Four-center Transition State Research Articles

Direct observation of β-alkynyl eliminations from unstrained propargylic alkoxide Cu(i) complexes by C-C bond cleavage.

β-Carbon eliminations of aryl, allylic, and propargylic alkoxides of Rh(i), Pd(ii), and Cu(i) are key elementary reactions in the proposed mechanisms of homogeneously catalysed cross-coupling, group transfer, and annulation. Besides the handful of studies with isolable Rh(i)-alkoxides, β-carbon eliminations of Pd(ii)- and Cu(i)-alkoxides are less definitive. Herein, we provide a comprehensive synthetic, structural, and mechanistic study on the β-alkynyl eliminations of isolable secondary and tertiary propargylic alkoxide Cu(i) complexes, LCuOC(H)(Ph)C[triple bond, length as m-dash]CPh and LCuOC(ArF)2C[triple bond, length as m-dash]CPh (L = N-heterocyclic carbene (NHC), dppf, S-BINAP), to produce monomeric (NHC)CuC[triple bond, length as m-dash]CPh, dimeric [(diphosphine)CuC[triple bond, length as m-dash]CPh]2, and the corresponding carbonyl. Selective β-alkynyl over β-hydrogen elimination was observed for NHC- and diphosphine-supported secondary propargylic alkoxide complexes. The mechanism for the first-order reaction of β-carbon elimination of (IPr*Me)CuOC(ArF)2C[triple bond, length as m-dash]CPh is proposed to occur through an organized four-centred transition state via a Cu-alkyne π complex based on Eyring analysis of variable-temperature reaction rates by UV-vis kinetic analysis to provide ΔH ‡ = 24(1) kcal mol-1, ΔS ‡ = -8(3) e.u., and ΔG ‡ (25 °C) = 27 kcal mol-1 over a temperature range of 60-100 °C. Additional quantitative UV-vis kinetic studies conclude that the electronic and steric properties of the NHC ligands engendered a marginal effect on the elimination rate, requiring 2-3 h at 100 °C for completion, whereas complete β-alkynyl eliminations of diphosphine-supported propargylic alkoxides were observed in 1-2 h at 25 °C.

Theoretical investigations on photodissociation dynamics of deuterated alkyl halides CD3CH2F

The product branching ratio between different products in multichannel reactions is as important as the over-all rate of reaction, both in terms of practical applications (e.g. models of combustion or atmosphere chemistry) in understanding the fundamental mechanisms of such chemical reactions. A global ground state potential energy surface for the dissociation reaction of deuterated alkyl halide CD3CH2F was computed at the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ level of theory for all species. The decomposition of CD3CH2F is controversial concerning C−F bond dissociation reaction and molecular (HF, DF, H2, D2, HD) elimination reaction. Rice-Ramsperger-Kassel-Marcus (RRKM) calculations were applied to compute the rate constants for individual reaction steps and the relative product branching ratios for the dissociation products were calculated using the steady-state approach. At the different energies studied, the RRKM method predicts that the main channel for DF or HF elimination from 1,2-elimination of CD3CH2F is through a four-center transition state, whereas D2 or H2 elimination from 1,1-elimination of CD3CH2F occurs through a direct three-center elimination. At 266, 248, and 193 nm photodissociation, the main product CD2CH2+DF branching ratios are computed to be 96.57%, 91.47%, and 48.52%, respectively; however, at 157 nm photodissociation, the product branching ratio is computed to be 16.11%. Based on these transition state structures and energies, the following photodissociation mechanisms are suggested: at 266, 248, 193 nm, CD3CH2F→absorption of a photon→TS5→the formation of the major product CD2CH2+DF; at 157 nm, CD3CH2F→absorption of a photon→D/F interchange of TS1→CDH2CDF→H/F interchange of TS2→CHD2CHDF→the formation of the major product CHD2+CHDF.

An accurate full-dimensional H4O potential energy surface and dynamics of an exchange reaction.

