4 Sigmatropic Reactions

Theory of Sigmatropic Shifts Pericyclic Rearrangements Rearrangement processes proceed directly by concerted pericyclic mechanisms. (change in position of one s bond as well as of several p bonds) (example 1) Cope rearrangement (1,5-hexadiene) (example 2) Migration of hydrogen atoms in conjugated dienes

Theory of Sigmatropic Shifts Pericyclic Rearrangements Identification of the order of a particular sigmatropic shift (1) The two atom forming the bond being broken are both numbered 1 (2) The atoms in each direction from the bond being broken are numbered consecutively as atoms 2, 3, and so on

Theory of Sigmatropic Shifts Suprafacial and Antarafacial Shifts Sigmatropic shift might result in retention or inversion of geometry of the migrating group

Theory of Sigmatropic Shifts Suprafacial and Antarafacial Shifts Suprafacial rearrangement The migrating group remains on the original face of the p system

Theory of Sigmatropic Shifts Suprafacial and Antarafacial Shifts Antarafacial rearrangement The migrating group ends up on the opposite face of the p system (retention of the geometry)

Theory of Sigmatropic Shifts Suprafacial and Antarafacial Shifts Antarafacial rearrangement The migrating group ends up on the opposite face of the p system (inversion of the geometry)

Theory of Sigmatropic Shifts Suprafacial and Antarafacial Shifts Antarafacial rearrangement The migrating group ends up on the opposite face of the p system (inversion of the geometry) [1,5] antarafacial migration of the methyl group

Theory of Sigmatropic Shifts Application of Symmetry Conservation Rules Aromatic transition state approach Prediction of the stereochemistry of a sigmatropic rearrangement is by counting the number of electrons involved in the reaction and applying the aromatic transition state approach Sigmatropic shift involving 4n electrons proceed via an odd number of antarafacial interactions which will result in an odd number of inversions. Sigmatropic shift involving 4n+2 electrons proceed via an even number of antarafacial interactions which will result in an even number of inversions

Theory of Sigmatropic Shifts Application of Symmetry Conservation Rules [1,5] shift of a hydrogen is a six-electron process Three other stereoisomers, resulting from rearrangements with two inversions, could be formed by allowed processes However, their formation would require strained TS and is extremely unlikely.

Theory of Sigmatropic Shifts Frontier Orbital Approach Regard all the reacting bonds (p bonds and s bonds) and then to consider the symmetry of its HOMO. In a Cope rearrangement, consider two allylic radicals. HOMOs of the two radicals would have the same symmetry and would recombine in a suprafacial manner

Theory of Sigmatropic Shifts Frontier Orbital Approach In a [1,5] hydrogen shift, the TS would resemble a hydrogen atom associated with a pentadienyl radical Since HOMO (f3) is symmetric, suprafacial migration is possible. Suprafacial [1,5] shift would be allowed.

Theory of Sigmatropic Shifts Frontier Orbital Approach In a [1,3] hydrogen shift, the TS would resemble a hydrogen atom associated with a allyl radical Since HOMO (f2) is antisymmetric, suprafacial migration would be forbidden. Antarafacial [1,3] shift would be allowed.

Theory of Sigmatropic Shifts Frontier Orbital Approach In a [1,3] methyl group shift, suprafacial migration with Inversion of the configuration of the migrating group would be allowed. An antarafacial migration with retention would be unlikely to occur.

Experimental Observation [1,3] Sigmatropic Shifts

Experimental Observation [1,3] Sigmatropic Shifts

Experimental Observation [1,3] Sigmatropic Shifts

Experimental Observation Effects of Polar Substituents

Experimental Observation Effects of Polar Substituents Less forbidden in highly polar system than in other nonpolar molecules. The presence of strong EWG on one radical unit in the TS would lower the energy levels of its MO. Similarly, the presence of EDG on one radical would raise the energy of its HOMO closer th that of the LUMO of the other radical unit and would facilitate their interaction in the TS.

Experimental Observation Photochemical [1,3] Shifts - Photochemical [1,3] shifts, if cencerted, should yield products resulting from suprafacial migrations. - However, many of these reactions proceed via excited triplet states (diradicals) which then recombine to form the rearrangement products. (No stereospecific reaction) - [1,3] Shifts resulting from excited singlet states of alkene can also proceed via recombination of free radical intermediates. - In some cases rearrangement from singlet states do appear to proceed by concerted mechanisms.

Experimental Observation Photochemical [1,3] Shifts Above reactions proceed stereospecifically with retention of the configuration

Experimental Observation [1,5] Shifts Hydrogen migrations - Suprafacial pathway in small cyclic system is common at 200 oC and above.

Experimental Observation [1,5] Shifts Hydrogen migrations - Suprafacial pathway in open-chain system is also common at 200 oC and above.

Experimental Observation [1,5] Shifts Hydrogen migrations - When indenes are heated to 100 oC, they undergo entirely [1,5] shifts in stead of [1,3] hydrogen migrations.

Experimental Observation [1,5] Shifts Hydrogen migrations.

Experimental Observation [1,5] Shifts [1,5] migrations of alkyl, aryl and acyl groups - Alkyl and aryl groups have never been observed to undergo thermal [1,5] shifts in acyclic systems. - Thermal [1,5] shifts in cyclic systems are common.

Experimental Observation [1,5] Shifts Migratory aptitudes in [1,5] shifts - Carbonyl and carboxyl groups are the very best migrators, with hydrogen atoms second best. Aromatic rings and vinyl groups are also good migrators, but not as good as carbonyl groups or hydrogens Alkyl groups are the poorest migrators. benzyl>cyclopropylmethyl>isopropyl>ethyl>methyl

Experimental Observation [1,5] Shifts Migrations in blocked aromatic systems - Migrations can take place at relatively low temperature. Why? The blocked aromatic rings attain an appreciable degree of aromatic stabililty in the TS for the rearrangements.

Experimental Observation [1,5] Shifts Migrations in blocked aromatic systems (example 1)

Experimental Observation [1,5] Shifts Migrations in blocked aromatic systems (example 2)

Experimental Observation [1,5] Shifts Migrations in blocked aromatic systems (example 3) Exceptionally rapid [1,5] benzyl group migration take place when compound 17 is treated with base at 0 oC because of the fact that a new aromatic ring is formed.

Experimental Observation [1,5] Shifts Migrations in blocked aromatic systems (example 4) Although [1,5] migration of saturated alkyl groups in cyclohexadienes are very rare, gentle heating of 19 results in [1,5] alkyl shifts to form 20 because of the fact that a new aromatic ring is formed.

Experimental Observation [1,7] Shifts Thermal [1,7] shift of a hydrogen atom proceed entirely by antarafacial paths.

Experimental Observation [1,7] Shifts Thermal [1,7] shift of carbon atoms are extremely rare but do occur in the interconversion of the bicyclic nonatrienes.

Experimental Observation [1,7] Shifts Photochemical [1,7] shifts should proceed suprafacially. Hydrogen atom and alkyl groups have been observed in cycloheptatrienes.