Concerted disrotatory mechanism

In other studies, Clark and Smale have investigated the effects of a-phenyl substitution in the solvolyses of endo- and exo-bicyclo[3.1.0]hexyl through [5.1.0]octyl chlorides. For the endo derivatives, where the solvolyses proceed by a favored, concerted disrotatory mechanism in the parent systems, replacement of hydrogen by phenyl gives a relatively small rate enhancement (kph/ H = 1 )- other hand with the exo[3.1.0]... 

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). 

For nonsymmetrically substituted oxiranes, the fragmentation pathway depends on the nature of the substituents. Groups capable of stabilizing an anion become part of the carbene fragment. Conclusions may be drawn about the mechanism of photolysis from the mode of the fragmentation. It has been suggested that the C-C bond is split in a concerted disrotatory manner with retention of the orbital symmetry ylides are formed that are zwitterionic in structure (Eq. 369), and they cause the photochromic behavior exhibited by 2,3-diaryloxiranes at low temperature. 

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. 

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... 

Since the detailed calculation of these matrices is sufficiently described in the original literature [33, 58], it is possible to present directly the final results first for the case of concerted reactions for which there are two alternative reaction mechanisms, conrotatory and disrotatory. The first of these mechanisms is allowed by the Woodward-Hoffmann rules while the second one is forbidden. 

Bordwell and coworkers reported the stereospecific quantitative desulfurization of episulfides with organolithiums.39) The mechanism of this reaction, however, remained undefined. In addition to a disrotatory concerted sulfurane decomposition [i.e. Eq. 9), p elimination from a 2-alkylthioalkyl-lithium may be envisioned (see Eq. 10). However, independent stereo-... 

Fig. 6.4 Ring closure of cis-1,3, 5-hexatriene to cyc/o-hexadiene. In C2v symmetry there is a symmetry mismatch between the b and a occupied orbitals. Vibronic orbital coupling requires a concerted mechanism, based on a disrotatory ring closure, which conserves only the o plane...

The first step is a disrotatory cyclohexadiene-hexatriene isomerization. Its product, cf5-dihydrobenzocyclooctatetraene, is less stable than the trans dimer and is known to isomerize to it, [27] the isomerization presumably taking place via an a" displacement that reduces symmetry to Ci, in which no reaction is forbidden. At the higher temperatures at which fragmentation occurs, the first product should be in equilibrium with the reactant, and its eight-membered ring is sufficiently flexible that a similar desymmetrization would allow it to serve as an unstable intermediate. The activation parameters cited above, which - for the postulated mechanism - measure the enthalpy and entropy differences between the reactant and the transition state of the second step, are not inconsistent with concerted electrocyclic rupture of both bonds via a relatively unconstrained transition state. 

Structural constraints can prevent reactions that would be allowed by these rules. For example, the very strained bicyclo[2.2.0]hex-2-ene (Dewar benzene) opens slowly at 130°C to cyclohexadiene [31], while the less strained bicyclo[4.2.0]octa-2,4-diene is in rapid equiUbrium with cyclooc-tatriene at 80-100°C [32]. The allowed conrotatory movement in the first case would lead to cyclohexadiene with a trans double bond, which is impossible. However, the latter opening is disrotatory, giving all-cw double bonds in the cyclooctatriene. The higher-temperature reaction, disallowed by these rules, is a higher activation energy process, perhaps involving radicals, but not a concerted mechanism. 

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