B3LYP Diels-Alder reaction

They reported that the DFT calculations of 114 at the B3LYP/6-31G level showed that the ji-HOMO lobes at the a-position are slightly greater for the syn-n-face than for the anti face. The deformation is well consistent with the prediction by the orbital mixing rule. However, the situation becomes the reverse for the Jt-LUMO lobes, which are slightly greater at the anti than the syn-n-face. They concluded that the iyn-Jt-facial selectivity of the normal-electron-demand Diels-Alder reactions... 

FIGURE 2.3 Endo and exo TS geometries of the Diels-Alder reaction between o-QM and MVE, also in the presence of an ancillary CHC13 molecule (MEV-TSexoCHCl3), at B3LYP/ 6-31G(d,p) level, in the gas phase. Forming bond lengths are in A. 

AMI semi-empirical and B3LYP/6-31G(d)/AMl density functional theory (DFT) computational studies were performed with the purpose of determining which variously substituted 1,3,4-oxadiazoles would participate in Diels-Alder reactions as dienes and under what conditions. Also, bond orders for 1,3,4-oxadiazole and its 2,5-diacetyl, 2,5-dimethyl, 2,5-di(trifluoromethyl), and 2,5-di(methoxycarbonyl) derivatives were calculated <1998JMT153>. The AMI method was also used to evaluate the electronic properties of 2,5-bis[5-(4,5,6,7-tetrahydrobenzo[A thien-2-yl)thien-2-yl]-l,3,4-oxadiazole 8. The experimentally determined redox potentials were compared with the calculated highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies. The performance of the available parameters from AMI was verified with other semi-empirical calculations (PM3, MNDO) as well as by ab initio methods <1998CEJ2211>. 

Theoretical work on the gas-phase hetero-Diels-Alder reaction of A -sulfinyl dienophiles was used to study both endo- and o-modes of cycloaddition for both (E)-29 and (Z)-30 dienophiles at the B3LYP/6-31G level (Scheme 2) <2000JOC3997>. In summary, these calculations have predicted that (1) the A -sulfinyl dienophiles prefer the (Z)-30 orientation over (E)-29 stereochemistry by 5-7 kcalmoP, (2) the transition state is concerted but nonsynchronous, and (3) an lYo-transition state with diene 31 is favored over the fvo-approach both kinetically and thermodynamically. 

DFT calculations on the intramolecular Diels-Alder reaction of penta-l,3-dienyl acrylates predict stereoselectivities that are in good agreement with the experimen- (g) tal results.85 Another DFT study at the B3LYP/6-31G(d) level of the intramolecular Diels-Alder cycloaddition of 5-vinylcyclohexa-1,3-dienes has been reported. Reaction rates are influenced by dienophile twisting and substituent effects.86 The intramolecular dehydro-Diels-Alder reactions of ynamides (79) provides a new synthesis of benzo[fc]-, tetrahydrobenzo[fc]-, naphtho[l,2-/j -, and dibenzo[a,c]carbazoles... 

DFT theory at the B3LYP/6-31G(d,p) level was used to investigate the Diels-Alder reactions of o-quinone methides with various ethenes. Calculations show that solvent decreases the activation energy and increase the asynchronicity.116 The Diels-Alder reaction of ortho-quinone methides derived from 3//-1,2-benzoxathiole 2,3-dioxides with maleimides produces chroman 2,3-dicarboxylic acid derivatives.117... 

We first take up the issue of computing the activation barrier for this parent Diels-Alder reaction. Rowley and Steiner experimentally determined an activation barrier of 27.5 0.5 kcal mol for Reaction 4.1. Using thermal corrections computed at B3LYP/6-31G, Houk and coworkers estimate that Alfi(0 K) =... 

To summarize this section, for the Diels-Alder reaction, HF and CASSCF vastly overestimate the reaction barrier. Dynamical correlation is essential for the description of the Diels-Alder transition state. MP2 underestimates the barrier. MP4 and Cl methods both provide very good results, but triples configurations must be included. The preferred method, when one combines both computational efficiency and accuracy, is clearly DFT. It is likely the strong performance of B3LYP with pericyclic reactions, typified by the Diels-Alder results described here, that propelled this method to be one of the most widely used among computational organic chemists. 

On the other hand, DFT performs extraordinarily well. Spin contamination is a problem with DFT methods snch as B3LYP and so an unrestricted wavefunction must be used and spin correction appUed. Using this approach, Houk found 4 to lie 3.4 kcal mol" below 6 and 2.8 kcal mol below 7. Coupled with the computed KIEs, discussed next, the results of computational studies indicate that the Diels-Alder reaction proceeds by the concerted mechanism. 

Cyclopentadiene 75 can dimerize by a Diels-Alder reaction, whereby each molecule could act as either the diene or the dienophile, that is, in a [4-1-2] or a [2-1-4] fashion (Scheme 4.10). One might therefore expect two isoenergetic transition states leading to these two products 76a and 76b. Instead, to their surprise, Caramella and coworkers located a single transition state on the B3LYP/6-31G PES. TS 77, as shown in Figure 4.18, has C2 symmetry. The... 

Fig. 10.9. Asynchronous transition structures for Diels-Alder reactions of butadiene with maleic acid and 1,2,4-triazoline using B3LYP/6-31G calculations. Reproduced from Tetrahedron, 57, 5149 (2001) and J. Am. Chem. Soc., 120, 12303 (1998), by permission of Elsevier and the American Chemical Society, respectively.

Fig. 10.10. Alternate transition structures for Diels-Alder reaction of isoprene with propenal (a) structure without formyl hydrogen bond (b) structure with formyl hydrogen bond. Dimensions are from B3LYP/6-31G( f) computations in the gas phase and in PMC with s = 4.335 (shown in parentheses). Adapted from Org. Lett., 5, 649 (2003), by permission of the American Chemical Society.

Fig. 10.12. Activation and reaction energies (B3LYP/6-31G ) in kcal/mol for intramolecular Diels-Alder reactions of CH2 = CH(CH2) CH = CHCH = CH2 for n = 1 to 4. Adapted from Theochem, 618, 201 (2002).

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