Ab initio electronic structure calculations have been carried out on six hydrogen abstraction reactions of the form, X−H + Y• → X• + H−Y, where X, Y = CH3, NH2, and OH. Geometric structures for the reactants, reactant complexes, transition states, product complexes, and products of each reaction have been gradient optimized at both the UMP2 and DFT(B3LYP) theory levels using the 6-311++G(2d,p) basis set. The character of each stationary state as a minimum or saddle point was determined by a harmonic force field calculation. PMP2 and CCSD(T) energies were also calculated at the UMP2 optimized geometries. CCSD(T) geometry optimizations were carried out for selected cases to resolve differences in results between UMP2 and DFT. The calculated reaction energies, barrier heights, and weak complex stabilities are compared to experiment, where possible, and among the different theory levels. The geometric structures and wave function properties are compared between the UMP2, CCSD(T), and DFT(B3LYP) methods. In general, all the methods predict reaction energies to within several kcal/mol of experiment. Calculated DFT(B3LYP) activation barriers are usually equal to or smaller than fitted Arrehenius equation activation energies (E a), with the gap increasing as X and Y are more electronegative. The DFT(B3LYP) activation barrier for the CH4 + CH3 • reaction is, therefore, in the best agreement with experiment. The UMP2, PMP2//UMP2, and CCSD(T)//UMP2 methods give activation energies that are somewhat greater than E a, as expected. The geometric structures of the three exchange reaction (X ≠ Y) transition states (TS) generally agree reasonably well between the UMP2 and DFT(B3LYP) methods, with active site bond length differences reflecting variations in the calculated exothermicities of the reactions. The UMP2 and DFT(B3LYP) methods give different conformations for the NH3 + OH• transition state, and CCSD(T) endorses the DFT results. For the H2O + OH• reaction UMP2 gives the symmetric structure as an energy minimum. DFT(B3LYP) predicts the symmetric structure to be the TS, in agreement with the optimized CCSD(T) result. For the NH3 + NH2 • reaction the UMP2 transition state is symmetric but with an unusually low imaginary reaction path frequency, while the DFT(B3LYP) frequency is reasonable. CCSD(T)//UMP2 and DFT(B3LYP) hydrogen-bonding energies for the reaction and product complexes are very similar and generally smaller than the UMP2 and PMP2//UMP2 results. The equilibrium geometric structures at the UMP2 and DFT(B3LYP) levels generally agree reasonably well. However, for the NH2···H−OH product complex DFT(B3LYP) gives a planar conformation, while UMP2 predicts a C s nonplanar configuration.

No abstract

Ligands that coordinate to SmI through oxygen are prevalent in the literature and make up a significant portion of additives employed with the reagent to perform reactions of great synthetic importance. In the present work a series of spectroscopic, calorimetric and kinetic studies demonstrate that nitrogen-based analogues of many common additives have a significantly higher affinity for Sm than the oxygen-based counterparts. In addition, electrochemical experiments show that nitrogen-based ligands significantly enhance the reducing power of SmI . Overall, this work demonstrates that the use of nitrogen-based ligands provides a useful alternative approach to enhance the reactivity of reductants based on Sm .

No abstract

cycloheptatriene,8 but it is inert to the reaction conditions on our time scale. It may also be relevant that in addition to sodium carbonate, sodium bicarbonate and to a lesser extent sodium acetate9 and tribasic potassium phosphate also speed the reaction. Monobasic sodium phosphate has no effect.

New horizons: We have shown that the scope of photostimulated SmI2 reactions could be vastly extended by converting the bimolecular protonation step to a unimolecular one. This was achieved by using a proton donor, such as MeOH, which complexes to SmI2 (see scheme). Protonation within the ion pair prevents rapid back electron transfer, which limited the applicability of photostimulated SmI2 reductions.

Since cyclopropanes are much more reactive than cyclobutanes despite their nearly identical strain energy, it has become clear that rate enhancement in cyclopropanes is partly due to strain release but also partly due to an additional factor about whose nature there is no consensus. Activation and equilibrium energies for a series of nucleophilic reactions of MeO-, MeS-, MeNH-, Me2N-, and MePH- with strained rings ranging from epoxide to aza-cubane were computed at the HF/631 + G* level. Using the Marcus equation, in combination with the computed intrinsic barrier for the identity reactions of the same nucleophiles with the strain free reference compounds (e.g., MeO- with Me-O-Me and Me2N- with Me3N), enabled a quantitative determination of the individual contributions of the partial strain release and the additional factor to the overall lowering of the transition state energy. Analysis of the data reveals the following: (a) There is no contribution of the additional factor to rate enhancement in four membered rings for first row elements (O and N) and only a small contribution (approximately 2 kcal) for second row elements (S and P). This is to be compared with a contribution of 7-17 kcal for three membered rings. (b) A significant synergistic effect is observed. Thus, in housane, for example, the additional factor amounts to 12.7 kcal which is more (by nearly 5 kcal) than the sum of the individual contributions of the isolated three and four membered rings. (c) The magnitude of the additional factor was found to be Periodic Table row dependent.

Radical anions of activated olefins may serve as good models for the transition state of Michael addition reactions. 1,2 In our search for a suitable reducing agent to effect the generation of these radical anions 3 our attention was drawn to SmI 2 , which is well-known to have exceptional qualities as a single electron transfer reductant. 4,5 Our initial studies revealed, however, that SmI 2 displays an extremely interesting and multifaceted mechanistic chemistry of which very little is well understood. 6 We believe therefore that an exploration of the mechanistic chemistry of SmI 2 is justified on its own merits. Equation 1 outlines the reactions studied.In a competition experiment, a mixture of MA and DP in THF was reacted with SmI 2 in the presence of a proton donor. 7,8 The product ratio was found to depend on the concentration of the latter. At low proton donor concentration, the selectivity is relatively high and approaches the thermodynamic stability ratio of the two radical anions. 9 As the proton donor concentration increases, the selectivity decreases, finally reaching a constant value. For MeOH and trifluoroacetic acid (TFA), at high concentration, the product ratio approaches unity ( Figure 1). These results are consistent with a mechanism where the protonation competes with the equilibration of the radical anions. At the lower proton donor concentration range, the lifetime of the radical anions is long enough to permit equilibration resulting in a relatively high selectivity. As the proton donor concentration increases, the equilibration is suppressed, thus leading to a lower selectivity. Consistent with the suggested mechanism, the plateau for the much stronger acid TFA is achieved at a concentration (0.25 M) much lower than that of MeOH (2.5 M).Surprisingly, for PrOH and MeOD, proton donors having kinetic acidity lower than MeOH, the plateau level is achieved at a product ratio below unity (0.7 and 0.8, respectively). The absence of selectivity at the plateau region for TFA and MeOH indicates a fast and unselective reaction of SmI 2 with the two substrates. 10 The lack of dependence of the selectivity on the "external" concentration of the various proton donors at the plateau regions, combined with the fact that for MeOD and PrOH plateau is achieved below unity, suggests that protonation occurs internally, probably within a triple complex such as ROH‚SmI 2 ‚MA •-(or ROH‚SmI 2 ‚DP •-). 11 The triple complexes can either undergo equilibration (and gravitate to their thermodynamic distribution ratio) or undergo an internal protonation which will "lock" the product distribution. The height of the plateau is determined by a competition between two processes: internal protonation within the triple complex and equilibration of the radical anions of the two substrates. In the case of MeOH this protonation is fast enough to prohibit any appreciable equilibration. Slowing down the protonation rate by using acids with lower kinetic acidity (MeOD and PrOH) permits a certain extent of equilibration. The ...