Cyclobutenes torquoselectivity

On the other hand, hexatriene must take a helical structure.6 By analogy with the octatetraene case, it may be inferred that electronic effects will have a smaller influence and steric effects a larger influence than in the cyclobutene torquoselectivity. This is indeed what was found by Houk and co-workers.7... 

Electron-accepting R groups are best represented by two low-lying orbitals, one filled and one empty. Figure 6.1 shows that the interaction of the HOMOs will inhibit the in pathway, whereas the HOMO (cyclobutene)-LUMO (R) interaction will promote it. The LUMO (cyclobutene)-HOMO (R) also disfavors the in pathway. These results are summarized in Figure 6.3 interaction (1) favors the in mode and interactions (2) and (3) impede it. This means that the energy gap between the cyclobutene HOMO and the R LUMO must be as small as possible for an in reaction to occur. In other words, R must be a powerful acceptor. However, Rondan and Houk believe that even then, interaction (3) may largely cancel interaction (1) and the torquoselectivity will not be very pronounced. 

Explain the torquoselectivity in the following cyclobutene ring-opening reactions ... 

The conservation of orbital symmetry dictates that electrocycUc reactions involving An electrons follow a conrotatory pathway while those involving 4 -l-2 electrons follow a disrotatory pathway. For each case, two different rotations are possible. For example, 3-substituted cyclobutenes can ring open via two allowed conrotatory but diastereomeric paths, leading to E- or Z-1,3-butadienes, as shown in Scheme 4.11. Little attention was paid to this fact until Houk and coworkers developed the theory of torquoselectivity in the mid-1980s. They defined torquoselec-tivity as the preference of one of these rotations over the other. 

Buda, A. B. Wang, Y. Houk, K. N. Acid-base-controlled torquoselectivity theoretical predictions of the stoeochanical course of the electrocycUc reactions of cyclobutene-3-carboxyUc acid and the conjugate base and acid, J. Org. Chem. 1989, 54, 2264-2266. 

Niwayama, S. Houk, K. N. Competition between ester and formyl groups for control of torquoselectivity in cyclobutene elechocyclic reactions, Tetrahedron Lett. 1992, 33, 883-886. 

Niwayama, S. Wang, Y Houk, K. N. The torquoselectivity of electrocycUc reactions of 3-donor-3-acceptor-substituted cyclobutenes, Tetrahedron Lett. 1995, 36, 6201-6204. 

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. 

Figure 9.7 Examples of torquoselectivity in cyclobutenes, oxacyclobutenes and sterically crowded cyclobutene ring-openings.

The torquoselectivity of the 4 r electrocyclic ring-opening reaction of 2-azetines has been reported to be controlled by the Brpnsted acidity of the catalyst and the polarity of the solvent." A review of the origin of the torquoselectivity in the thermal ring opening of cyclobutenes based on the Cplex-isoelectronic theory and expanded Robinson electronic theory has been reported." Cplex-isoelectronic theory is said to provide a rational for the torquoselectivities in the ring opening of cyclobutenes." ... 

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