24 Sigmatropic Rearrangements Truong Nguyen

The May lab Sigmatropic Rearrangements 1. Introduction A class of concerted pericyclic reactions governed by orbital symmetry. One bond is being broken while another bond is being formed across a π system. Migrating group remains the same face with the system: suprafacial. Migrating group moves to opposite face of the system: antarafacial. Processes with 2- and 6-electron (Huckel, 4n+2) are allowed suprafacially, 4-electron processes must occur antarafacially (Mobius, 4n).1 Order [1,j] 1+j = 4n 4n + 2 Supra/retention Forbidden Allowed Supra/inversion Allowed Forbidden Antara/retention Allowed Forbidden Order [i,j] i+j = 4n 4n + 2 Supra/supra Forbidden Allowed Supra/antara Allowed Forbidden Antara/antara Forbidden Allowed Antara/inversion Forbidden Allowed Nomenclature of [n,m] sigmatropic rearrangement: numbering the atoms of the bond being broken as atom 1, then count the atoms from the broken bond in each direction to the atoms (n, m) of the new bond. 2. [1,2] Sigmatropic rearrangement: Often initialized by the formation of a reactive intermediate such as a carbocation or a carbanion to form a more stable intermediate. Many name reactions involve a [1,2] shift such as Wagner-Meerwein, Beckmann, or Stevens rearrangement. - Wagner-Meerwein rearrangement in bicyclic terpenes The May lab 3. - Beckmann rearrangement - Stevens rearrangement [1,3] Sigmatropic rearrangement 3.1. Thermal [1,3] hydride shifts: The Mobius topology required in the transition state prohibits such a shift because it is geometrically impossible. 3.2. Thermal [1,3] alkyl shifts: Geometry of the transition state is prohibitive, but an alkyl group can invert its geometry, and therefore proceed via a suprafacial shift. - Antarafacial [1,3] alkyl shift: symmetry allowed, geometrically impossible - Suprafacial [1,3] alkyl shift with inversion: symmetry allowed The May lab 3.3. Photochemical [1,3] shift: should proceed suprafacially; however, most are nonconcerted (have a diradical mechanism, to which the Woodward-Hoffmann rules do not apply).2 4. [1,5] Sigmatropic rearrangement: involving the shift of 1 substituent (H, alkyl, or Ar) down 5 atoms of a π system. Suprafacial thermal shifts are quite common, while antarafacial photochemical shifts are rarely observed. 4.1. [1,5] hydride shifts: Hydrogen has been shown to shift in both cyclic and open chain systems at high temperature. These reactions proceed suprafacially, via a Huckel topology transition state. - Photochemical irradiation allows antarafacial [1,5] hydride shift.3 - 4.2. [1,5] alkyl shifts: thermally allowed, though very poor, and never been observed in an open-chain system. Studies showed the rate preferences for [1,5] alkyl shift in cyclic system: carbonyl > hydride > phenyl and vinyl > alkyl.4 The May lab 5. [1,7] Sigmatropic rearrangement: proceed through antarafacial fashion, via a Mobius topology transition state (predicted by Woodward-Hoffmann rules), observed in the conversion of lumisterol to vitamin D2. - In a bicyclic molecule, the migration of a divalent group, which is a part of 3-membered ring, is preferred as a “walk rearrangement”. 6. [2,3] Sigmatropic rearrangement: - Classified in 2 types: rearrangement of carbanions or rearrangement of ylides - Proceed through a 5-membered cyclic transition state. - High stereoselectivity: prefer (E)-alkene product. Generally, the (E)-alkene will favor the formation of anti product, while (Z)-alkene will favor formation of syn product. The May lab 6.1. Type 1: encompass anionic rearrangement. - [2,3]-Wittig rearrangement: allylic ether into a homoallylic alcohol via a concerted, pericyclic process. Often competes with [1,2]-Wittig rearrangement. Carbanion