Cycloadditions symmetry forbidden

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

According to the Woodw ard-Hofmann rules the concerted thermal [2n + 2n] cycloaddition reaction of alkenes 1 in a suprafacial manner is symmetry-forbidden, and is observed in special cases only. In contrast the photochemical [2n + 2n cycloaddition is symmetry-allowed, and is a useful method for the synthesis of cyclobutane derivatives 2. 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

H-Azepines 1 undergo a temperature-dependent dimerization process. At low temperatures a kinetically controlled, thermally allowed [6 + 4] 7t-cycloaddition takes place to give the un-symmetrical e.w-adducts, e.g. 2.231-248-249 At higher temperatures (100-200°C) the symmetrical, thermodynamically favored [6 + 6] rc-adducts, e.g. 3, are produced. These [6 + 6] adducts probably arise by a radical process, since a concerted [6 + 6] tt-cycloaddition is forbidden on orbital symmetry grounds, as is a thermal [l,3]-sigmatropic C2 —CIO shift of the unsym-metrical [6 + 4] 7t-dimer. 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

The sterically unbiased dienes, 5,5-diarylcyclopentadienes 90, wherein one of the aryl groups is substituted with NO, Cl and NCCHj), were designed and synthesized by Halterman et al. [163] Diels-Alder cycloaddition with dimethyl acetylenedicarbo-xylate at reflux (81 °C) was studied syn addition (with respect to the substituted benzene) was favored in the case of the nitro group (90a, X = NO ) (syrr.anti = 68 32), whereas anti addition (with respect to the substituted benzene) is favored in the case of dimethylamino group (90b, X = N(CH3)2) (syn anti = 38 62). The facial preference is consistent with those observed in the hydride reduction of the relevant 2,2-diaryl-cyclopentanones 8 with sodium borohydride, and in dihydroxylation of 3,3-diarylcy-clopentenes 43 with osmium trioxide. In the present system, the interaction of the diene n orbital with the o bonds at the (3 positions (at the 5 position) is symmetry-forbidden. Thus, the major product results from approach of the dienophile from the face opposite the better n electron donor at the (3 positions, in a similar manner to spiro conjugation. Unsymmetrization of the diene % orbitals is inherent in 90, and this is consistent with the observed facial selectivities (91 for 90a 92 for 90b). 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

The details of symmetry forbidden reaction will be clear when we study the cycloaddition of ethene described later. 

So summarizing we see that in thermal [2 + 2] cycloaddition, a supra-supra process is geometrically possible but symmetry forbidden. But in supra-antar process, symmetry is allowed but geometrically difficult. Now we have to explain how the photochemical formation of cyclobutane takes place from 2n components. 

In this way Hoffmann and Woodward have established the following simple rule a concerted m + n cycloaddition will be symmetry allowed in the ground state and symmetry forbidden in the excited state if m + n = 4q + 2 (q = 0, 1, 2...) if m + n = 4q the reaction will be symmetry allowed in the excited state and symmetry forbidden in the ground state. This rule applies to ms + ns cycloadditions and to ma + na processes. 

The mechanism involves a [2 + 2] cycloaddition reaction between an alkene and a transition metal carbene (Scheme 10.13). In the absence of a transition metal carbene catalyst, the reaction between two alkenes is symmetry forbidden and only takes place photochemically. However, the d-orbitals on the metal catalyst (typically Grubbs s catalyst as shown in Scheme 10.13), break the symmetry and the reaction is facile. 

For reason of symmetry conservation, a thermal concerted [2-1-2]-cycloaddition is forbidden as far as D2h symmetry can be assumed for the activated complex. This is not necessarily the case for the concerted dimerization of iminoboranes, in which the symmetry requirements seem to be essentially lowered. Nevertheless, a two-step mechanism according to Eq. (21) must be taken into account. The assumed intermediate in Eq. (21) contains a sextet boron atom with a linear... 

Although thermal [2 + 2] cycloadditions are forbidden as concerted reactions by the orbital symmetry conservation rules the same structural features which promote intermolecular cy-cioadditions will also promote intramolecular reactions. In addition, the proximity between two alkene moieties dictated by the tether length and rigidity would make these processes entropically favorable. A few reports have documented thermal intramolecular cycloadditions to cyclopropenes and activated alkenes. The thermal Cope rearrangement of allylcyclopropenes apparently proceeds by a two-step mechanism in which intramolecular [2 + 2] adducts have been observed.72-73... 

Few syntheses of cyclobutanes by the five minus one strategy starting from a metallacyclopen-tane by elimination of the metal fragment have been reported. Such metallacarbocycles have been postulated as intermediates in the cyclodimcrization of alkenes to give cyclobutanes as an alternative mechanistic interpretation for the orbital symmetry forbidden thermal [2 -1- 2] cycloadditions of alkenes (Houben-Weyl, Vol. E18, pp 843 - 873). 

Draw the frontier orbital interactions for the all-suprafacial cycloaddition of an allyl anion to an alkene and for an allyl cation to a diene showing that they match, and show that the alternatives, allyl cation with alkene and allyl anion with diene are symmetry-forbidden. 

One requirement for a successful HOMO-LUMO interaction is that the symmetry of the HOMO must match the symmetry of the LUMO (either both symmetric or both antisymmetric). If so, then the interaction is symmetry allowed and will lead to productive cycloaddition. If file symmetries do not match, then the HOMO-LUMO overlap is symmetry forbidden and cycloaddition will not proceed. Molecular orbitals can be classified by their phase symmetry with respect to a plane normal to the n system. The symmetry is related to the number of nodal planes which occur in each individual molecular orbital. For the olefin component, file it orbital (HOMO) is symmetric with respect to this plane and file it orbital... 

We ascribe the increased stability of 162 to its bis-homoaromatic nature and to the fact that the increase in strain relative to 161 is minimized, as the two pivotal cyclopropane bonds are only partially formed. The cyclization of 161 must be fast even at - 50 °C the barrier is estimated to lie near 5 kcal/mol. The case of cycloaddition is surprising since it is formally of the [3 + 2]-type and, therefore, symmetry forbidden (vide supra). The driving force for the conversion 160 - 161 lies in the bis-homoaromatic stabilization of 161, and the conversion must have an early transition state [425]. 

A recent computational investigation of the [2+2] cycloaddition between a hypothetical molybdenum carbyne complex, CbMo CI I, and ethyne suggests that the formation of the molybdacyclobutadiene, CI3Mo(7/2-C3l l (), is allowed by orbital symmetry. In contrast, the direct formation of the isomeric molybdatetrahedrane (i.e., the if -cyclopropene complex), ChMoft -CsID, is symmetry forbidden <19930M1289> such complexes presumably arise by isomerization subsequent to initial molybdacyclobutadiene formation (Section 2.12.5). 

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