Pericyclic eactions page 29 4 ELECTIO ULE and OBITL YMMETY We are now familiar with the terminology of cycloadditions and the selection rules for 'symmetry allowed' and 'symmetry forbidden' reactions based on the properties of FMOs (section 3.5). We will now take a look at the way these work in practice. 4.1 Cycloadditions exemplified 4.1.1 llyl cation with alkene [π2s π2s] This is a 4-electron (i.e. 4n) combination. The possible OMO- combinations are mismatched, so a concerted supra-supra reaction is 'forbidden.' OMO OMO OMO 4.1.2 llyl anion with alkene [π4s π2s] This is a 6-electron (i.e. 4n 2) combination, so the OMO- combinations are now matched for a concerted supra-supra process. lthough such reactions are 'allowed,' the conditions can make it difficult to prove that they are not stepwise in nature (030 is probably concerted). ote that the dominant interaction is of the OMO of the nucleophile (electrondonor) and the of the electron-acceptor component. OMO OMO OMO 030 TF 45 C then 2 O 40%
Pericyclic eactions page 30 4.1.3 llyl cation with diene [π4s π2s] This is another 4n 2 combination with the OMO- pairs matched for a concerted supra-supra process. In example 032 the cation is made from the corresponding iodoalkene by treatment with a silver salt. OMO OMO OMO 032 C 2 Cl 2 O 2 50 C to T 40% 4.1.4 Triene with diene [π6s π4s] This is a 10-electron (i.e. 4n 2) combination, so the OMO- combinations are matched for a concerted supra-supra process. In example 024 the π components are both cyclic, and therefore 'locked' in the conformation required for reaction. OMO OMO OMO 024 O T 3 days O exo
Pericyclic eactions page 31 4.1.5 Tetraene with alkene [π8s π2s] This 10-electron (i.e. 4n 2) combination is highly uncommon, but the orbitals are correctly matched for a concerted supra-supra process. In example 031 the reaction again benefits from the tetraene being 'locked' in the conformation required for reaction. OMO OMO OMO 031 CO 2 CO 2 C 2 Cl 2 20 C 40% CO 2 CO 2 4.1.6 One of a kind [π14a π2s] This 16-electron (i.e. 4n) combination is unique. For concerted bonding the orbital symmetry requirement is for one of the π systems to react anatarafacially and the other suprafacially. For 023 it was shown (by X-ray crystallography) that the 14π system, heptafulvalene, is the antarafacial component. Models confirm that heptafulvalene is not flat, and it is apparent that the conformational requirements of the [π14a π2s] reaction allow it to compete successfully with the various alternative [π4s π2s] and [π8s π2s] cycloaddition modes. OMO C groups omitted 023 C C C C [14 2] antara-supra C C C C
Pericyclic eactions page 32 4.2 Electrocyclic reactions a formal introduction Electrocyclic reactions are pericyclic reactions in which a ring is formed or broken. We will analyse one specific case in detail in pericyclic reactions once you know how to analyse one specific case of a particular reaction type, predicting the others is easy. It is also easier to understand a thermal reaction using the familiar OMO rather than that of the excited state. hould a reaction turn out to be thermally forbidden, then it will be photochemically allowed. B light B heat B X Y disrotatory ring-opening Y X conrotatory ring-opening Y X ecall that pericyclic reactions are reversible, and as this is a thermal reaction we expect the ring-opened diene to be favored because a cyclobutene ring is quite strained. owever, it does not matter which reaction we use for our mechanistic analysis ring-opening or ringclosure we must get the same result. The thermolysis of cis-3,4-dimethylcyclobutene has been thoroughly studied and it is stereospecific; the sole product is (E,Z)-2,4-hexadiene. 005 175 C (E) (Z) <0.1% of the (E,E)-isomer If we analyse the product we can immediately see the reason for this selectivity. There's only one reacting component, so we don't have to worry about a OMO pair. We simply use the OMO of the diene. In the ring-closure reaction the terminal p orbitals must combine to give a bonding sigma orbital, so this must involve in-phase overlap. The only way we can achieve this is to rotate them either both clockwise or both counterclockwise. When they rotate in the same sense, the process is classified as COOTTOY. eacting ends of the π system (E,E)-diene ψ 2 diene OMO when process is thermal conrotation groups trans The scheme above shows only the π orbitals whose overlap create the C(3) C(4) σ-bond of the cyclobutene. complete analysis of the electrocyclic process will reveal how the other orbitals of the starting material are transformed into those of the product, and provides us with an opportunity to see the Woodward-offmann approach to pericyclic reactions in operation.
Pericyclic eactions page 33 4.3 Orbital correlation diagrams for butadiene cyclobutene interconversion The selection rules shown in section 3.5, based on the electron count, provide a means of predicting whether a concerted supra-supra cycloaddition is allowed or forbidden. more fundamental principle the conservation of orbital symmetry that underpins all pericyclic reaction pathways was articulated in the 1960s by. B. Woodward and. offmann in a series of detailed analyses from which the Woodward-offmann rules emerged. The IUPC definition is: Conservation of orbital symmetry requires the transformation of the molecular orbitals of reactants into those of products to proceed continuously by following a reaction path along which the symmetry of these orbitals remains unchanged. We will see what this means for the cyclobutene-butadiene electrocyclic reaction. The procedure is as follows: 1. Identify any symmetry elements that are maintained throughout the course of the reaction. 2. List the orbitals in the usual order of increasing energy. 3. Draw the orbitals so as to show the signs of the coefficients. 4. Classify each of the orbitals with respect to the symmetry element. 5. Construct the correlation diagram following the principle that each orbital in the starting material must feed into an orbital of the same symmetry in the product. 6. Connect the orbitals of the starting material to those in the product that are the closest in energy and of the same symmetry. The FMO analysis above indicates that the thermal electrocyclic ring-opening of cyclobutene will proceed by conrotation, which preserves a C 2 axis of symmetry [running through the centres of the σ and π bonds] during the reaction. This is the symmetry element chosen in step 1, and it is an easy matter to classify each orbital in the diagram below according to its relationship with the C 2 axis: if the structure is unchanged after rotation through 180 then it is symmetric (), otherwise it is antisymmetric (). The correlation diagram indicates that the conrotatory mode is allowed (no symmetry-imposed energy barriers). σ conrotation symmetry with respect to C 2 axis ψ 4 π ψ 3 π ψ 2 σ ψ 1
Pericyclic eactions page 34 The correlation diagram for the disrotatory process is constructed as above, but this time the symmetry element maintained during the reaction is a mirror plane. The / correlations now have the π orbital of the cyclobutene evolving into the antibonding ψ 3 orbital of the diene, which would render the thermal disrotation symmetry forbidden. owever, under UV irradiation, which can induce the promotion of an electron to the next level, the reaction is allowed to proceed by disrotation. With one electron in the former of the starting material [as shown in the correlation diagram below], the disrotatory process is energetically neutral and therefore allowed (.B. for the photochemical reaction, ring closure is favoured). σ disrotation symmetry with respect to mirror plane ψ 4 π ψ 3 π ψ 2 σ ψ 1.B. the FMO approach, analysing only the OMO of the diene, gives the same prediction. eacting ends of the π system (E,E)-diene ψ 3 diene OMO when process is photochemical disrotation groups cis 006 (E) light (E) (compare this with 005!)