Disrotatory reaction

A complete mechanistic description of these reactions must explain not only their high degree of stereospecificity, but also why four-ir-electron systems undergo conrotatory reactions whereas six-Ji-electron systems undergo disrotatory reactions. Woodward and Hoifinann proposed that the stereochemistry of the reactions is controlled by the symmetry properties of the HOMO of the reacting system. The idea that the HOMO should control the course of the reaction is an example of frontier orbital theory, which holds that it is the electrons of highest energy, i.e., those in the HOMO, that are of prime importance. The symmetry characteristics of the occupied orbitals of 1,3-butadiene are shown in Fig. 11.1. 

Fig. 11.4. Correlation diagram for cyclobutene and butadiene orbitals (symmetry-forbidden disrotatory reaction).

Examples of this type of process are the conrotatory and disrotatory reactions of butadiene ... 

Thus outward disrotatory reaction is favoured over the inward process, if A and B are bulky groups. PR. Schleyer et al [J.Amer.Chem. Soc 88, 2868 (1966)] have confirmed this experimentally showing that the anti derivatives solvolyse faster than the syn derivatives. But the outward motion will be disfavoured over the inward process if A and B form a medium-sized ring. 

We see the same pattern, but with opposite stereochemistry in the disrotatory reactions of the hexatrienes 4.47 and 4.49. The direction the reaction takes is determined by thermodynamics, ring-closing in this case, but the stereochemistry is not, since the formation of the cis disubstituted cyclohexadiene 4.48 is counter-thermodynamic. If you try the frontier... 

With two more electrons in the conjugated system, making eight in all, the octatetraenes 4.51 and 4.54 show conrotatory closure giving the cyclo-octatrienes 4.52 and 4.55, However, the reaction can only just be stopped at this stage, for the products undergo a second electrocyclic reaction giving the bicyclic dienes 4.53 and 4.56 as a result of the allowed disrotatory reaction of the all-m hexatriene units. 

With a smaller ring in the bicycloheptene 4.60, a conrotatory reaction is virtually impossible, since it would put a trans double bond into a 7-mem-bered ring. No visible reaction occurs until the forbidden disrotatory reaction 4.62 gives the cis,cis-diene 4.61 at 400°, and even then only in low yield. 

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... 

Consider the electrocyclic ring-opening reaction of cyclobutene. The molecule is formally divided into two fragments the double bond and the single 0 bond which is cleaved.9 The frontier orbital interactions (0,7t ) and (0, it) relevant to the conrota-tory and disrotatory reactions are given in diagrams 4, 5, 6 and 7, respectively. The net overlap is positive for 4 and 5, but zero for 6 and 7. The conrotatory process is therefore allowed, and the disrotatory process forbidden. 

A priori, we would expect disrotatory reactions to show poorer torquoselectivity than conrotatory reactions for two reasons. Consider, for example, the hexatriene cyclohexadiene interconversion. On the one hand, the overlap between R and the distal carbon C6 is similar for the in and out pathways, as in the in mode, the major lobe at C6 is oriented away from R ... 

In disrotatory reactions, one group rotates clockwise and one anticlockwise... 

This process can occur in principle in two ways. In one the two ends of the open chain turn in the opposite direction into the transition state. This is called a disrotatory reaction. 

The Woodward-Hoffmann rules for electrocyclic reactions can also be formulated using the terms suprafacial and antarafacial (Table 4.3). A it system is said to react suprafacially in a pericyclic reaction when the bonds being made to the two termini of the it system are made to the same face of the 77 system. It reacts an-tarafacially when the bonds are made to opposite faces of the 7r system. In electrocyclic reactions, disrotatory reactions are suprafacial, and conrotatory reactions are antarafacial. 

Figure 6.4 The butadiene cyclobutene ring closure reaction, (a) For the conrotatory reaction, the loop constructed from butadiene, cyclobutene, and bicyclobutane is p (or pi ), and no conical intersection is enclosed, (b) For the disrotatory reaction, the loop is ip (or i ) and encloses a conical intersection.

Fig. 10.24. Correlation diagrams for interconversion of cyclobutene and 1,3-butadiene (left) symmetry forbidden disrotatory reaction (right) symmetry allowed conrotatory reaction.

The first anion A is formed by removal of the only possible proton one from the NCH2 group. This anion might be considered aromatic (six electrons from the three alkenes, two from N and two from the anion) but it is clearly unstable as it closes in an electrocyclic reaction at > -35 °C. This is a six-electron process and must therefore be disrotatory. The rotating hydrogens are shown on the structure of A. It is essential that the 5,5 ring closure must be cis and that demands a disrotatory reaction. Both anions A and B are extensively delocalized and it is a matter of choice where you draw the anion. 

Ring enlargement of gam-dibromocyclopropanes cf. 4, 432-433). Tosylated medium-sized rings can be obtained in high yield by reaction of em-dibromo-cyclopropanes with this Ag(I) salt. A single product, an (E)-allylic tosylate, is obtained (disrotatory reaction). ... 

Subsequently, the conversion of the transacts,cis.trans isomer at higher temperatures to the products derived from the trans,cis,cis,cis isomer was observed indicating that the disrotatory reaction was 11.1 kcal/mol higher in energy than the conrotatory, concerted electrocyclization. 

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