Reaction pathway conrotatory

Formation of a tr bond by ( + )-( +) overlap of p orbital lobes at the termini of i/>2 of 1,3-butadiene through a conrotatory reaction pathway (thermal reaction). 

In a similar process, tertiary enaminones react with alkynylcarbene complexes to give the corresponding pyranylidene complexes following a reaction pathway analogous to that described above. First, a [2+2] cycloaddition reaction between the alkynyl moiety of the carbene complex and the C=C double bond of the enamine generates a cyclobutene intermediate, which evolves by a conrotatory cyclobutene ring opening followed by a cyclisation process [94] (Scheme 49). 

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 ... 

Scheme 12. Different photochemical reaction pathways of czs-stilbene (1) (i) to phenanthrene (2) by conrotatory [n6] electrocyclization, and (ii) to tetraphenylcyclobutane (33) by a [2+2] photocycloaddition [67a-d, 68]...

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... 

Not only must we consider the symmetry properties of 1,3-butadiene orbitals, but we must also consider the s)mtunetry properties of both cyclobutene and the transition structure expected for the conversion of the reactant to product. The two pathways for the closure of 13-butadiene to cyclobutene are illustrated in Figure 11.16. The Cl—C2, C2—C3, and C3—C4 a bonds are shown as solid lines. The p orbitals of 1,3-butadiene and cyclobutene, as well as the sp orbitals of the C3—C4 cr bond of cyclobutene, are represented by the shapes of the atomic p or sp orbitals. This is therefore only a basis set representation, not an illustration of a particular molecular orbital. Although there are many symmetry elements present in the representations of both 1,3-butadiene and cyclobutene, in the conrotatory reaction the only symmetry element that is present continuously from reactant through transition structure to product is the C2 rotation. Similarly, only the a reflection is present from reactant through transition structure to product for the disrotatory pathway. 

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. 

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). 

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-... 

Recently, Rueping et al. [36] disclosed the first enantioselective Nazarov cyclization reaction organocatalyzed by a chiral Brpnsted acid. The proposed reaction pathway involves a conrotatory 4n electrocyclization of the divinyl ketone 83 leading to the formation of an enol intermediate which is then snbjected to enantioselective protonation by the chiral Brpnsted acid 82b. This electrocyclization-protonation reaction was conducted with various divinyl ketones 83 under optimized conditions, i.e. in the presence of the chiral A -triflyl phosphoramide 82b (5 mol%) in chloroform at -10°C, affording the corresponding cyclopentenones 84 with 67-78% ee (for examples, see 83a-c 84a-c, Scheme 3.44). 

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. 

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... 

Both the frontier-orbital and the Mobius-Hlickel 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 photochemical process, only the conrotatory. For example, for a 1,3,5-triene, the symmetry of the HOMO is... 

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