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Pericyclic reactions

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By B. R. Thorat, Organic Chemistry, SET-NET purpose

Pericyclic reactions

  1. 1. By Bapu R. Thorat M.Sc. SET. NET. MS-CIT Assit. Professor Dept of Chemistry &Dept of Biotechnology, Governmentof Maharashtra, IsmailYusufCollegeofArts,ScienceandCommerce,Jogeshwari (East), Mumbai-60 Pericyclic Reactions
  2. 2. Chemical reactions Bond formation or breaking Intermediate Ionic intermediates such as cation, anion, ion pair, etc. Free radical intermediate Transition State formation Electrophilic - cation Nucleophilic - anion Pericyclic reactions Rearrangements Free radical reactions Thermal Photochemical
  3. 3. Transition State formation Pericyclic Reactions Chemical reaction MechanismFree radical intermediate Ionic intermediates such as cation, anion, zwitter ion, etc 1. Proceeds in single cyclic transition state. 2. Reactions not influenced by any solvent, reagent, structural changes, catalysts, etc. 3. These are highly stereospecific in nature. 4. These reactions are initiated either by heat or light. Such class of reactions are called as pericyclic reactions. Reactions are either carried out thermally or photochemically. The chemical reaction in which starting material is converted into a single stereo-isomeric product is called as stereospecific or regiospecific reaction. Factors – solvent, Catalyst, reagent, Structural changes Not influenced Source of energy Heat Light
  4. 4. SNi SN1 SN2
  5. 5. Pericyclic Reactions Pericyclic reactions (pericyclic means shifting of electrons around the circle) are first explained by the scientist Hoffman and Woodward in 1965. Sigmatropic rearrangement Ene reactionsCycloaddition reactions Electrocyclic reactions- Ring closer and ring opening Pericyclic reactions Photochemical Reactions Activated by radiation energy Thermal Reactions Activated by heat activated molecule The chemistry of activated molecule in thermal and photochemical reactions is totally different. - different products are obtained.
  6. 6. Pericyclic Reactions +2 hv Ketonic Sensitizer Cis- Trans- 2 activated molecule Pericyclic Reactions Electrons and molecular orbitals takes part in reaction LUMO Bonding molecular orbitals Anti-Bonding molecular orbitals HOMO Process Thermal Photochemicall From mixture – one
  7. 7. Molecular orbital symmetry Molecular orbitals ψ Antibonding molecular orbitals having higher energy & are empty at G.S. Bonding molecular orbitals having lower energy & contains electrons in G.S. 1. Mirror plane symmetry (m): A and C 2. (C2) axis of symmetry: B and D CH2CH2 buta-1,3-diene CH2 CH2 (Bonding MO) (Antibonding MO) B C D A
  8. 8. 1. Mirror plane symmetry (m) is a plane passing through the center of the molecular orbital and perpendicular to the molecular orbital axis, it bisect the molecular orbital in two parts, one part is mirror image of other part. It is maintained by Dis rotatory process. 2. (C2) axis of symmetry is assume to present if the rotation of the molecular orbital around the axis by 1800 (360/2) gives raise to another orbital, then reflect it across a plane perpendicular to the axis. It is maintain by con rotatory process. C2 axis of symmetry C2 Plane of symmetry Bonding pi-orbital Anti-Bonding pi-orbital C2 axis of symmetry Plane of symmetry Sigma-bond Orbitals M C2 π S A π* A S σ S S σ* A A
  9. 9. carbocation alkene Allylic cation diene Diene cation cyclopentadiene cation Hexatriene
  10. 10. Molecular orbital symmetry 1,3-butadiene, total number of molecular orbitals are 4- π1, π2, π2*, π1* where π1 and π2 are bonding and π2*and π1* are antibonding molecular orbitals. They are also denoted as- (ψ1, ψ2, ψ3 and ψ4). 1,3,5-hexatriene, total number of molecular orbitals are six i.e. π1, π2, π3, π3*, π2*, π1* where π1, π2 and π3 are bonding and π3*, π2*and π1* are antibonding molecular orbitals. They are also denoted as- (ψ1, ψ2, ψ3, ψ4, ψ5 and ψ6) Orbitals ψ1 ψ2 ψ3 ψ4 M S A S A C2 A S A S Orbital ψ1 ψ2 ψ3 ψ4 ψ5 ψ6 M S A S A S A C2 A S A S A S
