The effect of replacing carbon by nitrogen in reductions with SmI2: reduction of azobenzene

The reduction of azobenzene by SmI(2) in THF to give hydrazobenzene was investigated. The kinetics are first order in the substrate and first order in SmI(2). The kinetic order in MeOH is ca. 0.56, and in TFE it is ca. 0.2. The fractional order in the proton donors is interpreted as being a result of their acting in two opposing manners. In one the proton donor enhances the reaction by protonation of the radical anion, and in the other it slows the reaction by binding to the lone pair electrons of the nitrogen in the azobenzene. This hampers the fast inner-sphere electron-transfer mode. Experiments conducted in the presence of low concentrations of HMPA show rate enhancement suggesting that the SmI(2), which is partly coordinated to HMPA molecules, has some free sites to bind to the substrate. When more HMPA is added, it prevents the fast inner-sphere mechanism and the rate decreases. In this system, the increase in the reduction potential of SmI(2) caused by HMPA is similar to the rate enhancement by an inner sphere mechanism. In general, the replacement of a skeletal carbon by a nitrogen atom causes a significant rate enhancement.     

Three-component coupling of acylphosphonates and two carbonyl compounds promoted by low-valent samariums: one-Pot synthesis of beta-hydroxyphosphonates

Three-component coupling of acylphosphonates and two carbonyl compounds leading to beta-hydroxyphosphonates has been achieved with low-valent samariums. Thus, acylphosphonates reacted with aldehydes in the presence of semicatalytic amounts of samarium metal or SmI(2) to give acyloxyphosphonates in good yields. The second coupling reaction of the acyloxyphosphonates with aldehydes or ketones promoted by SmI(2) afforded beta-hydroxyphosphonates instead of olefins. Moreover, these two reactions could be carried out in one pot.     

The reactions of some ortho-substituted anilines with various α,β-acetylenic ketones. A route to substituted quinolines

The reactions of some ortho-substituted anilines with various α,β-acetylenic ketones were investigated as a route to 4-alkyl-, 4-aryl-, 4-hydroxy-, and 4-amino-3-quinolyl ketones. The anilines examined were 2-aminoacetophenone ( 1 ), 2-aminobenzophenone ( 2 ), anthranilonitrile ( 3 ), methyl anthranilate ( 4 ), and ethyl anthranilate ( 5 ). The acetylenic ketones used were 1,4-diphenyl-2-butyne-1,4-dione ( 6 ), 3-butyn-2-one (7), 1,3-diphenyl-2-propyn-1-one ( 8 ), and 4-phenyl-3-butyn-2-one ( 9 ). The acetylenic ketones typically reacted with the anilines to give enamines; however, exceptions were found. Acetylene 6 reacts with 3 to give the enamine ( 13 ) along with a small amount of 2,3-dibenzoyl-4-quinolamine ( 14 ). The reactions of 1 or 2 with 6 give the respective quinoline derivatives directly. Acetylene 8 reacts with 2 to give 3-benzoyl-2,4-diphenylquinoline ( 22 ) directly, whereas no reaction occurs between 8 and 1 or 3 . Acetylene 9 does not react with 1, 2 , or 3 . The enamines exist as the intramolecularly hydrogen bonded isomers and usually undergo cyclization with 5 molar equivalents of methanolic sodium methoxide to give quinoline derivatives. The 4-quinolinols exist predominantly as the 4-quinolinone tautomer.     

Reduction of acetals with samarium diiodide in acetonitrile in the presence of Lewis acids

Transformation of acetals into ethers by partial reduction using a samarium diiodide-Lewis acids-acetonitrile system is described. The reaction with aromatic acetals occurred in good yields in the presence of aluminum chloride (2 eq) whereas the corresponding aliphatic, vinylic, and alkynyl derivatives did not afford ethers under the same conditions. Beta-elimination to give an enol ether becomes predominant when aliphatic acetals that possess a hydrogen at the 2-position are treated with iodotrimethylsilane in the presence of SmI2 or SmI3.     

