Torquoselectivity of the electrocyclic ringopening reaction of 3substituted cyclobutenes here we take a phenomenological approach to explain the torquoselectivity of the electrocyclicringopening reaction of 3substituted cyclobutenes to eliminate or minimize any bias. Chart and diagram slides for powerpoint beautifully designed chart and diagram s for powerpoint with visually stunning graphics and animation effects. Nbo analysis of the torquoselectivity of the electrocyclic. Jan 04, 2016 electrocyclic reactions an electrocyclic reaction is a reversible reaction that involves ring closure of a conjugated polyene to a cycloalkene, or ring opening of a cycloalkene to a conjugated polyene. Analyzing torquoselectivity in electrocyclic ring opening. When you look at a mechanism, make sure you can explain exactly what every component of the reaction is doing. One example is the conversion of norbornadiene to quadricyclene, described in section 3d. For example, ring closure of 1,3,5hexatriene forms 1,3 cyclohexadiene, a product with one more. What is a torquoselective reaction using substituted cyclobutenes as an example, both clockwise and counterclockwise rotation is possible and produces products of different ole. An introduction to molecular orbital theory and relevant stereochemical concepts is provided as background, with all four classes of pericyclic.
Pericyclic reactions represent an important class of concerted single step processes involving. When this e,e2,4hexadiene ring closes in an electrocyclic reaction. An introduction to molecular orbital theory and relevant stereochemical concepts is provided as background, with all four classes of pericyclic reactions discussed and illustrated with orbital. Orbital phase theory and the diastereoselectivity of some.
Pergamon 0040403995012494 tetrahedron letters, vol. In 1984, kirmse, rondan, and houk disclosed another theory in electrocyclic reactions, which was referred to as torquoselectivity or torquoelectronic effect,3 to describe the. Since numerous examples have recently surfaced in the literature, we feel that the time has come to make such an attempt. Torquoselectivity simple english wikipedia, the free. Torquoselectivity on the vinylallenes thermal electrocyclic ring. In these cases it serves as a powerful predictive tool. The phenomenological model involves finding regularity. Torquoselectivity is a special kind of stereoselectivity observed in electrocyclic reactions in organic chemistry, defined as the preference for inward or outward. The electrocyclic reactions of polyenes lxtolo transformations involve the rotation of the terminal carbons of the conjugated polyene. Pericyclic reactions are reactions in which a cyclic, conjugated system of electrons is created in the transition state. Electronic control of stereoselectivities of electrocyclic reactions of.
Walker mj1, hietbrink bn, thomas be 4th, nakamura k, kallel ea, houk kn. We will consider two categories of pericyclic reactions. A new term, torquoselectivity, was created to describe these rotational preferences as described briefly in figure 9. Analyzing torquoselectivity in electrocyclic ring opening reactions of trans3,4dimethylcyclobutene and. Torquoselectivity in the electrocyclic ringopening of. Torquoselectivity in thermal electrocyclic reactions of.
The formation of cyclohexa1,3diene by heating hexa1,3,5triene is an example of ringclosure electrocyclic reaction. A large preference for inward rotation of the nitrogen lone pair and outward rotation of the n. Accelerated electrocyclic ringopening of benzocyclobutenes. For the sake of simplicity, we use the term electrocyclizations both for the forward reaction and electrocyclic ring openings. Density functional theory was used to investigate the role of the 3,3geminal diester groups and the origin of torquoselectivity in these electrocyclic reactions. Some examples are dielsalder and ozonolysis reactions, which are described below. The electrocyclic ring closure is designated by blue arrows, and the ring opening by red arrows. Molecular orbital explanation for the selection rules. The validity of maximum hardness, minimum electrophilicity and minimum polarizability principles is assessed to explain the phenomenon of torquoselectivity inward and outward preference in the conrotatory ring opening reactions of trans3,4dimethylcyclobutene into z,z. Pvp orbital stereoselectivity princeton university.
Four major types of pericyclic reactions cycloadditions cycloadditions are characterized by formation of a cyclic product when two components come together. So, the torquoselectivity essentially refers to, the preferential twisting of the. Ellis department of chemistry, university of british columbia 2036 main mall, vancouver, british columbia, canada v6t iz1 summary. Predicting competitive and noncompetitive torquoselectivity in. Torquoselectivity also happens in selective electrocyclic reactions that break open rings. Figure 2 torquoselectivity of various groups during the ring opening of cyclobutenes. Im thinking now about the torquoselectivity of the ringopening because in an exercise we had an example with 1ethyl1ethoxyedgcyclobut2ene. Request pdf origins of inward torquoselectivity by silyl groups and other acceptors in electrocyclic reactions of cyclobutenes recent reports of inward. A measure of selectivity in the direction of the conrotatory or disrotatory mode in an electrocyclic reaction. Torquoselectivity in the electrocyclic interconversions of 1azapolyenes and their heterocyclic isomers was investigated theoretically. It is characterised by two fragments coming together to form two new. We show that this contradictory torqueselectivity is under the control e of g minal bond participation 21.
