Chemistry
Sigmatropic Rearrangement
Sigmatropic rearrangement is a type of organic reaction in which a sigma bond is rearranged within a molecule. It involves the movement of a substituent or a group of atoms from one part of the molecule to another, resulting in a new arrangement of the atoms. This reaction is often used in the synthesis of complex organic molecules.
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eBook - ePubUnderstanding Organometallic Reaction Mechanisms and Catalysis
Computational and Experimental Tools
- Valentin P. Ananikov(Author)
- 2014(Publication Date)
- Wiley-VCH(Publisher)
Chapter 5 Computational Studies on Sigmatropic Rearrangements via π-Activation by Palladium and Gold CatalystsOsvaldo Gutierrez and Marisa C. Kozlowski5.1 Introduction
5.1.1 Sigmatropic Rearrangements
A sigmatropic shift or rearrangement is an intramolecular pericyclic reaction along a π-framework where one σ-bond is formed at the cost of breaking another σ-bond. When a fragment migrates, this transformation is typically referred to as a rearrangement . Canonical forms of the most frequent Sigmatropic Rearrangements are listed in Figure 5.1 , along with the standard naming convention. Heteroatom variants are common and virtually any center along the rearranging framework can be a heteroatom.Prototypical Sigmatropic Rearrangements.Figure 5.1Many Sigmatropic Rearrangements have been studied both experimentally and computationally [1]. Perhaps the most widely studied variant is the [3,3]-Sigmatropic Rearrangement, where X, Y, Z can be all carbons (Cope rearrangement) or various heteroatoms (Eq. (5.1 )). For example, Y = O and X, Z = C corresponds to the Claisen rearrangement. Many of these processes can occur thermally without catalysts, but catalysis via Lewis acid activation or π-activation has been demonstrated. Most of these rearrangements proceed via a concerted mechanism, but other radical and ionic pathways can intervene depending on the substitution and the presence of catalysts (Figure 5.2 ) [1].5.1Different possible mechanisms for the [3,3] Sigmatropic Rearrangement.Figure 5.25.1.2 Metal-Catalyzed Sigmatropic Rearrangements
Many metals catalyze Sigmatropic Rearrangements including several that likely function by π-activation such as mercury, palladium, platinum, and, most recently, gold. For the most part, computational studies have focused on understanding the mechanism and stereochemical-determining factors of [3,3]-sigmatropic shifts that do not involve π-activation [2]. Herein, we present a survey of those computational studies focusing on palladium- and gold-catalyzed sigmatropic shifts in which the transformation was promoted by π-activation, that is, complexation to either carbon-carbon double or triple bonds.
eBook - ePubPericyclic Reactions
A Mechanistic and Problem-Solving Approach
- Sunil Kumar, Vinod Kumar, S.P. Singh(Authors)
- 2015(Publication Date)
- Academic Press(Publisher)
3.4.4 Aza-Cope Rearrangement 1133.4.5 The Claisen Rearrangement 1153.4.6 Some Clever Variants of the Claisen Rearrangement 1293.5 [5,5] Sigmatropic Shift 1323.5.1 Solved Problems 1333.6 [2,3] Sigmatropic Rearrangements 1363.6.1 Solved Problems 1373.7 Peripatetic Cyclopropane Bridge: Walk Rearrangements 1393.7.1 Solved Problem 1413.8 Sigmatropic Rearrangements Involving Ionic Transition States 1423.8.1 Solved Problems 144Many thermal (or photochemical) rearrangements involve the shifting of a σ-bond, flanked by one or more π-electron systems, to a new position [i,j] within the molecule in an uncatalyzed intramolecular process. Since it is rearrangement of a σ-bond, these reactions are called Sigmatropic Rearrangements of order [i,j]. These reactions are often classified with two numbers, i and j, set in brackets [i,j] and the system is numbered by starting with the atoms forming the migrating σ-bond. These numbers [i and j] indicate the new positions of the σ-bond whose termini are i − 1 and j − 1 atoms removed from the original bonded loci (Scheme 3.1 ).Scheme 3.1 Sigmatropic Rearrangements of order [1,3] and [1,5].Very often, the migrating σ-bond is situated in between two π-bond systems as in the Cope and the Claisen rearrangements (Scheme 3.2 ).Scheme 3.2 Sigmatropic Rearrangements of order [3,3].3.1. Suprafacial and Antarafacial Processes
