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The mechanism, reactivity and regioselectivity of Diels–Alder cycloaddition reactions of 2-methyl-1,3-butadiene with four dienophiles, ( E)-2-benzylidene-3-oxobutanenitrile, ( E)-2-(furan-2-ylmethylene)-3-oxopentanenitrile, ( E)-2-benzylidene-4,4-dimethyl-3-oxopentanenitrile, and (E)-2-benzoyl-3-(furan-2-yl) acrylonitrile, have been studied by means of several theoretical approaches. The regioselectivity of these reactions has been confirmed by their activation energies, the Gazquez–Mendez rule, Houk's rule and global descriptors of the products. All the studied reactions have normal electron demand character and a polar nature. All calculations were done at the DFT-B3LYP/6-31G(d) level of theory.

The [3,3]-sigmatropic rearrangement reactions of α-methyl allyl acetate (1), and α-methyl allyl trifluoroacetate (2) have been studied using ab initio molecular orbital (MO) methods at the HF, B3LYP, B3PW91, and BHandHLYP levels with a 6-31++G** basis set. Among the methods used in this study, the values for activation parameters obtained with the BHandHLYP/6-31++G** method are in good agreement with experimental values. The calculated data demonstrate that in the rearrangement reactions of the compounds studied, the polarisation of the O3–C4 bond is rate determining. Optimised transition states at the BHandHLYP/6-31++G** level were used for calculating the nucleus-independent chemical shift (NICS) and also a natural bond orbital (NBO) analysis was carried out at the same level. Based on the optimised ground state geometries using the BHandHLYP/6-31++G** method, the NBO analysis of donor–acceptor (bonding–antibonding) interactions revealed that the stabilisation energies associated with electronic delocalisation from the σO(3)–C(4) bonding orbital to the π*C(5)=C(6) antibonding orbital, increase from compounds 1 to 2. The σO(3)–C(4) →π*C(5)=C(6) resonance energies for compounds 1 and 2 are 1.59 and 1.62 kcal mol−1, respectively. Also, analysis of the donor–acceptor interactions σO(3)–C(4) →π*C(5)=C(6) suggests that in these rearrangements, the TS structure in compound 2 has more aromatic character than for compound 1. The NBO results revealed that in these rearrangements, activation energies are not controlled by the σO(3)–C(4) →π*C(5)=C(6) resonance energy. NICS results indicate that the aromaticities of the transition states are controlled by the out-of-plane component. The predicted high-pressure-limit rate constants for the rearrangement reactions of compounds 1 and 2 are represented as 6.12 × 1012 exp(–21604/T), and 1.04 × 1013 exp(–17846/T) s−1, respectively. The fall-off pressures for the rearrangement reactions 1 and 2 are found to be 9.56 × 10−5 and 1.09 × 10−3 mmHg, respectively. The fall-off pressures of compounds 1 and 2 in both reactions are in the following order: P1/2(2) > P1/2(1) and the rates follow as: rate(2) > rate(1). Analysis of bond order, NBO charges, bond indices and synchronicity parameters suggest the Claisen rearrangement reactions of 1 and 2 occur through a concerted and slightly asynchronous six-membered cyclic transition state type of mechanism.

Theoretical studies of the Claisen and thio-Claisen allylic (3,3-sigmatropic) rearrangements and decomposition reactions of allyl formate (1) and allyl dithioformate (2), have been performed using density functional theory (DFT) and B3LYP and MPW1PW91 methods with a 6-31++G** basis set. Of these DFT, the values for activation parameters obtained with the B3LYP/6-31++G** method are in good agreement with the available experimental data. Transition states optimised at the B3LYP/6-31++G** level of theory were used for calculating the nucleus-independent chemical shift (NICS). Our results indicate that the magnitudes of the aromatic characters of the transition states are not in agreement with the activation energies and therefore these reactions are not controlled by aromaticity, but are controlled kinetically by the aromaticity of the transition states. KEywoRdS: Claisen, thio-Claisen, rearrangement, decomposition, aromaticity, density functional theory, nucleus-independent chemical shift

