2-) has successfully been used to simulate the observed dynamical behavior. Simulation with this simple scheme shows that oscillations can be calculated even if only about 1% of sulfur (IV) is oxidized to S2O6(2-) along with the main product SO4(2-). Agreement between calculated and measured dynamical behavior is found to be quite good. Increasing temperature decreases both the period length of oscillations in a CSTR and the Landolt time measured in a closed reactor. No temperature compensation of the oscillatory frequency is found in this reaction.     

The factors that influence the elution order for the resolution of amino acids on vancomycin phase using the polar-organic mobile phases after their pre-column derivatization with electrophilic reagents

A variety of amino acids were enantioresolved on a vancomycin bonded chiral phase using the polar-organic mobile phases after their pre-column derivatization with electrophilic reagents in alkaline medium. The resolution was highly dependent on the analyte's structure and was enhanced as the aromatic side-chain group on the skeleton of analyte for pi-pi interaction with the chiral selector became available. The steric hindrance resulting from the bulky side-chain group on the analyte also affected the resolution. Elution reversal, not found on the teicoplanin phase under the same chromatographic conditions, was possible through altering the type of reagent used in the derivatizing reaction (e.g. 2,4-difluorophenylisothiocyanate to 2,4-difluorophenylisocyanate). It is believed that the steric hindrance, as a result of the bulky sulfur atom in reagents such as methylisothiocyanate and others examined in this study, was responsible for the reversed elution order. The bulkiness of the substituent on the aromatic ring of derivatizing reagents (i.e. 2,3- and 3,5-dichlorophenylisothiocyanate) was observed to affect the resolution and alter the elution order as well.     

原位生成的高价碘试剂在有机合成中的应用进展

原位生成的高价碘试剂具有原子经济性、性能温和和绿色环保等优点,在诸多合成和不对称催化等反应中表现活跃.详细介绍了原位生成高价碘的概念以及反应机理,根据不同的反应类型分别对原位生成的三价碘、五价碘以及手性高价碘试剂在有机合成反应中的应用进行了归纳总结,分析了原位生成的高价碘试剂目前面临的问题,并对今后的发展趋势作了展望.     

Coulometric Karl Fischer titration of trace water in diaphragm-free cells

Factors influencing the accuracy and precision for diaphragm-free Karl Fischer coulometric determinations of low μg-amounts of water have been studied using the Metrohm 756 (pulsed current) coulometer and eight different types of commercial coulometric reagents and some modifications of these. As in the case of diaphragm-free coulometric titration of large amounts of water, the positive errors, due to the formation of oxidizable reduction products (of sulfur dioxide) in the cathode reaction (besides hydrogen), were found to be minimized by the use of highest possible pulse current (in the range 100–400 mA) in combination with the fastest possible titration rate. Most accurate (102–103%) and precise results (typical relative standard deviation 1.8%) were obtained for reagents containing very large concentrations of imidazole in combination with the presence of modifiers like hexanol, chloroform and propylene glycol (i.e. the HYDRA-POINT reagents). Similar results were obtained when this type of reagent was mixed 60/40 with xylene according to the ASTM recommendation for water determinations in petroleum products like crude and lubricating oils. Addition of decanol to this type of reagent mixture was found to reduce the influence from the oxidative reduction products significantly. A reduction of the error from 3.6% relative to 1.6% was achieved by addition of 9% (v/v) of decanol to a 60/40 reagent mixture of HYDRA-POINT Coulometric Gen (containing hexanol as modifier) and xylene. For larger concentrations of decanol the pulse current had to be lowered to 100 mA and this might explain why no further improvement was observed. An additional attempt to minimize the interference by lowering the concentration of sulfur dioxide in the reagents gave no significant effect. However, by means of a home-built computer-controlled coulometric instrumentation based on continuous instead of pulsed current (including a large cathodic current density) it was possible to achieve recovery rates close to 100% for the best reagents investigated. The reason for this improvement is discussed.     

Ligand exchange reactions of aryl pyridyl sulfoxides with grignard reagents: Convenient preparation of 3- and 4-pyridyl grignard reagents and examination of the leaving abilities of pyridyl anions

Aryl 3- and 4-pyridyl sulfoxides undergo ligand exchange in reactions with aryl Grignard reagents to generate 3- and 4-pyridyl Grignard reagents, which, upon treatment with aldehydes or ketones, give the corresponding addition products in moderate-to-good yields. The mechanism for the exchange reaction was investigated by treating optically active 3- and 4-pyridyl p-tolyl sulfoxides with a phenyl Grignard reagent. Inversion of the configuration of the sulfur atom was the stereochemical result of the reactions. In the reactions of phenyl 2-pyridyl sulfoxide with Grignard reagents, the leaving ability of the 2-pyridyl group competes with that of the phenyl group. Both the experimental and MO calculated enthalpy values for deprotonation of α-, β-, and γ-protons of pyridine in the gas phase [1] are in accordance with the following order of the leaving abilities of aryl and pyridyl Grignard reagents: 4-PyMgBr > 3-PyMgBr » PhMgBr > p-TolMgBr > 2-PyMgBr.     

Synthesis of Sultones from Chlorosulfates by a Complex Cascade Reaction Occurring under Mild Thermal Conditions

Chlorosulfate derivatives are interesting reagents that have been traditionally used to get other sulfur-containing compounds by formal nucleophilic substitution of the chlorine atom. This work describes a different mode of reactivity of alkyne-containing chlorosulfates to get sultones, the sulfur analogues of lactones. The complex skeletal rearrangement observed in this transformation is comparable to those intricate processes promoted or catalyzed by organometallic compounds. However, the reaction here described does not require any reagent or additive, being just a thermal process. Computational calculations support a mechanism based on a series of cascade reactions where almost every step is counterintuitive. Some of these steps include original processes related to classical reactions, thus adding complementary views to traditional organic chemistry.     

A Catalyst‐Free Amination of Functional Organolithium Reagents by Flow Chemistry

Reported is the electrophilic amination of functional organolithium intermediates with well‐designed aminating reagents under mild reaction conditions using flow microreactors. The aminating reagents were optimized to achieve efficient C?N bond formation without using any catalyst. The electrophilic amination reactions of functionalized aryllithiums were successfully conducted under mild reaction conditions, within 1 minute, by using flow microreactors. The aminating reagent was also prepared by the flow method. Based on stopped‐flow NMR analysis, the reaction time for the preparation of the aminating reagent was quickly optimized without the necessity of work‐up. Integrated one‐flow synthesis consisting of the generation of an aryllithium, the preparation of an aminating reagent, and their combined reaction was successfully achieved to give the desired amine within 5 minutes of total reaction time.