Since a sigmatropic reaction involves the migration of a σ-bond across the π-electron system, there are two different stereochemical courses by which the process may occur. When the migrating σ-bond moves across the same face of the conjugated system, it is called a suprafacial process whereas in antarafacial process the migrating σ-bond is reformed on the opposite π-electron face of the conjugated system . The following [1,5] sigmatropic shifts illustrate both these processes and their stereochemical consequences (Figure 3.1
eBook - ePubOrganic Reaction Mechanisms 2008
An annual survey covering the literature dated January to December 2008
- A. C. Knipe(Author)
- 2011(Publication Date)
- Wiley(Publisher)
CHAPTER 13 Molecular Rearrangements: Part 1. Pericyclic Molecular Rearrangements S. K. ARMSTRONG Formerly at the Department of Chemistry, University of Glasgow[3,3]-Sigmatropic RearrangementsAll-carbon Pericyclic Systems and [3,3]-Sigmatropic Cascades One Heteroatom Two or More Heteroatoms[2,3]-Sigmatropic RearrangementsOther [n,m ]-Sigmatropic Rearrangements [1,n ]-Sigmatropic RearrangementsDiradical and Carbene RearrangementsElectrocyclic RearrangementsGroup Transfer ReactionsTandem Pericyclic RearrangementsReferencesBifurcations on potential energy surfaces have been reviewed, including Cope and electrocyclic rearrangements and ene reactions, and also cycloadditions.1 The new Cplex-isoelectronic theory has been applied to electrocyclization, Sigmatropic Rearrangements, cheletropic reactions, and antiaromaticity, and has given results consistent with experimental data, and mechanistic predictions that, in a number of cases, differ from those of quantum chemical calculations.2[3,3]-Sigmatropic Rearrangements All-carbon Pericyclic Systems and [3,3]-Sigmatropic CascadesAb initio calculations on 2,8(4,6)-disubstituted semibullvalenes and 2,8:4,6-tetrasubstituted semibullvalenes have found that π-bonding substituents can reduce or even remove the energy barrier to Cope rearrangement.3Various 7-allyloxy-8-ketocoumarin derivatives (1) have been shown to rearrange in a series of [3,3]-Sigmatropic Rearrangements as shown in Scheme 1 . With a 7-cyclohexenyloxy substituent, or with an 8-benzoyl group, the sole products were found to be 6-allylic coumarins of type (2). With a 7-allyloxy substituent, further rearrangements competed as shown; the proposed mechanisms, as illustrated in Scheme 1
eBook - PDFOrbital Symmetry
A Problem - Solving Approach
- Roland Lehr(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
PROBLEM 111.12 The following reaction has been observed: Chapter IV THE STEREOCHEMISTRY OF SIGMATROPIC REACTIONS Extensive studies of the stereochemistry of concerted, sigmatropic carbon-carbon bond rearrangements of orders [1,3] and [1,5] have been carried out by Professor J. A. Berson at Yale University and by Professor Η. E. Zimmerman at the University of Wisconsin. In this chapter, we shall consider their work in this area to exemplify the constraints imposed on the stereochemical outcome of concerted Sigmatropic Rearrangements in cases where orbital symmetry is conserved. PROBLEM IV.1 By way of orientation, the reader should review the sections in Chapter I which deal with the applications of orbital symmetry relationships to Sigmatropic Rearrangements. (a) Consider first suprafacial Sigmatropic Rearrangements of order [ 1 , 3 ] : What is the stereochemical fate of the migrating group, R, if this reaction is to proceed thermally in accordance with the Woodward-Hoffmann rules? (I.e., does R retain its configuration or does it suffer inversion in the thermally allowed suprafacial [1,3] Sigmatropic Rearrangement?) (b) Repeat your analysis of part (a), above, for a thermal, suprafacial Sigmatropic Rearrangement of order [1,5]. 6 0 P R O B L E M IV.3 61 PROBLEM IV.2 (a) Suggest a mechanism for the following transformation: (b) Consider the following thermal [1.3] Sigmatropic Rearrangements: The rearrangement of eA7tfaexo -6-acetoxy-7-methylbicyclo[3.2.0] hept-2-ene (I) is seen to proceed with predominant inversion, whereas rearrangement of the corresponding endo, endo isomer proceeds with predominant retention of configuration of the migrating center. Furthermore, exo ^ endo epimeriza-tion of the methyl group does not compete with rearrangement during the pyrolysis of I, but this process is found to occur 60% as fast as the rearrange-ment of II to A and B. Offer a detailed explanation to account for these observations.
