Cyclobutene, conrotatory ring

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. 

In this section we will present results33 from the MD simulations along the IRC performed for five model reactions the HCN CNH isomerization reaction, the conrotatory ring opening of cyclobutene, ethylene-butadiene cycloaddition, the prototype SN2 reaction Cl +CII3CI C1CH3- -Cr, and the chloropropene isomerization Cl - CH2 - CH=CH2 - CH2=CH - CH2C1. 

Figure 4-13. Conrotatory ring opening of cyclobutene the reactant, TS, and die product geometries...

Figure 4-14. Conrotatory ring opening of cyclobutene IRC energy profile and free energy profile from the MD simulation at 300K...

Figure 4-15. Conrotatory ring opening of cyclobutene the changes in the interatomic distances (a), angles (b), and torsional angles (c) along the IRC and from the 300K MD simulation...

Figure 7-19. Correlation diagram for the conrotatory ring closure in the butadiene-cyclobutene isomerization. Adaptation of Figure 10.12 from reference [82] with permission.

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). 

The ligand reaction A B, then, exactly mirrors the allowed transformation in a pure organic reaction. Examples include the s5unmetry-allowed Cope rearrangement [a2g+ n2s] (Eq. 3), the Diels-Alder cycloaddition [lAs + n2,] (Eq. 4) and conrotatory ring-opening of cyclobutene [, 2 -)- g2a]... 

In a lengthy theoretical paper, Rondan and Houk considered the available data, described ab initio calculations and discussed earlier explanations concerning the stereochemical aspects of the ring openings of substituted cyclobutenes.These authors came to the following conclusions. The stereochemistry of the thermal electrocyclic conrotatory ring opening of 3- and 4-substituted cyclobutenes is controlled by... 

In conrotatory ring opening, the reoriented a orbitals derived from cyclobutene look like part of the butadiene molecular orbitals 1 2 and %. The orbitals derived from the double bond of cyclobutene look like part of the butadiene molecular orbitals and 3. Because the signs of the cyclobutene orbitals can be correlated with bonding orbitals of butadiene by... 

Fig. 18.4. The 1,3-diene-cyclobutene interconversion. The orbitals shown are not molecular orbitals, but a basis set of p-atomic orbitals, (a) Disrotatory ring closure gives zero sign inversion, (b) Conrotatory ring closure gives one sign inversion. We could have chosen to show any other basis set (e.g., another basis set would have two plus lobes above the plane and two below, etc.). This would change the number of sign inversion, but the disrotatory mode would stiU have an even number of sign inversions, and the conrotatory mode an odd number, whichever basis set was chosen.

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).

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