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Neighbouring Group Participation.pptx
(i) Non-classical carbocations display delocalization of sigma bonds through 3-center-2-electron bonds in bridged systems. Neighboring group participation can assist reactions by donating electrons through lone pairs, pi bonds, aromatic rings, or sigma bonds. (ii) The pinacol-pinacolone rearrangement involves the migration of an alkyl group from one carbon to another after the loss of a leaving group from a vicinal diol. The migration is assisted by delocalization of the carbocation intermediate onto the oxygen atom. (iii) In asymmetrical glycols, the group with greater ability for carbocation delocalization, such as phenyl, will migrate preferentially over
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Neighbouring Group Participation.pptx
- 1. Neighboring Group Participation Dr.GurumeetC Wadhawa , Assistant Professor, Department of Chemistry. Rayat Shikshan sansthas Veer Wajekar ASC College,Phunde,Uran
- 2.
- 3. NH2 HO 2 N + HO2 HO CH + HNO2 -N2 OH OH O
- 4. III. Non-classical Carbocations非经典碳正离子 Non-classical ions are a special type of carbonium ions displaying delocalization of sigma bonds in 3-center-2-electron bonds of bridged systems. S. Winstein (1949): Acylation (solvoysis) of norbornyl brosylate endo- exo- A key observation is that in this nucleophilic displacement both isomers give the same reaction product an exo-acetate 2. Also the reaction rate for the exo-reaction is 350 times the reaction rate for the endo reaction.
- 5. In a relatedexperiment both enantiomers 1 and 2 of the exo-brosylate on solvolysis give the same racemic reaction product. The optical activity of the reaction disappears at the same reaction rate as that of the solvolysis.
- 6. Sigma electrons inthe C1-C6 bond assist by neighbouring group participation with the expulsion of the leaving group 1 2 7 4 6 exo- 1 2 6 endo- 3. A non-classical ion: pentavalent, symmetrical In a non-classical carbocations, the positive charge is delocalized by a double or triple bond that is not in the allylic position or by a single bond.
- 7. George A Olah(1964): direct evidencefor the norbornyl cation by NMR analysis Olah, G.A., J. Am.Chem.Soc. 104, 7105(1982)
- 8. IV. Neighbouringgroupparticipation(NGP) Neighbouring groupparticipation or NGP has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a bond or bond. When NGP is in operation it is normal for the reaction rate to be increased. It is also possible for the stereochemistry of the reaction to be abnormal (or unexpected) when compared with a normal reaction. A. NGP by heteroatom lone pairs The rate of reaction is much higher for the sulfur mustard and a nucleophile than it would be for a primary alkyl chloride without a heteroatom.
- 9. B. NGP byan alkene (C=C as a neighboring group) H TsO H TsO The rate of acetolysis 100 1 O Configuration retention retention Ts = H3C S O
- 13. Even if thedouble bond is more remote from the reacting center the alkene can still act in this way. H OBs H OBs O S O O Br BsO : -(syn-7-norbornenyl) ethyl brosylate (A): Relative rate of acetolysis (A) 140,000 (B) 1
- 14. DISCUSSION (i)Evidence for thenon-classical cations. (ii)A neighboring group lends anchimeric assistance only when there is sufficient demand for it. (iii)The ability of C=C to serve as a neighboring group can depend on its electron density. H SO2 MosO R1 Mos = MeO R2 Relative rates of the solvolysis R1 = R2 = H R1 = H, R2 = CF3 R1 = R2 = CF3 1.4x1012 1.5x106 1
- 15. C. NGP byan aromatic ring An aromatic ring can assist in the formation of a carbocationic intermediate called a phenonium ion by delocalising the positive charge. the tosylate reacts with acetic acid in solvolysis then rather than a simple SN2 reaction forming B,a 48:48:4 mixture of A, B and (C+D) was obtained. QUESION: please give a mechanism which forms A and B.
