Electrocyclic reactions conrotatory 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... 

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 ... 

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. 

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. 

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. 

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. 

The usual notion of the coordinate of electrocyclic reactions is associated with the rotation of the end groups about double bonds. The conrotatory motion is thermally allowed for the reaction XXI-XXIII. Semiempirical [2,36-42] and ab initio [43-45] calculations of the critical regions of the PES of this reaction and of the still simpler cyclization of the allyl cation to the cyclopropyl cation have greatly refined the overall picture of the intrinsic mechanism and revealed some important distinguishing features common to all electrocyclic transformations. 

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