Disrotatory motion

The reverse reaction, closure of butadiene to cyclobutene, has also been explored computationally, using CAS-SCF calculations. The distrotatory pathway is found to be favored, although the interpretation is somewhat more complex than the simplest Woodward-Hoffinann formulation. It is found that as disrotatory motion occurs, the singly excited state crosses the doubly excited state, which eventually leads to the ground state via a conical intersection. A conrotatory pathway also exists, but it requires an activation energy. 

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 ... 

The same conclusion may again be reached by considering only the HOMO orbital. Figure 15.24. For the conrotatory path the orbital interaction leads directly to a bonding orbital, while the orbital phases for the disrotatory motion lead to an anti-bonding orbital. 

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... 

The other possibility disrotatory motion) would have one moving clockwise while the other moves counterclockwise the cis isomer would have given the cis-cis diene (shown) or the trans-trans diene ... 

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. 

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 thermal cleavage of cyclohexadienes, then, the positive lobes must lie on the same side of the plane, requiring disrotatory motion ... 

Disrotatory motion is also necessary for the reverse reaction, in order that the orbitals that overlap may be of the same sign ... 

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 the following disrotatory motion, the movement can be in any of the ways, either by inward or outer ward rotation. (Opening by movements in different directions). 

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

In con-rotatory motion C2 symmetry is maintained throughout while in disrotatory motion m symmentry is maintained. 

Correlation diagram for disrotatory motion of cyclohexadiene-hexatriene system symmetry maintained. 

For example in cyclopropyl cations, it can be seen that groups situated on the same side of the ring as the leaving group will move inwardly. The same process will impose an ontoward disrotatory motion to the groups situated in anti as follows. 

The linear cheletropic reactions in which the polyene is a suprafacial component (i.e., involving disrotatory motion of the termini) will be allowed if it has a total of (4n + 2) electrons. But linear cheletropic reactions in which the polyene is an antarafacial component (i.e., involving conrotatory movement of the termini) are allowed if it has a system of 4n electrons. 

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. 

Irradiation causes a disrotatory motion by exciting an electron from ttj-the HOMO [Fig. 9-18(6)]. 

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. 

Molecular orbital calculations at the extended Hiickel level indicate that the two disro-tatory modes of ring cleavage (but not the symmetry-forbidden conrotatory mode) require similar activation energies for a model methylenecyclopropane-PdCl2(HCN) complex. The disrotatory motion of the carbon-carbon bond breaking away from the metal (dis-... 

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). 

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