Tetra methyl glycol is rearranged into methyl ter-butyl ketone in the presence of moderate H2SO4. Base catalysed process in which alkyl migrates and tosylation ...
Rearrangements Due to Electron Deficient Carbon (C+) Atom Wagner-Meervain
Pinacol-Pinacolone
Semi-Pinacol
Demajnove
Statement
Alkyl group migrates to cationic centre. There is change in basic skeleton of molecule.
Tetra methyl glycol is rearranged into methyl ter-butyl ketone in the presence of moderate H2SO4.
Base catalysed process in which alkyl migrates and tosylation occurs at less hindered hydroxyl group of diol.
Carbonium ion formed by diazotization of amine in the presence of HNO2.
Allylic compounds undergo acid or base catalysed functional groups and double bond migration yield new compound.
Key Points
1. 2. 3.
Catalysed by Lewis acids. 1⁰ → 3⁰ carbonium ion Migratory group moves opposite to leaving group Reaction proceeds faster in polar solvents. Order of reactivity for solvents is, SO2 ˃ MeNO2 ˃ MeCN ˃ PhOMe ˃ PhBr ˃ PhH ˃ Br> Diethyl ether.
1. 2. 3. 4.
1. 2. 3.
Variant of pinacol reaction Base catalysed process 1,2-diol converts into monotosylate Formation of tosylate Decreases in the order1⁰ > ⁰2 > 3⁰ (removal of tosyl) Opposite to acid catalysed reaction Tosylation occurs at less hindered hydroxyl group
1. 2. 3. 4. 5.
Deionization on alicyclic amine. HNO2 used in reaction mixture. Ring expansion 3C-C8. Yield is better with smaller rings. Ring contraction takes place on 4 membered ring and to rings of C-6 to C-8. Prime Point: Positive charge is formed on alicyclic carbon. Prime Point: Positive charge formed outside the ring.
1. 2.
Camphinol is natural product which can be rearranged into sentene by protonation. Α piene hydrochloride into isobornylchloride.
1.
Synthesis of camphene. Synthesis of bornylchloride. Used for ring expansion.
1.
Used for ring opening of epoxide (cyclic ether). Used for ring contraction by elimination of tosylates in presence of a base.
1.
Cyclopropylmethaol and cyclobutyl methanol can be generated. Higher membered ring of alicyclic ketones can be prepared. Cyclic bromohydrin on treatment with Grignard reagent gives alkoxide ion which upon refluxing undergo ring expansion.
1.
4.
Classical Examples
1.
2.
Applications
1. 2. 3.
Migratory aptitude of groups Removal of hydroxyl group Steric factor. Steriochemical requirements for rearrangement.
4. 5. 6. 7.
2.
2. 3. 4.
6. 7.
Allylic
3. 4. 5. 6.
via SN1, SN2, SN1' and SN2' SN2 – substitution proceeds normally. SN1 – mixture of products. Et-Na-ethoxide-in rec mix. Small conc. of Na-ethoxide gives 1st order kinetics. Large conc. of Na-ethoxide gives 2nd order kinetics.
Cis 1,2-dimethylcyclohexane 1,2-dio is rearranged to 2,2dimethyl cyclohexanone. Trans 1,2dimethylcyclohexane 1,2-diol transforms into 1-acetyl 1methyl cyclopentane. β halohydrin and β aminohydrin may be produced. Ring expansion of 7 membered to 8 membered. Preparation of 3⁰ ketone. Ring expansion in presence of nitrous acid.
1. 2.
2. 3.
2. 3.
Preparation of 1⁰ & 3⁰ alcohol. Used for conversion of 1⁰ → 3⁰ alcohols. This can be used for the synthesis of natural products. Such as Citral, Ionones, Retinol (Vitamin-A)
