Electrocyclic rearrangements disrotatory

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... 

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

On irradiation the diazepinone 7 rearranges to the bicyclic ketone 8 by a symmetry allowed disrotatory 4rc-electrocyclization.9 5... 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

How can we account for the stereoselectivity of thermal electrocyclic reactions Our problem is to understand why it is that concerted 4n electro-cyclic rearrangements are conrotatory, whereas the corresponding 4n + 2 processes are disrotatory. From what has been said previously, we can expect that the conrotatory processes are related to the Mobius molecular orbitals and the disrotatory processes are related to Hiickel molecular orbitals. Let us see why this is so. Consider the electrocyclic interconversion of a 1,3-diene and a cyclobutene. In this case, the Hiickel transition state one having an... 

The rules for disrotatory electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are summarized in the accompanying chart ... 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocyclic reactions, the rules regarding conrotatory and disrotatory cyclizations are reversed. 

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 value of the terms suprafacial and antarafacial is that, unlike disrotatory and con-rotatory, they can also be used to describe the way that 7r systems react in cycloadditions and sigmatropic rearrangements. Most importantly, when a 77 system reacts suprafacially, its out groups become cis in the product when it reacts antarafacially, they become trans. Note that in disrotatory electrocyclic reactions, the out groups become cis, and in conrotatory electrocyclic reactions, the out groups become trans. 

The Nazarov cyclization is an example of a 4iT-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear p ictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constmeted to allow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with the pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatory) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

Tropilidene (1,3,5-cycloheptatriene) is a non-planar, tub-like molecule with only a 6.3 kcal/mol barrier to ring inversion. It is converted to toluene upon heating with log = 13.54 - 51 100/2.3/ T or log = 13.9 - 52200/2.3/ r. Themost reasonable course of this rearrangement is disrotatory electrocyclization to norcaradiene followed by homolytic cleavage of an external cyclopropane bond to a 1,3-biradical and subsequent vicinal hydrogen shift (Scheme 8.8). 

The key concept for the formulation of the rules in question is the concept of the transition vector (see Sect. 1.3.3.1). This vector may be regarded as a quite short, albeit finite, portion of the part of the reaction path whose beginning lies at the transition state point. A displacement of the system in the direction defined by the transition vector lowers its potential energy. Figure 1.10 shows as an example the form of the transition vector for the electrocyclic reaction of disrotatory rearrangement of the cis-Dewar benzene into benzene [49]. 

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