  11. 11. Node When we move in molecular orbital, the sign of the orbital change (above the plane or consider all orbitals below the plane) is called as node. For a linear conjugated π-system the wave function ψn will posses (n-1) nodes.  If (n-1) is zero or even integer, ψn will be said to be symmetric with respect to m and antisymmetric with respect to C2.  If (n-1) is an odd integer ψn will posses the C2 axis of symmetry and antisymmetric with respect to m. Node = 0 Node = 1 Node = 2 Node = 3 ψ1 ψ2 ψ3 ψ4
  12. 12. Pericyclic Reactions Electrocyclic reactions- Process in which two π bonds convert into one π & one σ bond and vice versa. Ring opening- Ring closer- Two atomic orbitals forming a σ-bond may be rotating in opposite directions, one in clockwise and other in anticlockwise manner is called as dis-rotatory process Two atomic orbitals forming a σ-bond may be rotating in same direction either in clockwise or anticlockwise manner is called as con-rotatory process. dis ratatory con rotatory If the substitutions are present on the rotating carbons also rotate in same direction
  13. 13. Frontier Molecular Orbital [FMO] Method Molecular orbital model for analyzing pericyclic reactions has been proposed by Kenichi Fukui of Japan. The correlation diagram is useful for the detail analysis of an Electrocyclic reactions. A B C D X Y W Z CH2CH2 A B C D X Y W Z Bonding MO Antibonding MO Mirror (m) of symmetry C2 axis of symmetry A S A S A S A S A B C D X Y W Z A S A S A S A S 1,3-butadiene cyclobutene
  14. 14. 1,3,5-hexatriene1,3-cyclohexadiene C2 C2 M M E A S A S A S A A S A S S A S A S A S S A S A S A antibonding molecular orbital bonding molecular orbital
  15. 15. Frontier Molecular Orbital [FMO] Method The correlation diagram is useful for the detail analysis of an Electrocyclic reactions. A similar conclusion is obtained by considering the symmetry of highest occupied molecular orbital [HOMO] of open chain partner in Electrocyclic reactions. If the HOMO having C2-axis of symmetry (node is odd), then reaction will follow con- rotatory path. If HOMO posses a mirror plane symmetry (node is zero or even number), a reaction will follows dis-rotatory path. Thermal Reactions Transition State Configurational Preference 4n + 2 (aromatic) Disrotatory 4n (antiaromatic) Conrotatory Photochemical Reactions Transition State Configurational Preference 4n + 2 (aromatic) Conrotatory 4n (antiaromatic) Disrotatory
  16. 16. Frontier Molecular Orbital [FMO] Method A similar conclusion is obtained by considering the symmetry of highest occupied molecular orbital [HOMO] of open chain partner in Electrocyclic reactions. A B C D X Y W Z CH2CH2 A B C D X Y W Z Bonding MO Antibonding MO Mirror (m) of symmetry C2 axis of symmetry A S A S A S A S A B C D X Y W Z A S A S A S A S Thermal HOMO Con-rotatory
  17. 17. Frontier Molecular Orbital [FMO] Method A similar conclusion is obtained by considering the symmetry of highest occupied molecular orbital [HOMO] of open chain partner in Electrocyclic reactions. A B C D X Y W Z CH2CH2 A B C D X Y W Z Bonding MO Antibonding MO Mirror (m) of symmetry C2 axis of symmetry A S A S A S A S A B C D X Y W Z A S A S A S A S Photochemical HOMO Dis-rotatory
  18. 18. Process Total Electrons T.S. Symmetry HOMO Node Process allowed hυ 4 Antiaromatic m ψ3 2 Dis Process Total Electrons T.S. Symmetry HOMO Node Process allowed ∆ 4 Antiaromatic C2 ψ2 1 Con
  19. 19. Process Total Electrons T.S. Symmetry HOMO Node Process allowed 6 Aromatic m ψ3 2 Dis Process Total Electrons T.S. Symmetry HOMO Node Process allowed hv 6 Aromatic C2 ψ4 3 Con ∆
  20. 20. Examples Process Total number of electrons T.S. HOMO node symmetry process allowed Process Total number of electrons T.S. HOMO node symmetry process allowed Process Total number of electrons T.S. HOMO node symmetry process allowed
  21. 21. Examples 7. Suggest the mechanism of following reaction. D D DD
  22. 22. D D D D D DIt is electrocyclic ring opening reaction. Process Total ele HOMO Node Symmetry Process allowed Thermal 4 ψ2 1 C2 Con DD D D
  23. 23. Examples 8. In the following concerted reaction, the product is formed by a-. H H 1. 6-pi disrotatory electrocyclisation 2. 4-pi disrotatory electrocyclisation 3. 6-pi conrotatory electrocyclisation 4. 4-pi conrotatory electrocyclisation