is often generated from direct lithiation or tin transmetallation. - A highly enantioselective [2,3]-Wittig rearrangement using chromium carbonyl complexes.5 - The substrate must contain acidic hydrogens adjacent to the ether oxygen, which was a significant limitation. The development of transmetallation methods that allowed the selective generation of carbanions represents a profound advance.6 6.2. - Type 2: encompasses ylide formation (carbon ylide or heteroatom ylide) Mislow-Evans rearrangement: allylic sulfoxides to allylic alcohol in a [2,3]-shift as a singlepot reaction The May lab - In a selenium analogue of the Mislow-Evans arrangement, aryl selenides could be oxidized to selenate, which underwent [2,3] rearrangement.7 - Another “heteroatom ylide” formation catalyzed by [Pd].8 - Sommelet–Hauser rearrangement: forming ammonium ylide. - Doyle-Kirmse reaction: an allyl sulfide reacts with diazo compounds to form the homoallyl sulfide compound. The May lab - Many reactions using diazo compounds to generate ylides9 - Hypervalent iodine is an alternative to diazo compounds for use in the tandem ylide formation/[2,3] rearrangement.10 7. [3,3] Sigmatropic rearrangement: proceed suprafacially, via a Huckel topology transition state. 7.1. - Cope rearrangement: Found by Cope in 1940,11 a reversible process. - Many strategies employed to drive the rearrangement toward the desired product (introduction of conjugation or aromaticity). - Introducing a strained-ring (cyclopropane or cyclobutane) makes a Cope arrangement irreversible. The May lab Rh-catalyzed asymmetric cyclopropanation/Cope arrangement12 Tandem Wolff/Cope rearrangement13 Cyclobutane strain-release Cope rearrangement14 7.2. - Oxy-Cope Rearrangement: discovered in 1964 by Jones and Berson.15 The product is favored by incorporating an oxygen. Due to keto-enol tautomerization, the reaction is irreversible in most case. - Anionic alkoxides accelerate the reaction rate: known as “anionic oxy-Cope rearrangement” Base-mediated anionic oxy-Cope16 The May lab Base-catalyzed dianionic oxy-Cope rearrangement17 7.3. - Claisen rearrangement: The first example of a [3,3] sigmatropic rearrangement (1912).18 A heteroatom variant of the Cope arrangement. - The formation of carbonyl group makes this reaction, unlike other sigmatropic rearrangements, irreversible. - Involving in many tandem pericyclic processes, isomerization, and cyclization. BASF continuous Citral process including a Claisen/Cope rearrangement19 The May lab Acid-catalyzed Johnson-Claisen/Diels-Alder cyclization20 Rhodium-catalyzed Bamford–Stevens/Claisen rearrangement21 Platinum-catalyzed cycloisomerization/Claisen rearrangement22 Palladium-catalyzed aza-Claisen23 The May lab - Charge substrates can accelerate Claisen rearrangement. Ammonium-Claisen rearrangement24 Thio-Claisen rearrangement25 - Ireland-Claisen rearrangement: convert allylic esters with a strong base to give γ,δunsaturated carboxylic acids Employed in the synthesis of inhibitor (–)-ebelatone A26 The May lab 7.4. Hetero-Cope rearrangement - Aza-cope rearrangement: usually initiated by acid-catalyzed condensation to form a reactive iminium ion. Lewis acid-mediated tandem aza-Cope/Petasis-type reaction.27 - Oxonia-Cope rearrangement: the oxygen analogue of the aza-Cope rearrangement, often competes with oxocarbenium reactions. Isomerization of allylation adducts by oxonia-Cope rearrangement.28 7.5. Carroll rearrangement: an analogue of Claisen rearrangement of β-keto allyl ester in the presence of a base, followed by decarboxylation.29 The May lab 8. [5,5] Rearrangement: similar to [3,3] shift, proceed suprafacially, Huckel topology transition state. - [5,5] shift of Phenyl pentadienyl ether30 The end.