eBook - PDFOrganic Chemistry
A Mechanistic Approach
- Penny Chaloner(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
Don’t be alarmed because the reaction is going in the opposite direction from the one you have seen before—the orbital symmetry considerations are just the same. If you feel less confident opening than closing the rings, that’s not unusual; most students do. Check your result by running the reaction the other way, under the same conditions, and make sure that you get back to the SM. Δ 18.6.2 Sigmatropic RearrangementS Sigmatropic Rearrangements are a class of reaction in which an σ-bond is moved effectively “across” a π-system to a new position. Figure 18.35 shows a Cope rearrangement and is a 3,3-sigmatropic process (note this is not an equally balanced equilibrium; the material with the disubstituted double bond is more stable). Figure 18.36 shows a 1,5-sigmatropic shift, of a hydrogen atom, from one end of the molecule to the other. The numbering of these rearrangements often seems confusing. The simplest way to approach the problem is to number the atoms from the σ-bond that is being broken toward its new position (ignore substituents). The numbers at the site of the new σ-bond give the so-called order of the reaction. So for the Cope rearrangement ( 18.9), we see where the 3,3-designation arises. This also works for the 1,5-proton shift—there is only one atom, the hydrogen, to be numbered in the migrating unit, and it moves to atom 5 in the chain ( 18.10). 1 2 3 1 2 3 σ-bond 18.9 H 1 1 2 3 4 5 σ-bond 18.10 FIGURE 18.35 Cope.rearrangement,.a.3,3-sigmatropic.process. CD 2 uni0394 CHD 2 H FIGURE 18.36 1,5-Hydride.shift. 876 18.6 Neutral Rearrangements The first reactions to consider are 1,3- and 1,5-hydrogen shifts. Although the 1,5-shift shown in Figure 18.36 works well, the 1,3-hydogen shift is disallowed thermally. In analyzing these reac- tions, the convention is that we consider one LUMO and one HOMO, and the HOMO is always the HOMO of the hydride ion, a 1s orbital. The LUMO is the lowest unoccupied molecular orbital of the allyl cation.
eBook - PDF- R. B. Woodward, R. Hoffmann(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
7. Theory of Sigmatropic Reactions We denned as a sigmatropic change of order [i,j] the migration of a a bond, flanked by one or more π electron systems, to a new position whose termini are ;-l and/-l atoms removed from the original bonded loci, in an uncatalyzed intramolec-ular process. Thus, the well-known Claisen and Cope rearrangements are sigma-tropic changes of order [3,3]. Suprafacial C Antarafacial C Figure 28. Suprafacial and antarafacial [1,5] shifts of a hydrogen atom. A priori, there are two topologically distinct ways of effecting a sigmatropic migra-tion. These are illustrated in Figure 28 for the [1,5] shifts of a hydrogen atom. In the first, suprafacial process, the transferred hydrogen atom is associated at all times with the same face of the π system. In the second, antarafacial process, the migrat-ing atom is passed from the top face of one carbon terminus to the bottom face of the other. For the analysis of these reactions correlation diagrams are not relevant since it is only the transition state and not the reactants or products which may possess molec-ular symmetry elements. We shall present several equivalent methods for ana-lyzing these reactions. 1. The use of the principle of conservationof orbital symmetry is here illustrated for the case of a suprafacial [1,3] hydrogen shift. The relevant correlations are shown in Figure 29. Clearly, two electrons can enter a bonding orbital, either a or π, of the product, but the other two must be placed either in a σ* or a π* orbital, if orbital symmetry is to be conserved. The reaction is symmetry-forbidden. 7. Theory of Sigmatropic Reactions 115 c=c f, ·:? a-b Isolated orbitale a+b Interacting orbitals before transfer of hydrogen Interacting orbitals after transfer of hydrogen Figure 29. Conservation of orbital symmetry in a suprafacial [1,3] hydrogen shift.
