Thermally allowed reaction

The ground state bonding orbitals of cyclohexadiene is correlated with the ground state bonding orbitals of hexatriene and so it will be a thermally allowed reaction. 

The reason that thermally allowed reactions are photochemically forbidden and vice versa also becomes more readily apparent (van der Lugt and Oosterhoff, 1969). As we have just noted the existence of a large ground state barrier for a particular reaction (i.e. the reaction is thermally forbidden ) implies that there is another potential surface of the same symmetry that has a well not far above this barrier (as in Fig. 21a). Photoinduced reactions may occur through the funnelling of the reaction complex from the excited surface well on to the ground state surface in a radiationless transition. Since the... 

Figure 12.2. Three classifications of pericyclic reactions, with examples of thermally allowed reactions. Cheletropic is a special case of electrocyclic.

The bicyclo[4.4.1]undecane derivative 311 was obtained by [6+4] cycloaddition of 310 with a conjugated diene [72]. The [6+4] cycloaddition is a thermally allowed reaction, and free cycloheptatriene undergoes the [6+4] cycloaddition by heating. However, the [6+4] cycloaddition of cycloheptatriene coordinated by Cr(CO)3 proceeds at 0°C under irradiation. These results show the profound effect of coordination of Cr(CO)3 on reactivity. 

The HOMO of the excited ethylene molecule has the same symmetry as the LUMO of a ground-state ethylene. An excited molecule can react with a ground-state molecule to give cyclobutane (Figure 15-22). The [2 + 2] cycloaddition is therefore photo chemically allowed but thermally forbidden. In most cases, photochemically allowed reactions are thermally forbidden, and thermally allowed reactions are photochemically forbidden. 

The cycloadditions are supra-facial—they occur on one face only of each tc system—and for a thermally allowed reaction there should be An + 2 electrons in the mechanism, but An in a photochemical cycloaddition. These rules are dictated by orbital symmetry. 

Although these surface scans are approximate, and probably suffer somewhat from the requirement of D3h symmetry, they do reveal that this remarkably exothermic, and thermally allowed, reaction has an unusually high activation barrier. Acetylene is neither a good donor nor a good acceptor, and the approach of three acetylenes, even in a geometry which produces both in-plane and out-of-plane aromatic sextets, results in no strong HOMO-LUMO interactions. Repulsive interactions due to the overlap of filled orbitals of the three molecules occur, but the filled and vacant orbitals of the acetylenes are too far apart in energy for any appreciable stabi-... 

Give the product of this thermally allowed reaction. 

As discussed in Chapter 12, electrocyclic reactions may proceed in a conrotatory or disrotatory fashion, that is, the n system cyclizes in an antarafacial or suprafacial manner, respectively. Since there is only a single component, it should be counted according to the general component analysis for a thermally allowed reaction, that is, Ti rn = conrotatory and = disrotatory. Thus, the cyclization of butadienes... 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

Thus, the Diels-Alder reaction is a thermally allowed reaction in which the two components both add suprafacially and in which one has four n-electrons and one has two (a [4s + 2s] reaction). The Woodward-Hoffmann... 

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. 

Dewar [16] and Zimmerman [17] proposed selection rules based on the cyclic transition state. A Hiickel transition state is one in which there are zero or an even number of phase transitions perpendicular to the plane of the reaction and is the allowed condition for thermal cases in which there are 4n-n2 electrons. These reactions occur in a suprafacial manner. A Mobius transition state, in which there is one or an odd number of phase transitions perpendicular to the plane of the reaction, predicts thermally allowed reactions when there are An electrons involved. These reactions have an antarafacial component. The advantage of this transition state analysis is that all types of concerted reactions are covered by basically one selection rule. If a continuous loop of overlap through all the... 

TVansition state 1 node, 4 electrons, aromatic, thermally allowed reaction... 

The Diels-Alder (DA) reaction is classified as a [4 -f 2] and thermally allowed reaction, and the reverse reaction, namely retro-Diels-Alder (rDA) reaction, is also thermally allowed. 

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