Selection rules cycloaddition

Termmology of Cycloadditions Selection Rules for Cycloadditions Secondary Interactions Cycloadditions of Charged Species Sigmatropic Rearrangements Termmology... 

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 ... 

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... 

This is because of retro-cycloaddition. In retro-cycloadditions, the reverse reactions are more favoured and the same selection rules apply. This is also because that X is normally a small inorganic molecule and of high thermodynamic stability. 

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... 

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. 

Verify that the selection rules found for the two-component cycloadditions (Section 11.3, p. 594) agree with the general pericyclic selection rule. What can be said about all-antara 2 + 2 + 2 +. .. cycloadditions About all-supra 2 + 2 + 2 +. .. cycloadditions ... 

There are few 6 + 2 cycloadditions known an example is given in Equation 12.33.87 The selection rules require either that the nitroso group enter... 

Cheletropic reactions are cyclizations - or the reverse fragmentations - of conjugated systems in which the two newly made o bonds terminate on the same atom. However, a cheletropic reaction is neither a cycloaddition nor a cycloreversion. The reason is that the chelating atom uses two AOs whereas in cycloadditions, each atom uses one and only one AO. Therefore, Dewar-Zimmerman rules cannot apply to cheletropic reactions. Selection rules must be derived using either FO theory or correlation diagrams 38 The conjugated fragment39 of 4n + 2 electron systems reacts in a disrotarory (conrotarory) mode in linear (nonlinear) reactions. In 4n electron systems, it reacts in a disrotarory (conrotarory) mode in nonlinear (linear) reactions. 

The rules based on the Hiickel-Mobius concept have their counterpart among the Woodward-Hoffmann selection rules. There was a marked difference between the suprafacial and antarafacial arrangements in the application of the Woodward-Hoffmann treatment of cycloadditions. The disrotatory and conrotatory processes in elec-trocyclic reactions presented similar differences. The suprafacial arrangement in both of the reacting molecules in the cycloaddition as well as the disrotatory ring closure in Figure 7-25 correspond to... 

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... 

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. 

Designate the following cycloadditions according to the number of PROBLEM 6.6 electrons contributed by each component. Is the stereochemistry shown in accordance with that predicted by the selection rules ... 

For sigmatropic reactions, as for electrocyclic reactions and cycloadditions, the course of reaction can be predicted by counting the number of electrons involved and applying the selection rules. A comprehensive rationalization of all the stereochemical aspects of these reactions requires application of the frontier orbital or orbital symmetry approaches, and, at this point, we will content ourselves with pointing out the salient features of the more common reactions of this class. 

The thermal desulfonylation of episulfones is highly stereospecific e.g., cis-2,3-dimethyl-thiirane-1,1-dioxide gives cw-2-butene , and trans- and cw-2,3-diphenyl thiirane 1,1-dioxides give trans and cis stilbenes, respectively . This result is remarkable in that the Woodward-Hoffman symmetry selection rules for inter-molecular cycloadditions (and their reversals) appear to exclude a concerted thermal decomposition of thiirane-l,l-dioxides . ... 

Cycloaddition reactions are transformations involving the fusion of open-chain substrates to cyclic products. Woodward and Hoffmann have divided all concerted cycloaddition reactions into allowed and forbidden categories defined by a complete set of selection rules (5). We address ourselves here to the catalytic operations required of a transition metal to switch the forbidden transformations to allowed. Our attention, therefore, will be directed exclusively to the forbidden reactions. Forbidden-to-allowed catalysis will be discussed as it applies to the simplest, and perhaps most important cycloaddition, the concerted, suprafacial, 1,2-addition of two olefins. 

Woodward and Hoffmann had been privately informed of the method of correlation diagrams 3> and published in the same issue of the same journal a study of concerted intermolecular cycloaddition reactions, for which selection rules were established by means of correlation diagrams for molecular orbitals 4>. 

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,...) ... 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

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. ... 

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