eBook - ePub- Michael Harmata(Author)
- 2011(Publication Date)
- Wiley(Publisher)
Chapter 3 Sigmatropic Rearrangements and Related Processes Promoted by Silver Jean-Marc Weibel, Aurélien Blanc, and Patrick Pale Laboratory for Organic Synthesis and Reactivity, Institute of Chemistry, University of Strasbourg, France 3.1 IntroductionSigmatropic shifts represent a large class of reactions involving the migration of at least one sigma bond. Therefore, such migrations lead to skeletal rearrangements of the carbon frame within the molecule undergoing this reaction.Sigmatropic Rearrangements usually involve σ bonds adjacent to a π system or a σ bond included in a strained system. As other transition metals, but with specific properties due to its d 10 electronic configuration, f orbitals and a relativistic effect,1 silver easily interacts with such systems. Silver salts have thus been explored as catalysts to facilitate and promote Sigmatropic Rearrangements.3.2 Wolff and Arndt–Eistert Rearrangements and Related ReactionsThe Wolff and Arndt–Eistert rearrangements are probably among the earliest known reactions promoted by silver ions.2, 3 Discovered at the turn of the nineteenth/twentieth century, the Wolff rearrangement allows the transformation of α-diazoketones to carboxylic acids,4 while the Arndt–Eistert rearrangement is a similar sequence also leading to carboxylic acids, but including the preparation of α-diazoketones from a shorter acid chloride (Scheme 3.1 ).5Scheme 3.1Numerous conditions have been developed for this transformation, but reproducible yields have usually been obtained by mixing a silver salt with a coreagent, such as silver nitrate associated with wet ammonia, silver oxide with triethylamine or sodium thiosulfate, and silver benzoate with triethylamine. Nonbasic conditions have also been described by Koch and Podlech using silver trifluoroacetate deposited on silica.6
eBook - PDF- James Morrison(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
F < B U (R)-(-)-248 100 °/o ee Η ^>CH 3 H C = C -C > t r i 3 — • H ^ C = C = C X H y , X OSAr A r ^ (-) Ch H H C ^ C -C ^ C h3 N 0H O W -W -2 49 ^ H H C = C -C ^ C h3 — • H C = C = C ^ H N 0-SO-Ar ArS0 2 r_j C H i Scheine 58 ality less easily than can 3,3-rearrangements, examples of aliène products from acetylenic precursors are known (277) (Scheme 58). On treatment with P C I 3 , the acetylenic alcohol 247 formed the rearranged phosphonic acid chloride 248 with quantitative asymmetric induction (206). Both the sulfenate and sulfinate rearrangements convert alcohol 249 to optically active aliènes (263). IV. Summary Sigmatropic Rearrangements other than 3,3- and 2,3-rearrangements are beyond the scope of this chapter but it should be noted that transfer of chirality has been observed in 1,3-(167b, 264), 1,4-(265), and 1,5-sigma-tropic rearrangements (266); the stereochemical emphasis in these cases was aimed at elucidating reaction mechanisms rather than practical syn-thetic routes to chiral molecules. It is clear that, for the latter purpose, concerted 3,3- and 2,3-Sigmatropic Rearrangements are hardly rivaled in their capacity to construct chiral centers of predictable relative and abso-lute configuration and to prepare molecules of high optical purity. 8. Sigmatropic Rearrangements 563 Acknowledgments I am indebted to J. A. Berson, J. E. Baldwin, D. A. Evans, J.J. Gajewski, R. E. Ireland, W. D. Ollis, S. Raucher, G. Saucy, E. Vedejs, Α. Viola, and F. Ε. Ziegler for kindly providing me with unpublished information and copies of their papers. The examples chosen for this chapter are surely incomplete, and I apologize to those authors whose results may have been inadvertently omitted. This chapter is dedicated, on the occasion of his retirement from a long and fruitful career, to Professor Richard T. Arnold, who recognized very early the versatility of thermal organic reactions and the consequences of cyclic transition states.