H2O and H2 are ubiquitous in the universe and their collisions play a crucial role in astrophysical processes. The reaction between the two also provides a prototype for understanding dynamics through four-center transition states. In this work, we adopted a strategy of combing two ab initio methods, CCSD(T)-F12a/AVTZ and MRCI-F12 + Qrot/AVTZ, to provide a balanced description for all regions of the configuration space, including one hydrogen exchange channel and two dissociation channels, namely H2 + H2O, H + H + H2O, and H + OH + H2. About 40 500 points were sampled to cover all dynamically relevant space, and the permutation invariant polynomial-neural network (PIP-NN) method was employed to develop a high-precision full-dimensional potential energy surface (PES), with a total fitting error of 0.055 kcal mol-1. Using a quasi-classical trajectory (QCT) method, the reaction dynamics and mode specificity of the hydrogen exchange channel were studied on the PES. It has been found that the stretching mode of H2, the bending, the symmetric stretching, the asymmetric stretching mode of H2O, and the translational mode can all promote reactivity. The strongest promotion effect comes from the H-H stretching mode. The Sudden Vector Projection (SVP) model was applied to predict mode specificity effects and rationalize the product energy partitioning. In both cases, the QCT and SVP results are generally consistent with each other. Furthermore, the hydrogen exchange channel was found to be dominated by sideways scattering.

Dissecting Transmetalation Reactions at the Molecular Level: Role of the Coordinated Anion in Gas-Phase Models for the Transmetalation Step of the Hiyama Cross-Coupling Reaction

Palladium-catalyzed cross-coupling protocols have become a cornerstone in organic synthesis. Here, a gas-phase model of the Hiyama cross-coupling reaction was designed to shed light on the roles of coordinated anions (fluoride versus chloride) in transmetalation from Si to Pd. A combination of mass spectrometry experiments and DFT calculations was used. The ligated palladium fluoride and chloride cationic complexes, [(phen)Pd(X)]+ (X = F and Cl), readily react with vinyltrimethylsilane, Me3Si(CH═CH2), via transmetalation to give [(phen)Pd(CH═CH2)]+ as the major product. DFT calculations reveal that this transmetalation reaction is concerted and proceeds via a four-centered transition state, illustrating the role of coordinated halide in this gas-phase system. Two minor side products are observed corresponding to transmetalation to give [(phen)Pd(CH3)]+ and [(phen)Pd(SiMe2X)]+. DFT calculations suggest that these arise from the same initial Si to Pd methyl transmetalation pathway to give the [(phen)Pd(CH3) + Me2(CH═CH2)SiX]+ intermediate, which either then loses Me2(CH═CH2)SiX or reacts via C–C bond coupling to ultimately yield propene and [(phen)Pd(SiMe2X)]+. [(phen)Pd(CH═CH2)]+ undergoes a reaction with a second molecule of vinyltrimethylsilane to form an adduct, which upon collision-induced dissociation liberates 1,3-butadiene to form [(phen)Pd(SiMe3)]+. DFT calculations suggest a mechanism in which C–C bond formation is followed by migration of SiMe3 from C to Pd. Links between the observed gas-phase chemistry and solution-phase Pd-mediated homocoupling reactions of vinyltrimethylsilanes are explored.

Mechanism of Reactions of 1-Substituted Silatranes and Germatranes, 2,2-Disubstituted Silocanes and Germocanes, 1,1,1-Trisubstituted Hyposilatranes and Hypogermatranes with Alcohols (Methanol, Ethanol): DFT Study.

The mechanism of reactions of silatranes and germatranes, and their bicyclic and monocyclic analogues with one molecule of methanol or ethanol, was studied at the Density Functional Theory (DFT) B3PW91/6-311++G(df,p) level of theory. Reactions of 1-substituted sil(germ)atranes, 2,2-disubstituted sil(germ)ocanes, and 1,1,1-trisubstituted hyposil(germ)atranes with alcohol (methanol, ethanol) proceed in one step through four-center transition states followed by the opening of a silicon or germanium skeleton and the formation of products. According to quantum chemical calculations, the activation energies and Gibbs energies of activation of reactions with methanol and ethanol are close, their values decrease in the series of atranes–ocanes–hypoatranes for interactions with both methanol and ethanol. The reactions of germanium-containing derivatives are characterized by lower activation energies in comparison with the reactions of corresponding silicon-containing compounds. The annular configurations of the product molecules with electronegative substituents are stabilized by the transannular N→X (X = Si, Ge) bond and different intramolecular hydrogen contacts with the participation of heteroatoms of substituents at the silicon or germanium.

