Shmaryahu Hoz

Guidelines for the Use of Proton Donors in SmI2 Reactions: Reduction of α-Cyanostilbene

The reduction of a series of alpha-cyanostilbenes with SmI(2) was studied in THF in the presence of various proton donors. No reaction occurred in the presence of the alcohols TFE, i-PrOH and t-BuOH. In the presence of MeOH, water and ethylene glycol the reactions occurred; however in the presence of water and ethylene glycol they were too fast for kinetic determinations (tau(1/2) < 1 ms). Reactions with MeOH were first order in SmI(2) and first order in the substrate. The order in MeOH varies as a function of its concentration and the plot of log k vs log [MeOH] is sigmoidal. Comparison of the kinetic isotope effect and the incorporation isotope effect suggests that, counterintuitively, protonation of the radical anion takes place on the carbon beta to the cyano group. It is concluded that proton donors that form complexes with SmI(2) expand the range of substrates that can be reduced by SmI(2). This is due to their proximity to the radical anion as it is formed. This short-lived radical anion cannot be efficiently trapped by a proton donor from the bulk medium. A protocol is herein suggested as to when proton donors which complex to SmI(2), e.g. MeOH, water and ethyleneglycol should be used, and when it is recommended to use noncomplexing proton donors, e.g. TFE, i-PrOH and t-BuOH, to induce reaction.

Can ground-state destabilization of an α-nucleophile induce an α-effect?

Abstract The possible importance of ground-state destabilization as the origin of the enhanced reactivity of α-nucleophiles is critically examined and it is concluded that, in general, this factor will contribute only slightly, if at all, to the manifestation of α-effects.

Reduction of Benzophenones with SmI2. Post Electron Transfer Processes

The kinetics of the reaction of several substituted benzophenones with SmI(2) in THF was studied by using stopped flow spectrometry. The electron transfer takes place during the dead time of mixing and for most derivatives it is nearly quantitative. In the presence of an excess of substrate and in the absence of proton donors the dimerization reaction to pinacol is second order in the radical anion and has a negative order in SmI(2). Similarly, in reactions in the presence of excess SmI(2), the reaction shows negative order in the concentration of the substrate. It is concluded that the radical anions exist as a mixture of monomeric ion pairs and Streitwieser dimers. In the presence of trifluoroethanol, protonation of the Streitwieser dimer occurs with a rate constant an order of magnitude larger than that of the monomer.

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.

Reductions with SmI2: mechanistic probe for distinguishing between two operational modes of proton transfer

The reduction of 1,1-diaryl-2,2-dicyanoethylenes with SmI(2) in THF was studied in the presence of four proton donors: H(2)O, MeOH, i-PrOH and trifluoroethanol (TFE). The kinetic order for the first two is nearly unity at low proton donor concentrations and approaches four at high concentrations, whereas, for i-PrOH and TFE, the log-log plot is linear with a slope smaller than one. Detailed analysis shows that a curved log-log plot such as for H(2)O and MeOH is indicative of a major contribution by protonation within an ion pair of the radical anion and Sm(+3) complexed to a variable number of proton donor molecules, whereas a linear plot is a result of protonation from the bulk solution.

The performance of density function theory in describing gas-phase SN2 reactions at saturated nitrogen

Abstract The prototype gas-phase identity S N 2 reactions at neutral nitrogen X − +NH 2 X→NH 2 X+X − (where X=F, Cl, Br, I) were studied using the three hybrid Hartree–Fock DFT methods B3LYP, MPW1PW and MPW1K in conjunction with the 6-31+G(d,p) basis sets. Comparison of the results with the high-level G2(+) theory indicated that all of the three hybrid DFT methods can give reasonable values of the complexation energies (Δ H comp ) for the ion–molecule complex formed in the reaction X − +NH 2 X (X=Cl, Br, I). The overall barriers (Δ H ovr ≠ ) and central barriers (Δ H cent ≠ ) for all of the reactions calculated using B3LYP functional are significantly underestimated. The best transition structures were obtained using MPW1K/6-31+G(d,p) level, which appears to exhibit the best performance in describing the potential energy surface for S N 2 reactions at neutral nitrogen. Some correlations of central barriers found at the G2(+) level of theory are reproduced by the results of three hybrid DFT methods for the reactions X − +NH 2 X→NH 2 X+X − (X=Cl, Br, I).

