Diels-Alder reaction topology

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... 

After a brief discussion of the notion of molecular topology and the analogy principle as related to topology/reactivity relationships more recent developments in the field of reactivity indices for polynuclear benzenoid hydrocarbons are reviewed. Reaction mechanisms and correlations of reactivity indices with rates of electrophilic substitution and Diels-Alder reactions, thermally induced polymerization, and biochemical transformations of benzenoid hydrocarbons are discussed. 

The Diels-Alder reaction of benzenoid aromatic hydrocarbons with dienophiles was discovered more than fifty years ago [61, 62]. A classical example is the reaction of anthracene with maleic anhydride (Scheme 5). Although this type of reaction, termed endogenic or endocyclie Diels-Alder reaction, could be expected to be particularly well suited for correlating structure (topology) of benzenoid hydrocarbons with kinetic data, the problem has been systematically studied only very recently. Biermann and Schmidt in a series of publications [12, 29, 45, 63, 64] reported second-order rate constants (k2), measured under standard conditions (1,2,4-trichlorobenzene, 91.5 + 0.2 °C), for the endocyclie Diels-Alder reaction between maleic anhydride and 102 benzenoid hydrocarbons. Each rate constant was measured twice, the values usually... 

The chirality at sulfur also confers to (Z)-3-carbonylmenthoxy vinyl sulfoxides a useful topological bias which will be discussed in context with asymmetric Diels-Alder reactions (Section 4.1.6.2.1 (vi)). 

Figure 13.18 The Evans principle applied to the Diels-Alder reaction and the [2-l-2]cycloaddition. The delocalized system of electrons in the transition state of the Diels-Alder reaction is topologically equivalent to the 3t system of benzene. The transition state is stabilized by aromaticity, and therefore the reaction is thermochemically...

Figure 13.21 Ring current in the transition structure of the Diels-Alder reaction (ethy-leneH-butadiene cyclohexene), (a) the six basis orbitals exhibit a cyclic overlap that is topologically equivalent with the % system of delocalized electrons In benzene.

The basis of this concept [32] is a simple parallel intuitively felt by Evans [154], between the ease of certain reactions and the arrangement of corresponding transition states. Thus, e.g., the ease of a majority of Diels-Alder reactions is related to the fact that transient structure created by approaching the diene and dienophilic components is isoconjugated, or in other words, topologically equivalent, with the aromatic benzene and as a such should be therefore stabilized, at least in part, as the benzene itself. This simple idea was revived by Dewar [32] who also generalized it into the form of simple rule that (thermally) allowed reactions proceed via aromatic transition states. The proposed theoretical justification of the above criterion arises from a simple idea of direct quantitative evaluation of the resemblance of electron structure of expected transition states with the appropriate aromatic standards. The quantitative measure of this resemblance is the similarily index (102), where Q and ref represent the density matrices of the expected transition state and the appropriate reference standard respectively. 

Cycloaddition processes can be described by a symbolism which describes the type and number of electrons involved in the reaction and the topology of the reaction. Thus a Diels-Alder reaction is a [477 + 277 ] process, signifying addition of a four-77-electron and a two-77-electron system, with both sets of orbitals reacting in a suprafacial mode. The allowed 2 + 2 addition would be described as 2iTs + 277 ]. 

Some novel C60-based assemblies were recently synthesized by [4+2] Diels-Alder cycloaddition reactions. Thus, fused tetrathiafulvalene-C60 dyads and C60-tetra-thiafulvalene-C60 dumbbell triads, in which the fullerene acceptor is doubly tethered to the donor tetrathiafulvalene through a rigidified cyclohexene ring [108], were prepared. With this novel approach, control of the relative orientation as well as the distance between the donor and acceptor units was achieved. Thereby, through-space interactions were expected to dominate because of the special topology of the constructed molecules. More examples of such donor-acceptor hybrid systems are discussed in the appropriate following sections, with their potential use in innovative technological applications. 

Now, we may consider the TS structure for a Diels-Alder cycloaddition reaction. The Huckel TS structure of zero node is aromatic in nature. Thus, the reaction takes place in suprafacial mode following the Huckel topology, whereas in Mobius... 

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