Conrotatory cyclobutene

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). 

As a first example of an electrocyclic reaction illustrating stereochemistry, let us take the pair of conrotatory cyclobutene openings, showing that the reactions are stereospecific. 

Fig. 6.42 7t-Energy changes for a conrotatory cyclobutene opening with a substituent on C-3...

With the conrotatory cyclobutene ring opening, secondary electronic effects contribute. Electron-releasing groups prefer to rotate outward, and electron-withdrawing groups prefer to rotate inward. 

Scheme 1. The reactions that were studied to demonstrate the reactivity-selectivity principle in conrotatory cyclobutene ring opening.

Thus, cis,trans,cis,cis-l,3,5,7-cyclononaXcirsiQnQ could be trapped during the thermolysis of the 6.1.0 triene. Subsequently it was found that this tetraene could be trapped with high concentrations of TCNE in a 2 + 2 fashion with a rate independent of TCNE concentration, which indicates rate-determining formation of the tetraene.Adducts of this tetraene were isolated earlier, and these were subsequently shown to be of the trans stereochemistry. Further, it was also found that the syn and anti isomers of 9-deuterio-cw-bicyclo[6.1.0]nonatriene interconvert prior to the isomerization to the dihydroindenes. The most likely pathway to the tetraene is a divinylcyclopropane 3,3-shift to bicyclo[5.2.0]nona-2,5,8-triene followed by a conrotatory cyclobutene ring opening (Scheme 10.11). ... 

Q 14. While benzo analogues II—IV undergo cycloreversion of the central cyclohexane ring by all disrotatory opening, the triscyclobutenocyclohexane I does so by stepwise conrotatory cyclobutene rupture. How do you explain these observations ... 

Electi ocyclic reactions are examples of cases where ic-electiDn 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 fomred 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 HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

SUBSTITUTED BUTADIENES. The consequences of p-type orbitals rotations, become apparent when substituents are added. Many structural isomers of butadiene can be foiined (Structures VIII-XI), and the electrocylic ring-closure reaction to form cyclobutene can be phase inverting or preserving if the motion is conrotatory or disrotatory, respectively. The four cyclobutene structures XII-XV of cyclobutene may be formed by cyclization. Table I shows the different possibilities for the cyclization of the four isomers VIII-XI. These structmes are shown in Figure 35. 

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. 

Fig. 11.5. Correlation diagram for cyclobutene and butadiene orbitals (symmetry-allowed conrotatory reaction).

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

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. 

Symmetry-allowed reaction (Section 10.14) Concerted reaction in which the orbitals involved overlap in phase at all stages of the process. The conrotatory ring opening of cyclobutene to 1,3-butadiene is a symmetry-allowed reaction. 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

Ring-opening of the frans-cyclobutene isomer shown takes place at much lower temperature than a similar ring-opening of the cis-cyclouiitciie isomer. Explain the temperature effect, and identify the stereochemistry of each reaction as either conrotatory or disrotatory. 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

The reverse reaction is also conrotatory. In contrast, the photochemical cyclobutene 1,3-diene interconversion is disrotatory in either direction." On the other hand, the... 

Construct a state correlation diagram for the conrotatory butadiene-cyclobutene system. 

This time it is conrotatory movement that results in a bonding situation, and formation of the trans dimethylcyclobutene (18). For the photochemical interconversion (which tends to lie over in favour of the cyclobutene), irradiation of the diene will result in the promotion of an electron into the orbital of next higher energy level, i.e. 3, and the HOMO to be considered now therefore becomes i/r3 (23) ... 

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

The thermal ring-closure of butadienes to cyclobutenes proceeds in a conrotatory fashion (equation 2) but this reaction is only observed in special cases because, in general, the equilibrium lies on the side of the open-chain isomer. 

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