Thermal ring-closure reactions, rate

The authors attribute the B2 and B states to the isomers TC and TT and support this claim with the statement that the thermal ring-closure reaction for this compound that lasts for tens of seconds shows two components. This fact alone means that their assignment is unlikely because any species equilibrating in 21 psec would not show two decay components over tens of seconds. Also, the isomerization between TT and TC forms of typical napthopyrans has been shown to result in a spectral shift [76,77], which is not observed in this case. Further, for the same and similar compounds, the two mero-forms have clearly resolved noncoalesced H-NMR peaks [78,79] and the rate constant for... 

This finding would support a quinoidal structure (Scheme 14) in the thermal ring-closure reaction because if the nonpolar species were stabilized, it would increase its rate of formation and decrease its rate of ring closure by raising the overall activation barrier. Also the double 3-bond in the zwitterionic form would be hard to twist. 

Conlin and coworkers reported a rate constant for the thermal electrocyclic ring closure of the transient l,3-(l-sila)butadiene derivative 8a in cyclohexane at 25 C, k = (1.19 0.06) x 105 s l31. The rate constant corresponds to the inverse of the lifetime of the silene in the dry solvent, as measured by laser flash photolysis of the isomeric silacyclobutene derivative 7a, the stable product of the thermal ring closure reaction... 

TABLE 14 Rate Constants and Activation Volumes for the Thermal Ring-Closure Reactions of M(CO)5(N—N)]in Several Solvents, where M = Mo or W and N—N = 1,10-phenan throKne"... 

Table 2 The experimental values of absorption, M(III) SP formation rate constant (kj), thermal ring closure rate constant (k p, equilibrium (K ) constant of the reversible reaction, relaxation time and colorability in...

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

Photolytically generated 1-silabuta-l,3-dienes undergo a thermal reverse reaction to 2-silacyclobutenes. Thus 2-phenylsilacyclobut-2-ene 360 is easily opened to the 2-phenylsilabuta-1,3-diene 361 by irradiation in 3-methylpentane matrix at 77 K or by flash photolysis at ambient temperature (equation 97)183. The rate for the thermal reverse reaction was measured at room temperature and the activation energy for the 1 -siladiene ring closure was estimated to be 9.4 kcalmol-1183. 

Interpretation of the rate constants and Arrhenius parameters for thermal ,Z-isomeriza-tion is not always straightforward mechanistically, however, since sequential electrocyclic ring closure/ring opening reactions can often provide a lower energy route to the same products as formal rotation about a single C=C bond. This can be illustrated by Brauman and coworkers classic study of the thermal isomerization of E,E-, E,Z- and Z,Z-1,4-dideuterio-1,3-butadiene At 637 °C, the E,E- and Z,Z-isomers intercon-... 

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