Stereochemistry disrotatory process

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

These selection rules can also be obtained from an analysis of polyene cyclizations. If the interaction between the terminal atoms Ca and Cn is bonding (antibonding), it will favor (disfavor) the cyclization. Figure 4.5 shows how the contribution of any given MO changes as a function of the reaction stereochemistry. When p is odd (even) the conrotatory process is disfavored (favored) and the disrotatory process is favored (disfavored). Obviously, the preferred pathway can be deduced by summing the contributions of all of the occupied MOs, up to and including the HOMO ... 

To obtain the stereochemistry observed in the product the long chain (carbon A) must move behind the plane of the page and hydrogen B (Hg) must move in front of the plane of the page. This is a disrotatory process. 

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

Figure 4.41 (a) Orbital overlaps in conrotatory and disrotatory o bond formation in a ring-closure reaction, (b) Stereochemistry of preferred products in photochemical and thermal (dark) processes... 

Although the stereochemistry could not be seen in the last example, it is evident in the migration of the hydrogen atom in the zwitterion intermediate 5.46, which is symmetry-all owed as a suprafacial [1,4] shift. Although the first step is a photochemical disrotatory 6-electron process, the second step is... 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

The stereochemistry of a thermally induced 10e electrocyclization (predicted to be disrotatory) has not been firmly established and the main synthetic application is found in the formation of azulenes and ring-fused azulenes as in the transformation (452) to (453). Thermolysis of (454) with spontaneous elimination of dimethylamine from intermediate (455) afforded the fused azulene structure (456). The chemistry of even higher order (12e to 20e") pericyclic processes has been recently reviewed. An example of an unusual sequence of pericyclic processes is the transformation of heptahendecafulvadiene (457) to the pentacyclic hydrocarbons (462) and (463) in a 2 1 ratio. The pathway for this transformation can be viewed as an initial conrotatory 20e electrocyclization followed by a cascade of 10e and 6e pericyclic processes. ... 

Problem 6.1 a. For the stereochemistry shown, the process must be disrotatory. This is a four-electron process and, thus, is symmetry-forbidden thermally. The term symmetry-forbidden does not mean the reaction is impossible, merely that so much energy is required that other processes usually occur instead. 

Suffert et reported the first examples of palladium-catalyzed 4-exo-dig cyclization-anion capture processes (Scheme 5.6.3). In an ingenious sequence, which incorporates two separate electrocyclic steps, the vinyl starter species 10, in the presence of Pd(0) and iranx-vinyl distannane 11 in benzene at 90 °C for 30 min, affords 12, which on further heating undergoes disrotatory electrocyclization to the strained tricyclic diol 13 (Scheme 5.6.3). The electrocyclization process establishes the expected anti-stereochemistry of the 6-H and tri-n-butylstannyl group. Intermediate 13 undergoes elimination-... 

We have considered three viewpoints from which thermal electrocyclic processes can be analyzed symmetry characteristics of the frontier orbital, orbital correlation diagrams, and transition state aromaticity. All arrive at the same conclusions about the stereochemistry of electrocyclic reactions. Reactions involving 4n + 2 electrons are disrotatory and involve a HUckel-type transition structure, whereas those involving 4n electrons are conrotatory and the orbital array are of the Mobius type. These general principles serve to explain and correlate many specific experimental observations. The chart that follows summarizes the relationship between transition stmcture topology, the number of electrons, and the feasibility of the reaction. 

Generalization of either the frontier orbital, the orbital symmetry, or the transition-state aromaticity analysis leads to the same conclusion about the preferred stereochemistry for concerted thermal electrocyclic reactions The stereochemistry is a function of the number of electrons involved. Processes involving 4n + 2 electrons will be disrotatory those involving 4n electrons will be conrotatory for Hiickel transition states. The converse holds for Mobius transition states. 

A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode of electrocyclic reaction is called disrotatory,... 

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