Electrocyclic Ring Opening of Cyclobutene

From a first inspection of the experimental and theoretical energetic data for stationary points along the reaction path compiled in Table 13-1, we see that Hartree-Fock theory 

59 The planar crv-butadiene is not a minimum but rather represents the transition structure for the degenerate isomerization of the two identical gauche isomers. 

13 Chemical Reactivity Exploring Potential Energy Surfaces 

Barrier Exp.a G2 HF SVWN SLYP BVWN BLYP B3LYP 

60 The stability of a Slater determinant can be checked by means of the stable keyword in Gaussian 98. For an extension to DFT, see Bauemschmitt and Ahlrichs, 1996a. 

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Table 13-1. Computed reaction barriers and isomer stabilities [kcal/mol] for the electrocyclic ring opening of cyclobutene (relative to cyclobutene 1, including zero-point vibrational contributions). Except for G2, the results were obtained using the 6-311+G(d,p) basis set.

We are also carrying out calculations 40) for the electrocyclic ring opening of cyclobutene (27) and bicyclobutane (28) to 1,3-butadiene (29). While.this work is not yet complete, we have established that 27 opens preferentially by... 

Intermediates (6) can yield to the corresponding [3-lactams via conrotatory electrocyclic closures (Fig. 3). The main features of these transition structures closely resemble those found for the thermal electrocyclic ring opening of cyclobutenes [34]. 

Fig. 1. Transition structure for the electrocyclic ring opening of cyclobutene to rrans-l,3-butadiene...

Table 1. Transition structure geometries, activation energies, and reaction energies for transition structures of electrocyclic ring opening of cyclobutene... 

The electrocyclic ring opening of cyclobutenes has found considerable use as a method for the synthesis of 1,3-dienes. The reaction has recently been reviewed by Marvell and is also the subject of... 

Fig. 4-17 The electrocyclic ring-opening of cyclobutene seen as a cycloaddition...

Finally, the preferred direction of disrotatory ring closure has been addressed just as in the electrocyclic ring opening of cyclobutene. In the six-electron case, tt acceptor substituents control the direction of rotation in the same sense as in cyclobutene but the torquoselectivity effect is attenuated. Further, steric effects tend to play a bigger role, and both effects were attributed to the disrotatory nature of the transition state. 

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