Frontside versus Backside S(N)2 substitution at group 14 atoms: origin of reaction barriers and reasons for their absence

Backside versus Frontside SN2 Reactions of Alkyl Triflates and Alcohols

Nucleophilic substitution reactions are elementary reactions in organic chemistry that are used in many synthetic routes. By quantum chemical methods, we have investigated the intrinsic competition between the backside SN2 (SN2-b) and frontside SN2 (SN2-f) pathways using a set of simple alkyl triflates as the electrophile in combination with a systematic series of phenols and partially fluorinated ethanol nucleophiles. It is revealed how and why the well-established mechanistic preference for the SN2-b pathway slowly erodes and can even be overruled by the unusual SN2-f substitution mechanism going from strong to weak alcohol nucleophiles. Activation strain analyses disclose that the SN2-b pathway is favored for strong alcohol nucleophiles because of the well-known intrinsically more efficient approach to the electrophile resulting in a more stabilizing nucleophile-electrophile interaction. In contrast, the preference of weaker alcohol nucleophiles shifts to the SN2-f pathway, benefiting from a stabilizing hydrogen bond interaction between the incoming alcohol and the leaving group. This hydrogen bond interaction is strengthened by the increased acidity of the weaker alcohol nucleophiles, thereby steering the mechanistic preference toward the frontside SN2 pathway.

Exploring borderline SN1-SN2 mechanisms: the role of explicit solvation protocols in the DFT investigation of isopropyl chloride

Nucleophilic substitution at saturated carbon is a crucial class of organic reactions, playing a pivotal role in various chemical transformations that yield valuable compounds for society. Despite the well-established SN1 and SN2 mechanisms, secondary substrates, particularly in solvolysis reactions, often exhibit a borderline pathway. A molecular-level understanding of these processes is fundamental for developing more efficient chemical transformations. Typically, quantum-chemical simulations of the solvent medium combine explicit and implicit solvation methods. The configuration of explicit molecules can be defined through top-down approaches, such as Monte Carlo (MC) calculations for generating initial configurations, and bottom-up methods that involve user-dependent protocols to add solvent molecules around the substrate. Herein, we investigated the borderline mechanism of the hydrolysis of a secondary substrate, isopropyl chloride (iPrCl), at DFT-M06-2X/aug-cc-pVDZ level, employing explicit and explicit + implicit protocols. Top-down and bottom-up approaches were employed to generate substrate-solvent complexes of varying number (n = 1, 3, 5, 7, 9, and 12) and configurations of H2O molecules. Our findings consistently reveal that regardless of the solvation approach, the hydrolysis of iPrCl follows a loose-SN2-like mechanism with nucleophilic solvent assistance. Increasing the water cluster around the substrate in most cases led to reaction barriers of ΔH‡ ≈ 21 kcal mol-1, with nine water molecules from MC configurations sufficient to describe the reaction. The More O'Ferrall-Jencks plot demonstrates an SN1-like character for all transition state structures, showing a clear merged profile. The fragmentation activation strain analyses indicate that energy barriers are predominantly controlled by solvent-substrate interactions, supported by the leaving group stabilization assessed through CHELPG atomic charges.

Inter- and Intra-Molecular Organocatalysis of SN2 Fluorination by Crown Ether: Kinetics and Quantum Chemical Analysis

We present the intra- and inter-molecular organocatalysis of S_N2 fluorination using CsF by crown ether to estimate the efficacy of the promoter and to elucidate the reaction mechanism. The yields of intramolecular S_N2 fluorination of the veratrole substrates are measured to be very small (<1% in 12 h) in the absence of crown ether promoters, whereas the S_N2 fluorination of the substrate possessing a crown ether unit proceeds to near completion (~99%) in 12 h. We also studied the efficacy of intermolecular rate acceleration by an independent promoter 18-crown-6 for comparison. We find that the fluorinating yield of a veratrole substrate (leaving group = −OMs) in the presence of 18-crown-6 follows the almost identical kinetic course as that of intramolecular S_N2 fluorination, indicating the mechanistic similarity of intra- and inter-molecular organocatalysis of the crown ether for S_N2 fluorination. The calculated relative Gibbs free energies of activation for these reactions, in which the crown ether units act as Lewis base promoters for S_N2 fluorination, are in excellent agreement with the experimentally measured yields of fluorination. The role of the metal salt CsF is briefly discussed in terms of whether it reacts as a contact ion pair or as a “free” nucleophile F⁻.

