Electrocyclic reaction cyclobutene-butadiene

As an example, let us consider the electrocyclic reaction of cyclobutene to butadiene transformation. The cyclobutene ring can break up either in a conrotatory... 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Electrocyclic reactions are examples of cases where n-electron 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 formed 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 HC1 + H system. The reaction is now Mobius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mobius-Hiickel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

The simplest example of an electrocyclic reaction involving 4n electron system is the thermal opening of cyclobutenes to 1,3 butadienes. The reaction can be done thermally or photochemically and under either conditions, it is stereospecific. 

Sometimes reaction coordinates are studied that involve substantial changes in bonding. In such an instance, it is critical that a consistent choice of orbitals be made. For instance, consider the electrocyclization of 1,3-butadiene to cyclobutene (Figure 7.2). The frontier orbitals of butadiene are those associated with the tt system, so, as just discussed, a (4,4) approach seems logical. However, the electrocyclization reaction transforms the two jt bonds into one different jr bond and one new a bond. Thus, a consistent (4,4) choice in cyclobutene would involve the jr and jt orbitals and the a and <7 orbitals of the new single bond. 

Electrocyclic reactions, 163, 165 butadienes to cyclobutenes, 164-165 component analysis, 168 stereochemistry, 165 Electron... 

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... 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

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)... 

Similarly, thermal (at 150°C) electrocyclic opening of cyclobutenes forms conjugated butadienes this mode of reaction is favoured by relief of ring strain. However, the reverse ring closure is not normally observed. Photochemical ring closure can be affected, but the stereospecificity is opposite to that of thermal ring opening. 

The cyclobutene-butadiene interconversion involves four v electrons and is designated a process. Note that by the principle of microscopic reversibility, the number of tt electrons involved in the transformation is the same for ring opening as for ring closing. Once we know the number of tt electrons involved in an electrocyclic reaction and the method of activation, the stereochemistry of the process is fixed according to the rules outlined in Table 6.1. 

Simple examples of electrocyclic reactions are the formation of cyclobutene from butadiene and cyclohexadiene from hexatriene ... 

The presence of a 1,3-butadiene or a cyclobutene in the starting material or the product may indicate a four-electron electrocyclic reaction. 

Electrocyclic Reactions. Electrocyclic reactions are those pericyclic reactions in which a ring is formed (or opened). Thus, cyclobutene (157), on heating, gives butadiene (158), and hexatriene (159) gives cyclohexadiene (160). 

In contrast to cycloadditions, which almost invariably take place with a total of (4 2) electrons, there are many examples of electrocyclic reactions taking place when the total number of electrons is a (An) number. However, those electrocyclic reactions with (An) electrons, like the butadiene-cyclobutene equilibrium, 6.50 6.51, differ strikingly in their stereochemistry from those reactions mobilising (An+2) electrons, like the hexatriene-cyclohexadiene equilibrium, 6.52 —> 6.53. This is only revealed when the parent systems are... 

Electrocyclic reactions can also be analyzed on the basis of the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground state molecules. A stabilized aromatic TS results in a low activation energy, i.e., an allowed reaction. An antiaromatic TS has a high energy barrier and corresponds to a forbidden process. The analysis of electrocyclizations by this process consists of examining the array of basis set orbitals that is present in the transition structure and classifying the system as aromatic or antiaromatic. For the butadiene-cyclobutene interconversion, the TSs for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, that is, the complete set of 2p orbitals involved in the reaction process, not the individual molecular orbitals. The tilt at C(l) and C(4) as the butadiene system rotates toward the TS is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

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