Rapid SmI2-mediated reductions of alkyl halides and electrochemical properties of SmI2/H2O/amine

Mixtures of SmI(2)/H(2)O/amine have been found to reduce alkyl halides more efficiently than SmI(2)/HMPA/alcohol mixtures at room temperature. Alkyl and aryl iodides were quantitatively reduced in <1 min and alkyl bromides in 10 min, while alkyl and aryl chlorides required more than 5 h for completion. Determination of the reaction order of Et(3)N in the reduction of 1-chlorodecane showed that the reaction order is one. Water was shown not to participate in the rate-determining step of this reduction. There was a significant change of the UV-vis spectrum and color of SmI(2) upon addition of either PMDTA or water, while no effect was observed with the addition of Et(3)N or TMEDA. Although the combination of SmI(2), water, and amines produces a very efficient reducing system, cyclic voltammetric experiments showed that the redox potential is nearly identical with that of SmI(2) alone. These results are consistent with precipitation providing the driving force for reduction. Taken together, the results of these experiments show that the combination of SmI(2)/H(2)O/amine provides a fundamentally novel and useful approach to enhance the reactivity of SmI(2).     

Preparation of samarium(II) iodide: quantitative evaluation of the effect of water, oxygen, and peroxide content, preparative methods, and the activation of samarium metal

Samarium(II) iodide (SmI(2)) is one of the most important reducing agents in organic synthesis. Synthetic chemistry promoted by SmI(2) depends on the efficient and reliable preparation of the reagent. Unfortunately, users can experience difficulties preparing the reagent, and this has prevented realization of the full synthetic potential of SmI(2). To provide synthetic chemists with general and reliable methods for the preparation of SmI(2), a systematic evaluation of the factors involved in its synthesis has been carried out. Our studies confirm that SmI(2) is a user-friendly reagent. Factors such as water, oxygen, and peroxide content in THF have little influence on the synthesis of SmI(2). In addition, the use of specialized glovebox equipment or Schlenk techniques is not required for the preparation of SmI(2). However, our studies suggest that the quality of samarium metal is an important factor and that the use of low quality metal is the main cause of failed preparations of the reagent. Accordingly, we report a straightforward method for activation of "inactive" samarium metal and demonstrate the broad utility of this protocol through the electron transfer reductions of a range of substrates using SmI(2) prepared from otherwise "inactive" metal. An investigation into the stability of SmI(2) solutions and an evaluation of commercially available solutions of the reagent is also reported.     

吡咯茂钐络合物的合成及其催化的丙交酯聚合

本文合成了吡咯修饰的二茂稀土金属有机化合物,并研究了该化合物对丙交酯的聚合,结果显示该化合物可以在很宽的温度范围内实现对丙交酯的高转化率聚合．     

A highly stereoselective samarium diiodide-promoted aldol reaction with 1'-phenylseleno-2'-keto nucleosides. Synthesis of 1'alpha-branched uridine derivatives

Since 1'-branched nucleosides are biologically important targets in medicinal chemistry, more efficient methods for preparing them are required. The 1'alpha-branched uridine derivatives were successfully synthesized via a samarium diiodide (SmI(2))-promoted aldol reaction. Treatment of the 1'alpha-phenylseleno-2'-ketouridine derivative 6, readily prepared from uridine, with SmI(2) at -78 degrees C in THF reductively cleaved the anomeric Se-C bond to generate the corresponding samarium enolate, which was highly stereoselectively condensed with aldehydes, such as PhCHO, MeCHO, i-PrCHO, or (CH(2)O)(n)(), to give the corresponding 1'alpha-1' 'S-branched products 12a-d. This is the first time an enolate has been generated by reductively cleaving a C-Se bond. The highly selective stereochemical results suggest that the aldol reaction proceeds via a chelation-controlled transition state. When an excess of aldehyde was used and the reaction mixture was gradually warmed, the tandem aldol-Tishchenko reaction proceeded to give the "arabino-type" nucleosides 14a-c, having a 2'-"up" hydroxyl and 1'alpha-1' 'S-branched chain. 1'alpha-Hydroxymethyluridine (21), which is the uracil version of the antitumor antibiotic angustmycin C, was synthesized from the aldol reaction product 10.     