eBook - PDFOrganic Reaction Mechanisms 1973 Reprint A
An annual survey covering the literature dated December 1972 through November 1973
- A. R. Butler, M. J. Perkins, A. R. Butler, M. J. Perkins(Authors)
- 2008(Publication Date)
- Wiley-Interscience(Publisher)
A similar conclusion was reached in the case of the rearrangement of tris- (2-methylallyl)borane.138 The thermal rearrangement of the benzylated pyrazine (148) to (149) represents the first clear-cut example of a [ 1,3]-sigmatropic shift with inversion involving nitrogen at the migration origin. The overall process showed minimal rate dependence on solvent, Molecular Rearrangements 439 proceeded with 295% stereospecificity and displayed a small extra-cage free-radical component.139 [1,3]-Sigmatropic C + N alkyl shifts have been reported in the cleavage reactions of some 1,4-diazocines.l40 These two approaches have been combined141 to examine the stereochemistry of [1,3]-sigmatropic alkyl shifts from nitrogen to carbon and their reverse in certain pyrazine-based heterocyclic systems. The shifts involved were all of the allowed suprafacial [1,3]-types. The base-induced conversion of o-dipropargylbenzeneand 2,3-&propargylnaphthalene to the corresponding allenes has been reported.141 Other Sigmatropic Migrations [ 1,5]-Migrations. An intramolecular [1,5]-shift of a formyl group has been detected in the thermolysis of methyl bicyclo[3.2.O]hept-2-en-7-ones.142 From rate studies on the thermolysis of 1 -methylcyclohexa-2,4-dienes, the formyl group undergoes [ 1,5]-sigma- tropic shift faster than hydrogen by more than two orders of magnitude, whereas the methoxycarbonyl group is slower by a factor of about 70 and acetyl shows a migration aptitude comparable to that of hydrogen.143 Thermolysis ( 150-190°, in decalin) of arylallenes causes their rearrangement via a [ l,b]-hydrogen shift to yield o-quinodi- methanes which then may cyclize to give dihydronaphthalenes and/or undergo [ l,7]-sigmatropic H-shifts to give arylbutadienes.
- Julian A Davies, C M Hockensmith, Yu N Kukushkin(Authors)
- 1996(Publication Date)
- World Scientific(Publisher)
Chapter 11. MOLECULAR REARRANGEMENTS OF COORDINATION COMPOUNDS In coordination chemistry, as in organic chemistry, syntheses are sometimes complicated by molecular rearrangements. In this Chapter, these processes are discussed and classified. A molecular rearrangement is defined here as a transformation resulting in a change in connectivity without any change in atomic composition. Thus, geometric isomerization is an example of a molecular rearrangement. The isomerization of square-planar and octahedral complexes is discussed in Chapters 1 and 12 and is not considered further here. Changes in spin state, i.e. transformations of high-spin complexes into low-spin complexes, or vice versa, and related electronic transformations are not included in the definition of molecular rearrangement employed here. Enantiomerization, ligand rotations about metal-ligand bonds, and transformations due to conformational changes of coordinated ligands similarly are excluded. Haptotropic rearrangements and sigmatropic shifts are subjects of current interest but they are very specific types of molecular rearrangement applicable to a relatively narrow class of coordination compounds. These processes are not considered here but are discussed in detail in a number of comprehensive reviews. 1 7 11.1 Rearrangements involving changes in metal-ligand binding sites The most important member of this class of rearrangement is linkage isomerism which is a process that may take place in complexes containing ambidentate ligands, i.e., ligands with more than one donor site but which bind in monodentate fashion. In 1894, Jorgensen 8 first reported linkage isomers in his studies of cobalt nitro-complexes and the isomerization reactions of these species have subsequently been investigated in detail. The rates of O-O and O-N isomerization in the cobalt(III) complexes [Co(N02)(NH 3 )5] 2+ and [Co(ONO)(NH 3 )5] 2+ , for example, have been studied by NMR spectroscopy, 9 Scheme 11.1.
eBook - ePubOrganic Reaction Mechanisms 2010
An annual survey covering the literature dated January to December 2010
- A. C. Knipe(Author)
- 2012(Publication Date)
- Wiley(Publisher)
Chapter 13 Molecular Rearrangements: Part 1. Pericyclic Reactions J. M. CoxonDepartment of Chemistry, University of Canterbury, Christchurch, New Zealand[3,3]-Sigmatropic, Claisen, and Cope Rearrangements [2,3]-Reactions Vinyl Cyclobutane and Vinyl Cyclopropane Rearrangement 1,2-Migration Ene Reaction Bergman Reaction Electrocyclic Reactions Cyclization 4 + 2-Cycloadditions 3 + 2-Cycloadditions Metathesis Metal-catalysed Reactions Miscellaneous References[3,3]-Sigmatropic, Claisen, and Cope Rearrangements
The diastereoselectivity of the [3,3]-rearrangement of 1,1-disubstituted allyl carboxylates (Scheme 1 ) has been reported to be a consequence of the transition state having a boat-like transition structure because of the participation of the lone pairs and the secondary orbital interaction.1 Although the transition structure for the [1,3]-rearrangement has a higher barrier, it is said not to proceed in the usual antarafacial manner due to the cyclic orbital interaction among two lone pairs of the carboxylate and the allylic lumo. The geminal bond participation controls the stereoselectivity in the [3,3]-rearrangement. The bond model analysis showed that electron-withdrawing σ-bond substituents prefer to occupy the Z
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