Cyclobutene disrotatory ring opening

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

Figure 7.17 Orbital correlation diagram for conrotatory and disrotatory ring openings of cyclobutenes.

Figure 7-21. The symmetry of the reaction coordinate in the conrotatory and disrotatory ring opening of cyclobutene.

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. 

Let us consider a disrotatory ring opening of cyclobutene to butadiene in which a mirror plane symmetry (m) is maintained. In the ground state, the orbitals of the reactant do not... 

Thus, conrotatory ring closing of butadiene to cyclobutene is symmetry allowed under thermal conditions. However, under photochemical conditions the LUMO becomes HOMO and the disrotatory ring opening is symmetry allowed (Fig. 8.48). 

Figure 4.7. Disrotatory ring opening of cyclobutene to butadiene a) orbital correlation diagram, b) configuration correlation diagram (dotted lines) and effect of configuration interaction, which converts the diagram into a state correlation diagram (solid lines). The triplet state is indicated by a broken line.

Figure 4.8. Calculated state correlation diagram for the disrotatory ring opening of cyclobutene to butadiene (by permission from Grimbert et al., 1975).

In Figure 4.12 the natural orbital correlation is shown once again for the disrotatory ring opening of cyclobutene. The n and r MOs correlate with the butadiene MOs and , for which the LCAO coefficients are largest for the two inner carbon atoms and have either equal or opposite signs, re-... 

Figure 4.13. Configuration correlation diagram (dotted lines) and the resulting state correlation diagram (solid and broken lines, respectively) for the disrotatory ring opening of cyclobutene, based on natural orbital correlations (by perihission from Bigot, 1980).

In this correlation diagram, the orbitals that mix upon disrotatory ring opening have been included in the orbital description of cyclobutene. In fact, little or no interaction exists between this a and n set in the planar cyclobutene. Since mixing between the indicated couples begins with incipient butadiene formation, they have been included for continuity. 

Merk and Pettit have reported the facile disrotatory ring opening of a number of cyclobutene derivatives (e.g., XXV- XXVI XXVII XXVIII) catalyzed by cuprous and silver salts (3S). 

Figure 3.16 shows the correlation diagram for the peiicyclic four-center reaction via a Hiickel-type transition complex as mentioned above. It contains two crossing correlation lines (72 - und 72-), which are indicative of a thermaUy forbidden transition (2). Examples for such a reaction via an antiaromatic four-membered Htickel ring are the dimerization of ethylene and the disrotatory ring opening of cyclobutene (41), both of which occur only photo-chemically but not thermaUy. 

Figure 7.16 Conrotatory and disrotatory ring opening of cyclobutenes and cyclohexadienes...

Ben-Nun and Martinez studied the photochemical opening of cyclobutene to 1,3-butadiene with ab initio molecular dynamics calculations and found that the motion leading to disrotatory ring opening is established within the first 15 fs after the electronic excitation. Ben-Nun, M. Martinez, T. J. /. Am. Chem. Soc. 2000,122, 6299. 

The ring-chain tautomerism between cyclobutene and butadiene is perhaps the most famUiar example of an allowed Woodward-Hoffmann process. This transformation invariably is discussed in every attempt to rationalize or teach the Woodward-Hoffmann orbital symmetry concepts. This popularity is due in large part to the existence of a geometrically well defined (and easily visualized) alternate, forbidden, electrocyclic pathway. Thus it is exceedingly simple to set up a nonaUowed strawman, the disrotatory ring opening, and... 

Leigh, W. J. Zheng, K. /. Am. Chem. Soc. 1991,113,4019 reported evidence for photochemical disrotatory ring opening in one bicyclic system incorporating a cyclobutene ring, but the stereospecificity in such cases seems to depend on the structural features incorporated into the reactants. For a discussion of the relationship of orbital symmetry to the photochemistry of cyclobutene, see Leigh, W. J. Can. ]. Chem. 1993, 71,147. 

Irradiation of cyclobutene produces the first excited state in which an electron is promoted from tt to tt orbital, and in this case a, tz, and Tt orbitals of cyclobutene correlate with W2, and 3 orbitals of butadiene. In other words, the first excited state of cyclobutene correlates with the first excited state of butadiene, and hence disrotatory ring opening (ring closing) is photochemically a symmetry-allowed process (Eqn 2.2). 

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