A G2(+) level investigation of the gas-phase identity nucleophilic substitution at neutral oxygen

Abstract G2(+) level molecular orbital calculations have been carried out for the identity nucleophilic substitution at saturated oxygen, X−+HOX→HOX+X− (X=F, Cl, Br, I). A comparison with data for the analogous reactions at saturated nitrogen, X−+NH2X→NH2X+X−, and at saturated carbon, X−+CH3X→CH3X+X−, indicate that the substitution reaction at saturated oxygen proceeds via a classic SN2 pathway. The calculated intrinsic barriers ΔHcent≠ for substitution at oxygen are found to be much higher than the corresponding barriers for substitution at carbon and nitrogen, decreasing in the order F (106.3 kJ / mol )> Cl (92.5 kJ / mol )> Br (70.3 kJ / mol )> I (58.6 kJ / mol ) . Stabilization energies of the ion–molecule complexes decrease in the order F (187.9 kJ/mol )> Cl (97.5 kJ/mol )> Br (81.2 kJ / mol )> I (66.5 kJ / mol ) , that are also significantly higher than the corresponding values at carbon and nitrogen, and correlate well with the halogen electronegativities. The overall barriers relative to the reactants (ΔHovr≠) are negative for all halogens F (−81.7 kJ / mol ) , Cl (−5.1 kJ / mol ) , Br (−10.7 kJ / mol ) , I (−8.1 kJ / mol ) . These trend is similar to that for the analogous reaction at nitrogen, but contrasts to that for the reactions at carbon where the ΔHovr≠ are negative only for X=F.

Pitfalls in the determination of the α-effect by a two-point analysis. The effect of solvent on the α-effect.

Abstract The methodology for evaluation of the α-effect is examined and it is shown that the two-point analysis method is generally of limited value. This leads to a re-examination of the effect of solvent on the α-effect.

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.

Broadening the Scope of Photostimulated SmI2 Reductions

However, in contradistinction to the HMPA method, the range of applicability of this method is surprisingly limited. To the best of our knowledge, the reaction types shown in Scheme 1 are the only examples of the application of this method. Once the reason for the limited applicability of the photostimulation technique is understood, knowledge of the intimate mechanistic details of SmI2 reactions may assist in the design of a strategy to broaden its scope. The common denominator for the photostimulated reactions shown in Scheme 1 is that there is no bimolecular ratedetermining step following the electron transfer. These reactions involve either a dissociative electron attachment process, such as in the mesolytic cleavage of alkyl halides, or, as may be the case for C6H5X leaving groups, the radical anion may have a short lifetime before it either donates the electron back to the SmI2 or undergoes cleavage. None of these reactions necessitate a bimolecular step, which should, for successful reaction, effectively compete with back electron transfer. It should be noted that in the ground state there is no appreciable electron transfer from SmI2 to the substrate in these cases. It is only after the SmI2 is excited that the process becomes viable (excitation at around 600 nm is equivalent to approximately 2.2 eV, which is much more than the 0.72 V increase effected by HMPA (4 equiv).) Yet, once electron transfer has taken place Sm is no longer excited and, therefore, back electron transfer from the radical anion to the ground state of Sm has a very high thermodynamic driving force. Hence, rapid back electron transfer prevents any bimolecular encounter, such as the protonation of the radical anion in the Birch-type reduction sequence (electron–proton–electron–proton transfer). The easiest way to overcome the deficiency of the extremely short lifetime of the radical anion caused by rapid back electron transfer is to convert the consequent bimolecular process, for example, protonation, into a unimolecular one. We have shown that the fact that MeOH complexes to SmI2 [8] enables protonation to occur from within the ion pair generated by electron transfer. Protonation within the ion pair is a unimolecular process, which may therefore effectively trap the radical anion as it is formed and, therefore, the need for a bimolecular encounter between the shortlived radical anion and a proton donor from the bulk solution is rendered unnecessary. Herein, we show that exploiting this proximity effect significantly broadens the scope of photostimulated SmI2 reduction. [a] Dr. M. Amiel-Levy, Prof. S. Hoz Department of Chemistry, Bar-Ilan University Ramat-Gan 52900 (Israel) E-mail : shoz@mail.biu.ac.il Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200902198. Scheme 1. SmI2 reduction under irradiation conditions. Ts= tosyl.