The torquoselectivity of electrocyclic reactions of 3. Physical chemistry chemical physics 2015, 17 35, 2310423111. Torquoselectivity in the electrocyclic ringopening of cyclopropyl anions article in journal of physical organic chemistry 225. Likewise, torquoselectivity theory was found to be applicable to. Studies have shown that the selectivity can also be changed by the presence of electron donating and electron withdrawing groups.
Ellis department of chemistry, university of british columbia 2036 main mall, vancouver, british columbia, canada v6t. Pdf lewis acid reversal of the torquoselectivity of the. Electrocyclic reaction simple english wikipedia, the free. However, there are two different conrotatory modes which can lead to isomeric products if the. Analyzing torquoselectivity in electrocyclic reactions. The reverse reaction may be called electrocyclic ring opening. The general rules for electrocyclic reactions are very easily derivable from the nodal properties of polyenes and polyenyl ions.
Analyzing torquoselectivity in electrocyclic ring opening reactions of. Electron withdrawing group is in trans position when i start with the product. Lets first see the forward reaction the electrocyclic ring closure as woodward and hoffmann found which goes by disrotatory cyclization. An electrocyclic reaction is the concerted cyclization of a conjugated. Torquoselectivity is not to be confused with the normal diastereoselectivity seen in pericyclic reactions, as it represents a further level of. Our new crystalgraphics chart and diagram slides for powerpoint is a collection of over impressively designed datadriven chart and editable diagram s guaranteed to impress any audience. May 01, 2009 read torquoselectivity in the electrocyclic ring. Pericyclic reaction involves a cyclic redistribution of bonding electrons through a concerted process i.
The fact that the reactions are concerted gives fine stereochemical control of the product, however, this page is more concerned with the general types of pericyclic reaction, than with regio and stereochemical control. Hyperconjugative interactions substituentcyclobutene occurring at the transition state seem to be responsible of these reactions torquoselectivity. Torquoselectivity is a special kind of stereoselectivity observed in electrocyclic reactions in organic chemistry, defined as the preference for inward or outward rotation of substituents in conrotatory or disrotatory electrocyclic reactions. The torquoselectivity of electrocyclic reactions of 3donor3. Torquoselectivity in the electrocyclic conversion of. Orbital mechanisms and stereochemistry is a complete guide to the topic that is ideal for graduate students, advanced undergraduate students and researchers in organic chemistry. Torquoselectivity induced by lonepair conjugation in the electrocyclic reactions of 1azapolyenes. Synthesis by cycloaddition and electrocyclic reactions. Nbo analysis of the torquoselectivity of the electrocyclic reactions of monosubstituted cyclobutenes article pdf available in journal of computational methods in sciences and engineering 162. The fmo theory can rationalize or predict the reaction mode conrotation versus disrotation of an electrocyclic reaction. Im interested in the reverse reaction, the ring opening 6 electrons, and therefore disrotatory.
Figure 9 orbital interactions explaining inward or outward rotation in summary, orbital symmetry provides us with the basic rules to predict the product of electrocyclic reactions. New insights into the torquoselectivity of the staudinger. The ring openings of 1,2dihydroazete, 1,2dihydropyridine, and 1,2dihydroazocine were examined using hf, mp2, and b3lyp calculations. Origins of inward torquoselectivity by silyl groups and other. Chelotropic reactions are a specific type of cycloaddition, where the two bonds are made or broken at the same atom. Find materials for this course in the pages linked along the left. The reaction product is a very unstable orthoquinodimethane but this molecule can be trapped in an endo addition with a strong dienophile such as maleic anhydride to the dielsalder adduct. The stereoselectivity of these reactions has been elucidated on the basis of the natural bond orbital analysis and correlation diagrams.
Analyzing torquoselectivity in electrocyclic ring opening reactions of trans3,4dimethylcyclobutene and 3formylcyclobutene through electronic structure principles. The rules of torquoselectivity still hold, with a calculated 68 kcalmol preference for. Huckel molecular oribital, conrotation, disrotation, torquoselectivity. Electrocyclic reactions we define as electrocyclic transformations the formation of a single bond between the termini of a linear system containing k. The torquoselectivity in an electrocyclic reaction refers to the direction that the substituents rotate. Electrocyclic reaction simple english wikipedia, the.
