Disrotatory ring closure

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

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. 

The reaction of 1-aminopyridinium iodide (429) with dimethyl chlorofumarate in ethanol/K2C03 to form, ultimately, a pyrazolo[l,5-n]pyridine also occurs via a 1,5-dipolar mechanism. The initially formed 1 1 adduct (430), stabilized by delocalization of the negative charge, underwent disrotatory ring closure as shown to give (431) in which the 3... 

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. 

Display the HOMO for cis-l,3,5-hexatriene. Which motion (conrotatory or disrotatory) insures bonding overlap Examine the geometry of the transition state for ring closure (hexatriene to cyclohexadiene). Is it consistent with the anticipated (conrotatory or disrotatory) motion of the terminal methylenes ... 

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. 

Active Figure 30.5 Thermal cyciizaltons of 2,4,6-octatrienes occur by disrotatory ring closures. Sign in atwww.thomsonedu.com... 

E,4Z,6Z,8 )-2,4,6,8-Decatetraene has been cyclized to give 7,8-dimethyl-1,3,5-cycloodati icne. Predict the manner of ring closure—comoiatory or disrotatory— for both thermal and photochemical reactions, and predict the stereochemistry of the product in each case. 

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohex-adienes in agreement with the Woodward-Hoffmann rule (delocalization band in Scheme 8) [41 3]. Photo-irradiation of hexatrienes is known to give bicylic products in a stereospecific [4n +2nJ manner (delocalization band in Scheme 8) [40] in contrast to this rule. 

Bridging of a different type has been observed in 3-oxido-l-phenylpyri-dinium (213) which on irradiation in ethyl acetate affords the isomer 214 by way of a photochemically allowed disrotatory ring closure.158 Similarly, stable diaziridines (215) have been isolated on irradiation of 3-oxidopyrida-zinium betaines (216).159 Other related transformations in 2-alkylcin-nolinium-4-olates160 and in 5-oxidopyridazinium betaines161 have been... 

The stereochemical significance of ring opening or ring closure becomes marked by taking substituted reactants. The identification of the product will indicate whether the reaction has taken place in con or disrotatory manner. 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

The first electrocyclic ring closure involves eight electrons, so it is conrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the tetraene, which are both in, become trans. The second electrocyclic ring closure involves six electrons, so it is disrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the triene, which are both out, become cis. This is the arrangement observed in the natural product. 

The HOMO of the pentadienyl cation is j/, which is antisymmetric, so a conrotatory ring closure occurs, consistent with the four electrons involved in this reaction. The HOMO of the pentadienyl anion is /2, which is symmetric, so a disrotatory ring closure occurs, consistent with the six electrons involved in this reaction. 

Other aliphatic acyclic dienes such as isoprene (2)102,111, 2-isopropyl-1,3-butadiene (24)102, and , -2,4-hexadiene (5)78 also yield the corresponding cyclobutene, all via excitation of the s-cis conformer. The latter yields the disrotatory ring closure product, c -3,4-dimethylcyclobutene (54), stereospecilically78. 

As mentioned in the Introduction, the ring closure of s-cis butadiene to cyclobutene has been at the very center of the evolution of theoretical understanding of polyene photochemistry to its current state25,87-89,151. Early ab initio calculations recognized the crucial role of the 21Ag state in the isomerization, and successfully accounted for the disrotatory stereospecificity of the reaction in terms of a two-dimensional model in which the planarity of the carbon framework is more or less maintained throughout12,13,15. 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers corresponding to disrotatory 47T-electrocyclization (185) and bicyclo[3.1.0]pentene (186) formation along with small amounts of the reduced product 187 (equation 69)279-281. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms)282, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184282 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy49. 

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

Upon exposure to UV light, a-tropolone methyl ether (142), included within chirally modified Y zeolite, has been found to undergo 4 7r-electron disrotatory electrocyclic ring closure to afford " the bicyclic photo-isomer (143). 

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). 

Fig. 20 Energy diagram that illustrates the S0, T, S, and S2 states for the photochemical disrotatory ring closure of butadiene. (Diagram adapted from Salem, 1982)...

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