Photostimulated exoemission and reduction of CeO2

The thermovacuum and photochemical reduction of CeO₂ was studied by the method of photostimulated exoemission. Analysis of the kinetics and temperature dependence of photostimulated exoemission permits one to suggest a possible mechanism of electron and ion phenomena accompanying the reduction processes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 838–844, May, 2000.     

Autocatalysis and surface catalysis in the reduction of imines by SmI2

The reduction of the three imines, N-benzylidene aniline (BAI), N-benzylidenemethylamine (BMI), and benzophenone imine (BPI), with SmI(2) gives the reduced as well as coupled products. The reactions were found to be autocatalytic due to the formation of the trivalent samarium in the course of the reaction. When preprepared SmI(3) was added to the reaction mixture, the reaction rate increased significantly. However, the kinetics were found to be of zero order in SmI(2). This type of behavior is typical of surface catalysis with saturation of the catalytic sites. Although no solids are visible to the naked eye, the existence of microcrystals was proven by light microscopy as well as by dynamic light scattering analysis. Although HRTEM shows the existence of quantum dots in the solid, we were unable to make a direct connection between the existence of the quantum dots and the catalytic phenomenon. In the uncatalyzed reaction, the order of reactivity is BPI > BMI > BAI. This order does not conform to the electron affinity order of the substrates but rather to the nitrogen lone pair accessibility for complexation. This conclusion was further supported by using HMPA as a diagnostic probe for the existence of an inner sphere electron transfer reaction.     

Electron transfer reduction of unactivated esters using SmI2-H2O

The reduction of unactivated esters using samarium diiodide is reported for the first time. The optimised protocol allows for the reduction of primary, secondary and tertiary alkyl esters in excellent yields and is competitive with reductions mediated by metal hydrides and alkali metals.     

A general electron transfer reduction of lactones using SmI2-H2O

Herein we describe a strategy for the selective, electron transfer reduction of lactones of all ring sizes and topologies using SmI(2)-H(2)O and a Lewis base to tune the redox properties of the complex. The current protocol permits instantaneous reduction of lactones to the corresponding diols in excellent yields, under mild reaction conditions and with useful chemoselectivity. We demonstrate the broad utility of this transformation through the reduction of complex lactones and sensitive drug-like molecules. Sequential electron transfer reactions and syntheses of deuterated diols are also described.     

Sm°/Cat. I2 and SmI2 induced reductive coupling of nitriles with azides

The intermolecular reductive coupling of nitriles with azides induced by Sm°/Cat. I₂ or SmI₂ was studied. Aromatic and aliphatic amidines were prepared in moderate to good yields under neutral and mild conditions respectively.     

The reduction of osmium carbonyl clusters by nitriles

The clusters Os₆(CO)₁₈ Os₇(CO)₂₁ and Os₈(CO)₂₃ are reduced to the anoins [Os₆(CO)₁₈]^2?, [Os₇(CO)₂₀]^2? and [Os₈(CO)₂₂]^2?, respectively, by the action of nitriles RCN (R = Me, Et, CH₂C(Me)); the kinetics of the reaction of Os₆(CO)₁₈ with EtCN have been examined and reveal a third order dependence on nitrile concentration.     

Mechanistic study of beta-substituent effects on the mechanism of ketone reduction by SmI(2)

The rate constants for the reduction of 2-butanone, methylacetoacetate, N, N-dimethylacetoacetamide, and a series of 4'- and 2'-substituted acetophenone derivatives by SmI(2) were determined in dry THF using stopped-flow absorption decay experiments. Activation parameters for the electron-transfer processes in each series of compounds were determined by a temperature-dependence study over a range of 30 to 50 degrees C. Two types of reaction pathways are possible for these electron-transfer processes. One proceeds through coordination (Scheme 1) while the other involves chelation (Scheme 2). The results described herein unequivocally show that both coordination and chelation provide highly ordered transition states for the electron-transfer process but the presence of a chelation pathway dramatically increases the rate of the reduction of these substrates by SmI(2). The ability of various functional groups to promote a chelated reaction pathway plays a crucial role in determining the rate of the reaction. Among the 2'-substituted acetophenone series, the presence of a fluoro, amino, or methoxy substituent enhances the rate of reduction compared to the 4'-analogues. In particular, the presence of a 2'-fluoro substituent on acetophenone provides a highly ordered transition state and considerably enhances the rate of ketone reduction.     

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.     

Electron transfer reduction of carboxylic acids using SmI2-H2O-Et3N

The first general method for efficient electron transfer reduction of carboxylic acids has been developed. The protocol using SmI(2)-H(2)O-Et(3)N allows for reduction of a variety of carboxylic acids in excellent yields and provides an attractive alternative to processes mediated by reactive alkali metals, lithium aluminum hydride, and boron hydrides. Of broader significance, the method allows acyl radical equivalents to be generated from carboxylic acids under mild reaction conditions.     

An Aldol-Like Reaction Mediated by SmI2

α-Haloketone reacted with aldehydes to form α, β-unsaturated ketones accompanied by dehalogenation in the presence of SmI₂. Mechanism involving samarium enolate intermediate formed in situ from α-haloketone with both SmI₂ and Sm(III)/I species was proposed.     

