Cycloadditions, potential energy surfaces

Houk K. N., Wilsey S. L., Beno B. R., Kless A., Nendel M., Tian J. Retro-Cycloadditions and Sigmatropic Shifts The C7H8 and C7H10 Potential Energy Surfaces Pure Appl. Chem. 1998 70 1947-1952... 

A theoretical study based on PM3 frontier molecular orbital (FMO) and potential energy surface (PES) analysis at the restricted Hartree-Fock (RHF)/6-31+G level was performed to examine the reaction of l-amino-2-ethoxycarbonyl-pyridinium mesitylenesulfonate and acrylonitrile in the presence of Hilnig s base leading to the formation of l,2-dihydropyrido[l,2-A]pyridazinium inner salt 17 <1999JOC9001>. The calculations indicated that both the [3+2] cycloaddition reaction and the ring expansion occurred in a concerted manner rather than through a stepwise mechanism via a zwitterionic intermediate 16 (Scheme 1). 

Earlier, [3+ 2]-cycloaddition reactions of nitronates have been described in terms of the FMO theory. For example, French researchers studied reactions of olefins containing EWG groups with nitronates by the FMO—INDO method (248, 338b, 419). Recently, more modem methods have been used for calculations of FMO and the potential energy surfaces for several analogous reactions (87, 399,... 

Another attempt to perform a general mechanistic consideration of [3 + 21-cycloaddition reactions of nitronates with olefins has been made relatively recently by Prof. Denmark and coworkers (162) using modern quantum-chemical methods, which allow one to correctly calculate the potential energy surfaces for model substrates. Since these data have been summarized in the recent review (335), it is not necessary to consider them as comprehensively as the study in (337). 

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. 

FIGURE 2. Schematic potential energy surfaces for [2+2] cycloaddition in the ground (G), singlet (S), and doubly excited (D) states. E is the exciplex and P the pericyclic minimum. 

The potential energy surface for the cycloaddition of hydrazoic acid to ethylene by the MINDO/2 method indicates a four-center, one-step process with the calculated enthalpy of formation of the transition state complex... 

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

The DA reactions between the electron-deficient diene 39 and the electron-rich ethylenes 33 and 40 and the electron-deficient ethylenes 11 and 15 have been reported by Spino et al. (see Scheme 2).37 The FMO theory was used to predict the reactivity of these reagents in DA reactions. These authors conclude that in the NED-DA reaction, the FMO theory could predict the relative reactivity, while in the case of the IED one, it could not.37 The high reactivity of the electron-deficient ethylene 11 with the electron-deficient diene 39 was studied using both the analysis of the potential energy surface for these cycloadditions and analysis of the global and local reactivity indexes.38 The analysis of... 

Acetaldehyde, 119-20, 380-82 Acetone, chemical titration. 428 oxetane formation, 318, 427 singlet and triplet states, 382. 428 Acetophenone. 407. 467 Acetylene, 203. 348 cycloaddition, 415, 423 excited state geometry. 45-46 2-Acetylnaphthalene, 398 6-Acetyloxycyclohexadienones. 463 Acidity, excited states, 48-52 Acrolein, 34, 382-83, 433 Acrylonitrile, 328, 414-15, 417 Activation energy, 382-83, 400 Acyl radical. 3S2-S5, 380-82, 460 Adiabatic. See Potential energy surface ... 

Bernardi F, Bottoni A, Olivucci M, McDouall JJW, Robb MA, Tonachini G. Potential energy surfaces of cycloaddition reactions. J Mol Struct (TFIEOCHEM) 1988 165 341-351. 

The potential energy surfaces for the addition and insertion reactions of PH with C2H4 were studied by an ab initio SCF-CI calculation with a 4-31G basis set. The cycloaddition of PH to the double bond yields phosphirane and was predicted to proceed without an activation energy. The insertion of PH into a C-H bond with formation of vinylphosphane was predicted to have an activation energy of about 6 kcal/mol. Both reaction channels were computed to be exothermic by about 75 kcal/mol [7]. 

The thermal cycloaddition of two ethylenes is one of the textbook examples used in the illustration of the Woodward-Hoffmann rules of orbital symmetry control in concerted reactions. Therefore, the related potential energy surface can provide various types of information of chemical and theoretical interest. A first question is associated with the mechanistic question of whether this reaction proceeds via diradical or concerted pathways. Since this reaction is an example of a concerted thermally forbidden process, it can be expected that the flavoured path be the diradical one. However, it is important to have a detailed description of the structural and energetic features of these different pathways. A second question is associated... 

Fig.2 Potential energy surface for the 2s+2s cycloaddition of two ethylene molecules. One axis (diagonal top left to bottom right) is the interfragment distance r and the other...

Fig.5 Potential energy surface for the 1,3 dipolar cycloaddition of Fulminic acid (HCNO) and acetylene molecules. The two axes are defined as in Fig. 1 Each division on the r axis corresponds to an increment of. 2ao (.lOSA) and the first division...

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