- 17. D. Cyclopropyl asa neighboring group Where cyclopropyl lends considerable anchimeric assistance, the developing p orbital of the carbocation is orthogonal to the participating bond of the cyclopropane ring.
- 18. E. The C-Csingle bond as a neighboring group i. The 2-norbornyl system +
- 21. ii The CyclopropylmethylSystem 2 CH Cl EtOH/H2O 2 CH OH 48% 17% OH + CH OH + 2 homoally alcohol 5% CH2X or 2 H C H2C CH 2 CH + H2C CH CH2 CH2 H2C CH CH2 CH2 OH or CH2=CHCH2CH2Cl The carbocationic intermidate is delocalised onto many different carbons through a reversible ring opening.
- 24. 3 3 iii. Methyl orHydrogen as Neighboring Group 3 3 3 3 3
- 25. CH3CH2CDCD3 CH3CHDCHCD3 CH3CHCDHCD3 CH3CDCH2CD3 opencations in equilibrium no NGP by hydrogen H OTs C CD3 H3C C H D O S- O Ts= H3C CD3 NGP by hydrogen H CH3 CH CD (e)
- 26.
- 27. Conversion ofan alcohol having two adjacent OH groups (pinacol) to a ketone (pinacolone) by the action of acid Pinacol-pinacolone Rearrangements
- 28. H C 3 CH3 OH OH CH3 + CH3 OH CH3 CH3 H3C HO CH3 CH3 CH3 CH3 CH3 H C CH H C 3 3 3 OH OH2 CH3 CH3 CH3 O H3C 2 H SO4 + _ H _ OH 2 pinacol pinacolone + + CH3 CH3 CH3 CH3 + H3C .. H O Mechanism Mechanism of the pinacol rearrangement of a symmetrical glycol 1. Protonation of a hydroxyl group 2. Loss of water Resonance-stablized carbocation 3. Methyl migration 4. Deprotonation
- 29. Facts 1, 2shift takes place in carbonium ion which is already a tertiary carbonium ion It is probably due to stabilization of rearranged ion that can be affected by delocalization involving the electron pair on the oxygen atom, and ready loss of proton
- 30. The stable cationformed superior for stabilization H+ Ph C CH 2 2 OH OH Ph C CH 2 + 2 OH Ph2CH CH +OH H+ Ph2CH CH O + Ph2CH CH OH
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- 32.
- 33. Facts It seemsthat migration of alkyl group is extremely fast on the loss of leaving group, taking place simultaneously That migrating group is from the opposite side of the leaving group – Because rearrangement is much more easy in compounds having trans configuration than that having cis configuration
- 34. Pinacol rearrangement ofan asymmetrical glycol HO OH HO Me Ph Me Me Me O Ph H SO 2 4 OH Ph Me B Ph C Ph D Me Ph A Me + Me Ph + Ph HO OH Et Ph Et Ph HO Et Et Et Et O Ph Ph H SO 2 4 H + _ F E Ph + Ph
- 35. Which group migrates? Phenylgroup move first because of greater chances of stabilization due to delocalization PhHC CHPh H+ PhHC CHPh OH + Ph2CH CH O H+ Ph2CH CH +OH Ph2CH CH OH OH OH +
- 36.
- 37. The stericchemistry of Pinacol Rearrangements It is found that o-MeOPh migrate more than a 1000 time less readily that p-substituent and even much less than phenyl. This is due to interference in the transition state with non-migrating group
- 38. Solved problem Explain thefollowing experimental facts ： R R R O R O R C R H2SO4 + OH OH (A) (B) (C) When R=CH3, （ B ） and （ C ） are formed ； When R=Ph, only （ C ）is formed.
- 39. -H + + R R :OH (6) + CR R :OH (3) C+ R R OH (5) + C R R OH (2) R C R OH + OH 2 (4) -H + C R R OH +OH 2 (1) (B) +H + R C R O O R (C) R OH OH (A) R C R When R=Me
- 40.