Reaction pathway disrotatory

Theoretical studies have been carried out on the conrotatory and disrotatory reaction pathways of hexa-l,3,5-triene to cyclohexadiene, and the effect of solvent and salt effects on the rates of the electrocyclic ring closure of (IZ, 3Z, 5 )-l,2,6-triphenylhexa-l,3,5-triene has been determined. An ab initio study of the electro-cyclization of (Z)-hexa-l,3,5-triene and its hetero-substituted analogues has been undertaken.The involvement of a lone pair on the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the ring. It has been reported that the intramoiecular aza-Diels-Alder reaction of an a./S-unsaturated hydrazones to a quinone, as in (197), is followed by an unprecedented rearrangement in which the aminoaryi moiety formally undergoes a 1,2-shift to yield benzo- or pyrido-[fc]acridine-6,11-dienes (198). 

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 ... 

Fig. 10 (a) Reaction scheme for the ring opening of cyclohexadiene along the disrotatory (allowed) pathway. The atoms indicated with asterisks were used to apply F. (b) Reaction barriers as a function of F applied at the CASSCF(6,6)/6-31G(d,p) level and using (6), (10), and (12)... 

A disrotatory, non-concerted cyclobutene ring opening to give a biradical was proposed for the reaction pathway. 

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... 

Figure 15.18 shows several examples of electrocyclic processes. Since the reactions are always allowed in either a conrotatory or disrotatory manner, the key issue is the control of stereochemistry. Electrocyclic reactions provide a good example of the power of pericyclic reactions in this regard. In all cases, the reaction proceeds as predicted from the various theoretical approaches. The restrictions placed by the orbital analysis on the reaction pathway are nicely demonstrated by examples D and E in Figure 15.18 only the stereochemistry given is found. An instructive example of the fact that it is the number of electrons that controls the process, not the number of atoms or orbitals, is the conrotatory ring closure of the four-electron pentadienyl cation prepared by protonation of a divinyl ketone (example G). 

In the following two reactions, denoted as (A) and (B), cyclodecapentaene cyclizes into trans- and c/s-9,10-dihydronaphthalene. Apply Woodward-Hofimann rules to determine the reaction condition (thermal or photochemical) and reaction pathway (conrotatory or disrotatory) of each process. 

For each of the following five reactions, (A) to (E), write down the reaction pathway (conrotatory or disrotatory) of the process. In addition, for bicydic species I, II, III, and ly determine the relative orientation (ds or trans) of the two bridgehead hydrogens bonded to the carbon atoms at which the two rings are fused. 

Computational chemistry also allows one to assess the symmetry-forbidden, disrotatory reaction. Experimental studies showed this pathway to be about 46-63 kJ mol less favorable than the conrotatory process. Although no transition structure has yet been located for this reaction pathway, a two-configuration SCF study estimated the energy difference between the allowed and the forbidden pathway to be at least 62 kJ mol . ... 

This corresponds to the electrocyclic ring-opening of the cyclopropyl cation the preferred reaction pathways should involve + 2 ] or [ jOa + 2a] interactions, (Equation 6.1). In simple unfused ring systems the evidence for disrotatory scission comes from kinetic measurements (see below). The stereochemical test is not available because of the interception of the allyl cation by a counter-ion, (Equation 6.2). However, when the cyclopropyl cation is part of a bicyclic system, for example (1), it is found that electrocyclic cleavage occurs readily even when the number n has the small value of 3 or 4. In these circumstances the conrotatory mode, which would yield the unstable trans-... 

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. ... 

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. 

Both the frontier-orbital and the Mobius-Hiickel methods can also be applied to the cyclohexadiene 1,3,5-triene reaction in either case the predicted result is that for the thermal process, only the disrotatory pathway is allowed, and for the... 

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. 

Spin-coupled theory has been used to smdy the changes that occur in the electronic wavefunction as a system moves along the intrinsic reaction coordinate for the case of the conrotatory and disrotatory pathways in the electrocyclization of cyclobutene to c/x-butadiene. Against intuitive expectations, conrotatory opening of cyclobutenes was found to be promoted by pressure. Ab initio MO and density functional calculations have indicated that the ring opening of the cyclobutene... 

This conversion is very slow thermally (i.e., without the catalyst) because the reaction must take place by a disrotatory pathway, which is disallowed thermally.388... 

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