Disrotatory motion, and

Depending on the relative symmetry of the p orbitals, a bonding (that is, a positive overlap) is obtained either for the conrotatory or for the disrotatory motion, and these result in different stereochemistries if the terminal atoms carry distinct substituents, as shown in Figure 4.41(b). 

How can we predict whether conrotatory or disrotatory motion will occur in a given case According to frontier orbital theory, the stereochemistry of an electro-cyclic reaction is determined by the symmetry of the polyene HOMO. The electrons in the HOMO are the highest-energy, most loosely held electrons, and are therefore most easily moved during reaction. For thermal reactions, the ground-state... 

Disaccharide, 997-999 1—>4 link in, 997-998 synthesis of, 1002 Dispersion forces, 62 alkanes and, 92 Disrotatory motion. 1183 Distortionless enhancement by... 

On the other hand, in the photochemical process, the HOMO of the product is now the x,3 orbital (Fig. 18.1), and in order for the p orbitals to achieve this symmetry (the two plus lobes on the same side of the plane), the substituents are forced into disrotatory motion. 

Figure 13. Snapshots of a typical excited state trajectory of cyclobutene. Values of the C-C bond distance and HCH hybridization angle are indicated. Immediately after the electronic excitation (at t = 0) the C-C bond begins to stretch. This is followed by a change in hybridization of the methylene carbons (from sp3 to sp2) and a pronounced disrotatory motion. (Figure adapted from Ref. 214.)...

Figure 15. Snapshots of the two frontier excited-state natural orbitals (computed using the HF-OA-CAS(4/4) S wavefunction) of the excited-state trajectory of cyclobutene shown in Fig. 13. Left panels Before the onset of disrotatory motion, the excited-state wavefunction can be described using a single determinant with one electron in a tt-like orbital (4>a) and one in a 7t -like orbital (4>b). Middle panels During the disrotatory motion the simplest description of the electronic wavefunction requires two determinants. In one determinant both electrons are in the (j)a orbital, and in the other they are both in the (j)b orbital. Both orbitals (<()a and 4>b) show significant cj—it mixing, which is a consequence of the significant disrotatory motion. Right panels When the disrotatory motion is completed, the excited-state wavefunction is described by a single determinant in which both electrons are in the <()b orbital. Note how the shape of the orbitals changes as the initial bonds are broken and the two new tc bonds are formed.

In disrotatory motion one p orbital will rotate in clockwise and the other in anticlockwise direction. 

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. 

The stereochemistry of the reaction has been examined both theoretically92 and experimentally.93 It has been found to be stereospecific, with disrotatory motion of the methylene groups. Cyclopropyl anions also undergo thermal rearrangements to allyl anions.94... 

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. 

The thermally allowed cyclization of a triene to form a cyclohexadiene occurs by a disrotatory motion, as illustrated in the following equation. In this case the product is favored at equilibrium because it has one more sigma bond and one fewer pi bond than the reactant. (Sigma bonds are stronger than pi bonds.)... 

As another example, the electrocyclic interconversion of 1,3,5-cycloheptatriene and norcaradiene also occurs by a disrotatory motion ... 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

Fig. 8. Two-dimensional potential energy surface and static reaction path for the synchronous disrotatory motion of the terminal methylene groups. 2 a represents the value of the carbon ring angle. The abcissa gives the common absolute value of both rotational angles 9 = Sj = -02. TS denotes the position of a transition state. The energies are in kcal/mol...

Five different values of have been studied, namely 61, 62,63, 64 and 65 kcal/mol. For tot = 61 kcal/mol, the only available chatmel is the synchronous conrotatory motion (transition state at 59.8 kcal/mol). For > 62 kcal/mol, the rotation of a single group (transition state at 61.6 kcal/mol) and the concerted disrotatory motion (transition state at 61.9 kcal/mol) both become feasible motions, at least in principle. For each value of jEtot, j rot has been varied stepwise from 2 to 50 kcal/mol, with a step of 2 kcal/mol. In addition, for given values of E o and has been varied from 45° (conrotatory motion, ie. antisymmetric vibra-... 

Fig. 13. Evolution of the reactive bands versus ( ( and rot for (a) a conrotatory motion of the terminal groups and (b) a disrotatory motion...

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