The role of CuI in the siloxane-mediated Pd-catalyzed cross-coupling reactions of aryl iodides with aryl lithium reagents

Experiments indicate that a catalytic amount of CuI plays an important role in the siloxane-mediated Pd-catalyzed cross-coupling reactions with the direct use of organolithium reagents. Addition of organolithium to the siloxane transfer agent generates an organosilicon intermediate. DFT calculations indicate that CuI initially accelerates the Si-Pd(II) transmetalation of the organosilicon intermediate by the formation of CuI2 -. Subsequently, CuI2 - works as a shuttle between the Si-Cu(I) and Cu(I)-Pd(II) transmetalation processes.

On the SN2 reactions modified in vibrational strong coupling experiments: reaction mechanisms and vibrational mode assignments

Recent experiments have reported modified chemical reactivity under vibrational strong coupling (VSC) in microfluidic Fabry-Pérot cavities. In particular, the reaction rate of nucleophilic substitution reactions at silicon centers (S_N2@Si) has been altered when a vibrational mode of the reactant was coupled to a confined light mode in the strong coupling regime. In this situation, hybrid light-matter states known as polaritons are formed and seem to be responsible for the modified chemical kinetics. These results are very encouraging for future applications of polaritonic chemistry to catalyze chemical reactions, with the ability to manipulate chemical phenomena without any external excitation of the system. Still, there is no theory capable of explaining the mechanism behind these results. In this work we address two points that are crucial for the interpretation of these experiments. Firstly, by means of electronic structure calculations we report the reaction mechanism in normal conditions of the two recently modified S_N2@Si reactions, obtaining in both cases a triple-well PES where the rate-determining step is due to the Si-C and Si-O bond cleavage. Secondly, we characterize in detail the normal modes of vibration of the reactants. In the VSC experiments, reaction rates were modified only when specific vibrations of the reactants were coupled to a cavity mode. We find that these vibrations are highly mixed among the different fragments of the reactants leading to a completely new assignment of the IR peaks coupled to cavity modes in the original experimental works. Our results are fundamental for the interpretation of the VSC experiments given that in the absence of a theory explaining these results, the current phenomenological understanding relies on the assignment of the character of the vibrational IR peaks.

Full Conformational Analyses of the Ultrafast Isomerization in Penta-coordinated Ru(S2C2(CF3)2)(CO)(PPh3)2: One Compound, Two Crystal Structures, Three CO Frequencies, 24 Stereoisomers, and 48 Transition States

The stereodynamics of an ultrafast (picosecond) isomerization in a penta-coordinated ruthenium complex, Ru(S₂C₂(CF₃)₂)(CO)(PPh₃)₂, were characterized by density functional theory (DFT). The ruthenium complex crystallizes in two almost-square pyramidal (SP) forms. The violet form has an apical PPh₃ ligand, the orange form has an apical CO ligand, and their solution displays three CO stretching frequencies. With 4 possible centers of chirality (1 ruthenium, 2 phosphines, and 1 dithiolate), there are 24 stereoisomers. DFT calculations of these stereoisomers show structures ranging from almost-perfect SP (τ₅ ≈ 0) to structures significantly distorted toward trigonal bipyramidal (TBP) (τ₅ ≈ 0.6). The stereoisomers fall neatly into three groups, with ν_CO ≈ 1960 cm^-1, 1940 cm^-1, and 1980 cm^-1. These isomers were found to interconvert over relatively small barriers via Ru-S bond twisting, CF₃ rotation, phenyl twisting, PPh₃ rotation, and, in some cases, by coupled motions. The composite energy surface for each CO frequency group shows that interconversions among the low-energy structures are possible via both the direct and indirect pathways, while the indirect pathway via isomers in the ν_CO ≈ 1980 cm^-1 group is more favorable, which is a result consistent with recent experimental work. This work provides the first complete mechanistic picture of the ultrafast isomerization of penta-coordinated, distorted SP, d⁶-transition-metal complexes.

