Transition conrotatory

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

Analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. The basis set orbitals for the conrotatory and disrotatory transition states are shown below. 

We have now considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbitals, orbital correlation diagrams, and transition-state aromaticity. All arrive at the same conclusions about stereochemistiy of electrocyclic reactions. Reactions involving 4n + 2 electrons will be disrotatory and involve a Hiickel-type transition state, whereas those involving 4n electrons will be conrotatory and the orbital array will be of the Mobius type. These general principles serve to explain and correlate many specific experimental observations made both before and after the orbital symmetry mles were formulated. We will discuss a few representative examples in the following paragraphs. 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

This is evidence for a four-membered cyclic transition state and arises from conrotatory motion about the C-3—C-4 bond. It is called conrotatory because both movements are clockwise (or both counterclockwise). Because both rotate in the same direction, the cis isomer gives the cis-trans diene. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Figure 9.13. Transition states for (a) conrotatory and (b) disrotatory closure.

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

The retro-Diels-Alder reaction has been reviewed.A fully concerted cyclic transition state has been proposed for conrotatory opening of cyclobutenes, in order to account for the low activation entropy and unexpected activation volume of ca —2 to —3cm mol . ... 

Recent calculations, using more powerful computational methods, have shown that the conrotatory path does not pass through a C2-symmetric transition structure269 270 the conrotatory route diverges as one methylene rotates in conrotatory fashion slightly faster than... 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

The hexatriene reaction is slow, because the unstrained transition structure, a graceful spiral bringing the p-orbitals easily within bonding distance, corresponds to the forbidden [rt6a] conrotatory process ... 

The reaction of the octatetraene is faster, because the easily achieved spiral transition structure corresponds to the allowed [K8a] conrotatory process ... 

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