Theoretical study of the thermal decomposition kinetics of 2,2-dimethyloxetane has been carried out at the B3LYP/6-311 + G**, B3PW91/6-311 + G**, and MPW1PW91/6-311 + G** levels. The MPW1PW91/6-311 + G** method was found to give a reasonably good agreement with the experimental kinetics and thermodynamic parameters. Two parallel unimolecular reactions occur to give either isobutene and formaldehyde [reaction (1)] or ethene and acetone [reaction (2)]. The calculated data demonstrate that the polarization of the O1-C2 bond in reaction (1) and C4-O1 bond in reaction (2), are rate determining. The analysis of bond order and NBO charges, bond indices, synchronicity parameters, and IRC calculations suggest reactions (1) and (2) occur through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

The gas phase elimination kinetics of 2-chloroethyltrichlorosilane (1), 2-chloro- ethylethyldichlorosilane (2), 2-chloroethyldiethylchlorosilane (3), and 2-chloro- ethyltriethylsilane (4), have been studied at the ab initio B3LYP/6-311 + G**, B3PW91 /6-311 + G** and MPW1PW91/6-311 + G** levels of theory. The B3LYP/6-311 +G** method is in good agreement with the experimental data for the activation parameters. The calculated data demonstrate that, in the elimination mechanism of 2-chloroethylsilane and derivatives, the polarization of the C1-C13 bond in the sense C1δ+ C1-δ3 is a determining factor. Based on the optimized ground state geometries using the B3LYP/6-311 + G** method, the natural bond orbital analysis (NBO) of donor - acceptor (bonding-antibonding) interactions revealed that in accordance with the increase of activation energy, the HOMO-LUMO energy-gaps in the ground state structures increase. Moreover, the order of energy barriers obtained could be explained by the number of electron-releasing chlorine atoms substituted at the silicon atom. NBO analysis shows that the occupancies of σc1-c13 bonds decrease for compounds 1-4as 4σ*C1-C13 bonds increase in the opposite order (4>3>2>1). This trend explains the comparatively easier thermal decomposition of the

A theoretical study on the regioselectivity of 1,3-dipolar cycloaddition reactions between methyleneamine N-oxide (nitrone) and cyclopent-3-Ene-1,2-Dione (DPhl), pyrrole-2,3-dione (DPh2), furan-2,3-dione (DPh3) and thiophene-2,3- dione (DPh4) has been carried out by means of several theoretical approaches, namely, activation energies, Houk's rule based on FMO theory and DFT reactivity indices. The calculations were performed at the DFT-B3LYP/6-31G(d) level of theory using Gaussian 03. The present analysis shows that these reactions have an NED character. Moreover, the results obtained from energetic and electronic approaches, with the exception of Gazquez - Mendez rule based on NPA analysis, confirm that 1,5-regioisomer is the major product.

Ab initio density functional theory (DFT) calculations have been performed on the [3,3] sigmatropic rearrangement of substituted hexa- 1,5-dienes (the Cope rearrangement) in the gas phase. The barrier heights calculated at the B3LYP/6-311G** level of theory were in a good agreement with experimental data. Optimized transition states at the B3LYP/6-311G** level were used to calculate nucleus-independent chemical shifts (NICS) and also natural bond orbital analysis (NBO) at the same level. Our results indicate that the aromaticities of the transition states are controlled by the out-of-plane component and these reactions are controlled kinetically by the aromaticity of the transition states. Based on the NBO analysis, the greater resonance energy for σ →π* and σ →σ* delocalization in R2 (compared to R1 and R3) could facilitate the Cope rearrangement of R2. The greater σ*3-4 anti-bonding orbital occupancy and the lower σ3-4 bonding orbital occupancy in R1 could facilitate the Cope rearrangement of R1 compared to R3. Comparative examination between substituted and un-substituted hexa-1,5- dienes show that in these reactions, steric parameters play no significant rule in the general mechanism of the Cope rearrangement.