Rotation disrotatory

When an aziridine ring is fused at the 2,3-bond to another ring system, the only ring opening structurally allowed is a disrotatory rotation, which must be photolytically performed as discussed above. Thus, the fused aziridine 8 on irradiation generates syn-azomethine ylide 9, which is captured by an acetylene to give a bicyclic cycloadduct (68CPB764). 

The states involved in the photochemical butadiene-to-cyclobutene conversion are if/u ( 2, and il/ of the first excited state of butadiene, and a, tt, and tt for the lowest excited state of cyclobutene. The correlation diagram for this reaction is shown in Fig. 11.4. The appropriate elements of symmetry are the plane of symmetry and the axis of symmetry corresponding to conrotatory and disrotatory rotation ... 

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. 

Conservation of orbital symmetry is a general principle that requires orbitals of the same phase (sign) to match up in a chemical reaction. For example, if terminal orbitals are to combine with one another in a cyclixation reaction as in pattern. A, they must rotate in the same dii ection (conrotatory ovei lap). but if they combine according to pattern H. they must rotate in opposite directions (disrotatory). In each case, rotation takes place so that overlap is between lobes of the it orbitals that are of the same sign. 

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. 

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 is an example of an electrocyclic reaction, and involves rotation of the terminal methylene groups either in the same way ( conrotatory ) or in opposite ways ( disrotatory ). 

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. 

For a bond to form, the outermost tt lobes must rotate so that favorable bonding interaction is achieved—a positive lobe with a positive lobe or a negative lobe with a negative lobe. If twro lobes of like sign are on the same side of the molecule, the two orbitals must rotate in opposite directions—one clockwise and one counterclockwise. This kind of motion is referred to as disrotatory. 

Disrotatory (Section 30.2) A term used to indicate that p orbitals rotate in opposite directions during electrocvclic ring-opening or ring closing. 

Use of substituted systems has shown that the reaction is stereospecific.300 The groups on C(2) and C(5) of the pyrroline ring rotate in the disrotatory mode on going to product. This stereochemistry is consistent with conservation of orbital symmetry. 

As there are 6 ir electrons to accommodate—two per orbital—the HOMO will be ip3 (11). To form the C—C a bond on cyclisation, the orbital lobes on the terminal carbon atoms of the conjugated system (C2 and C7—the C atoms carrying the Me substituents) must each rotate through 90° if mutual overlap is to occur (p/sp2— sp3 re-hybridisation must also occur). This necessary rotation could be either (a) both in the same direction—conrotatory (12), or (b) each in opposite directions—disrotatory (13) ... 

In the following disrotatory motion, the movement can be in any of the ways, either by inward or outer ward rotation. (Opening by movements in different directions). 

In disrotatory motion one p orbital will rotate in clockwise and the other in anticlockwise direction. 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Liebeskind favors a mechanism for the formation of 114 that cites the i72-alkene complex 115 as a crucial intermediate. The cobalt moiety coordinates on the opposite face to the R1 substituent, which means that upon disrotatory opening98 of the carbocycle the R1 group rotates inwards, accounting for the favored anti stereochemistry. The possibility of the vinylke-... 

Dynamics calculations have also provided new approaches to the stereochemical modes through which cyclopropanes and trimethylene intermediates may be related. Full quantum dynamics calculations for the trimethylene diradical based on a reduced dimensionality model that followed wave packet densities and time constants for formation of products led to the conclusion that conrotatory and disrotatory double rotations of both terminal methylene groups are favored over a single rotation of just one by a 2.2 1 ratio." °... 

On the other hand, it is conceivable that, if 7a undergoes disrotatory ring opening to 8a, energy remains in the disrotatory mode of methylene rotations for the short time that 8a takes to cross the TS for disrotatory closure to 7a. In this dynamical model, disrotatory ring opening to 8a, rather than resulting in preferential... 

To achieve this stereospecificity, both terminal C s rotate 90° in the same direction, called a conrotatory motion. Movement of these C s in opposite directions (one clockwise and one counterclockwise) is termed disrotatory. 

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

For a second example I take the ring opening of butene to cis butadiene. Considerations of symmetry allow a distinction to be made between paths which are distinguished by the modes of rotation of the CHj groups conrotatory and dis-rotatory as shown in Fig. 9. In the conrotatory mode the molecule retains a 2-fold axis of symmetry so that orbitals or states can be characterized by the symmetry labels A or 5, and in the disrotatory mode there is a plane of symmetry so that the symmetry labels. <4 and A , can be used. 

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