Conrotatory pathway

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. 

In the Mobius-Hiickel approach, diagrams similar to Figure 18.4 can be drawn for this case. Here too, the disrotatory pathway is a Hiickel system and the conrotatory pathway a Mobius system, but since six electrons are now involved, the thermal reaction follows the Hiickel pathway and the photochemical reaction the Mobius pathway. 

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. ... 

Conversely, for wavefunction 3, the terminal atomic orbitals

4 have the relative orientations shown in Fig. 3.5.8. Here a conrotatory pathway yields an antibonding interaction between the terminal atomic orbitals, while a disrotatory step leads to a stabilizing bonding interaction. Hence the disrotatory process wins out in this case. 

The cyclobutere radical cation l + has a C2v symmetry and a 2B ground state. Following a conrotatory pathway leads to the first excited state of cis-2 +, which also has a C2v symmetry and a 2BX state. Conversely, the 2A2 ground state of cis-2 + correlates to the second excited state of 1 +. Thus, a direct, symmetry conserving reaction of the group state of l + to the ground state of cis-2 + is not possible. 

Woodward-Hoffmann orbital symmetry rules (Section 30.9) a series of rules for predicting the stereochemistry of pericyclic reactions. Even-electron species react thermally through either antarafacial or conrotatory pathways, whereas odd-electron species react thermally through either suprafacial or disrotatory pathways. 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

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. 

Consider now a six membered ring as the transition state in the ring closure of 55. Now the Mobius six membered ring (the conrotatory pathway) has a smaller sixth... 

MD simulations in which F was applied explicitly yield results consistent with these experiments [29-31, 34]. For instance, an SMD study [30], in which 20 trajectories of cyclobutene were subjected to F using the hydrogen atoms on the carbon atoms of the scissile as APs, showed that the conrotatory pathway was consistently followed if the APs were in a trans configuration, whereas the... 

As the system progresses along the conrotatory pathway, rotation occurs about the central carbon-carbon bond, leading to the formation of trans-1,3-butadiene. 

Fig. 7 Portions of the FMPESs associated with rotating methylene groups of cyclobutene at different PP separations under constrained distance conditions for (a) the disrotatory pathway and (b) the conrotatory pathway. The energies on the surfaces are plotted relative to that of the structures on the disrotatory surface with a PP separation of 3.0 A and torsion of 180°. Solid lines indicate the MEP on each surface. The dashed line indicates the path the system follows upon moving from the disrotatory to conrotatory surface after the dissociation of the scissile bond. Locations of transition states and products are indicated as TS and P, respectively...

The stereospecificity of the cyclobutene isomerization was rationalized in the first of the Woodward-Hoffmann papers in 1965 which outlined the principle of Conservation of Orbital Symmetry in Concerted Reactions. This isomerization was defined as a conrotatory electrocyclic process. Note should be made of the possibility that the conrotatory pathway could also result in a cw,cw-2,4-hexadiene from a ran -3,4-disubstitued cyclobutene, but unfavorable steric interactions apparently intervene when a sterically large group rotates inward on the molecular system. Thus ran.y-l,2,3,4-tetramethylcyclobutene undergoes rearrangement with log k = 13.85 — 33 600/2.3R7 while the cis isomer reacts with log A = 14.1 — 31000/2.3RT so at 177°C the trans material reacts 49 times faster than the cis isomer. 

Remarkable effects of substituents were reported on the clockwise/counterclockwise competition in the conrotatory pathway. Without exceptions, electron donors rotated outwards whereas the acceptors preferred an inward rotation (Figure 9.7). This selectivity is particularly impressive since an often perfect electronic control can be achieved in these systems in the absence of charged intermediates, i.e. under the conditions of seemingly weak electron demand. 

As discussed in Chapter 4, the highest occupied molecular orbital (HOMO) of 1,3-butadiene in the ground state is ip2- Simultaneous rotation about both the C1-C2 bond and the C4-C3 bond in a clockwise fashion (as shown in Figure 11.7) would allow the two -I- lobes on the termini of 2 to overlap in a bonding mode, so the reaction is predicted to take place by a conrotatory pathway. Excitation of 1,3-butadiene to its lowest excited state... 

If the cyclobutene has an alkyl substituent at C3, both Z and E isomers are possible from the conrotatory pathway. Both steric effects and electronic effects of substituents elsewhere can influence the product distribution. See the discussion on page 757. 

The thermal conversion of Dewar benzene (20, Figure 11.12) to benzene is another example of an electrocyclic reaction that has a high steric barrier for the reaction pathway allowed by orbital symmetry. The allowed conrotatory pathway would produce geometrically strained cis, ds,frflns-l,3,5-cyclohexatriene. This isomerization was long thought to occur by the disrotatory pathway because, even though it has a higher electronic... 

Some electrocyciic reactions can be analyzed in different ways. For example, the conversion of o-xylylene (21) to benzocyclobutene (22) may be analyzed as either a 4-electron or an 8-electron electrocyciic process. Both analyses predict that the conrotatory pathway should be allowed, a result supported by ab initio calculations. ... 

Note that the four carbon atoms do not lie in the same plane in the calculated trai ition structure for the thermal conrotatory opening of cyclobutene. Lee, P. S. Sakai, S. Horstermatm, P. Roth, W. R. Kallel, E. A. Houk, K. N. /. Am. Chem. Soc. 2003,325,5839 found that the energy gap between the forbidden disrotatory pathway and the allowed conrotatory pathway decreases as the transition structure for cyclobutene ring-opening becomes more planar. 

---