Kinetically controlled asymmetric synthesis of silicon-stereogenic methoxy silanes using a planar chiral ferrocene backbone

We present a mechanistic proposal for the desymmetrisation of dimethoxy silanes with alkyllithiums. The stereochemical pathway is highly defined by the coordination of the metal lithium, which suppresses the reversible interconversion of the various pentavalent intermediates. The predicted kinetic control of the stereoinduction is verified by the experiments using a planar chiral ferrocene backbone.

Theoretical insight into the regioselective ring-expansions of bicyclic aziridinium ions

Transient bicyclic aziridinium ions are known to undergo ring-expansion reactions, paving the way to functionalized nitrogen-containing heterocycles. In this study, the regioselectivity observed in the ring-expansion reactions of 1-azoniabicyclo[n.1.0]alkanes was investigated from a computational viewpoint to study the ring-expansion pathways of two bicyclic systems with different ring sizes. Moreover, several nucleophiles leading to different experimental results were investigated. The effect of solvation was taken into account using both explicit and implicit solvent models. This theoretical rationalization provides valuable insight into the observed regioselectivity and may be used as a predictive tool in future studies.

Nucleophilic Substitution (SN 2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

The reaction potential energy surface (PES), and thus the mechanism of bimolecular nucleophilic substitution (SN 2), depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the SN 2 mechanism. One of the most striking effects is the transition from a double-well to a single-well PES when the central atom is changed from a second-period (e. g. carbon) to a higher-period element (e.g, silicon, germanium). Variations in nucleophilicity, leaving group ability, and bulky substituents around a second-row element central atom can then be exploited to change the single-well PES back into a double-well. Reversely, these variations can also be used to produce a single-well PES for second-period elements, for example, a stable pentavalent carbon species.

An interacting quantum atom study of model SN 2 reactions (X- ···CH3 X, X = F, Cl, Br, and I)

The quantum chemical topology method has been used to analyze the energetic profiles in the X-  + CH3 X → XCH3  + X- SN 2 reactions, with X = F, Cl, Br, and I. The evolution of the electron density properties at the BCPs along the reaction coordinate has been analysed. The interacting quantum atoms (IQA) method has been used to evaluate the intra-atomic and interatomic energy variations along the reaction path. The different energetic terms have been examined by the relative energy gradient method and the ANANKE program, which enables automatic and unbiased IQA analysis. Four of the six most important IQA energy contributions were needed to reproduce the reaction barrier common to all reactions. The four reactions considered share many common characteristics but when X = F a number of particularities occur. © 2017 Wiley Periodicals, Inc.

Ion-Pair SN 2 Reaction of OH- and CH3 Cl: Activation Strain Analyses of Counterion and Solvent Effects

We have theoretically studied the non-identity S_N 2 reactions of M_n OH^(n-1) +CH₃ Cl (M⁺ =Li⁺ , Na⁺ , K⁺ , and MgCl⁺ ; n=0, 1) in the gas phase and in THF solution at the OLYP/6-31++G(d,p) level using polarizable continuum model (PCM) implicit solvation. We want to explore and understand the effect of the metal counterion M⁺ and solvation on the reaction profile and the stereoselectivity of these processes. To this end, we have explored the potential energy surfaces of the backside (S_N 2-b) and frontside (S_N 2-f) pathways. To explain the computed trends, we have carried out analyses with an extended activation strain model (ASM) of chemical reactivity that includes the treatment of solvation effects.

Analyzing Reaction Rates with the Distortion/Interaction-Activation Strain Model

The activation strain or distortion/interaction model is a tool to analyze activation barriers that determine reaction rates. For bimolecular reactions, the activation energies are the sum of the energies to distort the reactants into geometries they have in transition states plus the interaction energies between the two distorted molecules. The energy required to distort the molecules is called the activation strain or distortion energy. This energy is the principal contributor to the activation barrier. The transition state occurs when this activation strain is overcome by the stabilizing interaction energy. Following the changes in these energies along the reaction coordinate gives insights into the factors controlling reactivity. This model has been applied to reactions of all types in both organic and inorganic chemistry, including substitutions and eliminations, cycloadditions, and several types of organometallic reactions.