Stereoselective Bromoboration of Acetylene with Boron Tribromide: Preparation and Cross-Coupling Reactions of (Z)-Bromovinylboronates.

The mechanism of acetylene bromoboration in neat boron tribromide was studied carefully by means of experiment and theory. Besides the syn-addition mechanism through a four-center transition state, radical and polar anti-addition mechanisms are postulated, both triggered by HBr, which is evidenced also to take part in the Z/E isomerization of the product. The proposed mechanism is well supported by ab initio calculations at the MP2/6-31+G* level with Ahlrichs' SVP all-electron basis for Br. Implicit solvation in CH2Cl2 has been included using the PCM and/or SMD continuum solvent models. Comparative case studies have been performed involving the B3LYP/6-31+G* with Ahlrichs' SVP for Br and MP2/Def2TZVPP levels. The mechanistic studies resulted in development of a procedure for stereoselective bromoboration of acetylene yielding E/Z mixtures of dibromo(bromovinyl)borane with the Z-isomer as a major product (up to 85%). Transformation to the corresponding pinacol and neopentyl glycol boronates and stereoselective decomposition of their E-isomer provided pure (Z)-(2-bromovinyl)boronates in 57-60% overall yield. Their reactivity in a Negishi cross-coupling reaction was tested. An example of the one-pot reaction sequence of Negishi and Suzuki-Miyaura cross-couplings for synthesis of combretastatin A4 is also presented.

Kinetic and mechanistic studies of the transformation of the catalyst, tris(pentafluorophenyl)borane, in the presence of silyl and germyl hydrides

Tris(pentafluorophenyl)borane catalyzed Si-H bond activation opens the door to numerous transition-metal-free reduction processes and is widely used in organic and polymer chemistry. However, chemical stability of B(C6F5)3 in the presence of silyl hydrides is limited, which can strongly affect its catalytic activity. Transformations of B(C6F5)3 in the presence of phenyldimethylsilane, triethylsilane and triethylgermane were studied by 19F NMR and UV spectroscopy, GC/MS and quantum-mechanical calculations. The observed exchange of pentafluorophenyl group attached to boron to hydrogen results in the formation of bis(pentafluorophenyl)borane, which has a strongly reduced ability to activate the Si-H bond. The substitution kinetics were studied by following the disappearance of absorption of the B(C6F5)3 charge transfer peak in the UV spectrum. Complementary quantum mechanical calculations allowed us to propose a mechanism of the ligand exchange reaction, which involves electrophilic substitution of the pentafluorophenyl group through a four-center transition state.

On the Role of Hydrogen Atom Transfer (HAT) in Thermal Activation of Methane by MnO+: Entropy vs. Energy

Abstract The temperature dependent kinetics and product branching fractions of first-row transition metal oxide cation MnO+ with CH4 and CD4 at temperatures between 200 and 600 K are measured using a selected-ion flow tube apparatus. Likely reaction mechanisms are determined by comparison of temperature dependent kinetics to statistical modeling along calculated reaction coordinates. The data is well-modeled with the reaction proceeding over a rate limiting four-centered transition state leading to an insertion intermediate, similar to reactions of NiO+ and FeO+, and showing characteristics of proton-coupled electron transfer (PCET). However, a more direct pathway traversing a transition state of hydrogen atom transfer (HAT) character to a hydroxyl intermediate is found to possibly be competitive, especially with increasing temperature. While uncertainties in calculated energetics limit quantitative assessment of the role of HAT at thermal energies, it is clear that this mechanism becomes increasingly prevalent in higher energy regimes.

