Disrotatory process

Figure 11.3 illustrates the classification of the MOs of butadiene and cyclobutene. There are two elements of symmetry that are common to both s-cw-butadiene and cyclobutene. These are a plane of symmetry and a twofold axis of rotation. The plane of symmetry is maintained during a disrotatory transformation of butadiene to cyclobutene. In the conrotatory transformation, the axis of rotation is maintained throughout the process. Therefore, to analyze the disrotatory process, the orbitals must be classified with respect to the plane of symmetry, and to analyze the conrotatory process, they must be classified with respect to the axis of rotation. 

Analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. The basis set orbitals for the conrotatory and disrotatory transition states are shown below. 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

In the thermal conrotatory process the molecule maintains a Ca axis of symmetry throughout the entire reaction, while the photochemical disrotatory process maintains a plane of symmetry as shown in Figure 9.13 for butadiene. 

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

In the above example although the ground state orbital of cyclobutene, o, correlates with the ground state orbitals of butadiene /b the n orbital of the former does not correlate the /2 orbital of the latter, but rather it correlates with the /3 which is an excited state. So in such a case the thermal transformation by disrotatory processes will be a symmetry forbidden reaction. 

On the other hand in a photochemical transformation by a disrotatory process, one electron is promoted from jt to n orbital and so the o, n and it orbitals of cyclobutene would correlate with /. /2 and /3 orbitals of butadiene. Thus the first excited state of cyclobutene, since it correlates with the first excited state of butadiene, therefore, the process would be a photochemically symmetry allowed process. 

Note that we predict ring opening of the cyclopropyl cation to require activation this at first sight seems to be at variance with evidence that rearrangement occurs as a concerted process in the solvolysis of cyclopropyl esters and indeed acts as a driving force 44). Moreover the evidence shows very clearly that this is so only for one of the possible disrotatory processes, i.e. that indicated in 25 ... 

As in the case of the bicyclo-octane the isomerizations must occur by a disrotatory process. It is clear that, owing to the rigid nature of these bicyclobutenes, considerable stretching of the bridgehead bond is necessary before appreciable twisting of the cyclobutene ring can... 

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

The thermal retro-reaction of 3-sulfolenes to dienes and sulfur dioxide occurs under mild conditions (about 120-200°C), and is, as predicted from the Woodward-Hoffmann rules, a disrotatory process so for 2,5-dimethyl-3-sulfolenes [541,542] ... 

The closed and open forms, 4 and 5, respectively, represent the formal starting and end points of an electrocyclic reaction. In terms of this pericyclic reaction, the transition state 6 can be analysed with respect to its configurational and electronic properties as either a stabilized or destabilized Huckel or Mobius transition state. Where 4 and 5 are linked by a thermally allowed disrotatory process, then 6 will have a Hiickel-type configuration. Where the process involves (4q + 2) electrons, the electrocyclic reaction is thermally allowed and 6 can be considered to be homoaromatic. In those instances where the 4/5 interconversion is a 4q process, then 6 is formally an homoantiaromatic molecule or ion. 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

Instead, the transition structure must involve the lobes on the top surface tilting towards one another, in order to achieve an allowed [n6s] disrotatory process ... 

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. 

These selection rules can also be obtained from an analysis of polyene cyclizations. If the interaction between the terminal atoms Ca and Cn is bonding (antibonding), it will favor (disfavor) the cyclization. Figure 4.5 shows how the contribution of any given MO changes as a function of the reaction stereochemistry. When p is odd (even) the conrotatory process is disfavored (favored) and the disrotatory process is favored (disfavored). Obviously, the preferred pathway can be deduced by summing the contributions of all of the occupied MOs, up to and including the HOMO ... 

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

In some processes, the cyclopropyl group behaves as though it were functionally equivalent to a double bond (Hoffmann and Woodward, 1965b). In (29), we have an excited state disrotatory process to one cyclopropyl ring and one double bond. With one more double bond in... 

Finally, the photochemical mechanism of bullvalene formation in (30) substitutes two cyclopropyl groups for two double bonds in an allowed disrotatory process (Schroder and Oth, 1967). 

It should be pointed out that our description of electrocyclic reactions thus far has been qualitative. Woodward and Hoffmann (1965a) do refer to unpublished HMO calculations which back up the almost intuitive symmetry arguments. Nevertheless, Fukui (1965,1966) and Zimmerman (1966) outlined HMO treatments in which they obtained changes in energy for conrotatory and disrotatory processes. On the basis that paths involving minimum energy between reactants and transition states were favored, their predictions were in essential agreement with those of Woodward and Hoffmann. 

For wavefunction if 2, the terminal atomic orbitals 4> 1 and 4 have the relative orientations as shown in Fig. 3.5.7. It is evident that a conrotatory process leads to a bonding interaction between

4, while a disrotatory process leads to an antibonding interaction between (p and 4>4. Clearly the conrotatory process prevails in this case. 

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