Electrocyclic reactions disrotatory motion

Electrocyclic reactions are examples of cases where n-electron bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are formed along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HC1 + H system. The reaction is now Mobius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mobius-Hiickel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

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. 

This intuitive parallel can be best demonstrated by the example of electrocye-lic reactions for which the values of the similarity indices for conrotatory and disrotatory reactions systematically differ in such a way that a higher index or, in other words, a lower electron reorganisation is observed for reactions which are allowed by the Woodward-Hoffmann rules. In contrast to electrocyclic reactions for which the parallel between the Woodward-Hoffmann rules and the least motion principle is entirely straightforward, the situation is more complex for cycloadditions and sigmatropic reactions where the values of similarity indices for alternative reaction mechanisms are equal so that the discrimination between allowed and forbidden reactions becomes impossible. The origin of this insufficiency was analysed in subsequent studies [46,47] in which we demonstrated that the primary cause lies in the restricted information content of the index rRP. In order to overcome this certain limitation, a solution was proposed based on the use of the so-called second-order similarity index gRP [46]. This... 

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. 

Both reactions are [2 + 2] photochemical electrocyclic reactions, which occur with disrotatory motion. 

A six-electron cychzation will proceed with disrotatory motion. Torquoselectivity in a six-electron electrocyclic reaction was first examined in the ring opening... 

How can we predict whether conrotatory or disrotatory motion will occur in a given case According to hontier orbital theory, tke stereochemistry <4 an electrocyclic reaction is determined hy the symmetry of the polyene HOMO. The electrons in the HOMO arc the highest-energy, most loosely held elec- irons, and are therefore most easily moved durir reaction. For thermal I... 

Note that for every electrocyclic reaction there are two con-rotatory and two disrotatory motions that may or may not be distinguishable. For example, the two conrotatory motions for trans 3,4-dimethylcyclobut-l-ene lead to cis-cis and trans-trans-1,4-dimethyl-butadiene ... 

When we recall the symmetry patterns for linear polyenes that were discussed in Chapter 1 (see p. 29), we can further generalize the predictions based on the symmetry of the polyene HOMO. The HOMOs of the An systems are like those of 1,3-dienes in having opposite phases at the terminal atoms. The HOMOs of other An + 1 systems are like trienes and have the same phase at the terminal atoms. Systems with An tt electrons will undergo electrocyclic reactions by conrotatory motion, whereas systems with An+ 2 a electrons will react by the disrotatory mode. 

Since this problem is a photochemical electrocyclic reaction involving four tt electrons, the motion must be disrotatory ... 

The opening of a cyclopropyl carbocation to form an allyl carbocation is an electrocyclic reaction involving two electrons. Because it involves an odd number of electron pairs, disrotatory opening is thermally allowed. The allyl carbocation is much more stable than the cyclopropyl carbocation because of resonance stabilization and relief of ring strain, so the allyl carbocation is favored. The thermal ring opening of the c/s-dimethylcyclopropyl carbocation occurs with a disrotatory motion. 

How do the symmetry properties of 1 2 influence electrocyclic reactions For convenience, let us examine the microscopic reverse of the ring-opening of a cyclobutene to a butadiene, realizing that any factors that appear on this reaction path also appear on the forward reaction path. For bonding to occur between the carbon atoms at the end of the ir-system, the positive lobe on C(l) must overlap with the positive lobe on C(4) (or negative with negative). This overlap can be accomplished only by conrotatory motion. Disrotatory motion causes overlap of orbitals of opposite sign, and precludes bond formation. Since similar symmetry properties of the HOMO exist for other 4n -systems, the conrotatory mode will also be preferred for all thermal electrocyclic reactions in these systems. 

Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology.

An electrocyclic reaction is as easy to analyze as that. Identify the HOMO, and then see whether conrotatory or disrotatory motion is demanded of the end carbons by the lobes of that molecular orbital. All electrocyclic reactions can be understood in this same simple way. The theory tells us that the thermal interconversion of cyclobutene and 1,3-butadiene must take place in a conrotatory way. For the cyclobutene studied by Vogel, conrotation requires the stereochemical relationship that he observed. The cis 3,4-disubstituted cyclobutene can only open in conrotatory fashion, and conrotation forces the formation of the cis,trans diene. Note that there are always two possible conrotatory modes (Fig. 20.12), either one giving the same product in this case. 

Orbital symmetry considerations dictate that in 4n-electron reactions the thermal process must use a conrotatory motion, whereas the photochemical reaction must be disrotatory.Just the opposite rules apply for reactions involving 4re + 2 electrons. The key to analyzing electrocyclic reactions is to look at the way the p orbitals at the end of the open-chain K system must move in order to generate a bonding interaction in the developing G bond. 

This is 6-electron electrocyclic reaction which proceeds by disrotatory motion under thermal conditions. Geometry of II presents no steric hinderance in disrotation. 

The type of motion the orbital of the terminal carbon undergoes in an electrocyclic reaction can be detected only if substituents are bonded to these atoms. Substituents move as orbitals move. The thermal cyclization of (2A,4Z,6T)-octatriene provides an example of this effect. We consider 713 because it is the HOMO. It contains the highest energy electrons, so it is the frontier molecular orbital. As outlined above, disrotatory motion is required for a O bond to form at the ends of a conjugated triene. Disrotatory motion of the terminal 2p orbitals causes simultaneous disrotatory motion of the C-1 and C-8 methyl groups and yields r-5,6-dimethyl-l,3-cyclohexadiene (Figure 25.6a). 

H.l According to the Woodward-Hoffhiann rule for electrocyclic reactions of 4 r-electron systems (Section H.2A), the photochemical cyclization of cis, trani -2,4-hexadiene should proceed with disrotatory motion. Thus, it should yield trani -S,4-dimethylcyclobutene ... 

H.4 (a) This is a photochemical electrocyclic reaction of an eight r-electron system—a 4n r-electron system where n = 2. It should, therefore, proceed with disrotatory motion. 

---