Activation Strain Analysis of SN2 Reactions at C, N, O, and F Centers

Fundamental principles that determine chemical reactivity and reaction mechanisms are the very foundation of chemistry and many related fields of science. Bimolecular nucleophilic substitutions (S_N2) are among the most common and therefore most important reaction types. In this report, we examine the trends in the S_N2 reactions with respect to increasing electronegativity of the reaction center by comparing the well-studied backside S_N2 Cl^– + CH₃Cl with similar Cl^– substitutions on the isoelectronic series with the second period elements N, O, and F in place of C. Relativistic (ZORA) DFT calculations are used to construct the gas phase reaction potential energy surfaces (PES), and activation strain analysis, which allows decomposition of the PES into the geometrical strain and interaction energy, is employed to analyze the observed trends. We find that S_N2@N and S_N2@O have similar PES to the prototypical S_N2@C, with the well-defined reaction complex (RC) local minima and a central barrier, but all stationary points are, respectively, increasingly stable in energy. The S_N2@F, by contrast, exhibits only a single-well PES with no barrier. Using the activation strain model, we show that the trends are due to the interaction energy and originate mainly from the decreasing energy of the empty acceptor orbital (σ*_A–Cl) on the reaction center A in the order of C, N, O, and F. The decreasing steric congestion around the central atom is also a likely contributor to this trend. Additional decomposition of the interaction energy using Kohn–Sham molecular orbital (KS-MO) theory provides further support for this explanation, as well as suggesting electrostatic energy as the primary reason for the distinct single-well PES profile for the FCl reaction.

Quantum Mechanical and Molecular Dynamics Studies of the Reaction Mechanism of the Nucleophilic Substitution at the Si Atom

The mechanism of the nucleophilic substitution at the Si atom, SiH3Cl + Cl*(-) → SiH3Cl* + Cl(-), is examined by both quantum mechanical (QM) and molecular dynamics (MD) methods. This reaction proceeds by two steps with the inversion or retention of the configuration passing through an intermediate with the trigonal bipyramid (TBP) structure, although the conventional SN2 reaction at the C atom proceeds by one step with the inversion of the configuration passing through a transition state with the TBP structure. We followed by the QM calculations all the possible paths of the substitution reaction that undergo the TBP intermediates with the cis and trans forms produced by the frontside and backside attacks of Cl(-). As a result, it was thought that TBPcis1 produced with a high probability is readily transformed to the energetically more stable TBPtrans. This fact was also shown by the MD simulations. In order to obtain more information concerning the trajectory of Cl(-) on the dissociation from TBPtrans, which we cannot clarify on the basis of the energy profile determined by the QM method, the MD simulations with and without the water solvent were conducted and analyzed in detail. The QM-MD simulations without the water solvent revealed that the dissociation of Cl(-) from TBPtrans occurs without passing through TBPcis1'. The ONIOM-MD simulations with the water solvent further suggested that the thermal fluctuation of the water solvent significantly affects the oscillation of the kinetic and potential energies of the substrate to facilitate the isomerization of the TBP intermediate from the cis form to the trans form and the subsequent dissociation of Cl(-) from TBPtrans.

Addition-Elimination or Nucleophilic Substitution? Understanding the Energy Profiles for the Reaction of Chalcogenolates with Dichalcogenides

We have quantum chemically explored the mechanism of the substitution reaction between CH3X(-) and the homo- and heterodichalcogenides CH3X'X″CH3 (X, X', X″ = S, Se, Te) using relativistic density functional theory at ZORA-OLYP/TZ2P and COSMO for simulating the effect of aqueous solvation. In the gas phase, all substitution reactions proceed via a triple-well addition-elimination mechanism that involves a stable three-center intermediate. Aqueous solvation, in some cases, switches the character of the mechanism to double-well SN2 in which the stable three-center intermediate has become a labile transition state. We rationalize reactivity trends and some puzzling aspects of these elementary reactions, in particular, vanishing activation energies and ghost three-center intermediates, using the activation strain model (ASM).

Ion-Pair SN 2 Substitution: Activation Strain Analyses of Counter-Ion and Solvent Effects

The ion-pair SN 2 reactions of model systems MnF(n-1) +CH3Cl(M(+) =Li(+), Na(+), K(+), and MgCl(+); n=0, 1) have been quantum chemically explored by using DFT at the OLYP/6-31++G(d,p) level. The purpose of this study is threefold: 1) to elucidate how the counterion M(+) modifies ion-pair SN 2 reactivity relative to the parent reaction F(-) +CH3Cl; 2) to determine how this influences stereochemical competition between the backside and frontside attacks; and 3) to examine the effect of solvation on these ion-pair SN2 pathways. Trends in reactivity are analyzed and explained by using the activation strain model (ASM) of chemical reactivity. The ASM has been extended to treat reactivity in solution. These findings contribute to a more rational design of tailor-made substitution reactions.