Recent reports of inward torquoselectivities in thermal electrocyclic ringopening reactions of 3silylcyclobutenes have revealed that saturated silyl substituents, just like the extensively studied. Special topics categories of pericyclic reactions among the reactants and products usually at least one molecule is unsaturated the reactions involve the formation or scission of sigmabonds and the consumption or generation of pi. The principle of conservation of orbital symmetry applies only to concerted pericyclic reactions. The different directions of rotation produce distinct structural isomers. Dielsalder reaction discovered by professor otto diels and his student kurt alder in 1928 and received nobel prize in 1950 we explicitly reserve for ourselves the application of the reaction developed by us to the solution of such problems. Nbo analysis of the torquoselectivity of the electrocyclic reactions of monosubstituted cyclobutenes. May 20, 2009 this torquoselectivity is controlled by the electronic nature of the c3 substituent. It is thought that the mechanism of the reactions proceeds via a series of intermediates b f, the structures of two of which d and e are shown below. The dielsalder reaction is one of the most well know example of cycloadditions.
Torquoselectivity is control of the direction of ring closure in a conrotatory or disrotatory mechanism scheme 7. Torquoselectivity is not to be confused with the normal diastereoselectivity seen in pericyclic. Pericyclic reactions require light or heat and are completely stereospeci. Electrocyclic reactions in organic synthesis an often studied electrocyclic reaction is the conrotatory thermal ringopening of benzocyclobutene. Torquoselectivity in thermal electrocyclic reactions of substituted 7. Analyzing torquoselectivity in electrocyclic ring opening reactions of trans3,4 dimethylcyclobutene and 3formylcyclobutene through electronic structure. The origins of torquoselectivity for substituents lacking. Based on fourteen years of teaching a graduate course, this long awaited textbook presents dielsalder reactions, electrocyclic reactions, sigmatropic rearrangements, ene reactions and their asymmetric versions, organometallic pericyclic reactions plus many more topics in a highly didactic way. The stereochemical outcome of an electrocyclic reaction depends on the number of participating electrons and wether it is a thermal or photochemical process. Analyzing torquoselectivity in electrocyclic ring opening reactions of trans3,4dimethylcyclobutene and 3formylcyclobutene through electronic structure principles alejandro moralesbayuelo,1, sudip pan2, julio caballero1, and pratim k. Pericyclic reactions electrocyclic reactions examples of 3, 4 and 5 membered.
Exam 4 100 points this examination covers material. It was found that, in the electrocyclic ringopening of substituted cyclobutenes, donor. Stereoelectronic effects in torquoselectivity of 4. An often studied electrocyclic reaction is the conrotatory thermal ringopening of benzocyclobutene. In these cases, steric strain is often the driving force for the selectivity. Stereoselectivity pericyclic reactions electrocyclic reactions torquoselectivity, basic rondanhouk treatment r r conout conin r et conout conin et et 1 2. I think many people have not been connecting the mechanisms with the overall reactions we have been discussing. Electrocyclic reactions are a subtype of pericyclic reaction which is unimolecular and in which the termini of a conjugated system become sigma bonded to each other to form a. This examination covers material contained in pericyclic reactions chapter 22, and natural products chapter 28. Burton 1 practiceproblemsrelatingtopericyclicreactions allofthefollowing reactions involveatleastonepericyclic. Analyzing torquoselectivity in electrocyclic ring opening reactions of trans3,4dimethylcyclobutene and 3formylcyclobutene through electronic structure.
If the substituent effect controls the torquoselectivity, it should result in the same torquoselectivity. Pdf pericyclic reactions download full pdf book download. In this section the syntheses of 1,3dienes via pericyclic reactions,14. Electrocyclic reactions a type of pericyclic reaction. Thermodynamic control of the electrocyclic ring opening of.
Houk department of chemistry and biochemistry, university of. Lewis acid reversal of the torquoselectivity of the electrocyclic ring opening of 3acetylcyclobutene. We will begin to look at how to predict pericyclic reactions. Many pericyclic reactions take place photochemically, that is, by irradiation with ultraviolet light. The validity of maximum hardness, minimum electrophilicity and minimum polarizability principles is assessed to explain the phenomenon of torquoselectivity inward and outward preference in the conrotatory ring opening reactions of trans3,4dimethylcyclobutene into z,z and e,ebutadienes and 3formylcyclo.
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