DFT Studies on the Polymerization of Functionalized Styrenes Catalyzed by Rare-Earth-Metal Complexes: Factors Affecting C–H Activation Relevant to Step-Growth Polymerization

The polymerization mechanism of functionalized styrene derivatives, viz., p-NMe2 (pMNS)-, o-/p-OCH3 (o/pMOS)-, and p-SCH3 (pMTS)-substituted styrene, catalyzed by cationic rare-earth-metal catalysts has been comparatively studied through density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it is found that large steric hindrance as a main factor prevents oMOS from undergoing o-C–H activation relevant to step-growth polymerization, while the nonoccurrence of C–H activation of pMNS and pMTS can be explained by the relatively less dispersion of charge distribution in the four-center transition states. In addition, the electron-donating pyridine side arm weakened the interaction between pMOS and the metal center and meanwhile increased the steric hindrance, preventing the C–H activation. Therefore, the simultaneous occurrence of C–H activation (step-growth polymerization) and C═C insertion (chain-growth polymerization) are affected by multiple factors such ...

The Phosphinoboration of N‐Heterocycles

The addition of phosphinoboronate ester Ph2 PBpin (pin=1,2-O2 C2 Me4 ) (1) to a number of different N-heterocycles has been investigated. Reaction of 1 with pyridine resulted in highly selective formation of the corresponding 1,4-addition product, with addition of the electron-deficient Bpin group to the pyridine nitrogen atom and the phosphido group to the para carbon atom. Conversely, reactions of para-substituted pyridine derivatives occurred predominately to afford 1,2-addition products while quinoline reacted to afford the 1,2-adduct which ultimately isomerized to afford the corresponding 1,4-addition product. Preliminary computational studies have been undertaken to explore possible pathways for these transformations including transfer of the PPh2- anion from [B(PPh2 )2 pin]- to the 4-position of a borenium/boronium activated pyridine and concerted pathways for 1,2-addition via intramolecular nucleophilic attack of PPh2 at C2 of a Ph2 PBpin-coordinated pyridine via a four-centered transition state and intramolecular transfer of PPh2 to the 2-position of a boron-activated pyridine in a phosphido-bridged dimer involving a six-centered transition state.

Cl2 Elimination in 248 nm Photolysis of (COCl)2 Probed with Cavity Ring-Down Absorption Spectroscopy.

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time.

Alkene Insertions into a Ru–PR2 Bond

An unusually broad series of discrete alkene insertion reactions has provided the opportunity to examine the mechanism(s) of this fundamental carbon–heteroatom bond-forming process. Ethylene, electron-rich and electron-poor (activated) alkenes all react with the Ru–P double bond in Ru(η5-indenyl)(PCy2)(PPh3) to form κ2-ruthenaphosphacyclobutanes. Thermal decomposition of these metallacycles in solution, via alkene deinsertion and β-hydride elimination, is particularly favored for electron-rich alkenes, and hydride-containing decomposition products are implicit intermediates in the observed isomerization of 1-hexene. Kinetic studies, including a Hammett analysis of the insertion reactions of para-substituted styrenes, suggest that two distinct inner-sphere pathways operate for the insertion of electron-rich versus activated alkenes. DFT analyses have identified one pathway involving simple cycloaddition via a four-centered transition state and another that proceeds through an η2-alkene intermediate. Such an intermediate was observed spectroscopically during formation of the ethylene metallacycle, but not for substituted alkenes. We propose that “pre-polarized”, activated alkenes participate in direct cycloaddition, while rate-determining η2-adduct formation is necessary for the activation of electron-rich alkenes toward migratory insertion into the Ru–P bond.

Un-catalyzed peptide bond formation between two monomers of glycine, alanine, serine, threonine, and aspartic acid in gas phase: a density functional theory study