  24. 24. Process Total electrons HOMO Node Symmetry Process allowed Thermal 6 ψ3 2 m Dis The triene skeleton of cyclo octatetraene undergoes electrocyclic ring closer reaction. The total number of electrons takes part in the reaction are 6, therefore HOMO of the reaction is ψ3. The node of reaction is 02, therefore m plane of symmetry is maintained. The m (plane of symmetry) has been maintained by dis-rotation. Process Total electrons HOMO Node Symmetry Process allowed Thermal 4 ψ2 1 C2 Con  con rotation H H dis rotation H H
  25. 25. The major product formed by photochemical reaction of (2E,4Z,6E)-decatriene is -
  26. 26. The major product formed by the following reaction of
  27. 27. Cycloaddition Reactions It is class of pericyclic reactions in which the system having mπ electrons is added into the system having nπ electrons forming cyclic product. It is versatile route for the synthesis of cyclic compounds having high degree of stereo selectivity under thermal and photochemical reaction conditions. Depending on the number of π electrons taking part in the reaction, is called as (m+n) or (m+n+….) Cycloaddition reactions. Or A concerted combination of two π-electron systems to form a ring of atoms having two new σ bonds and two fewer π bonds is called a cycloaddition reaction.
  28. 28. Cycloaddition Reactions In cycloaddition reactions, addition of two systems having double bonds take- place either in same or opposite side of the system. Such mode of addition is very important to decide the stereo chemistry of product. These different numbers of modes are named as suprafacial (on the same side) and antrafacial (on the opposite side). This specification is usually carried out by keeping a suitable subscript (s or a) after the number referring to the pi-components. E.g. The Diels-Alder reaction is also called as (4s+2s) cycloaddition. [suprafcial] [antrafacial] R R R R H H + heat (4s+2s)
  29. 29. Frontier Molecular Orbital [FMO] Method [Cycloaddition Reactions] A cycloaddition reaction is allowed or not which can be found to depends on the symmetry properties of the highest occupied molecular orbital (HOMO) of one reactant and the lowest unoccupied molecular orbital (LUMO) of other molecule. The favorable interaction can be predicted from the sign of HOMO and LUMO. e.g. During Cycloaddition of ethylene to cyclobutane (2s+2s) addition is thermally forbidden because lopes of HOMO of one ethylene molecule and that of other LUMO of another ethylene molecule are not having corresponding similar signs, it (2a+2s) thermally allowed. But when one ethylene molecule is irradiated, on electron get excited to excited state (antibonding molecular orbital) which is now become HOMO which is then correlated to LUMO of other unexcited ethylene molecule. Therefore, it is photochemically (2s+2s) allowed. HOMO LUMO HOMO of excited state. LUMO of unexcited state. (2s + 2s) (Ground state) [ , forbidden] (Excited state) [ hv, allowed]
  30. 30. Frontier Molecular Orbital [FMO] Method [Cycloaddition Reactions] In Diels-Alder reaction as sign of the 1,4-lopes of butadiene HOMO have been found to be matching those in the LUMO of ethylene. The (4s+2s) addition is thermally allowed and photochemically forbidden whereas (4a+2s) or (4s+2a) is photochemically allowed and thermally forbidden. The Diels-Alder reaction of cyclpentadiene to forming dicyclopentadiene. Invariably, the endo-dimer is formed rather that exo-dimer because of favorable secondary force due to interaction of frontier orbitals of diene and dienophile compounds which leads to lower the energy of the transition. (4s + 2s, , allowed) (4s + 2s, , allowed) (4s + 2a or 4a + 2s, hv , allowed) HOMO LUMO HOMO + LUMO (endo form) (exo form)
  31. 31. Cycloaddition Reactions The most common cycloaddition reaction is the [4π+2π] cyclization known as the Diels- Alder reaction in which cyclic product is formed from alkene and a diene. The stereochemistry of the substituent attached to double bonded carbon atom is maintained. The diene containing electron donating group while dienophile containing electron withdrawing groups are easily undergo (4+2) cycloaddition reaction. O O O CN CNNC NC O O O CN CNNC NC + + (4+2) cycloaddition (2+2+2) cycloaddition + one step mechanism + or+ - . two step mechanism zwitterions diradicals
  32. 32. Cycloaddition Reactions OAc OAc COOCH3 COOCH3
  33. 33. The diene containing electron donating group while dienophile containing electron withdrawing groups are easily undergo (4+2) cycloaddition reaction.
