Cyclobutene, conrotatory ring symmetry

Symmetry-allowed reaction (Section 10.14) Concerted reaction in which the orbitals involved overlap in phase at all stages of the process. The conrotatory ring opening of cyclobutene to 1,3-butadiene is a symmetry-allowed reaction. 

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. 

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 4.11. Derivation of the orbital correlation diagram for the conrotatory ring opening of cyclobutene a) intended correlation, b) correlation including interaction between a and n and between and o MO s, respectively. Orbiial symmetry labels n and a apply strictly only at the planar cyclobutene and butadiene geometries. Labels S and A, or solid and broken correlation lines respectively, indicate the symmetry behavior with respect to the twofold-symmetry axis (by permission from Michl, 1974b).

Obviously, while the disrotatory ring closure with conservation of mirror plane symmetry will take place thermally, the conrotatory ring closure with conservation of C2 axis of symmetry will be photochemical. Whether indeed there is such a predicted mode of ring closure, and it certainly is, can be deduced easily from product s stereochemistry. It must be noted that there is a switch in the mode of ring closure in moving from transformation 1,3-butadiene > cyclobutene to the transformation... 

Therefore, compounds IV and V undergo a different symmetry-allowed photochemical reaction, namely 47t-electron disrotatory cyclization to yield cyclobutene derivatives VI and VII. These compounds in turn are comparatively thermally stable because the thermally allowed 47t-electron conrotatory ring openings would have to put a rranj-double bond into ring B. 

Fig. 11.5. This reaction is symmetry-allowed, since the bonding orbitals of butadiene correlate with the bonding orbitals of cyclobutene, and vice versa. Detailed MO analysis of the transition state for conrotatory ring opening using high-level MO methods fully supports the conclusion that the reaction proceeds by a concerted process.

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

Fig, 4,18 The stereochemistry of many reactions is easily predicted from the symmetry of molecular orbitals, usually the highest occupied n MO (n HOMO). In the ring closure of 1,3-butadiene to cyclobutene the phase (+ or —) of the HOMO (i//2) at the end carbons (the atoms that bond) is such that closure must occur in a conrotatory sense, giving a definite stereochemical outcome. In the example above there is only one product. The reverse process is actually thermodynamically favored, and the cis dimethyl cyclobutene opens to the cis, trans diene. No attempt is made here to show quantitatively the positions of the energy levels or to size the AOs according to their contributions to the MOs... 

The ring opening of cyclobutadiene is predicted by symmetry rules to proceed as a conrotatory process. The kinetic product of the reaction is gauche- 1,3-butadiene with deviation from planarity of ca. 30°. The structures of the cyclobutene, TS, and gauche- 1,3-butadiene are shown in Figure 4-13. 

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

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