In the present report, un-catalyzed peptide bond formation between two monomers of glycine (Gly), alanine (Ala), serine (Ser), threonine (Thr), and aspartic acid (Asp) has been investigated in gas phase via two steps reaction mechanism and concerted mechanism at B3LYP/6-31G(d,p) and M062X/6-31G(d,p) level of theories. The peptide bond is formed through a nucleophilic reaction via transition states, TS1 and TS2 in stepwise mechanism. The TS1 reveals formation of a new C-N bond while TS2 illustrate the formation of C=O bond. In case of concerted mechanism, C-N bond is formed by a single four-centre transition state (TS3). The energy barrier is used to explain the involvement of energy at each step of the reaction. The energy barrier (20–48 kcal/mol) is required for the transformation of reactant state R1 to TS1 state and intermediate state I1 to TS2 state. The large value of energy barrier is explained in terms of distortion and interaction energies for stepwise mechanism. The energy barrier of TS3 in concerted mechanism is very close to the energy barrier of the first transition state (TS1) of the stepwise mechanism for the formation of Gly-Gly and Ala-Ala di- peptide. However, in case of Ser-Ser, Thr-Thr and Asp-Asp di-peptide, the energy barrier of TS3 is relatively high than that of the energy barrier of TS1 calculated at B3LYP/6-31G(d,p) and M062X/6-31G(d,p) level of theories. In both the mechanisms, the value of energy barrier calculated at B3LYP/6-31G(d,p) level of theory is greater than that of the value calculated at M062X/6-31G(d,p) level of theory.

ChemInform Abstract: Synthesis of α,β‐Unsaturated N‐Aryl Ketonitrones from Oximes and Diaryliodonium Salts: Observation of a Metal‐Free N‐Arylation Process.

An efficient transition-metal-free method for the preparation of α,β-unsaturated N-aryl ketonitrones under mild conditions has been developed. This reaction shows good functional group tolerance for both electron-rich and electron-deficient substituents on both oximes and diaryliodonium salts. Two examples of gram-scale preparations have been realized in good yields. Further transformations of these nitrones to different N-heterocycles have been demonstrated. DFT calculations suggest that N-arylation products are formed by [1,3]-phenyl migration of an O-coordinated oximate complex via a four-centered transition state, while the O-arylation products are formed by [1,3]-phenyl migration of a N-coordinated oximate complex.

Synthesis of α,β-Unsaturated N-Aryl Ketonitrones from Oximes and Diaryliodonium Salts: Observation of a Metal-Free N-Arylation Process.

An efficient transition-metal-free method for the preparation of α,β-unsaturated N-aryl ketonitrones under mild conditions has been developed. This reaction shows good functional group tolerance for both electron-rich and electron-deficient substituents on both oximes and diaryliodonium salts. Two examples of gram-scale preparations have been realized in good yields. Further transformations of these nitrones to different N-heterocycles have been demonstrated. DFT calculations suggest that N-arylation products are formed by [1,3]-phenyl migration of an O-coordinated oximate complex via a four-centered transition state, while the O-arylation products are formed by [1,3]-phenyl migration of a N-coordinated oximate complex.

Mechanistic Insights into Grubbs-Type Ruthenium-Complex-Catalyzed Intramolecular Alkene Hydrosilylation: Direct σ-Bond Metathesis in the Initial Stage of Hydrosilylation

Grubbs-type ruthenium-complex-mediated intramolecular alkene hydrosilylation of alkenylsilyl ethers has been developed to provide cyclic silyl ethers with high regioselectivity. This non-metathetical use of such ruthenium complexes for alkene hydrosilylation via preferential Si–H bond activation over alkene activation is notable, where the competing alkene metathesis dimerization was not detected. In addition to the synthesis of organosilicon heterocycles from readily available olefins, this study provides fundamental mechanistic insights into the non-metathetical function of Grubbs-type ruthenium catalysts. In the initial stage of hydrosilylation within a ruthenium coordination sphere, evidence for activation of a ruthenium complex by direct σ-bond metathesis between Si–H and Ru–Cl via a four-centered transition state is presented. This study counters the traditionally accepted Chauvin-type mechanism, specifically the addition of R3Si–H across the π-bond of a Ru-benzylidene.

The ligand effect on the selective C–H versus C–C bond activation of propane by NiBr+: a theoretical study

Density functional theory has been employed to investigate the ligand effect in the reaction of ligated NiBr+ with propane. Both initial C–H and C–C bond activation mechanisms for losses of HBr, H2, and CH4 are analyzed in terms of the topology of the potential energy surface. Losses of HBr and H2 involve three C–H activation mechanisms, that is, α,β-H, α,γ-H, and β,α-H abstractions, where the last β,α-H abstraction is the most favorable mechanism. Loss of CH4 involves initial C–C activation, but it is prevented by the high-energy barrier. When propane reacts with the open-shell ligated NiBr+, the ligand of Br in the initial C–H activation could direct abstract a H atom from propane substrate via a four-center transition state, without forming multi-σ-type bonding of Ni+, whereas the metal center in the initial C–C activation needs to experience an unfavorable three σ-type bonding (with Br, CH3, and CH2CH3), which explains why HBr and H2 are formed in the reaction of BrNi+/C3H8 and CH4 not.