SmI2-promoted Reformatsky-type reaction and acylation of alkyl 1-chlorocyclopropanecarboxylates

In the presence of HMPA in THF, highly stereoselective SmI(2)-promoted substitutions of alkyl 1-chlorocyclopropanecarboxylates 1 using various ketones, aldehydes (Reformatsky-type reaction), and acyl chlorides (acylation) proceeded to give trans-adducts (2 or 5) in good to high yield with excellent trans-stereoselectivity (trans-add/cis-add = > 99/1). The Reformatsky-type reaction of 1 with aldehydes and unsymmetrical ketones proceeded with moderate diastereoselectivity (re-face-adduct/si-face-adduct = 60/40-75/25).     

Intramolecular, reductive cyclization of beta-ketoisothiocyanates promoted by using samarium diiodide

A novel samarium diiodide (SmI2) promoted intramolecular cyclization of beta-ketoisothiocyanate, derived from alpha,beta-unsaturated esters and ammonium thiocyanate led to alpha-hydroxythiolactams and/or thiolactams in high yields. Treatment of beta-ketoisothiocyanate with two equivalents of SmI2 gave a mixture of alpha-hydroxythiolactam and thiolactam. Four equivalents of SmI2 afforded only thiolactam in high yields. The intramolecular cyclization took place with high to complete stereoselectivity. A mechanism to explain this transformation is proposed.     

Lactone Radical Cyclizations and Cyclization Cascades Mediated by SmI(2)-H(2)O

Unsaturated lactones undergo reductive radical cyclizations upon treatment with SmI(2)-H(2)O to give decorated cycloheptanes in a single highly selective operation during which up to three contiguous stereocenters are generated. Furthermore, cascade processes involving lactones bearing two alkenes, an alkene and an alkyne, or an allene and an alkene allow "one-pot" access to biologically significant molecular scaffolds with the construction of up to four contiguous stereocenters. The cyclizations proceed by the trapping of radical anions formed by electron transfer reduction of the lactone carbonyl.     

Cyclization of samarium diiodide-generated vinyl radicals in 6-(pi-exo)-exo-dig mode

Radical cyclization of vinyl iodides in 6-(pi-exo)-exo-dig mode were effected by SmI2 to give exo-cyclic dienes fused to six-membered rings.     

The first samarium(II)-mediated aryl radical cyclisation onto an aromatic ring

Intramolecular arylation of aryl radicals was mediated by SmI(2)/HMPA in the presence of i-PrOH to give spirocycles and/or reduced cine-cyclised products, while the reaction in the absence of i-PrOH gave the rearomatised fused rings.     

Double N-arylation of primary amines: carbazole synthesis from 2,2'-biphenyldiols

The double N-arylation of primary amines with 2,2'-biphenylylene ditriflates was investigated for the synthesis of multisubstituted carbazoles. Palladium complexes supported by 2-dicyclohexylphosphino-2'-methylbiphenyl or Xantphos [4,5-bis(diphenylphosphino)-9,9-dimethylxanthene] were found to be efficient catalysts for the reaction. The catalysts allow the use of anilines with an electron-donating or electron-withdrawing substituent and multisubstituted 2,2'-biphenylylene ditriflates as substrates. Ammonia equivalents, such as O-tert-butyl carbamate, are also employable as a nitrogen source to give the N-protected carbazoles which can easily give the corresponding N-unsubstituted carbazoles after deprotection. By using this methodology, a carbazole alkaloid, mukonine, is synthesized in 40% yield for five steps, in comparable efficiency to the recent precedents.     

