Electro cyclic reactions

How can we predict whether conrotatory or disrotatory motion will occur in a given case According to frontier orbital theory, the stereochemistry of an electro-cyclic reaction is determined by the symmetry of the polyene HOMO. The electrons in the HOMO are the highest-energy, most loosely held electrons, and are therefore most easily moved during reaction. For thermal reactions, the ground-state... 

The polyene character of 1 /7-azcpines makes them susceptible not only to a variety of electro-cyclic reactions, but also to cycloaddition with a variety of dienophiles, and to dimerization by [6 + 4] 7i-pericyclic reactions. 

A special method, with only two examples, starts from 1,2,4-triazines.20 21 Diels-Alder reaction with the strained dienophile dimethyl tricyclo[4.2.2.02,5]deca-3,7,9-triene-7,8-dicarboxylate (14) is followed by an elimination of nitrogen via a retro-Diels-Alder process. The formed product, however, cannot be isolated, but reacts via another retro-Diels-Alder reaction and an electro-cyclic reaction to provide the azocine derivative 15. The sequence order of the reactions is not clear, but both pathways lead to the same product. 

The application of the Woodward-Hoffmann theory 22> of electro-cyclic reactions to chemiluminescence has proved a very useful and productive approach, suggested independently by E. H. White and M. J. C. Harding 23>, F. Me Capra, D. G. Richardson, and Y. C. Chang u> 24>, and M. M. Rauhut and coworkers 25>. 

The requirement for a favorable bonding overlap of the orbitals forming the new sigma bond in the HOMO allows the preferred rotation to be predicted for any electro-cyclic reaction. Let s consider the photochemical conversion of a butadiene to a cyclobutene. Because the reaction occurs through the excited state, the HOMO is it. As can be seen, disrotation is necessary here for the new overlap to be bonding ... 

What we have just been telling you should convince you that the two reactions below are electro-cyclic reactions, not least because the stereochemistry reverses on going from thermal to photochemical reaction. 

While the Oosterhoff model that follows from the state correlation diagrams discussed in Section 4.2.3 describes the stereochemistry of electro-cyclic reactions correctly and in agreement with the Woodward-Hoffmann rules, it is oversimplified in that it does not attempt to actually locate the bottom of the pericyclic minimum and simply assumes a planar carbon framework. It therefore predicts a nonzero S -So gap at perfect biradicaloid geometry. 

Before discussing specific reactions, it is important to learn how to describe pericyclic reactions. There are four major classes of pericyclic reactions electro-cyclic reactions (ring openings or ring closings), cycloadditions, sigmatropic rearrangements, and ene reactions. 

Ene reactions make one new cr bond at the expense of one it bond, like electro-cyclic reactions, but they are fairly easy to distinguish from electrocyclic reactions otherwise. Look for allylic transposition of a double bond and transfer of an allylic H atom. In a retro-ene reaction, a nonallylic H is transferred to an allylic position. 

N4-monosubstituted hydrazones were the most reactive, independent of the steric or electronic nature of the substituent group. An electro-cyclic reaction mechanism, similar to the one operating in the isothiosemi-carbazone cyclization (81JOC3956), has been proposed for conversion of 41a to 43a (85CPB2678). No intermediate analogous to 45 has been detected (85CPB2678). 

We discuss some prototypical reactions, such as cis-trans isomerization, electro-cyclic reactions, and cycloadditions in terms of the structures of excited states and conical intersections. For most other reactions, we represent the reaction changes in terms of structures that depict the unpairing and re-pairing events. Such representations identify key structural features that influence the outcome of the reaction, even though they may leave much uncertainty about the detailed structure of the excited states and intermediates. For several reactions, the mechanism is discussed in terms of the structure and reaction paths for CIs. Structures of specific CIs derived from computation are depicted. At this time, it is perhaps too early to fully judge the accuracy of these structural representations. However, just as for depiction of TS structures, visualization of the Cl can help understand the course of a photochemical reaction. 

Electrocyclic reactions are reversible. The cyclic compound is favored for electro-cyclic reactions that form six-membered rings, whereas the open-chain compound is favored for electrocyclic reactions that form four-membered rings because of the angle strain and torsional strain associated with four-membered rings (Section 2.11). 

