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Molecular Rearrangementsuse only use internal for only Molecular rearrangementsuse SERP course A. Mickiewicz University, Based on Poznań 2016 internal Organic Chemistry Jan Milecki 5th Edition for Paula Yurkanis Bruice Neighboring group participation Some reactions proceed just too easy! 6 only O Cl Reacts with Nu: 10 x faster then Cl Cl Cl Hydrolyses 600x fasteuser then S TsO TsO Reacts with AcOH 1011 faster then internal What is the reason? for Oxygen atom lone pair „pushes away” chloride ion, creating resonance-stabilized cation HO R R O O Cl onlyO O O O Lone pair on the sulfur atom (strong nucleophile)use expels chloride ion giving rise to three-membered cyclic cation Cl O Ph HO R Ph S Ph S S R internal This mechanism is responsible for alkylating Cl Cl S activity (and hencefor toxicity) of mustard gas! Other examples of the lone pair assistance: Me Me O O OAc O O OAc = only OTs OTs OAc AusecOH Retention of configuration in the SN2 substitution indicates the neighboring group assistance! Assisting electrons do not have to come from the lone pair – p orbital assistance TsO AcO AcOH internal + Appropriate for structure O LUMO Ts HOMO only Rearrangements use What happens, when the participating group becomes trapped and remains in the place, which was the aim of electron attack? In this case isomeric product is formed – result of REARRANGEMENT „Simple” substitution: Cl OH NEt2 NaOH, H2O Et N 2 internalEt2N HO Expected product Real product (57% yield) . for Cl NEt2 Me Et2N Et2N HO Me Me OH Cl Good leaving group. Good nucleophile, only bad leaving group Et N 2 Secondary reaction center - slow Me substitution by an external nucleophile. Alkyl group can migrate too use Me Me Me Me Me Me I Me AgNO3, Me I X OH H2O Me Me Me Me Toocrowded for SN2 Primary cation, too unstable for SN1 H = H Me Me H C Me Ag Me Ag Me I I Me + Me Me Me internal Me Me H2O Transition state, rather than intermediate OH Me for Me Me H Me Molecule rearranges to form Me H more stable cation Me onlyMe Secondary Tertiary LUMO empty p orbital HOMO H H use H H Me H H Me H H LUMO Me H migrates =_ H Me H H Me H migrates Me H H HOMO filled Me H H H orbital H internal for Carbocations rearrange easily! How to produce a carbocation? 1. Dissociation of halogenides (promoted by silver ions) RX AgonlyR AgX 2. Protonatiion of alcohols use CH3 CH3 H2 HONO +N + 2H O 3. Nitrosation of amines H3C C C NH2 H3C C CH2 2 2 (aliphatic) CH CH internal3 3 for only Aryl amines – dissociation only (stable salts) use H 4. Protonation of alkenesinternal H for How to predict the direction of rearrangement? Ph H C C CH Ph shift only3 2 Ph Ph C4H9 CH3 shift C CH2 CH3 H3C C CH2 Ph C H C H 4 9 4 9 C H shift H3C C CH2 C4H9 use4 9 Migration of phenyl group – very stable intermediate (benzil and tertiary carbon atoms in the three H3C H membered ring, charge spread over phenyl ring). internalFavors this direction of migration C4H9 H for Wagner-Meerwein rearrangement The rearrangement was first discovered in bicyclic terpenes for example the conversion of isoborneol to camphene only OH H OH2 + use H -H2O The story of the rearrangement reveals that many scientists were puzzled with this and related reactions and its close relationship to the discovery of carbocations as internalintermediates for Rearrangement of camphenilol to santene only use Cl Ring strain release can be a driving HCl force for rearrangement internal Four-membered ring Five-membered for ring Pinacol Rearrangement only use In the conversion that gave its name to this reaction, the acid-catalyzed elimination of water from pinacol gives t-butyl methyl ketone internal for Mechanism of the Pinacol Rearrangement This reaction occurs with a variety of fully substituted 1,2-diols, and can be understood to involve the formation of a carbonium ion intermediate that subsequently undergoes a rearrangement. The first generated intermediate, an α-hydroxycarbonium ion, rearranges through a 1,2-alkyl shift to produce the carbonyl compound. If two onlyof the substituents form a ring, the Pinacol Rearrangement can constitute a