Stereospecificity electrocyclic reactions

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Apart from their intrinsic interest, these electrocyclic reactions have considerable synthetic carbon-carbon bond-forming importance because of their rigid stereospecificity, which is much greater than in the vast majority of other, non-concerted reactions involving biradical or bipolar intermediates. 

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. 

Although a few other acyclic examples of stereospecific isomerisation of hexatrienes are known, specially in the field of natural product like steroid chemistry, the commonest reactions of this type are in cyclic hexatrienes. Cyclooctatriene and cyclooctatetraene are systems in which the electrocyclic reaction goes very readily and they show an interesting trend. 

Thermal and photochemical electrocyclic reactions are both stereospecific, with the two processes giving rise to stereospecific reactions in the opposite sense. 

A striking feature of thermal electrocyclic reactions that proceed by concerted mechanisms is their high degree of stereospecificity. Thus when cis-3,4-dimethylcyclobutene is heated, it affords only one of the three possible... 

The cyclization step of Equation 28-8 is a photochemical counterpart of the electrocyclic reactions discussed in Section 21-10D. Many similar photochemical reactions of conjugated dienes and trienes are known, and they are of great interest because, like their thermal relatives, they often are stereospecific but tend to exhibit stereochemistry opposite to what is observed for formally similar thermal reactions. For example,... 

As a first example of an electrocyclic reaction illustrating stereochemistry, let us take the pair of conrotatory cyclobutene openings, showing that the reactions are stereospecific. 

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

This chapter follows on from chapter 12 where we introduced some basic ideas on stereocontrol. Since then we have met many stereospecific reactions such as pericyclic reactions including Diels-Alder (chapter 17), 2 + 2 photochemical cycloadditions (chapter 32), thermal (chapter 33) cycloadditions, and electrocyclic reactions (chapter 35). Then we have seen rearrangements where migration occurs with retention at the migrating group such as the Baeyer-Villiger (chapters 27 and 33), the Amdt-Eistert (chapter 31) and the pinacol (chapter 31). 

Let s begin by considering the simplest electrocyclic reaction, the thermally induced interconversion of a diene and a cyclobutene. As illustrated in the following example, the reaction is remarkably stereospecific, occurring only by a conrotatory motion ... 

Electrocyclic reactions of conjugated polyenes create chiral molecules through stereospecific conrotatory or disrotatory processes. In solution, the two enantiom-... 

Stereochemistry Electrocyclic reactions, like all pericyclic processes, exhibit great stereospecificity. The stereospecificity of such reactions is demonstrated by thermal closure of fransjds,trans-2,4,6-octatriene (8.12) to ds-5,6-dimethyl-1,3-cyclohexadiene (8.13) and of the isomeric frans,cis,czs-octatriene (8.14) to frans-5,6-dimethyl-1,3-cyclohexadiene (8.15). 

The easiest way to rationalize the stereospecificity in electrocyclic reactions is by examining the symmetry of the HOMO of the open (non-cyclic) molecule, regardless of whether it is the reactant or the product. For example, the HOMO of hexatriene is 3, in which orbital lobes (terminal) that overlap to make the new a-bond have the same phase (sign of the wave function). Thus, in this case, the new cr-bond between these two terminal orbital lobes can be formed only by the disrotation suprafacial overlap) (Fig. 8.45). If the terminal orbital lobes of HOMO of hexatriene were to close in a conrotatory antarafacial overlap) fashion, an antibonding interaction would result. 

The stereoselective synthesis of (+)-trichodiene was accomplished by K.E. Harding and co-workers. The synthesis of this natural product posed a challenge, since it contains two adjacent quaternary stereocenters. For this reason, they chose a stereospecific electrocyclic reaction, the Nazarov cyclization, as the key ring-forming step to control the stereochemistry. The cyclization precursor was prepared by the Friedel-Crafts acylation of 1,4-dimethyl-1-cyclohexene with the appropriate acid chloride using SnCU as the catalyst. The Nazarov cyclization was not efficient under protic acid catalysis (e.g., TFA), but in the presence of excess boron trifluoride etherate high yield of the cyclized products was obtained. It is important to note that the mildness of the reaction conditions accounts for the fact that both of the products had an intact stereocenter at C2. Under harsher conditions, the formation of the C2-C3 enone was also observed. 

The property that the stereochemical result of an electrocyclic reaction is absolutely predictable is called stereospecificity. A stereospecific reaction will give you one stereochemical result when a cis starting material is used, and the opposite result when a trans starting material is used. Other examples of stereospecific reactions include Sn2 substitutions, catalytic hydrogenation of alkynes or alkenes, and dihydroxylation and bromination of alkenes. 

A few months after the communication by Woodward and Hoffmann, H. C. Longuet-Higgins and E. W. Abrahamson 3> suggested a different approach to the problem of stereospecificity of electrocyclic reactions, based on correlation diagrams between the orbitals, the electron configurations, and the states of the reactant and the product. 

Orbital Symmetry Basis for the Stereospecificity of Electrocyclic Reactions... 

Solutiorn Electrocyclic reactions are concerted, therefore they are completely stereospecific. The exact stereochemistry of the product depends upon the number of double bonds in the polyene molecular orbital theory allows us to predict this stereochemistry. Let us look at the electron configuration of butadiene, a four-ir-electron system, in the ground state and in the first excited state (achieved by the absorption of radiation) ... 

Heating this product leads to a retro Diels-Alder reaction cyclopentadiene is released and a cyclobutene is formed stereospecifically trans. This now decomposes by a four-electron conrotatory electrocyclic reaction that could give either the E,E- or the Z,Z- diene. 

The molybdenum complex (184) is an effective catalyst for the transformation of the dienes (185 X = H2, O) into the tetrahydroazepines (186 X = H2, O) with concomitant loss of ethylene <92JA7324>. Dihydroazepines (188) are prepared from the a-allylamino dienenitriles (187) in a stereospecific conrotatory electrocyclization reaction <89JOC481>. 

The Rules for the stereospecificity apply only to pericyclic reactions which are concerted. The Rules apply neither to non-concerted reactions nor to those that are not pericyclic. Pericyclic reactions are those which involve a monocyclic transition state having a conjugated array of interacting orbitals, one per atom. Three types of pericyclic processes have been recognized electrocyclic reactions, cycloadditions, and sigmatropic shifts. 

Another example of an electrocyclic reaction that exhibits a high degree of stereospecificity is the ring-closure of hexatrienes to cyclohexadienes ... 

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