B3LYP calculations Diels-Alder reactions

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... 

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... 

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.

Combined quantum mechanical/molecular mechanical methods are not, of course, restricted to studies of reactions but can also be used to study association processes and conformational transitions. Most implementations use a two-zone model as described above, but Morokuma and colleagues have described a multilayered approach called ONIOM [Svensson et al. 1996]. ONIOM is a particularly apt name given that a typical calculation is constructed from a series of layers For example, a three-layer ONIOM calculation on the Diels-Alder reaction involved an inner core treated with the B3LYP density functional approach, the intermediate layer with a Hartree-Fock level of theory and the outer layer with MM3. A particular feature of ONIOM and its related methods is that they provide rigorous gradients and second derivatives, so enabling properties such as vibrational frequencies to be calculated [Dapprich et al. 1999]... 

Ishihara, et al. reported an organocatalytic enantioselective Diels-Alder reaction of dienes a-(A,iV-diacylamino)acroleins, Scheme 3.44 [59], DFT calculation (with B3LYP) of catalyst revealed that the (Z)-aldiminium salt derived from a-(N,N-diacylamino)acrolein and catalyst has 2.8 kcal/mol lower energy than the fi-form conformer. Based on this fact, diene should approach enantiomerically the Si face of the electron-deficient enimide to give the ent/<9-(25)-adduct as a major product. Earlier, catalyst 134-Tf2NH as well as l,l -binaphthyl-2,-2 -diammonium salt (catalyst-TfjNH) were used in the enantioselective Diels-Alder reaction with a-acyloxyacroleins [60]. 

B3LYP/6-311G (PCM) calculations have shown that the 4 + 2-cycloaddition of cyclopentadiene and -/ -nitrostyrene in nitromethane solution proceeded in a stepwise mechanism. Sulfanyl-methylene-5(4//)-oxazolones and -sulfanyl-a-nitroacrylates have been used as dienophiles in the Diels-Alder reactions to synthesize norbornene/ane amino acid derivatives suitable for peptide synthesis. Organoammonium salts of chiral triamine catalyse the Diels-Alder reaction of a-(carbamoylthio)acroleins with acyclic 1,3-dienes to produce cyclohexenes with sulfur-containing chiral quaternary carbons.The helical-chiral hydrogen-donor... 

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>. 

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. 

All energy optimizations, frequency analyses, and solvation computations were carried out by using the B3LYP/6-31G level of theory, which has been shown to produce realistic structures and energies for pericyclic reactions [37,40], and properly treat the butadiene and acrolein Diels-Alder gas phase reaction [17,72,76], Vibrational frequency calculations at the same level of theory were used to confirm all stationary points as either minima or transition structures, and provide thermodynamic and zero-point energy corrections. 

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