Conrotatory motion

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. 

FIGURE 18.2 Thermal ring opening of 1,2-dimethylcyclobutene. The two hydrogens and two methyls are forced into conrotatory motion so that the resulting p orbitals have the symmetry of the HOMO of the diene. 

FIGURE 18.3 Thermal ring closing of a 1,3-diene. Conrotatory motion is required for two + lobes to overlap. 

We may also look at this reaction from the opposite direction (ring closing). For this direction the rule is that those lobes of orbitals that overlap (in the HOMO) must be of the same sign. For thermal cyclization of butadienes, this requires conrotatory motion (Fig. 18.3). In the photochemical process the HOMO is the %3 orbital, so that disrotatory motion is required for lobes of the same sign to overlap. 

In the photochemical process the lobes of the same sign must overlap and this is possible by conrotatory motion. 

Calculations on the ring opening of fran -cyclopropylidene (113) to 1,3-dimethylallene predicted a barrier of 4.2 kcal mor via initial disrotatory motion of the substituents followed by a change to conrotatory motion. The di-cyclopropylidene rearrangement is barrierless and, in agreement with the elusive nature of 1,2-cycloheptadiene, the barrier to ring opening of bicyclic cyclopropylidene (114 n = 2) cannot be overcome at low temperatures. 

To achieve this stereospecificity, both terminal C s rotate 90° in the same direction, called a conrotatory motion. Movement of these C s in opposite directions (one clockwise and one counterclockwise) is termed disrotatory. 

The HOMO for the thermal reaction then requires a conrotatory motion [Fig. 9-18(a)]. 

Figure 14.3. (a) Orbital correlation diagram for electrocyclic reaction of butadienes (b) Orbital correlation diagram for electrocyclic reaction of hexatrienes. Solid lines and S, A denote correlation for conrotatory motion dashed lines and S, A denote correlation for disrotatory motion. 

Let s begin by considering the simplest electrocyclic reaction, the thermally induced interconversion of a diene and a cyclobutene. As illustrated in the following example, the reaction is remarkably stereospecific, occurring only by a conrotatory motion ... 

The analysis in Section 22.3 indicates that the thermal interconversion of a diene with a cyclobutene should occur by conrotation. The reaction is allowed in both directions, as long as a conrotatory motion is followed. However, usually only the conversion of the cyclobutene to the diene is observed because the cyclobutene is destabilized by angle strain and is present only in trace amounts at equilibrium. An example of the opening of a cyclobutene to form a diene is provided by the following equation ... 

The decatetraene cyclizes at — 10°C. by a conrotatory motion, to produce the cyclooc-tatriene with the two methyl groups in a trans orientation. When the cyclooctatriene is warmed to 20°C, it undergoes a thermally allowed disrotatory ring closure to give the final product. 

Once the theory of pericyclic reactions was developed, it was recognized that the conversion of Dewar benzene to benzene is an electrocyclic reaction. This conversion involves two pairs of electrons one pair of pi electrons and one pair of sigma electrons of the Dewar benzene. (The third pair of electrons is located in exactly the same place in both the reactant and the product and so is not involved in the reaction.) An electrocyclic reaction involving two pairs of electrons must occur by a conrotatory motion if it is to be thermally allowed. However, the conrotatory opening of Dewar benzene is geometrically impossible, because it would result in a benzene with a trans double bond, a compound with too much angle strain to exist. 

In the skin of animals, 7-dehydrocholesterol is converted to vitamin D, by the reaction sequence that follows. The first step in this process, the conversion of 7-dchy-drocholesterol to pre-cholecalciferol, requires light. This is an electrocyclic reaction and must occur by a conrotatory motion to avoid the formation of a highly strained trans double bond in one of the rings. Conrotation involving three pairs of electrons must occur photochemically to be allowed. 

The HOMO for the thermal reaction then requires a conrotatory motion [Fig. 9-16(a)], Irradiation causes a disrotatory motion by exciting an electron from n2 - nf. which now becomes the HOMO [Fig. 9-16(6)]. 

Four electron pairs undergo reorganization in this electrocyclic reaction. The thermal reaction occurs with conrotatory motion to yield a pair of enantiomeric rra/w-7,8-dimethyI-1,3,5-cyclooctatrienes. The photochemical cyclization occurs with disrotatory motion to yield the cis-1,8-dimethyl isomer. 

Fig. 7. Two-dimensional potential energy surface and static reaction path for the synchronous conrotatory motion of the terminal methylene groups. 2 a represents the value of the carbon ring angle. The abcissa gives the common value of both rotational angles 6 = dj = dj. TS denotes the position of a transition state. The energies are in kcal/mol...

---