Intermolecular hydroamination of vinylarenes by iminoanilide alkaline-earth catalysts: a computational scrutiny of mechanistic pathways.

A thorough computational exploration of the mechanistic intricacies of the intermolecular hydroamination (HA) of vinylarenes by a recently reported class of kinetically stabilised iminoanilide [{N^N}Ae{N(SiMe3)2}⋅(THF)n] alkaline-earth amido compounds (Ae = Ca, Sr, Ba) is presented. Two distinct mechanistic pathways for catalytic HA mediated by alkaline-earth and rare-earth compounds have emerged over the years that account equally well for the specific features of the process. On one hand, a concerted proton-assisted pathway to deliver the amine product in a single step can be invoked and, on the other, a stepwise σ-insertive pathway that comprises a rapid, reversible migratory olefin insertion step linked to a less facile, irreversible Ae-C alkyl bond aminolysis. The results of the study presented herein, which employed a heavily benchmarked and reliable DFT methodology, supports a stepwise σ-insertive pathway that involves fast and reversible migratory C=C bond insertion into the polar Ae-N pyrrolido σ bond. This proceeds with strict 2,1 regioselectivity via a highly polarised four-centre transition state (TS) structure, linked to irreversible intramolecular Ae-C bond aminolysis of the alkaline-earth alkyl intermediate as the energetically favourable mechanism. Turnover-limiting aminolysis is consistent with the significant KIE measured; the DFT-derived effective barrier matches the Eyring parameter empirically determined for the best-performing {N^N}Ba(NR2) catalyst gratifyingly well. It also predicts the observed trend in reactivity (Ca<Sr<Ba) correctly and the computationally estimated primary KIE is close to the observed values. Non-competitive kinetic demands militate against the operation of the alternative concerted proton-assisted pathway, which describes N-C bond formation triggered by concomitant amino proton transfer at the C=C linkage via a multi-centre TS structure. A detailed comparison of {N^N}Ae(NR2) catalysts revealed that the variation in the Ae-pyrrolido bond strength together with the degree of protection of the alkaline earth by a sterically encumbering iminoanilide ligand scaffold not only profoundly influences the performance in HA catalysis, but also the likelihood of traversing rival mechanistic pathways.

Roaming dynamics in radical addition-elimination reactions.

Radical addition-elimination reactions are a major pathway for transformation of unsaturated hydrocarbons. In the gas phase, these reactions involve formation of a transient strongly bound intermediate. However, the detailed mechanism and dynamics for these reactions remain unclear. Here we show, for reaction of chlorine atoms with butenes, that the Cl addition-HCl elimination pathway occurs from an abstraction-like Cl-H-C geometry rather than a conventional three-centre or four-centre transition state. Furthermore, access to this geometry is attained by roaming excursions of the Cl atom from the initially formed adduct. In effect, the alkene π cloud serves to capture the Cl atom and hold it, allowing many subsequent opportunities for the energized intermediate to find a suitable approach to the abstraction geometry. These bimolecular roaming reactions are closely related to the roaming radical dynamics recently discovered to play an important role in unimolecular reactions.

Addition Reaction of Cyclopropane with Magnesium Dihydride (MgH2): A Theoretical Study

The addition reaction of cyclopropane with <TEX>$MgH_2$</TEX> has been investigated using the B3LYP density functional method employing several split-valence basis sets. Both along the and perpendicular to the cyclopropane ring approach has been reported. It is shown that the reaction proceeds via a four-centered transition state. Calculations at higher levels of theory were also performed at the geometries optimized at the B3LYP level, but only slight changes in the barriers were observed. Structural parameters for the transition state are also reported.