You may begin to see a pattern. The Woodward-Hoffmann rules for electro-cyclic reactions are as follows ... 

According to the Woodward-Hoffman rules for electro-cyclic reactions, a photochemical reaction is required for concertedness in a disrotatory manner when the number of 7T electrons is 4n 4 ir electrons = 4n for n = 1), ... 

A cycloaddition reaction is actually a type of pericydic reaction, but the term peri-cyclic includes other types of reactions. The textbook definition of pericydic is a reaction whose transition state has a cyclic structure (i.e., the electrons are flowing in a closed loop). In addition to cycloaddition reactions (which exchange two Jt-bonds for two o-bonds, or vice versa), pericydic reactions include sigmatropic reactions, electro-cyclic reactions, and cheletropic reactions (and a few others which we ll ignore). 

Generally, a VB-treatment is used to teach chemistry (because it is more succinct and less complicated than MO-theory) until cases (like electro-cyclic reactions) are treated that cannot be understood properly within simple VB-theory. This need not be the case, but is usually a good compromise for teaching chemical bonding and reactivity effectively. 

Ratio of cyclopentadiene The formation of bicyclopentene is formally an electro-cyclic reaction like that which converts the benzene system to a Dewar benzene [47], and the formation of 3 is crossed cycloaddition like that which converts benzene to benzvalene [48]. Stmctural derivatives of 3 were known prior to the isolation of the parent Masamune prepared 1,5-diphenyl tricyclo[2.1.0.0 Jpentan-3-one by photolysis of a diazocyclopropenyl ketone [49] (X-ray analysis of a derivative confirmed the structure in the face of skepticism [50,51] and Closs and Larrabee prepared 2,4-dimethyltricyclo[2.1.0.0 ]pentane [52] by heating the sodium salt of a cyclobutenyl tosylhydrazone both kinds of reactions are known to generate carbenes or carbe-noids, and presumably involve insertion of a carbene-type carbon into a carbon-carbon double bond ... 

Scheme 12 Synthesis of hexahydrocannabinol analog by combination of 4jt-6jt electro-cyclic reactions and hetero Diels-Alder reaction [87]...

Predict the product for each of the following electro-cyclic reactions ... 

The mechanism of this cycloaddition can be explained by FMO theory, in which one component acts as a HOMO and other as LUMO in a favorable low-energy TS to afford a stereoselective product. The Woodward-Hofifmann mles for electro-cyclic reactions are also applied to this cycloaddition reaction. The reaction of an alkene with a carbene is considered as a 4n electron process and of a conjugated diene with an electrophilic molecule as a 4n+2 electron process. Therefore, for thermal reaction of 4n electron process, conrotatory motion of the substituents from the termini of the n system will favor a low-energy TS to afford the product and in photochemical process, the reverse disrotatory mode of motion will be the favored path. Similarly, for a 4n + 2 electron process, disrotatory mode is a symmetry allowed process in thermal reaction and conrotatory mode for its photochemical reaction. 

The difference between the conrotatory and disrotatory mechanisms for the electro-cyclic reactions of -cw-butadiene and disilylbutadiene has been explained in terms of the biradical character of the terminal carbon (or silicon) atoms. 

Darcy, P.J., Heller, H.G., Strydom, P.)., and Whittall, J. (1981) Photochromic heterocyclic fulgides. Part 2. Electro-cyclic reactions of (e)-2, 5-dimethyl-3-furylethylidene(alkyl-substituted methylene)succinic anhydrides. J. 

A new entiy to the total synthesis of isocryptolepine (cryptosanguinolentine), isolated from Cryptolepis sanguinolenta, was achieved by constructing a tetracyclic ring system through a microwave-assisted tandem Curtius rearrangement and electro-cyclic reaction of an aza 67c-electron system (Hayashi et al., 2012). The tetracyclic lactam was further converted to isocryptolepine in a four-step sequence. The acid is simply reacted with DPPA in toluene under microwave irradiation to dehver the desired fused lactam in 97% yield. 

---