ring-expansion or ring-contraction reaction. use internal for CH CH3 3 OH CH3 CH3 H H only O OH O CH3 CH3 trans group migrates use OH OH2 CH3 CH3 CH3 H CH CH CH OH 3 O 3 O 3 H H H O CH3 CH3 internal for OH OH2 H C CH 3 H 3 CH3 CH3 CH CH OH2 3 3 O O H CH3 OH OH CH3 H onlyCH 3 H O Ring contraction use CH3 H OH OH OH OH2 O H -H2O O internal H Ring expansion for Epoxides undergo similar rearrangement (pinacol-type) MgBr H MgBr O 2 O Ph O Ph Ph Ph Ph onlyPh Grignard reagents not always open epoxides in desired way! use OH OH RLi RMgBr O R R O OH O MgBr RMgBr O MgBr internal H R for only use Baeyer-Villiger Oxidation internal for Mechanism of Bayer-Villiger Oxidation BF3 BF O BF3 3 O O R R R R R R only BF3 O O O O O BF O BF3 3 R' R useR' R R R R' O O + R R H O O -H O O H H O O O BF -RCOO O O BF3 3 -BF3 R' R R' + R R O O O O R R R O internal Order of migration:for R= tertiary alkyl >secondary alkyl >aryl >primary alkyl >methyl Benzilic Acid Rearrangement only 1,2-Diketones undergo a rearrangement in theuse presence of strong base to yield α-hydroxycarboxylic acids. The best yields are obtained when the subject diketones do not have enolizable protons. The reaction of a cyclic diketone leads to an interesting ring contraction: Ketoaldehydes do not react in the same manner, where a hydride shift is preferred (see Cannizzarointernal Reaction) for only use Mechanism of Benzilic Acid Rearrangement internal for Cannizzaro Reaction only This redox disproportionation of non-enolizable aldehydesuse to carboxylic acids and alcohols is conducted in concentrated base. α-Keto aldehydes give the product of an intramolecular disproportionation in excellent yields. internal for Mechanism of the Cannizzaro Reaction only use The Cannizzaro Reactioninternal should be kept in mind as a source of potential side products when aldehydes are treated under basic conditions. for Favorskii Rearrangement Alpha-halogeno ketones O O O EtO O onlyOEt O Br EtO Br : OEt use O OEt internal for Hofmann Rearangement (Degradation) R R O R R H2O NH NaOH, X2 C 1 2 N C O 1 R X=Cl, Br R N OH -CO R1 NH R1 H 2 2 O only use internal for Mechanism O X O O O R X H2O R O onlyR X Na R H OH N N N N 1 R1 H R H -H R1 R1 H O use -X O C -X O N R X N R 1 N R R R1 H R1 Intermediate internal Nitrene for Isocyanates are versatile starting materials: only use Isocyanates are also of high interest as monomers for polymerization work and in the derivatisation of biomacromolecules. internal for Beckmann Rearrangement only use An acid-induced rearrangement of oximes to give amides. This reaction is related to the Hofmann and Schmidt Reactions and the Curtius Rearrangement, in that an electropositive nitrogen is formed that initiates internalan alkyl migration. for Mechanism of the Beckmann Rearrangement only use Oximes generally haveinternal a high barrier to inversion, and accordingly this reaction is envisioned to proceed by protonation of the oxime hydroxyl, followed by migration of the alkyl substituent "trans" to nitrogen. The N-O bond is simultaneously cleaved withfor the expulsion of water, so that formation of a free nitrene is avoided. Claisen Rearrangement The aliphatic Claisen Rearrangement is a [3,3]-sigmatropic rearrangement in which an allyl vinyl ether is converted thermally to an unsaturated carbonyl compound. The aromatic Claisen Rearrangement is accompaniedonly by a rearomatization: use The etherification of alcohols or phenols and their subsequent Claisen Rearrangement under thermal conditions makes possible an extension of the carbon chain of the molecule. internal for Mechanism of the Claisen Rearrangement The Claisen Rearrangement may be viewed as the oxa-variant of the Cope Rearrangement only Mechanism of the Cope Rearrangement use internal Mechanism of the Claisen Rearrangement for The aromatic Claisen Rearrangement is followed by a rearomatization: only use When the ortho-position is substituted, rearomatization cannot take place. The allyl group must first undergo a Cope Rearrangement to the para-position before tautomerization is possible. internal for .
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