  34. 34. Electron-donating and - withdrawing substituents in the product are arranged in pseudo- ortho and pseudo-para positions to each other.
  35. 35. Orbital interactions lower energy of TS more stable exo isomer Thermodymic control endo product – Kinetic control More steric interactions Thermodymic control Kinetic control
  36. 36. SO2 CH3 CH3 CH3 CH3  SO2 CH3 CH3 CH3 CH3  CH3 CH3 SO2 SO2+ LUMO HOMO Example The reaction containing 8 electrons therefore occurs with a conrotatory motion into triene termini. If the cheletropic reaction is linear, i.e. if the HOMO of SO2 reacts suprafacially as it does in the case of dienes, the triene must react antrafacially, which is consistent with the observed conrotation.
  37. 37. Examples 1. The concerted photochemical reaction between two olefins leading to a cyclobutane ring is- a. π2 s + π2 a cycloaddition. b. π2 s + π2 s cycloaddition. c. σ2 s + σ2 s cycloaddition. d. π2 s + σ2 a cycloaddition. HOMO HOMO LUMO LUMO supra-supra antra-antra
  38. 38. It is example (2+2) cycloaddition reaction. It is photochemical process, therefore consider first excited state (LUMO) of first olefine is HOMO which is then correlated to LUMO of second olefine (unexcited form). Therefore answer is (b). HOMO HOMO LUMO LUMO supra-supra antra-antra
  39. 39. Examples 2. The intermediate A and the major product B in the following conversion are- NH2 COOH NaNO2 /HCl A B NH2 1. A is carbocation and B is 2. A is carbanion and B is 3. A is free radical and B is 4. A is benzyne and B is
  40. 40. It is combination of two reactions- Benzyne formation and Diel’s Alder reaction. The anthranilic acid undergoes diazotizing followed by decarboxylation forming benzyne. It shows [4+2] cycloaddition reaction [Diel’s Alder reaction] forming cyclic product (4). NH2 COOH NaNO2 ClH HNO2 NaCl HNO2 N2 Cl COOH O O H N2 Cl + + + A + [4+2] cycloaddition rea. Diel's Alder reaction B
  41. 41. Examples 3. The structures of the major products X and Y in the following transformation are- O + hv X Y O O O O O O O O O 1. 2. 3. 4. X = Y = X = Y = X = Y = X = Y =
  42. 42. During Diel’s Alder reaction between cyclic diene and substituted dienophile forming endo product rather than exo because of favorable secondary force due to interaction of frontier orbitals of diene and dienophile compounds which leads to lower the energy of the transition (3). The X further undergoes photochemical [2+2] cycloaddition reaction of alkene and carbonyl group (Paterno-Buchi reaction) forming oxetane [four membered cyclic ether].
  43. 43. Example 6. Suggest the mechanism of following reaction. O O COOMe COOMe MeOOC COOMe + ? ?
  44. 44. O O COOMe O O COOMe +Step I HOMO LUMO Total electrons - 6, node- 2, Aromatic T.S., Supra-supra process allowed O O COOMe COOMe COOMe CO2 or It is electrocyclic ring opening reaction.Step II COOMe MeOOC COOMe COOMe COOMe +orStep III. It is [4+2] cycloaddition reaction called Diel's Alder reaction. The substituent on dienophile is endo when using cyclic diene.
