Activation energy 2+4 -cycloadditions

The simplest of all Diels-Alder reactions cycloaddition of ethylene to 1 3 butadi ene does not proceed readily It has a high activation energy and a low reaction rate Substituents such as C=0 or C=N however when directly attached to the double bond of the dienophile increase its reactivity and compounds of this type give high yields of Diels-Alder adducts at modest temperatures... 

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... 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

In an investigation by Yamabe et al. [9] of the fine tuning of the [4-1-2] and [2-1-4] cycloaddition reaction of acrolein with butadiene catalyzed by BF3 and AICI3 using a larger basis set and more sophisticated calculations, the different reaction paths were also studied. The activation energy for the uncatalyzed reaction were calculated to be 17.52 and 16.80 kcal mol for the exo and endo transition states, respectively, and is close to the experimental values for s-trans-acrolein. For the BF3-catalyzed reaction the transition-state energies were calculated to be 10.87 and 6.09 kcal mol , for the exo- and endo-reaction paths, respectively [9]. The calculated transition-state structures for this reaction are very asynchronous and similar to those obtained by Houk et al. The endo-reaction path for the BF3-catalyzed reaction indicates that an inverse electron-demand C3-0 bond formation (2.635 A... 

The Lewis acid-catalyzed reaction of nitrone 21 with ethyl vinyl ether 22 (Scheme 8.8) was also investigated for BH3 and AlMe3 coordinated to 21 [32]. The presence of BH3 decreases the activation energy for the formation of 23 by 3.1 and 4.5 kcal mol to 9.6 kcal mol for the exoselective reaction and 11.6 kcal-mol for the endo-selective reaction, respectively, while the activation energy for the formation of 24 increases by >1.4 kcal mol , compared to those for the uncatalyzed reaction. The transition-state structure for the BH3-exo-selective 1,3-dipolar cycloaddition reaction of nitrone 21 with ethyl vinyl ether 22 is shown in Fig. 8.19. 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

The kinetic stability of pentazole has been estimated by the activation energy of decomposition or retro-[3 -i- 2]-cycloaddition reaction of 19.8 kcal moL [107] and 19.5 kcal mol- [108] with a half-life of only 14 s at 298 K [108]. 

The energy and geometry data listed in Table 2.5 show that the effect of solvent bulk (computed for cyclohexane, toluene, diethylether chloroform, THF, and methanol by PCM model), decreases the activation energy, increasing asynchronicity for the [4 + 2] cycloaddition reactions. 

Ab initio Hartree-Fock and density functional theory (DFT) calculations were performed to study transition geometries in the intramolecular hetero-Diels-Alder cycloaddition reactions of azoalkenes 20 (LJ = CH2, NFI, O) (Equation 1). The order of the reactivities was predicted from frontier orbital energies. DFT calculations of the activation energies at the B3LYP level were in full agreement with the experimental results described in the literature <2001JST(535)165>. 

The ene reaction (or Alder reaction) is a cycloaddition which requires an activation energy higher than that of the Diels-Alder reaction [41]. Without a catalyst it usually occurs under pressure and/or at high temperature. The reaction with an allcene (ene) is much easier if the latter is more substituted (high HOMO) and the enophile is more electron-poor (low LUMO). 

Certain specific steric effects are operative on intramolecular nitrile oxide— olefin cycloadditions. These effects are governed by both ring size and character of substituents. Thus, cycloadditions to the exomethylene group are successful with substituted methylenecyclohexanones 334 (m = 1, 2 n = 2) and gave tricyclic 335 (m = 1, 2), but do not occur with methylenecyclopentanones 334 (m = 1, 2, 3 n = 1). Activation energies calculated by molecular mechanics are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives cA-decalone 336 (403). 

Fig. 19 Transition states involved in the cycloaddition (endo-mode) of the model diene with benzaldehyde, both in the absence (TS-endo) and the presence (TS-(Si)-4b, TS-(Si)-4b) of the TADDOL catalyst corresponding activation energies Occal mol ](B3LYP/6-31G(d)//B3LYP/6-31G(d) PM3)...

The reason for this difference is that if 16 were to undergo a concerted elimination, it would have to follow the forbidden (high-energy) [2ns + 2ns] pathway. For 17, the elimination can take place by the allowed [2ns + 4rcv] pathway. Thus, these reactions are the reverse of, respectively, the [2 + 2] and [4 + 2] cycloadditions, and only the latter is an allowed concerted process. The temperature at which 16 decomposes is fairly typical for strained azo compounds, and the decomposition presumably proceeds by a noncon-certed diradical mechanism. Because a C—N bond must be broken without concomitant compensation by carbon-carbon bond formation, the activation energy is much higher than for a concerted process. 

Theoretical calculations on the cycloaddition reactions of a range of 1,3-dipoles to ethene in the gas phase have been carried out (85) with optimization of the structures of these precursor complexes and the transition states for the reactions at the B3LYP/6-31G level. Calculated vibration frequencies for the orientation complexes revealed that they are true minima on the potential energy surface. The dipole-alkene bond lengths in the complexes were found to be about twice that in the final products and binding was relatively weak with energies <2 kcal mol . Calculations on the cycloaddition reactions of nitrilium and diazonium betaines to ethene indicate that the former have smaller activation energies and are more exothermic. 

Jursic (92) studied the cycloaddition reaction of a mtinchnone with acetylene from several theoretical standpoints using density functional theory on AMI geometries. The predicted activation energy for the 1,3-dipolar cycloaddition is 11.49 kcal/mol and the elimination of carbon dioxide from the cycloadduct to give a pyrrole is 5.82 kcal/mol. Both reactions are extremely exothermic as observed experimentally. 

Individual activation energies from BP, BLYP, EDFl and B3LYP density functional models are similar (and different from those of Hartree-Fock and local density models). They are both smaller and larger than standard values, but typically deviate by only a few kcal/mol. The most conspicuous exception is for Diels-Alder cycloaddition of cyclopentadiene and ethylene. Density functional models show activation energies around 20 kcaPmol, consistent with the experimental estimate for the reaction but significantly larger than the 9 kcal/mol value obtained from MP2/6-311+G calculations. Overall, density functional models appear to provide an acceptable account of activation energies, and are recommended for use. Results from 6-3IG and 6-311+G basis sets are very similar, and it is difficult to justify use of the latter. 

A good example for which experimental data are available, involves activation energies for Diels-Alder cycloadditions of different cyanoethylenes as dienophiles with cyclopentadiene, relative to the addition of acrylonitrile with cyclopentadiene as a standard. 

Table 9-4 Relative Activation Energies of Diels-Alder Cycloadditions of Cyclopentadiene and Electron-Deficient Dienophiles ... 

Diels-Alder cycloaddition of 1,3-butadiene and acrylonitrile is significantly slower than the analogous reaction involving cyclopentadiene. Might this simply be a consequence of the difference in energy between the ground-state trans conformer of butadiene and the cA like conformer which must be adopted for reaction to occur, or does it reflect fundamental differences between the two dienes That is, are activation energies for Diels-Alder cycloaddition of cA-butadiene and of cyclopentadiene actually similar ... 

A second set of comparisons assesses the consequences of use of approximate reactant and transition-state geometries for relative activation energy calculations, that is, activation energies for a series of closely related reactions relative to the activation energy of one member of the series. Two different examples have been provided, both of which involve Diels-Alder chemistry. The first involves cycloadditions of cyclopentadiene and a series of electron-deficient dienophiles. Experimental activation energies (relative to Diels-Alder... 

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... 

K. N. Houk, N. G. Randan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivity, Tetrahedron, 1985, 41, 1555. 

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