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11 Concerted Pericyclic Reactions 11 Concerted Pericyclic Reactions Introduction There are many reactions in organic chemistry that give no evidence of involving intermediates when subjected to the usual probes for studying reaction mechanisms. Highly polar transition states do not seem to be involved either,since the rates of the reactions are often insensitive to solvent polarity. Efforts to detect free-radical intermedi- ates in these reactions by spectroscopic or chemical means have not been successful,and the reaction rates are neither increased by initiators nor decreased by inhibitors of free- radical reactions. This absence of evidence of intermediates leads to the conclusion that the reactions are single-step processes in which bond making and bond breaking both contribute to the structure at the transition state,although not necessarily to the same degree. Such processes are called concerted reactions. There are numerous examples of both unimolecular and bimolecular concerted reactions. An important group of concerted reactions are the concerted pericyclic reactions.1 A pericyclic reaction is characterized as a change in bonding relationship that takes place as a continuous concerted reorganization of electrons. The word ``concerted'' speci®es that there is a single transition state and that there are no intermediates in the process. To maintain continuous electron ¯ow,pericyclic reactions must occur through cyclic transi- tion states. Furthermore,the cyclic transition state must correspond to an arrangement of the participating orbitals that can maintain a bonding interaction between the reacting atoms throughout the course of the reaction. We shall see shortly that these requirements make pericyclic reaction highly predictable in terms of such features as relative reactivity, stereospeci®city,and regioselectivity. The key to understanding the mechanism of the concerted pericyclic reactions was the recognition by Woodward and Hoffmann2 that the pathways of such reactions were determined by the symmetry properties of the orbitals that were directly involved. Their recognition that the symmetry of each participating orbital must be conserved during the 1. R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry,Academic Press,New York,1970. 2. R. B. Woodward and R. Hoffmann, J. Am. Chem. Soc. 87:395 G1965). 605 606 reaction process dramatically transformed the understanding of this family of reactions and stimulated much experimental work to test and extend their theories.3 Woodward and CHAPTER 11 CONCERTED Hoffmann's approach led to other related interpretations of orbital properties which are PERICYCLIC also successful in predicting and explaining the course of concerted pericyclic reactions.4 REACTIONS There has been a great deal of effort at modeling the transition states of concerted pericyclic reactions.5 Each of the major theoretical approachesÐsemiempirical MO, ab initio MO,and density functional theoryÐhave been applied to the problem,and some comparisons made.6 These approaches generally parallel the orbital symmetry rules in their prediction of stereochemistry and provide insight into relative activation energies and substituent effects. We will refer to speci®c examples for several reactions. 11.1. Electrocyclic Reactions There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The ®rst class that we will consider are electrocyclic reactions. An electrocyclic reaction is de®ned as the formation of a single bond between the ends of a linear conjugated system of p electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes: CH CH CH 3 CH 3 3 H 3 H H H H H 175°C 175°C Ref. 7 CH3 CH3 H H H H H CH3 H CH3 It is not surprising that thermolysis of cyclobutenes leads to ring opening,since the strain in the four-membered ring is relieved. The activation energy for simple alkyl- substituted cyclobutenes is in the range of 30±35 kcal=mol.8 What is particularly signi®cant about these reactions is that they are stereospeci®c. cis-3,4-Dimethylcyclobu- tene is converted to E,Z-2,4-hexadiene, whereas trans-3,4-dimethylcyclobutene yields the E,E-isomer. The stereospeci®city of such processes is very high. In the ring opening of cis- 3,4-dimethylcyclobutene, for example, only 0.005% of the minor product E,E-2,4- hexadiene is formed,even though it is more stable than the E,Z-isomer.9 3. For reviews of several concerted reactions within the general theory of pericyclic reactions,see A. P. Marchand and R. E. Lehr,eds., Pericyclic Reactions,Vols. I and II,Academic Press,New York,1977. 