  45. 45. The major product formed in the following reaction is -
  46. 46. T.S. of 1, 3 - not formed because alkyl and ester groups are trans together - stereochemistry does not change during cycloaddition reaction
  47. 47. Sigmatropic Rearrangement Many thermal and photochemical rearrangements are known which involve the shifting of a σ-bond flanked by one or more π-electron systems to a new position [i,j] with in the molecule. It is an uncatalyzed intramoleculer process. This rearrangement involve shifting of the σ-bond, hence called as sigmatropic rearrangement of the order [i,j]. The i and j are two number set in the square bracket and the numbering of system is done by starting with atoms from which the migration of the σ-bond started. In some rearrangements, the migrating σ-bonds lie between two conjugated bond systems. e.g. Cope & Claisen rearrangements. R1 R1 R1 R1 e.g. 1. 2. [1,3] shift [1,5] shift X X 1 2 3 1' 2' 3' (3,3) shift (X - C<, O)
  48. 48. Sigmatropic Rearrangement Suprafacial and Antrafacial processes In sigmatropic rearrangement, the σ-bond migrates across the π-bonds through the two different stereo-chemical sources.  Migrated σ-bond gets moved across the same face of the conjugated system, suprafacial process migrating σ-bond get reformed on the opposite π-electron face of the conjugated system, antrafacial process i.e. migrating group migrate at opposite face of the conjugated system. e.g. [1,5] sigmatropic shift shows both these stereo-chemical consequences as- BA A B H C D H C D BA A B HC D H C D suprafacial antrafacial As lengthing of the migrating system increases, antrafacial shift also increases. The antrafacial [1,3] shift is thermally impossible.
  49. 49. FMO Methods (Sigmatropic Rearrangement) The analysis of the sigmatropic rearrangement shows that the migrating bond gets cleaved homolytically resulting pair of radicals. The migrating group get migrates over the HOMO’s of the conjugated system. e.g. Analysis of suprafacial [1,5] sigmatropic shift of hydrogen in which the homolytic cleavage gives rise to the production of hydrogen atom and pentadienly radical. The ground state electronic configuration of pentadienly radical is ψ1 2ψ2 2ψ3 1 and HOMO is ψ3 1 of this radical possess same sign on the terminal lopes (plane symmetry). Therefore thermally [1,5] migration is suprafacially allowed. H H H H 3
  50. 50. FMO Methods (Sigmatropic Rearrangement) The first excited state electronic configuration of pentadienly radical is ψ1 2ψ2 2ψ4 1 and HOMO is ψ4 1 of this radical possess opposite sign on the terminal lopes (C2 axis of symmetry). Therefore photochemically [1,5] migration is antrafacially allowed. ψ1 2ψ2 2ψ3 1 ψ4 0ψ5 0 → ψ1 2ψ2 2ψ3 0 ψ4 1ψ5 0 Ground state Excited state A similar analysis shows that, for the system [i,j] possess i+j=4n+2 electrons, the thermal reaction should be suprafacial and photochemical process should be antrafacial but for those cases i+j=4n, the thermal reaction should be antrafacial and photochemical process should be suprafacial. These rules are summarized as follow- i+j Δ-allowed hυ-allowed 4n antrafacial suprafacial 4n+2 suprafacial antrafacial H H H 4
  51. 51. Examples 4. Suggest the mechanism of following reaction. O O
  52. 52. O O O H O Ans: 1,5-carbon sigmatropic shift 1,5-hydrogen sigmatropic shift 3 HOMO H 1,5-sigmatropic Hydrogen shift 3 HOMO C 1,5-sigmatropic Carbon shift Process Total Electrons HOMO Node T.S. Process allowed 6 ψ3 2 Aromatic Suprafacial
  53. 53. A concerted [1,3]-sigmatropic rearrangement took place in the reaction shown below. The structure of the resulting product is
  54. 54. FMO Methods No. of electrons involved No. of nodes Type of transition state Δ-allowed hυ-allowed 4n Zero or even Antiaromatic - Dis 4n Odd Aromatic Con - 4n+2 Zero or even Aromatic Dis - 4n+2 Odd Antiaromatic - Con No. of electrons involved No. of nodes Type of transition state Δ-allowed hυ-allowed 4n Zero or even Antiaromatic - Supra-supra Antra-antra 4n Odd Aromatic Supra-antra Antra-supra - 4n+2 Zero or even Aromatic Supra-supra Antra-antra - 4n+2 Odd Antiaromatic - Supra-antra Antra-supra No. of electrons involved No. of nodes Type of transition state Δ-allowed hυ-allowed 4n Zero or even Antiaromatic - supra 4n Odd Aromatic Antra - 4n+2 Zero or even Aromatic supra - 4n+2 Odd Antiaromatic - antra Cycloaddition addition Electrocyclic reaction Sigmatropic rearrangement
  55. 55. Ene Reaction The joining of a double or triple bond to an alkene reactant having transferable allylic hydrogen is called an ene reaction. The allylic hydrogen undergoes 1,5-migration with change in the position of the allylic double bond. Like Diels Alder reaction, it is also 6π electron electrocyclic reaction, but here two electrons of the allylic C-H σ- bond takes the place of two π- electrons of the diene in Diels Alder reaction. H X Y H X Y X Y H + X=Y - C=C, C=O, C=S, N=O, N=N, etc.
