Rotation conrotatory

The barrier for stereomutation is calculated to be close to 61 kcalmoT1 (CISD+Q/TZ + 2py 32,275 Conrotatory double rotation of the methylene groups is about 1 kcal mol-1 more favourable than single rotation of a methylene group. Calculations further reveal that dis-rotatory rotation is also possible with a barrier0.5 kcalmol-1 larger than conrotatory rotation (Figure 27). 

Disrotatory and conrotatory rotation The concerted rotation around two bonds in the same direction, either clockwise or counterclockwise, is described as conrotatory. In the electrocyclic ring opening, the terminal p-orbitals rotate (or twist, roughly 90°) in the same direction known as conrotation (comparable to antarafacial) to form a new ct-bond. In disrotatory cyclization (comparable to suprafacial) the terminal p-orbitals rotate in opposite directions. These two modes of the electrocyclic reaction are shown in Fig. 8.37. 

An elegant test of the Hoffmann prediction was executed by Berson and co-workers. They prepared the trans-l,2-dideuterocyclopropane of Figure 11.16 B enantiomerically enriched—this simple molecule is chiral by virtue of isotopic substitution. If, after ring opening at the C1-C2 bond, there is a coupled, conrotatory rotation about the C-C bonds, we will directly produce the enantiomer of the original trans starting material. Alternatively, if the C-C bonds of the biradical rotate independently, we expect formation of the cis-... 

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. 

Conversely, if lobes of like sign are on opposite sides of the molecule, both orbitals must rotate in the same direction, either both clockwise or both counterclockwise. This kind of motion is called conrotatory. 

Conrotatory (Section 30.2) A term used to indicate that p orbitals must rotate in the same direction during electro-cyclic ring-opening or ring closure. 

This is evidence for a four-membered cyclic transition state and arises from conrotatory motion about the C-3—C-4 bond. It is called conrotatory because both movements are clockwise (or both counterclockwise). Because both rotate in the same direction, the cis isomer gives the cis-trans diene. 

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) ... 

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. 

Tius and co-workers elegantly applied a variant of the Nazarov reaction to the preparation of cyclopentenone prostaglandins (Scheme 19.39) [46]. Moreover, it was demonstrated that the chirality of non-racemic allenes is transferred to an sp3-hybridized carbon atom. Preparation of allenic morpholinoamide 214 and resolution of the enantiomers by chiral HPLC provided (-)- and (+)-214. Compound (-)-214 was exposed to the vinyllithium species 215 to afford a presumed intermediate which was not observed but spontaneously cyclized to give (+)- and (—)-216 as a 5 1 mixture. Compound (+)-216 was obtained with an 84% transfer of chiral information and (-)-216 was obtained in 64% ee. The lower enantiomeric excess of (—)-216 indicates that some Z to E isomerization took place. This was validated by the conversion of 216 to 217, where the absolute configuration was established. The stereochemical outcome of this reaction has been explained by conrotatory cyclization of 218 in which the distal group on the allene rotates away from the alkene to give 216. 

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." °... 

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. 

The reaction was studied theoretically by Hoffmann who concluded that it should involve a double rotation of the methylene groups.81 A study of the kinetics of racemization vs. isomerization of optically active trans-l, 2-dideuteriocyclopropane provided evidence for this process.82 Further experimental studies have made the interpretation of the details of this reaction more difficult.83 The most recent calculations84 have concluded that the conrotatory double rotation is slightly preferred. It is likely that substituents could have a major effect on the course of the reaction. 

---