The activation strain model and molecular orbital theory

The activation strain model is a powerful tool for understanding reactivity, or inertness, of molecular species. This is done by relating the relative energy of a molecular complex along the reaction energy profile to the structural rigidity of the reactants and the strength of their mutual interactions: ΔE(ζ) = ΔE_strain(ζ) + ΔE_int(ζ). We provide a detailed discussion of the model, and elaborate on its strong connection with molecular orbital theory. Using these approaches, a causal relationship is revealed between the properties of the reactants and their reactivity, e.g., reaction barriers and plausible reaction mechanisms. This methodology may reveal intriguing parallels between completely different types of chemical transformations. Thus, the activation strain model constitutes a unifying framework that furthers the development of cross-disciplinary concepts throughout various fields of chemistry. We illustrate the activation strain model in action with selected examples from literature. These examples demonstrate how the methodology is applied to different research questions, how results are interpreted, and how insights into one chemical phenomenon can lead to an improved understanding of another, seemingly completely different chemical process. WIREs Comput Mol Sci 2015, 5:324-343. doi: 10.1002/wcms.1221.

Gas-phase reaction of ClO−with CHnCl4-n(n= 0, 1, 2, 3) and CX3H (X = F, Cl and Br): Substituent effect from a comparative study

Substituent effects on reactivity are studied using the hybrid B3LYP and BHandHLYP methods of density functional theory with the aug-cc-pVDZ basis set. The chosen testing models includes two very representative reactions in chemical research, the bimolecular nucleophilic substitution (SN2) reaction and the deprotonation reaction, in which the former is represented by ClO−+ CHnCl4-n(n = 0, 1, 2, 3), and the latter is based on reactions of ClO−with CX3H (X = F, Cl, and Br). Our theoretical findings suggest that a heavier substituent X in substrate results in a higher activation energy, a slower SN2 reaction, but a faster deprotonation reaction. Those are well confirmed by some presented results from bond orders, second-order perturbative energy E(2), and activation strain model analysis. Moreover, we have further explored the reactivity difference derived from substituent effects in term of the relationships of reactive barrier with the charges transferred and the leaving-bond distance in TSs, respectively, especially the TSs in SN2 reactions. Again, the rate constants at 298–1000 K are also evaluated for the SN2 reactions presented through the transition state theory.

Reactivity in nucleophilic vinylic substitution (S(N)V):S(N)Vπ versus S(N)Vσ mechanistic dichotomy

The intrinsic electronic factors that determine reactivity in prototypical identity nucleophilic vinylic substitution reactions, X(-) + ViX → XVi + X(-) (Vi = vinyl), have been studied by performing quantum chemical calculations (OPBE/6-311++G(d,p)). Of the two limiting reaction types envisaged--the S(N)Vπ and S(N)Vσ mechanisms--the former is preferred for most combinations of nucleophiles and substrates, except for the combination of unactivated substrates and poor nucleophiles, as seen for the much studied reactions Cl(-) + CH2CHCl and Br(-) + CH2CHBr. It was found that periodic trends for S(N)Vπ are essentially the same as those previously reported for nucleophilic aromatic substitution, S(N)Ar, while intrinsic S(N)Vσ nucleophilicity parallels aliphatic S(N)2. It is therefore concluded that S(N)V reactivity in general can be understood in terms of this mechanistic dichotomy. Furthermore, a few representative reactions were analyzed applying two complementary schemes for energy decomposition analysis.

