Disrotatory closure

Disrotatory cis-S,6-dimethyl- 1,3-cyclohexadiene conrotatory fraw-5,6-d Unethyl -1,3-cvclohexadiene. Disrotatory closure occurs. 

All the y-sultines were obtained as diastereomeric mixtures (ca 1 1, by NMR), and each one of y-sultines ( + )-49 and ( + )-51 (R = t-Bu) was separated into two diastereomers A and B by column chromatography. The oxidation of y-sultines (— )-49A and (+ )-49B to the corresponding optically active sultones (+ )-52A,B, which lack a chiral sulfur, may be taken as proof that the observed optical activity in the sultines is also due to the y-carbon. This result seems to exclude the intermediacy of vinylsulfene in the reaction mechanism, since its disrotatory closure would lead to racemic y-carbon in the product. 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Figure 9.13. Transition states for (a) conrotatory and (b) disrotatory closure.

In an interesting illustration of these reactions, the two disymmetric trienes (+)-33a and (—)-33b were found to preserve their chirality upon photolysis at 193 K and provide cyclohexadienes 34a and 34b, respectively (Scheme 8)18. Upon warming above 205 K, however, they lose their chiral integrity by competitive disrotatory cyclization to the achiral dienes 35a and 35b. The thermal disrotatory closure to the cis-fused ring isomer is generally found to be extremely facile in these systems. 

Many other examples of contrasting behaviour have been discovered. For example all-cis-cyclodecapentaene (VII) photochemically equilibrate at low temperatures with trans 9, 10 dihydronapthalene by a conrotatory six electron electrocyclic reaction but it is converted thermally into cis-9, 10 dihydronaphthalene by disrotatory closure. 

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. 

The photochemical disrotatory closure of butadiene to cyclobutene has been described with a state-correlation diagram, like that shown in Figure 21.4. It is based on the familiar orbital-correlation diagram of Woodward and Hoffmann," from which the intended correlations indicated by the dashed lines can readily be deduced. The solid lines indicate that there is an avoided crossing, which is put in as a result of the quantum mechanical noncrossing rule. It says that two states of the same total symmetry cannot cross. Instead, as they approach each other in energy, they will mix and separate, as the solid lines indicate. 

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... 

Figure 11.9 Symmetry at an intermediate stage of the disrotatory closure. Only the mirror plane a remains as a symmetry element.

Figure 11.10 Symmetry classification and correlation of orbitals for the disrotatory closure of butadiene. Closure with electron pairs remaining in their original levels would lead to the excited state indicated by the orbital occupancy on the right.

It is instructive now to turn to the correlation diagrams in Figures 11.11 and 11.12 for conrotatory and disrotatory closure of hexatriene, a six tt electron system. The disrotatory mode is now allowed, the conrotatory forbidden. If correlation diagrams for larger systems are constructed, it will be found that with each addition of two carbons and an electron pair the predicted selectivity will reverse. 

Figure 11.12 Correlation diagram for the allowed disrotatory closure of hexatriene.

The antibonding overlap in the HOMO when disrotation occurs makes the formation of the new sigma bond unfavorable. The disrotatory closure of a diene to a cyclobutene is thermally forbidden. 

Disrotatory closure of the substituted benzene to produce a Dewar benzene is photo-chemically allowed, as is, of course, the reverse process. However, because benzene is conjugated, it absorbs UV light at longer wavelengths than the Dewar benzene isomer. Therefore, it is possible to selectively excite the benzene chromophore and produce the less stable Dewar isomer. In this particular case the rm-butyl groups favor the reaction because they destabilize the benzene isomer somewhat, owing to steric hindrance. Because the two adjacent im-butyl groups in the Dewar isomer do not lie in the same plane, this steric strain is decreased in the product. Because of this steric effect and the forbidden nature of the conversion back to benzene, the Dewar isomer is relatively stable. However, when it is heated to 200°C, it is rapidly converted to the benzene isomer, probably by a nonconcerted pathway. 

Figure 7-18. Correlation diagram for the disrotatory closure of butadiene. Adaptation of Figure 10.14 from reference [81] with permission.

Extensive investigations of the thermal and photochemical reactions of [Pg.721]

The predicted photochemical disrotatory closure of protonated divinyl ketones has been documented in several laboratories, most notably by Nozaki, Noyori, and Cerfontain (equation 3). The stereochemical outcome in these reactions was discernible due to secondary processes which preserved the sense of electrocyclization. 

The theory of electrocyclic reactions predicts a disrotatory closure of 4 ir-systems in the excited state. This prediction was verified in fact by Lehr using the same biscyclohexenyl ketone (equation 14). The cyclization proceeded at various wavelengths in pentane as well as in benzene. The reaction could not be inhibited by naphthalene or piperylene. Sensitization with acetophenone did not accelerate the reaction. Finally, no deuterium was incorporated when (fa-benzene was used as solvent. -Damascone undergoes an apparent disrotatory closure with capture of the zwitterionic intermediate by solvent (equation 15). ... 

Subsequent to these developments Mishra and Crawford , in a further attempt to elucidate the decomposition mechanism and nature of intermediates, have investigated the stereometric distribution of products in the thermolysis of (3R 5R)-(+)-rranj-3,5-dimethyl-l-pyrazoline. If the reaction proceeds as postulated by Crawford et al., the tra/w-l,2-dimethylcyclopropane product would result from the disrotatory closure of the planar biradical and should be racemic, while the above two alternatives, step-wise elimination of nitrogen or the involvement of pyramidal diradicals by inversion at both asymetric centres would be expected to yield optically active tranj-1,2-dimethylcyclopropane. 

The stereochemistry of the initial bromination turns out to be irrelevant as it disappears when the. > yallyl cation is formed. We know the stereochemistry of the final product so we know the stereochemistry of the cyclopropanone it must be on the opposite face of the five-membered ring to lie methyl group. The disrotatory closure of the oxyallyl cation goes preferentially one way with the H and CMe2Br substituents going upwards and the carbonyl group going down. 

Disrotatory cis-5,6-dimethyl-1,3-cyclohexadiene conrotatory irons-5,6-dimethyl-1,3-cyclohexadiene. Disrotatory closure occurs. 

Disrotatory cis-5,6-dimethyU l,3cyclohexadiene conrotaiory f/-ons-5.6-dimethyl-l,3-cyclohexadiene-Disrotatory closure occurs. 

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