B3LYP calculations cycloaddition

Keating, A.E., Garcia-Garibay, M.A., and Houk, K.N., Origins of Stereoselective Carbene 1,2-Shifts and Cycloadditions of 1,2-Dichloroethylidene A Theoretical Model Based on CBS-Q and B3LYP Calculations,/. Am. Chem. Soc., 119, 10805,1997. 

Table 13-3. Basis set dependence of activation (AEa) and reaction energies (AEr) computed using the B3LYP functional for the concerted gas-phase cycloaddition of ethylene to trans-butadiene [kcal/mol]. All calculations include zero-point vibrational contributions evaluated at the B3LYP/6-311+G(d,p) level.

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>. 

Density functional theory calculations (B3LYP/6-31G level) have provided an explanation for the stereodivergent outcome of the Staudinger reaction between acyl chlorides and imines to form 2-azetidinones (/3-lactams). When ketene is formed prior to cycloaddition, preferential or exclusive formation of ct5-j6-lactam (50) is predicted. If, however, the imine reacts directly with the acid chloride, the step that determines the stereochemical outcome is an intramolecular 5n2 displacement, and preferential or exclusive formation of trans isomer (51) is predicted. These predictions agree well with the experimental evidence regarding the stereochemical outcome for various reactants and reaction conditions. 

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. 

The cycloaddition reaction of methyleneketene 25 and 5-methylene-l,3-dioxan-4,6-dione 26 was studied by DFT at the B3LYP/6-31G" level of theory both in the gas phase and in chloroform solution <1999JMT(488)187>. In the gas phase, the activation barriers for reactions to 27, 28, or 29 (Scheme 1) were calculated to be 21.81, 0.25, and 2.96 kcal mol respectively thus, the reaction leading to the 1,2-adduct 28 was lowest, in agreement with the... 

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. 

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 levels of calculation (including semi-empirical calculations) provide a qualitatively correct account of the experimental regio and stereochemical preferences. The only (apparent) exceptions are that both B3LYP/6-31G and MP2/6-31G models show modest preferences for meta products in cycloaddition of 2-methylcyclopentadiene and acrylonitrile. Note, in particular, the success of the calculations in properly assigning the more crowded syn product for the cycloaddition of 5-methoxycyclopentadiene and acrylonitrile. Also note the large magnitude for the preference. Clearly factors other than sterics are at work. 

Some intramolecular [2+2] cycloadditions of sulfonyl isocyanate-olefins have been studied by Metz et al. [116]. These authors found that the intramolecular reaction depicted in Scheme 36 was calculated to be endergonic at the B3LYP/6-31G and B3LYP(PCM)/6-31G levels, the corresponding free activation energies being in the... 

Semiempirical AMI and DFT (B3LYP/6-31G ) calculations were used to investigate the highly diastereoselective 1,3-dipolar cycloaddition of 1,4-dihydropyridine- (g) containing azomethine ylides to [60]fullerene (Prato s reaction). The activation energy for the four calculated transition state structures favours the formation of SSaS and... 

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... 

The dimerization of thioformylketene was investigated by B3LYP and G3MP2B3 methods. The 4 + 4-pathway has the lowest energy barrier and calculations suggest that the reaction is pseudopericyclic.231 The stereospecific intramolecular 4 + 4-cycloaddition reaction between cyclohexadiene iron tricarbonyl complex and appended dienes (198) generates cyclooctadiene tricyclic adducts (199) (Scheme 56).232 The first example of an asymmetric intermolecular 4 + 4-photocycloaddition reaction in solution between 9-cyanoanthracene and chiral 2-methoxy-l-naphthamides has been reported. The frozen chirality is effectively transferred to the optically active product.233... 

A DFT study at the B3LYP/6-31G level on the intramolecular 5 + 2-cycloaddition reactions of 3-OR (R = SiMe3, H, CHO, Me)-substituted / -hydroxy-y-pyrones tethered to alkenes and alkynes has been reported. Calculations indicate an initial transfer... 

The molecular geometries and the frontier orbital energies of heterophospholes 28-31 were obtained from density functional theory (DFT) calculations at the B3LYP/6-311- -G, level. The 1,3-dipolar cycloaddition reactivity of these heterophospholes in reactions with diazo compounds was evaluated from frontier molecular orbital (FMO) theory. Among the different types of heterophospholes considered, the 2-acyl-l,2,3-diazaphosphole 28, 377-1,2,3,4-triazaphosphole 30, and 1,3,4-thiazaphosphole 31 were predicted to have the highest dipolarophilic reactivities. These conclusions are in qualitative agreement with available experimental results <2003JP0504>. 

Analysis of 13C distribution in recovered alkynes using C4 atom as an internal standard led to experimental KIEs as collected in Table 4. For both catalysts significant isotope effect was observed for the terminal acetylenic carbon. Experimental KIEs are consistent with cyclopropenation via intact tetrabridged rhodium carbenoids and do so to support [2+2] cycloaddition. DFT calculations using B3LYP functional were complicated and did not give conclusive results. 

Theoretical and experimental studies revealed a mechanistic twist of the concerted [8+2] cycloaddition between dienylbenzo[c]furans and dimethyl acetylenedicarboxylate (DMAD). Thus, DFT calculation at the (U)-B3LYP/6-31+G(d) level had suggested a stepwise mechanism involving the formation of a zwitterionic intermediate for the [8+2] cycloaddition between DMAD and dienylbenzo[c]furans with electron-donating methoxy groups in the diene moiety. When no electron-donating substituents are present in the diene moiety of the dienylbenzo[c]furan, computational results indicated an alternative mechanism in which a [4+2] reaction occurs between the furan moiety and DMAD, and was followed by a [l,5]-vinyl shift <07JA10773>. 

Structural elements can also be incorporated into the furan starting materials so that intramolecular cycloadditions proceed at or below ambient temperature even with an unactivated dienophile, such as the example illustrated in Scheme 33 <20020L473, 2002JOC3412>. Based on B3LYP/6-31G calculations, the amidofuran substrate 60 was shown to be populated in a reactive conformation that was imparted by the amide carbonyl of the tether. 

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