Type-I dyotropic reactions: understanding trends in barriers

To understand the factors that control the activation barrier of type-I 1,2-dyotropic reactions (X-EH(2)-CH(2)-X*→X*-EH(2)-CH(2)-X, with E=C and Si, X=H, CH(3), SiH(3), F to I) and trends therein as a function of the migrating groups X, we have explored ten archetypal model reactions of this class using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity, which had to be extended from bimolecular to unimolecular reactions. Thus, the above type-I dyotropic reactions can be conceived as a relative rotation of the CH(2)CH(2) and [X···X] fragments in X-CH(2)-CH(2)-X. The picture that emerges from these analyses is that reduced C-X bonding in the transition state is the origin of the reaction barrier. Also the trends in reactivity on variation of X can be understood in terms of how sensitive the C-X interaction is towards adopting the transition-state geometry. A valence bond analysis complements the analyses and confirms the picture emerging from the activation strain model.

Density functional study of S(N) 2 substitution reactions for CH(3) Cl + CX(1) X(2•-) (X(1) X(2) = HH, HF, HCl, HBr, HI, FF, ClCl, BrBr, and II)

A systematic investigation on the S(N) 2 displacement reactions of nine carbene radical anions toward the substrate CH(3) Cl has been theoretically carried out using the popular density functional theory functional BHandHLYP level with different basis sets 6-31+G (d, p)/relativistic effective core potential (RECP), 6-311++G (d, p)/RECP, and aug-cc-pVTZ/RECP. The studied models are CX(1) X(2•-) + CH(3) Cl → X(2) X(1) CH(3) C(•) + Cl(-) , with CX(1) X(2•-) = CH(2) (•-) , CHF(•-) , CHCl(•-) , CHBr(•-) , CHI(•-) , CF(2) (•-) , CCl(2) (•-) , CBr(2) (•-) , and CI(2) (•-) . The main results are proposed as follows: (a) Based on natural bond orbital (NBO), proton affinity (PA), and ionization energy (IE) analysis, reactant CH(2) (•-) should be a strongest base among the anion-containing species (CX(1) X(2•-) ) and so more favorable nucleophile. (b) Regardless of frontside attacking pathway or backside one, the S(N) 2 reaction starts at an identical precomplex whose formation with no barrier. (c) The back-S(N) 2 pathway is much more preferred than the front-S(N) 2 one in terms of the energy gaps [ΔE cent≠(front)-ΔE cent≠(back)], steric demand, NBO population analysis. Thus, the back-S(N) 2 reaction was discussed in detail. On the one hand, based on the energy barriers (ΔE cent≠ and ΔE ovr≠) analysis, we have strongly affirmed that the stabilization of back attacking transition states (b-TSs) presents increase in the order: b-TS-CI(2) < b-TS-CBr(2) < b-TS-CCl(2) < b-TS-CHI < b-TS-CHBr < b-TS-CHCl < b-TS-CF(2) < b-TS-CHF < b-TS-CH(2) . On the other hand, depended on discussions of the correlations of ΔE ovr≠ with influence factors (PA, IE, bond order, and ΔE def≠), we have explored how and to what extent they affect the reactions. Moreover, we have predicted that the less size of substitution (α-atom) required for the gas-phase reaction with α-nucleophile is related to the α-effect and estimated that the reaction with the stronger PA nucleophile, holding the lighter substituted atom, corresponds to the greater exothermicity given out from reactants to products.

The activation strain model of chemical reactivity

Herein, we provide an account of the activation strain model of chemical reactivity and its recent applications. In this model, the potential energy surface DeltaE(zeta) along the reaction coordinate zeta is decomposed into the strain DeltaE(strain)(zeta) of the increasingly deformed reactants plus the interaction DeltaE(int)(zeta) between these deformed reactants, i.e., DeltaE(zeta) = DeltaE(strain)(zeta) + DeltaE(int)(zeta). The purpose of this fragment-based approach is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition-state geometries (e.g., early or late along the reaction coordinate) in terms of the reactants' properties. The usage of the activation strain model is illustrated by a number of concrete applications, by us and others, in the fields of catalysis and organic chemistry.

Mechanism of S(H)2 reactions of disulfides: frontside vs backside, stepwise vs concerted

Density functional theory calculations indicate that the S(H)2 reactions of disulfides with alkyl or aryl radicals take place via concerted backside displacement. The activation energies for reactions of Me* with RSSR (R = Me, Et, (i)Pr, (t)Bu) increase with the size of R, since larger R groups prevent the formation of an ideal geometry for SOMO-LUMO overlap. Frontside transition states can also be located, but these lie at least 11 kcal mol(-1) above the corresponding backside transition states.