  56. 56. Cope Rearrangement The 1,5-dienes isomerizes {[3,3] rearrangement} on heating up to 3000C. Reaction is normally reversible and gives mixture of starting material and product. The position of equilibrium is depends on the stability of the starting material and the products. The temperature needed for the reaction is depends on the substituents and relative strain of the double bonds of 1,5-system. If the substituent R is attached to double bonded carbon then energy of the transition state is lowered and reaction occurs at 165 – 1850C. 1 2 3 1 2 3 ' ' ' [3,3] rearrangement OH OH O 1 2 3 1 2 3' ' ' [3,3] rearrangement O O O H 1 2 3 1 2 3' ' ' [3,3] rearrangement + OH O OH O H H 1 2 3 1 2 3 ' ' ' [3,3] rearrangement4. 5. [3,3] rearrangement 6. [3,3] rearrangement 100% 90% 98% 150 C, 1hr 320 C KH, THF, reflux 18 hrs. 0 0
  57. 57. Cope Rearrangement A common example of Cope rearrangement involving [3,3] sigmatropic rearrangement in 1,5-diene (meso-3,4-dimethyl-1,5-hexadiene) on heating (pyrolysis) giving exclusively cis,trans-isomer of 2,6-octadiene. This reaction proceeds through six membered six electron transition state. CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 meso-3,4- dimethyl-1,5- hexadiene Z,E-2,6-octadiene E Z Bu-n Bu-n Bu-n 15 C 0 CH3 CH3 CH3 OH CH3 O CH3 CH3 CH3 CH3 CH3 OHC KH, THF, RT 18-crown-6 CH3 CH3 CH3 OH CH3 O CH3 CH3 CH3 CH3 CH3 OHC KH, THF, RT 18-crown-6
  58. 58. Examples 5. Suggest the mechanism of following reaction. A hv
  59. 59. Step I is [2+2] cycloaddition reaction forming cyclobutane. It is [2s + 2s] process. Step II is [3+3] sigmatropic rearrangement reaction called Cope rearrangement proceeds through six membered cyclic (chair form) transition state. H H hv A HOMO HOMO LUMO LUMO supra-supra antra-antra node 2, antiaromatic transition state, suprafacial allowed H H H H H H A Cyclic chair form six membered transition state
  60. 60. Claisen Rearrangement The Claisen rearrangement provides an excellent stereoselective route for the synthesis of γ,δ-unsaturated carbonyl compounds from allylic alcohols through allyl vinyl ethers. This reaction involves [3,3]-sigmatropic rearrangement and takes place through a cyclic six membered transition state. The importance of this reaction is the formation of carbon-carbon bond due to expense of carbon- oxygen bond. It is highly stereoselective leading predominantly E-configuration of the new double bond. The cyclic transition state preferred chair conformation with the substituent R1 in the less hindered pseudoequatorial position. O R1 R2 R1 O R2 O R2 R1 (R2 = H, alkyl, OR, OSiR3, NR2) Heat The aromatic Claisen rearrangement involves [3,3] sigmatropic rearrangement of allyl aryl ethers with migration of the allyl group (with allylic transposition) to the ortho position of the aromatic ring. If ortho-position of the ring is substituted then double rearrangement is occur and gives para-substituted product instead of ortho-substitution. Br CH3 O OMe Br CH3 O OMe Br CH3 OH OMe0 190 C decalin
  61. 61. Examples 9. The product formed and the process involved in the following reaction is - OK OK O O O 1. 2. 3. 4. [3,3] sigmatropic rearrangement [1,3] sigmatropic rearrangement [5,5] sigmatropic rearrangement [1,5] sigmatropic rearrangement
  62. 62. The following transformation involves sequential
  63. 63. CH2 CH2 OK  CH2 CH2 OK CH2 CH2 OK  OK O O CH2 CH2 OK CH2 CH2 O CH2 CH2 O 1 2 3 4 5 1 2 3 4 5 [3,3] sigmatropic rearrangement [5,5] sigmatropic rearrangement The selection rule for sigmatropic rearrangement is – The total number of electron involved for the sigma-tropic rearrangement is (4n +2) electron, then it is thermally suprafacial and photochemically antrafacial through the aromatic transition state.