Solvent-dependent diastereoselectivities in reductions of beta-hydroxyketones by SmI2

The reductions of a series of beta-hydroxyketones by SmI(2) were examined in THF, DME, and CH(3)CN using methanol as a proton source. Reductions in THF and DME typically lead to the syn diastereomer with DME providing higher diastereoselectivities. Reductions in CH(3)CN provided the anti diastereomer predominantly. This study reveals that solvation plays an important role in substrate reduction by SmI(2). [reaction: see text]     

A New Asymmetric α-Amido-hydroxyalkylation Mediated by SmI_2

Polyhydroxylated indolizidine alkaloids, such as castanospermine (I) and swainsonine (II) are of longstanding interest due to their powerful glycosidase inhibitory activity. Asymmetric hydroxyalkylation via α-amido carbanions to form C-C bands would provide a convenient approach to these compounds. HO HO H OH H OH HO OH N N HO I II In continuation of our efforts in developing asymmetric α-amido-hydroxyalkylation method,1 we report a new SmI2 mediated flexible…     

The effect of proton donors on the facial stereoselectivity in SmI2 reduction of norcamphor

The endo/exo product ratio in the reactions of SmI(2) with norcamphor in the presence of various proton donors was determined. The effect of MeOH, EtOH, trifluoroethanol (TFE), ethylene glycol (EG), and water was investigated at various concentrations of these proton donors. At low concentrations of EtOH, TFE, and EG, an endo/exo ratio near unit was found. This ratio increased as the concentration of the proton donor increased. However, MeOH and water gave a U-type curve, in a plot of the endo/exo ratio vs proton donor concentration. The difference between the two groups of proton donors was shown not to result from differences in their acidities or polarity effects. It is suggested that at low MeOH and water concentrations, the second electron transfer takes place from the dimer of SmI(2) rather than from the monomer. This bulky electron donor approaches the radical anion preferentially from the exo direction giving rise to the high endo/exo ratio at the left arm of the U-shaped curve. Comparison of kinetic and product H/D isotope effects shows that protonation on carbon, the step that locks the stereochemistry, is a post rate determining step.     

Reduction of activated olefins by SmI2. Detouring the classical Birch mechanism and a negative order in SmI2

The reaction of an excess of 1,1-diaryl-2,2-dicyanoethylenes (1) with SmI2 is biphasic for olefin with at least one available para position. The first phase is completed in less than 0.5 s with the second phase extending over a few hundred seconds. This phase is second order with respect to the radical anion, which is formed in the dead-time of the mixing in the stopped flow spectrophotometer and is overall of -1 order in the initial concentration of SmI2. In this phase, a dimer is formed between two radical anions with the formation of a C-C bond between a benzylic and a para position. The second phase is enhanced by proton donors and shows an H/D kinetic isotope effect with MeOH. Minute amounts of ethylene glycol accelerated the reaction to such an extent that the second phase is "absorbed" into the first, rendering it rate determining. In this phase, the dianionic dimer disproportionates after protonation to furnish the neutral species and the anion, which after second protonation provides the reduced product. When the two para positions are occupied by substituents, the reaction takes place by the traditional Birch reduction sequence of electron-proton-electron-proton-transfer steps. It is shown that the detour mechanism, coupling followed by disproportionation, should be typical of olefin but not of carbonyl reduction. This difference stems from the dissimilarity in protonation rate on carbon and oxygen.     

The role of proton donors in SmI2-mediated ketone reduction: new mechanistic insights

The effects of proton donors (alcohols and water) on the rate of reduction of acetophenone by SmI2 have been examined utilizing stopped-flow spectrophotometric studies. The rate orders with respect to proton source and the kinetic isotope effects were determined as well. The reaction was first-order in phenol, 2,2,2-trifluoroethanol, methanol, and ethanol and zero-order in 2-propanol and 2-methyl-2-propanol when 25 equiv of proton source were used in the reduction. Methanol, ethanol, 2,2,2-trifluoroethanol, and phenol also showed a direct correlation between the pKa of the alcohol and the rate of reduction. Under the same conditions, water had a fractional rate order of 1.4. Further studies showed that water has a rate order of 1 at lower concentrations (<8 equiv) and a rate order of 2 at higher concentrations (>80 equiv). These results clearly indicate that the nature of the proton donor and its concentration affects the rates of reduction. Water has a high affinity for SmI2 (compared to that of the alcohols), and the onset of coordination at relatively low concentrations channels the reaction through a mechanistically distinct pathway.     

Solvent‐free cyanosilylation of aldehydes catalyzed by SmI2

A novel method to obtain racemic cyanohydrin silylethers by reaction of trimethylsilyl cyanide with a variety of aldehydes promoted by catalysis of SmI₂ is reported. The corresponding cyanosilylethers were obtained in high yields (up to 99%) in solvent‐ free conditions at room temperature within a relatively short time using 0.01–0.5 mol% catalyst loadings. Copyright © 2007 John Wiley & Sons, Ltd.     

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).     

SmI2-mediated radical cyclizations directed by a C-Si bond

The use of a silicon stereocontrol element in cyclobutanol and cyclopentanol-forming cyclizations mediated by SmI(2) results in excellent diastereocontrol. The C-Si bond in the products of cyclization provides a versatile handle for further manipulation. An asymmetric route to cyclization substrates involving copper-catalyzed silyl transfer has also been developed.