Regioselectivity MNDO calculations

Regioselectivity in the LiTMP metalation of 2,4-dichloropyrimidine (109) was shown to be dependent on temperature and solvent (Scheme 33) (91JHC). In THF/Et20 at - 100°C, metalation followed by acetaldehyde quench gave the 5-substituted (kinetic) product 110, whereas in THF/ HMPA mixture at -70°C, treatment with the same electrophile afforded the corresponding 6-substituted (thermodynamic) derivative 111. Minimum neglect of differential overlap (MNDO) calculations support the observed results. 

In order to understand the nature of the regioselectivity of the amination reaction MNDO calculations of the ground state of some nitroquinoxalines... 

The Diels-Alder cycloaddition reaction of dihydropyran with acrolein was performed in the presence of various H-form zeolites such as H-Faujasites, H-p, H-Mordenites which differ both in their shape selective as well as their acidic properties. The activity of the different catalysts was determined and the reaction products were identified. High 3delds in cycloadduct were obtained over dealuminated HY (Si/Al=15) and Hp (Si/Al=25) compared to HM (Si/Al=10). These results were accounted for in terms of acidity, shape selectivity and microporosity vs mesoporosity properties. The activity and the regioselectivity were then discussed in terms of frontier orbital interactions on the basis of MNDO calculations for thermal and catalyzed reactions by complexing the diene and the dienophile with Bronsted and Lewis acidic sites. From these calculations, Bronsted acidic sites appeared to be more efficient than Lewis acidic sites to achieve Diels-Alder reactions. 

FMO calculations using the MNDO method and electron-density calculations were performed to establish which calculations are in agreement with the experimentally observed regioselectivity of the SnH amination of some nitronaphthyridines (see Section III,B). 

Also, the observed highly regioselective course in the SnFI substitutions in 2-R-3-nitro-l, 8-naphthyridines (R = FI, OFI, Cl, NFI2, OEt) with the anion of chloromethyl phenyl sulfone was explained by MNDO quantum-chemical calculations showing that, like in the aminations, the interaction of FIOMO of the nucleophile with LUMO of the nitronaphthyridines controls the regioselectivity (91JFIC1075). 

The transition-state geometries for the cyclization of a range of electrophilic radicals have been obtained by MNDO semiempirical calculations.42 The regioselectivities observed were rationalized using a frontier orbital approach. Semiempirical calculations were not suited for describing the cyclization of the pent-4-en-l-oxyl radical.43 Instead, results indicated that ab initio (UHF/6-31G ) and (UBP/DZVP) were more reliable. 

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

Nonempirical MO calculations were also used to predict the IR absorption spectrum of unsubstituted 2//-pyran (1) (90CCC10). Some MNDO and AMI studies were carried out to obtain a thermodynamic interpretation of the regioselectivity of the formation of pyrans from pyrylium cations... 

---