4. H. C. Longuet-Higgins and E. W. Abrahamson, J. Am. Chem. Soc. 87:2045 G1965); M. J. S. Dewar, Angew. Chem. Int. Ed. Engl. 10:761 G1971); M. J. S. Dewar, The Molecular Orbital Theory of Organic Chemistry, McGraw-Hill,New York,1969; H. E. Zimmerman, Acc. Chem. Res. 4:272 G1971); K. N. Houk,Y. Li,and J. D. Evanseck, Angew. Chem. Int. Ed. Engl. 31:682 G1992). 5. O. Wiest,D. C. Montiel,and K. N. Houk, J. Phys. Chem. 101:8378 G1997). 6. D. Sperling,H. U. Reissig,and J. Fabian, Liebigs Ann. Chem. 1997:2443; B. S. Jursic, THEOCHEM 358:139 G1995); H. Y. Yoo and K. N. Houk, J. Am. Chem. Soc. 119:2877 G1997); V.Aviente,Y. H. Yoo,and K. N. Houk, J. Org. Chem. 62:6121 G1997); K. N. Houk,B. R. Bono,M. Nendal,K. Black,H. Y. Yoo,S. Wilsey,and J. K. Lee, THEOCHEM 398:169 G1997); J. E. Carpenter and C. P. Sosa, THEOCHEM 311:325 G1994); B. Jursic, THEOCHEM 423:189 G1998); V. Brachadell, Int. J. Quantum Chem. 61:381 G1997). 7. R. F . K. Winter, Tetrahedron Lett. 1965:1207. 8. W. Kirmse,N. G. Rondon,and K. N. Houk, J. Am. Chem. Soc. 106:7989 G1984). 9. J. I. Brauman and W. C. Archie,Jr., J. Am. Chem. Soc. 94:4262 G1972). The reason for the observed stereospeci®city is that the groups bonded to the 607 breaking bond all rotate in the same sense during the ring-opening process. Such SECTION 11.1. motion,in which either all the substitutents rotate clockwise or all rotate counterclockwise, ELECTROCYCLIC is called the conrotatory mode. When such motion is precluded by some structural feature, REACTIONS ring opening requires a higher temperature. In the bicycloheptene shown below,the ®ve- membered ring prevents completion of a conrotatory ring opening because it would lead to a cis,trans-cycloheptadiene. The reaction takes place only at very high temperature, 400C,and probably involves the diradical shown as an intermediate. H H H H H3C H3C H3C H3C conro- 400°C 400°C H tation H H3C Ref. 10 H3C H3C H3C H H The principle of microscopic reversibility requires that the reverse process,ring closure of a butadiene to a cyclobutene,must also be a conrotatory process. Usually,this is thermodynamically unfavorable,but a case in which the ring closure is energetically favorable is conversion of trans,cis-2,4-cyclooctadiene G1) to bicyclo[4.2.0]oct-7-ene G2). The ring closure is favorable in this case because of the strain associated with the trans double bond. The ring closure occurs by a conrotatory process. H H H H 80°C Ref. 11 H 1 2 H Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospeci®city. The ring closure is normally the favored reaction in this case,because the cyclic compound,which has six s bonds and two p bonds,is thermodynamically more stable than the triene,which has ®ve s and three p bonds. The stereospeci®city is illustrated with octatrienes 3 and 4. E,Z,E-2,4,6-Octatriene G3) cyclizes only to cis-5,6-dimethyl-1,3-cyclohexadiene, whereas the E,Z,Z-2,4,6-octa- triene G4) leads exclusively to the trans cyclohexadiene isomer.12 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 10. R. Criegee and H. Furr, Chem. Ber. 97:2949 G1964). 11. K. M. Schumate,P. N. Neuman,and G. J. Fonken, J. Am. Chem. Soc. 87:3996 G1965); R. S. H. Liu, J. Am. Chem. Soc. 89:112 G1967). 12. E. N. Marvell,G. Caple,and B. Schatz, Tetrahedron Lett. 1965:385; E. Vogel,W. Grimme,and E. Dinne, Tetrahedron Lett. 1965:391; J. E. Baldwin and V. P. Reddy, J. Org. Chem. 53:1129 G1988). 608 of electrocyclic reaction is called disrotatory. CHAPTER 11 CONCERTED CH3 PERICYCLIC H H CH3 REACTIONS H3C H H CH3 H ≡ H 132°C H H H CH3 HH H H CH 3 H 3 CH3 H H CH3 H3C H H H H ≡ H 178°C CH H CH3 3 H HH H CH3 H 4 H A complete mechanistic description of these reactions must explain not only their high degree of stereospeci®city,but also why four- p-electron systems undergo conrotatory reactions whereas six-p-electron systems undergo disrotatory reactions. Woodward and Hoffmann proposed that the stereochemistry of the reactions is controlled by the symmetry properties of the HOMO of the reacting system.13 The idea that the HOMO should control the course of the reaction is an example of frontier orbital theory,which holds that it is the electrons of highest energy,i.e.,those in the HOMO,that are of prime importance. 14 The symmetry characteristics of the occupied orbitals of 1,3-butadiene are shown in Fig. 11.1. Why do the symmetry properties of c2 determine the stereochemistry of the electrocyclic reaction? For convenience,let us examine the microscopic reverse of the ring opening.
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