Selection Rules for Cycloaddition Reactions

From a more detailed analysis of MO and state correlation diagrams. Woodward and Hoffmann presented a set of selection rules for cycloaddition reactions, which are summarized in Table 11.1. ° Here p and q are the number of electrons in the two n systems imdergoing the cycloaddition reaction. When the sum of p and g is a member of the 4n series, then the reaction is thermally allowed to be suprafacial with respect to one of the n components and antarafacial with respect to the other one. When the sum of p and qisa member of the 4n -h 2 series, then Ihe reaction is thermally allowed when it is either suprafacial with respect to both components or antarafacial with respect to both. As usual, the selection rules are reversed for photochemical reactions. 

The Diels-Alder reaction may be the best known cycloaddition, but other types of cycloaddition reactions are also synthetically important. Consistent with the predictions of the selection rules, the [ 6 -f 4J cycloaddition was seen in the reaction of cyclopentadiene with tropone (61)  

The [ 8j + cycloaddition was observed in the addition of 8,8-dimethyl-isobenzofulvene (62) to dimethyl fumarate (63) ° °  

The [ 85 + 65] cycloaddition was seen in the addition of 8,8-dimethylisoben-zofulvene to tropone °  

Arynes may also participate in cycloaddition reactions. As shown in equation 11.25, thermal decomposition of benzenediazonium-2-carboxylate (64) leads to benz5me (65), which imdergoes the Diels-Alder reaction with the diene 66 to produce 67. 

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The selection rules for cycloaddition reactions can also be derived from consideration of the aromaticity of the transition state. The transition states for [2tc -f 2tc] and [4tc -1- 2tc] cycloadditions are depicted in Fig. 11.11. For the [4tc-1-2tc] suprafacial-suprafacial cycloaddition, the transition state is aromatic. For [2tc -F 2tc] cycloaddition, the suprafacial-suprafacial mode is antiaromatic, but the suprafacial-antarafacial mode is aromatic. In order to specify the topology of cycloaddition reactions, subscripts are added to the numerical classification. Thus, a Diels-Alder reaction is a [4tc -f 2 ] cycloaddition. The... 

The selection rules for cycloaddition reactions can also be derived from considering the basis set orbitals from which the transition state for the cycloadditions would arise (Fig. 10.11). For [4+2]-suprafacial addition, the transition state is aromatic for [2+2]-suprafacial addition, it is antiaromatic. On the other hand, a [2+2]-addition that is antarafadal in one component is an allowed process. 

Under photochemical conditions, however, the reaction can take place because the symmetry of the excited-state HOMO is the opposite of the ground-state HOMO. Therefore, overlap of the excited-state HOMO of one alkene with the LUMO of the second alkene involves symmetry-allowed suprafacial bond formation. The selection rules for cycloaddition reactions are summarized in Table 28.3. 

To apply the selection rules for cycloadditions, add the number of tt electrons from each component undergoing reaction and then apply the rules oudined in Table 6.2. 

The selection rules for cycloadditions (Table 11.1) apply to pericyclic reactions described as cycloadditions, but the rules may be generalized concise form as follows " ... 

Electrocyclic and Sigmatropic reactions can be considered in terms of cycloadditions. It has been shown that it is possible if we consider sigma bond as a component of cycloaddition. Selection rules for cycloadditions can be applied to electrocyclic as well as sigmatropic reactions. Involvement of obond in these reactions is possible in following manner ... 

Cycloadditions with 1,3-cyclohexadienes proceeding with very low activation energies (Table IV) bear a close relationship to thermal Diels-Alder reactions (see ref. 5 and references cited therein). Hoffmann and Woodward237 have developed selection rules for thermal and photochemical concerted cycloaddition reactions according to which Diels-Alder reactions can occur in a concerted fashion with singlet ground-state... 

This type of condensation is of great interest in connection with the Woodward-Hoffmann selection rules for symmetry-allowed concerted suprafacial and antarafacial cycloaddition reactions.284 The generalized rules for cycloaddition of an m- to an n-electron system predict that the concerted supra-supra or antara-antara dimerization is allowed in the excited state (i.e., photochemically) when m + n = 4q, and in the ground state (i.e., thermally) when to + n = 4q + 2, where to and n are the numbers... 

It has become clear from the Woodward-Hoffmann-rules how orbital symmetry controles in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction 239>. For cycloaddition reactions of a m-ji-electron system to a M-jr-electron molecule the following stereochemical selection rules have been established (q = 0,1,2,...) ... 

The selection rules for [tt4 + tt2 ] and other cycloaddition reactions can also be derived from consideration of the aromaticity of the TS3 In this approach, the basis set p orbitals are aligned to correspond with the orbital overlaps that occur in the TS. The number of nodes in the array of orbitals is counted. If the number is zero or even, the system is classified as a Htickel system. If the number is odd, it is a Mobius system. Just as was the case for ground state molecules (see p. 716), Htickel systems are stabilized with 4 + 2 electrons, whereas Mobius systems are stabilized with 4n electrons. For the [tt4 + tt2] suprafacial-suprafacial cycloaddition the transition state is aromatic. 

The selection rules that determine the outcome of electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements are summarized in Tables 29.1, 29.3, and 29.4, respectively. This is still a lot to remember. Fortunately, the selection rules for all pericyclic reactions can be summarized in one word TE-AC. ... 

We have developed a different set of selection rules for electrocyclic, sigma-tropic, cycloaddition, and other concerted reactions, but the fundamental principle—the conservation of orbital symmetry—is the same in all cases. Now we will see that all of these reactions can be considered to be variants of cycloaddition reactions. To do so, we must first note that a cr bond can participate in a cycloaddition process, just as can a n bond, with the following provisions ... 

The Woodward—Hofmann selection rules for the generalized case are the same as for cycloaddition reactions (1) For supra—supra or an antara—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p + q = 4n -I- 2, and it is photochemically allowed when p + q = 4n. (2) For supra—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p -I- q = 4n, and it is photochemically allowed when p - - q = 4n - - 2. 

Woodward RB, Hoffmann R (1965) Selection rules for concerted cycloaddition reactions. J Am Chem Soc 87 2046—2048... 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

If the analogy that is drawn between the Si=Si dimer on the Si(100)-2 x 1 surface and an alkene group is reasonable, then certain parallels might be expected to exist between cycloaddition reactions in organic chemistry and reactions that occur between alkenes or dienes and the silicon surface. In other words, cycloaddition products should be observed on the Si(100)-2 x 1 surface. Indeed, this prediction has been borne out in a number of studies of cycloaddition reactions on Si(100)-2x1 [14], as well as on the related surfaces of Ge(100)-2 x 1 (see Section 6.2.1) and C(100)-2 x 1 [192-195]. On the other hand, because the double-bonded description is only an approximation, deviations from the simple picture are expected. A number of studies have shown that the behavior differs from that of a double bond, and the asymmetric character of the dimer will be seen to play an important role. For example, departures from the symmetry selection rules developed for organic reactions are observed at the surface. Several review articles address cycloaddition and related chemistry at the Si(100)-2 x 1 surface the reader is referred to Refs. [10-18] for additional detail. 

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