Diels-Alder reactions theoretical treatment

In addition to the Hopf cydization of 176, there is a second pericydic reaction leading to 162, that is, the dehydro Diels-Alder reaction of butenyne with acetylene (Scheme 6.47). The theoretical treatment of this process by Johnson et al. [59] predicted a free reaction enthalpy and a free activation enthalpy, both at 25 °C, of -13.4and 42.0kcalmol-1, respectively. Ananikov [116] arrived at a similar result for the intramolecular case of non-l-en-3,8-diyne (202) and calculated the same quantities to be -15.3 and 30.9 kcal mol-1 for the formation of the isoindane 203. As already discussed regarding Scheme 6.40, the conversion of 162 into benzene and likewise that of 203 into indane have to be considered as a sequence of two [1,2]-H shifts 116, 117], whose highest transition state has a significantly lower energy than that for the formation of 162 and 203 by the dehydro Diels-Alder reaction. 

Further examples of Diels-Alder cycloaddition reactions with small or negligible rate solvent effects can be found in the literature [531-535], The thermolysis of 7-oxabicyclo[2.2.1]hept-5-ene derivatives is an example of a solvent-independent retro-Diels-Alder reaction [537]. For some theoretical treatments of the solvent influence on Diels-Alder cycloaddition reactions, which, in general, confirm their small solvent-dependence, see references [536, 797-799]. 

Since the disclosures that the thermal dimerizations of acrolein and methyl vinyl ketone provide the 3,4-dihydro-2//-pyrans (1, 2) derived from 4ir and 2Tt participation of the a,3-unsaturated carbonyl compound in a Diels-Alder reaction, an extensive series of related observations have been detailed. This work has been the subject of several comprehensive reviews - - including the Desimoni and Tacco-ni extensive tabular compilation of work through 1974. Consequently, the prior reviews should be consulted for thorough treatments of the mechanism, scope, and applications of the [4 + 2] cycloaddition reactions of a,3-unsaturated carbonyl compounds. The [4 + 2] cycloaddition reactions of 1-oxa-1,3-butadienes with their 4-it participation in the Diels-Alder reaction exhibit predictable regioselectivity with the preferential or exclusive formation of 2-substituted 3,4-dihydro-2W-pyrans (equation 1). The exceptions to the predicted regioselectivity that have been observed involve the poorly matched [4 + 2] cycloaddition reaction of an electron-deficient l-oxa-l,3-butadiene with an electron-deficient dienophile, e.g. methyl crotonate or methacrolein. - Rigorous or simplified theoretical treatments of the [4 + 2] cycloaddition reaction of 1-oxa-1,3-butadienes predict the preferential formation of 2-substituted 3,4-dihy-dro-2f/-pyrans and accommodate the preferred endo approach of the reactants in which the carbon-carbon bond formation is more advanced than carbon-oxygen bond formation, i.e. a concerted but nonsynchronous [4 + 2] cycloaddition reaction. ... 

A theoretical treatment of the photochemical Diels-Alder reaction has been published.58 Pusset and Beugelmans have measured the fluorescence spectra and singlet energies of several conjugated dienes (81).59... 

The occurrence of photochemical Diels-Alder reactions has always seemed slightly at variance with Woodward-Hoffmann concepts, so it is interesting that Epiotis and Yates have now described a theoretical treatment of these processes. 

Theoretical quantitative treatments of cycloadditions mainly concern the Diels-Alder reaction. A possible approach is that of calculating the para-localisation energy, that is the variation of Tr-electron energy of the conjugated diene system, when two Tr-electrons are localised upon the atoms, in 1,4-relation to each other, which must form the new tr-bonds. This has been done by Brown , using the molecular orbital method some successful predictions of reactivity and of the positions of addition for polycyclic aromatic hydrocarbons and polyenes were made. This method has also been used by other authors - . 

In the previous, and almost in every calculation on Diels-Alder reactions, it has been assumed that diene and dienophile lie on top of each other in parallel (or roughly parallel) planes. Under this condition, the endo approach is theoretically better than the exo approach only because of secondary interactions. However, it has been shown that, for cyclopentadiene dimerisation, if the two molecules are allowed to approach in a non-parallel way, the endo preference can be mainly attributed to a more favourable primary interaction, due to an approach at an angle of ca. 60°, which could be allowed only to the c/ido-oriented dienophile because of steric reasons . The pmo method has also been applied to a simplified treatment of some competing 1,4 and 1.2 thermal cycloadditions involving diradical intermediates . 

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