One—Pot Reaction of Aldehydes,α—Haloketones and （Phenylsulfonyl） acetonitrile Promoted by SmI3

One-pot reaction of aldehydes,α-haloketones and (phenylsulfonyl) acetonitrible promotes by SmI3 proceeded smoothyl to give 1-cyano-1-phenylsulfonyl-2-aryl-3-aroyl-propane derivatives in moderate good yields.     

Reactions of N- and C-Alkenylanilines: III. Synthesis and Cyclization of Substituted 2-(1-Methyl-2-butenyl)anilines

Reactions of substituted 2-(1-methyl-2-butenyl)anilines with iodine result in cyclization and formation of 3-iodo-1,2,3,4-tetrahydroquinolines; N-methylsulfonyl-2-(1-methyl-2-butenyl)anilines give rise exclusively to the corresponding 2-(1-iodoethyl)-3-methyl-2,3-dihydroindoles.     

SmI2-KI二元体系相图的研究

运用DTA差热分析法和X射线衍射粉末法研究了Sml₂-KI二元体系等压相图及该体系中KSm₂I_s,K₂SmI₄的性质和结构.借助J.Kutscher讨论REX₃-AX(RE=稀土,A=碱金属)体系形成化合物规律的方法,对SmI₂-AI,YbI₂-AI(A=Na,K,Rb,Cs)两个系列形成的规律进行了初步讨论。     

A novel photochromic system of 4,5-dialkenylthiophenes constructed by the smarium diiodide promoted coupling reactions of thiophene-2-carboxylate with aryl ketones

[reaction: see text] The SmI2-promoted coupling reaction of ethyl thiophene-2-carboxylate with aryl ketones (2 equiv), followed by acid-catalyzed dehydration and oxidative aromatization, gave dialkenylthiophenes 1b-d, which underwent electrocyclizations upon irradiation with 300-nm light in CH3CN solution to give the corresponding closed-ring species with absorption lambda(max) approximately 425 nm. The interconversion between dialkenylthiophenes and their corresponding closed-ring species constitutes a novel photochromic system bearing an ester group for potential uses in linkage and wavelength tuning.     

Reduction of ketones and alkyl iodides by SmI(2) and Sm(II)-HMPA complexes. Rate and mechanistic studies

The effect of HMPA on the electron transfer (ET) rate of samarium diiodide reduction reactions in THF was analyzed for a series of ketones (2-butanone, methyl acetoacetate, and N,N-dimethylacetoacetamide) and alkyl iodides (1-iodobutane and 2-iodobutane) with stopped flow spectrophotometric studies. Activation parameters for the ET processes were determined by temperature-dependence studies over a range of 30-50 degrees C. The ET rate constants and the activation parameters obtained for the above systems in the presence of different equivalents of HMPA were compared to understand the mechanism of action of HMPA on various substrates. The results obtained from these studies indicate that coordination or chelation is possible in the transition state geometry for SmI(2)/ketone systems even in the presence of the sterically demanding ligand HMPA. After the addition of 4 equiv of HMPA the ET rate and activation parameters for ketone reduction by Sm is unaffected by further HMPA addition while a linear dependence of ET rate on the equivalents of HMPA was found in the SmI(2)/alkyl iodide system. The results of these studies are consistent with an inner-sphere-type ET for the reduction of ketones by SmI(2) (and SmI(2)[bond]HMPA complexes) and an outer-sphere-type ET for the reduction of alkyl iodides by SmI(2) or SmI(2)[bond]HMPA complexes.     

Reformatsky-type reaction of chiral nonracemic alpha-bromo-alpha'-sulfinyl ketones with aldehydes. Synthesis of enantiomerically pure 2-methyl-1,3-diol moieties

Chiral nonracemic alpha-bromo-alpha'-sulfinyl ketones were shown to react with aldehydes in the presence of SmI(2) in a Reformatsky-type reaction to give the corresponding adduct with excellent syn diastereoselectivity. Further reduction of the Reformatsky adducts furnished anti- and syn-2-methyl-1,3-diol moieties in excellent yields and diastereoselectivities.