B3LYP/6-31G //AMl

AMI, B3LYP/6-31G //AMl, and B3LYP/6-31G computational studies have been used to investigate the molecular mechanism of the domino cycloadditions between DMAD and naphthalino- and anthraceno-furanophanes. The transannular Diels-Alder reaction of the furanophane (143) successfully produced the desired isomer (144), a key intermediate in the total synthesis of the diterpenoid chatancin (Scheme 55). ... 

In early 2000s, when routine DFT calculations have became feasible for the fullerene derivatives, Clare and Kepert reconsidered some of their earlier results by comparing AMI to B3LYP/6-31G //AMl. One of the main conclusions of these studies was the understanding that one should treat results of AMI calculations with caution. 

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>. 

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... 

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... 

HOF = heat of formation (kcal/mol) computed by AMI E = total energy (a.u.) computed with B3LYP/6-3lG(d)//AMl AEj = activation barrier (kcal/mol) computed by AMI AEn = activation barrier (kcal/mol) computed with B3LYP/6-31G(d)//AMl. 

A = AMI B = B3LYP/6-31G(d)//AMl AEj = energy difference between benzo[6]heterocycle and benzo[c]heterocycle AEn = imaginary enthalpy of reaction benzene + heterocycle into benzo[6]heterocycle plus ethylene AEm = imaginary enthalpy of reaction benzene + heterocycle into benzo[c]heterocycle plus ethylene. 

To determine the reliability of these computational approaches, we have computed activation barriers for these reactions at both AMI ab initio and B3LYP/6-31G(d) DFT theory levels (Table 30). Knowing that activation barriers for the addition of cyclopropene to furan computed at the B3LYP/6-31G(d)/AMl theory level were 18.7 and 18.4 kcal/mol for an endo and exo cycloaddition reaction (Table 8) and it is experimentally feasible, it becomes obvious that none of the cycloaddition reactions presented in Table 25 should be able to be accomplished experimentally. All activation barriers were around 40 kcal/mol or higher, with the exception of the cyclopropane addition to 1,2-oxazole (Table 31). The comparison... 

To confirm this assumption, we have computed activation barriers for acetylene, ethylene and cyclopropene addition to 4,4-dimethyl-[4H]-l,2-diazole. To our delight, the B3LYP/6-31G(d)/AMl computed activation barrier for ethylene addition to 4,4-dimethyl-[4//]-1,2-diazole is almost identical (Table 47) to the value obtained with full B3LYP/6-31+G(d) calculation on [4H]-1,2-diazole as dienophile (Table 44). The activation barrier for the acetylene addition is 22.3 kcal/mol indicating that this reaction should be also experimentally feasible. As indicated... 

AEi = activation barrier computed by AMI AEn = activation barrier computed with the B3LYP/6-31G(d)//AMl theory model. 

Comparative data for a few particularly interesting systems is provided in Table 5-15. STO-3G, 3-21G and 6-3IG Hartree-Fock models, local density models, BP, BLYP, EDFl and B3LYP density functional models all with the 6-3IG basis set, the MP2/6-31G model and MNDO,AMl andPM3 semi-empirical models have been examined. 

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