Electrocyclization kinetic studies

In addition to the above kinetics studies, the fluorene cyclization was studied using ab initio computational methods.323 It was found that the theoretically predicted barriers to the cyclizations for the dicationic intermediates agree well with the values obtained from the kinetic experiments. For example, geometry optimization and energy calculations at the B3LYP/6-31 level estimated that the activation energy (Ea) is 14.0 kcal/mol for the 4jt-electron conrotatory electrocyclization reaction involving compound 57 and the diprotonated intermediate (46, eq 13). 

The thermal isomerization of 5-acetyl-5-methylbicyclo[2,l,0]pentane (494) has been the subject of a detailed kinetic study. Rates and activation parameters have been determined for all the processes. The exo-endo interconversion and the rearrangement to the cyclopentene (495) evidently do not involve a common intermediate. The activation parameters for the rearrangement to cyclopentenyl ketone, AH = 22.2 kcal mol and AS = - 33.0 cal K mol are indicative of a highly ordered transition state and suggest that the cyclopropyl-allylic rearrangement is an electrocyclic process. 

The formation of cyclic nitrones (150) from pericyclic mechanism. Kinetic and computational studies have provided evidence for the involvement of a novel pseudo-pericyclic electrocyclization in the conversion of o-vinylphenyl isocyanates into quinolin-2-ones. " Such reactions have also provided evidence of torquoselectivity in a 6jt system. Hash vacuum thermolysis of triazoles (151) has been found to afford dihydroquinolines (155), presumably by generation of a-oxoketenimines (152) which can undergo a [1,5]-hydrogen shift to the o-quinoid imines (153)7(154) and subsequent electrocyclization (see Scheme 57). 

Heinze, MacNeil and coworkers developed the first methodology for the preparation of isomerically pure E- and Z-perfluoro-l,3-pentadienes, as described in equations 56 and 5749,50. Similar methodology was employed for the preparation of related perfluoro-3,5-octadiene systems49. These dienes were key components for studying the kinetic and thermodynamic electrocyclic interconversions of perfluorinated dienes and cyclobutenes,... 

Wiest, Montiel, and Houk (1997) have studied carefully a large number of TS structures for organic electrocyclic reactions and, based on comparison to experiment (particularly including kinetic isotope effect studies) and very high levels of electronic structure theory. 

Irradiation of the photochromic DHI 7 gives rise to the colored forms 8 and 9. The thermal back or fade reaction 9 —> 7 (i.e., the 1,5-electrocyclization), can be easily studied kinetically using UV spectroscopy. The rate constant k thus determined can be converted by the equation ... 

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