B3LYP 5 + 2-cycloaddition

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>. 

Facial selectivity in 1,3-dipolar cycloadditions to cis-3,4-dimethylcyclobutene (73) (Scheme 1.21) was studied. Only phenylglyoxylo- and pyruvonitrile oxides lacked facial selectivities (anti syn = 1 1). All other nitrile oxides formed preferably anti-74. The anti/syn ratio increased from 60 40 (R = P-O2NC6K4) and 65 35 (R = Ph) to 87 13 and 92 8 for bulky ten-Bu and mesityl substituents, respectively. The transition-state structure of the cycloaddition of formonitrile oxide was determined using both HF/6-31G and B3LYP/6-31G methods. The... 

Dipolar cycloaddition of C60 with nitrile oxides was modeled at the B3LYP/6-31G(d,p)//AMl level, and its mechanism and regiochemistry were investigated. Theoretically, the reaction can proceed by four types of additions, viz., closed [6,6], open [5,6], closed [5,6], and open [6,6] additions. Analysis of... 

Density functional theory methods using the hybrid B3LYP functionals have been performed to study geometries and energetics of several intramolecular [2+3] dipolar cycloadditions of azides to nitriles (Section 11.06.6.1) toward fused tetrazole formation, including tetrazoles 14 and 15 <2003JOC9076>. 

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. 

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)...

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. 

Table 12-19 Effect of Choice of Geometry on Relative Energies of Regio and Stereochemical Products of Diels-Alder Cycloadditions of Substituted Cyclopentadienes with Acrylonitrile/ B3LYP/6-31G Model... 

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... 

Further reports on the aromaticity investigated in conjunction with new synthesized heterophospholes are also in accord with their aromaticity (Scheme 46). The l-pheny-3,5-di-tert-butyl-l,2,4-azadiphosphole 88 (R t-Bu, R Ph) exhibited equalized bond lengths (PC bond lengths were between 1.697 and 1.746 A). A comparison of the B3LYP/6-311+G computed NICS(O), BI, and BDSHRT values of 88, 89, and 90 (R H, R H) with the corresponding parameters for pyrrole, furan, and thipohene, respectively, has shown a small, but consistent, decrease in all these aromaticity measures of the phosphorus derivatives [264], The reactivity of 89 (R Mes) has been compared to that of 90 (R /-Hu) in a [4 + 2] cycloaddition... 

The mechanism of the cycloaddition of singlet dibromocarbene to formaldehyde was studied using DFT at the B3LYP/6-31G level of theory 47 The energy barrier is estimated as 13.7 kJ mol-1. Reaction paths for the addition of dichlorocarbene to 1,2-disubstituted cyclopropenes were studied using the same level of theory as above.48 The addition gives 1,3-dienes or bicyclobutanes and was predicted to be concerted following an asymmetric approach. 

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... 

RSaS stereoisomers.29 The 1,3-dipolar cycloaddition of [60]fullerene with diazomethane, nitrile oxide, and nitrone afforded fullereno-pyrazolines and -isoxazolines. These reactions were modelled at the B3LYP/6-31G(d,p)//AMl level and the reaction mechanisms, regiochemistry, and nature of addition were investigated.30... 

The Lewis acid-mediated domino reaction between cyclohex-2-enone and methyl azide has been investigated at the B3LYP/6-31G and B3LYP/6-31-G //B3LYP/ 6-31G levels of theory. A domino process comprising three consecutive reactions yields a cyclopentanone derivative as the product.31 The Cp RuCl(PPh3)2-catalysed 1,3-dipolar cycloaddition of organic azides with terminal alkynes produces 1,5-disub-stituted 1,2,3-triazoles.32... 

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... 

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