Electrocyclic transformation

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

To date, only a few examples have been identified of electrocyclic transformations and ene reactions as the initiating event in a domino process. 

Vollhardt s investigations100 into electrocyclic transformations on a CpCo template produced the following unusual result. After photolysis of CpCo(CO)2 in the presence of the tosyl hydrazone of trans-4-pheny -3-buten-2-one, the only isolated product was the vinylketene complex 123. Note that the tricarbonyliron analogue of this complex has also been isolated.3,87 The mechanism of formation was not discussed, but it seems likely that the ketene carbonyl originated as a carbonyl ligand that replaced the hydrazone moiety, perhaps via a vinylcarbene intermediate. 

Thus, the study of the crown containing systems whose photochromism is based on the electrocyclic transformation demonstrates that the complex formation process can be successfully applied for controlling of its spectral and photochemical properties. 

The practical exploitation of the proposed criterion can be very simply demonstrated by the example of the electrocyclic transformation of cyclobutadiene to cyclobutene, for which the structure of the intermediate can be quite reliably estimated from the available results of quantum chemical calculations [74] (Scheme 2). 

As an example of the practical use of the above criterion, let us discuss again the electrocyclic transformation of butadiene to cyclobutene. The individual alternative reaction mechanisms are described, as in the previous chapter, by the scheme II. For evaluating relative ease of individual reaction paths, it is necessary to first calculate the density matrices PR, P, and FP which are then, in the next step converted into the similarity indices rRP, rRI and rIP. Such a calculation requires, however, the density matrices to be transformed into the common basis of atomic orbitals [33,43]. These transformations are described by the matrices TRP, TRI and TIP which has to be determined for each elementary step. 

Electrocyclic Reactions.—Peyerimhoff, Buenker, and co-workers have carried out very detailed studies of the electrocyclic transformations159 between cyclic and open-chain hydrocarbons. The calculations employ a large GTO set of s- and p-type basis functions. In every study, the necessity of including limited configuration interaction was carefully investigated. The prototype electrocyclic transformation of cyclobutene to cis-butadiene via the thermochemical process has been studied in detail.160-161 The same authors also give an analysis of the qualitative theories for such reactions based on their ab initio calculations.163 A similar study of the electrocyclic transformations of cyclopropyl and allyl systems has also been made.163... 

Hsu K, Buenker RJ, Peyerimhoff SD, Theoretical determination of the reaction path in the prototype electrocyclic transformation between cyclobutene and cis-butadiene Thermochemical process J Am Chem Soc 93, 2117-2127 (1971)... 

K. Hsu, R. J. Buenker, S. D. Peyerimholf, Theoretical Determination of the Reaction Path in the Prototype Electrocyclic Transformation between Cyclobutenebndci. -Butadicne. Thcrrnochcrnical Process. ./. Am. Chem. Soc. 1971,93,2117-2127. 

Condensed 4-nitropyrazoles have been produced from pyridinium N-imines and l,l-bis(methylmercapto)-2-nitroethylene 2-methylthio-l-nitropyrazolo[l,5-a]pyridines (54) (40-60%) are so obtained.49 After the electrocyclic transformation, the intermediate 53 does not lose the elements of nitrous acid, but rather undergoes dehydrogenation to yield the observed products (Scheme 11). Isoquinolinium (V-imine gives 2-methylthio-l-nitro-pyrazolo[5,1 -a]isoquino ine similarly.49... 

The nature of silver s role in the catalysis of s mimetry-forbidden reactions, however, is puzzling. Silver salts are kno-wn to catalyze symmetry-forbidden electrocyclic transformations with remarkable facility 3) when rhodium(I) complexes exhibit only marginal activity Rhodium(I), of course, readily inserts into the a bond of cubane giving the kind of intermediate required for an allowed [a2 -b [Pg.84]

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

Electrocyclic transformations involve intramolecular formation of a ring by bond formation at the ends of a conjugated tt system. The product has one more a bond and one less tt bond than the starting material. The... 

Transformation of starting material to 6-17 involves a concerted elec-trocyclic transformation, followed by anion-promoted ring opening of the epoxide. The other transformations are one-step concerted processes. Explain the stereochemical preferences of each reaction. Why is the electrocyclic transformation of 6-17 to 6-19 favored when such reactions usually occur more readily in the opposite direction ... 

Hint 6.15 In the frontier orbital approach to electrocyclic transformations, the course of... 

Initial interest in the solvolyses of cyclopropyl derivatives stemmed from the observation that they underwent solvolysis with concerted ring-opening , and that the reaction was strongly dependent on the nature and stereochemistry of substituents on the ring. This was explained by Woodward and Hoffmann who predicted from orbital symmetry considerations that the electrocyclic transformation in which a cyclopropyl carbocation is converted to an allyl cation should occur in a disrotatory fashion. Also, the particular disrotatory path a given system will take should be dependent on the stereochemistry of the leaving group. This is illustrated as follows. 

The electrocyclic transformation of the )5-lithiocyclopropyloxirane 336" also occurs in a conrotatory fashion as suggested by the formation of 338 (after protonation) from the... 

It was shown that the cyclopropyl cation is not an intermediate but that ring-opening occurs simultaneously with loss of tosylate. Prediction and experiments showed that for this electrocyclic transformation, susceptible to treatment by the Woodward-Hoffmann rules , the substituents cis to the leaving group rotate inwardly and substituents trans to the leaving group rotate outwardly (equation 2). 

Woodward and Hoffmann have defined electrocyclic transformations as the formation of a single bond between the termini of a linear system containing k ir electrons [XIX->XX], and the converse process (1). 

Electrocyclic transformations, in fact, proceed with high stereospecificity dictated by the number of n electrons in the open-chain it system (XIX). The reaction course taken by an electrocyclic transformation follows that in which the highest occupied molecular orbital in XIX has maximum bonding character throughout the transformation. The symmetry of this orbital, therefore, dictates the course of transformation and is the basis of the Woodward-HoiFmann selection rules (i). Consider, for example, the interconversion of butadiene (XXI) and cyclobutene (XXII). 

When considering a possible role for a catalyst in converting a symmetry-forbidden to a symmetry-allowed electrocyclic transformation, it is best to examine the correlation diagram of the entire set of transforming molecular orbitals in a manner similar to that described for cycloaddition reactions. The symmetry-forbidden disrotatory transformation of cyclobutene to butadiene is described in Fig. 8. 

Buenker, Robert J., Peyerimhoff, Sigrid D., 8c Hsu, Kang. 1971. "Analysis of Qualitative Theories for Electrocyclic Transformations Based on the Results of Ab Initio Self-Consistent-Field and Configuration-Interaction Calculations." Journal of the American Chemical Society 93 5005-5013. 

The explanation offered for the stereospecificity of electrocyclic transformations was in terms of the properties of the highest occupied Hiickel molecular orbital (HOMO) of the open chain molecule. 

Ab initio calculations including configuration interactions were reported for the addition of the NH2 radical to ethylene 177>, for dissociation of formaldehyde into radicals 178> and molecular products 179>, and for the electrocyclic transformation between cyclobutene and butadiene 180>. 

A variety of chemical functionalities undergo photoisomerization, including trans-cis isomerization around double bonds, electrocyclic transformations, sigmatropic rearrangements, cycloadditions and bond cleavage. Figure 3... 

---