  64. 64. Explain the following reaction - + H H CH2 CH2 CH2 CH2 + CH2 CH2 + 0 °C 97% 0 °C 37% 0 °C 0% CH2 CH2 O O + O O RT CH2 CH2 O O CH3 OCH3 + O O H3CO CH3 H and 100 °C CH2 CH3 CH3 CH3 CH3 NO2 CH2 + 80 °C Bu3SnH PhMe, AIBN CH3 CH3 CH3 CH3
  65. 65. Explain the following reaction - CH2 CH2 O O + O O RT CH2 CH2 O O CH3 OCH3 + O O H3CO CH3 H and 100 °C CH2 CH2 O O + O O RT CH CH O O CH2 CH2 CH2 CH2 O O CH3 OCH3 + O O H3CO CH3 H 100 °C C C O O CH2 CH2H3CO CH3 H H CH3 O O H3CO The dienophile containing electron withdrawing group and diene containing electron donor are undergoes Diels Alder reaction. Along with steric effect, electronic effect can also plays important role.
  66. 66. Explain the following reaction - + H H CH2 CH2 CH2 CH2 + CH2 CH2 + 0 °C 97% 0 °C 37% 0 °C 0% The isolated double bond and diene do not usually take part in intermolecular Diels Alder reactions, but number of cyclic alkenes ( and alkynes) with considerable angle strain are reactive dienophiles or dienes. The drawing force for these reactions is though to be the reduction in angle strain associated with the transition state for the addition.
  67. 67. Explain the following reaction - CH2 CH3 CH3 CH3 CH3 NO2 CH2 + 80 °C Bu3SnH PhMe, AIBN CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 HSteric repulsion CH2 - CH3 CH3 CH2 CH3 H + Total nine structures possible in which -CH 2 carbon of terminal methylene group having negative charge while only three structures are possible in which carbon of -CMe 2 group having negative charge CH2 CH3 CH3 CH3 CH3 Electron rich carbon NO2 CH2 N CH2 + O - O Electron deficient center But energy difference between LUMO of dienophile and HOMO of diene is relatively higher and interaction between them is poor therefore carried out at higher T. Substituents (bulky substituents) such as methyl group on the diene discourage the diene from adopting the cisoid conformation and hence hindered the reaction. CH2 CH3 CH3 CH3 CH3 NO2 CH2 + 80 °C Bu3SnH PhMe, AIBN CH3 CH3 CH3 CH3 CH3 CH3 CH3 CH3 NO2 CH3 CH3 CH3 CH3 NO2
  68. 68. Examples H3 OO CH3 O CH3 Et CH3 H CH3 O Cl Cl Cl Et3N PhCH=NPh O O O O CH3 CH3 CH3 CH3 CH3 CO2Me CO2Me O O MeO + i. ii. iii. i. ii. LDA a. c. b. d. ? ? ? ? ?e. f. ?+ * g. h. i. ? ? ? hv j. ? + +
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By B. R. Thorat, Organic Chemistry, SET-NET purpose

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