This already departs from the four‐arrow textbook mechanism, where the electron pair of the O−H σ bond is conventionally used for the C−O bond formation in the epoxide, matching instead the five‐arrow variation. The changes in dipole moment, as indicators of hidden ionic intermediates at the proton transfer stages, also reflect this (Figure 4). Please check your email for instructions on resetting your password. In the present case we can see in group I that the two C−O bonds in the epoxide are not formed in a synchronous fashion. We will demonstrate that a full description of the changes to bonding for the epoxidation reaction of alkenes using peracids is possible using IBOs. We may use the phenyl anion to make this point clear. Fax: +65 6779 1936 Whilst Robinson is generally credited with introducing the formalism, the initial example1 was restricted to illustrating resonance in hexatriene and it is the better‐known second attempt in 19242 to explain a reaction outcome that more closely resembles the modern mechanistic usage. In summary, we have demonstrated that the standard textbook mechanism for e. g. the closed shell epoxidation of an alkene by a peracid can be placed on a firmer theoretical foundation by deriving and analysing intrinsic bond orbitals (IBOs) along the reaction path. Submitted by Germán Fernández on Sun, 07/15/2012 - 16:17. The study first exhibits the images of σ* orbital of the peroxo bonds in neutral or protonated peroxy acids. Fetching data from CrossRef. Use the link below to share a full-text version of this article with your friends and colleagues. If we accept this connection, there are some important conclusions that can be made. In order to probe if an IBO and therefore a bond/lone pair is involved in a bond making or breaking event, we compute and plot the quantity orbital change (see Figure, However, the orbitals really are three‐dimensional functions φ(x,y,z), and the two‐dimensional iso‐surface representation is but one of multipe possibilities of condensing important parts of information they carry into a form accessible to human understanding. We should be able to probe our observation by introducing substituents other than Me on the alkene. A variety of alkenes was oxidized with this system, giving 87–95% analytical yield of the corresponding epoxides. Here we focus on applying IBOs to the epoxidation reaction shown in Scheme 1, including the evolution of σ and π bonding and showing how the results can be mapped to the use of curly arrows. Here we focus on applying IBOs to the epoxidation reaction shown in Scheme 1, including the evolution of σ and π bonding and showing how the results can be mapped to the use of curly arrows. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. The argument for this is simply that the orientation of this lone pair is perpendicular to the 6‐electron π system of the aromatic ring. Such grouping or choreography can be used to tease out other features of non‐synchronous reactions such as hidden ionic intermediates and the role of substituents on the alkene or acid. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Curly arrows for the epoxidation of propene by peracetic acid as derived from the IBO analysis. Whilst most practitioners of curly arrows tend to assume they represent just a convenient formalism,4 efforts continue to extract more formal curly arrow descriptions from quantum chemistry calculations.5. Intrinsic reaction coordinate for reaction of propene with peracetic acid at the M06‐2X/Def2‐TZVPPD/CPCM(CH2Cl2) level of theory showing (a) the energy profile, (b) the gradient norm profile illustrating the “hidden intermediate” at IRC=1.5 (see red arrow) and (c) the dipole moment response. and you may need to create a new Wiley Online Library account. It is crucial to appreciate that the major differences are a result of the orientation of bonds and Ione pairs featuring π symmetry and their changes. This procedure reveals the importance of including transformations between σ and π bonds. Overall, these changes identified by the IBO analysis of the electron flow only partially agree with conventional textbook mechanisms. This latter statement inadvertently renders curly arrows a formalism, which they do not have to be. Any queries (other than missing content) should be directed to the corresponding author for the article. For this particular mechanism at times the end‐point of the curly arrow indicating the formation of a given bond can differ such that the termination of (some) arrow heads ends directly at an atom (Scheme 1(b)).4, 8 An alternative depiction (Scheme 1(c))9 involves instead five curly arrows via oxygen lone pair participation, thus recognising the differing nucleophilic character of an oxygen lone pair versus a O−H covalent bond. Working off-campus? Instructions for using Copyright Clearance Center page for details. This approach has been proposed by Vidossich and Lledós.5a, 17 If this is done for all the IBOs computed along an IRC, then one obtains the loci of the complete curve representing that electron pair transformation and hence the curvature of what, with the addition of an arrow head indicating the direction of the reaction, would formally constitute a curly arrow (Figure 5).


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