Pyrolytic elimination reaction

Notes to College students

1. Pyrolytic Elimination Reactions ORGANIC CHEMISTRY – II (I-MSc) Subject Code: 33CT21 (UNIT-I) By Dr. G. Balakrishnan Assistant Professor Department of Chemistry Vivekananda College Madurai, Tamil Nadu

2. Pyrolytic elimination The  pyrolytic elimination has a common mechanistic feature: a concerted reaction via a cyclic transition state within which an intramolecular proton transfer is accompanied by  syn -elimination  to form a new carbon-carbon double bond . ( pyrolytic Ei (Elimination internal) reactions )   (430-480 C)

3. The term pyrolytic elimination literally means an elimination reaction occurring in the organic substrate due to the application of heat ( absence of solvent/reagent etc.,), and mostly carried out in the gaseous phase though can be performed in inert solvents. These type of eliminations are different from other types of eliminations (E1,E2 or E1cB) as all other types of elimination reactions require an added base (external base or solvent) to proceed.

4. Ester Pyrolysis

5. Ester pyrolysis reaction converting esters containing a β-hydrogen atom into the corresponding carboxylic acid and the alkene. The reaction is an Ei elimination and operates in a syn fashion. If more than 1 β hydrogen is present then mixtures of alkanes are generally formed.  Since this reaction involved cyclic transition states, conformational effects play an important role in determining the composition of the alkene product .

6. pyrolytic Ei eliminations in alicyclic systems 1,3 and 1,n- pyrolytic eliminations may also take place.

7. Cope Elimination

8. The Cope proceeds through a concerted  syn -elimination mechanism. The oxygen from the N-oxide acts a base, forming an O-H bond, while the C-H and C-N bonds break to form the new C-C pi bond.

9. Sulfoxide Elimination https://en.wikipedia.org/wiki/Ei_mechanism#Sulfoxide_Elimination

10. β- hydroxy phenylsulfoxides were found to undergo thermal elimination through a 5-membered cyclic transition state, yielding β- keto esters and  methyl ketones  after  tautomerization . Allylic alcohols  can be formed from β- hydroxy phenylsulfoxides that contain a β’-hydrogen through an E i  mechanism, tending to give the β,γ- unsaturation . 1,3-Dienes  were found to be formed upon the treatment of an allylic alcohol with an aryl sulfide in the presence of  triethylamine .  Initially, a sulfenate ester is formed followed by a  [2,3]- sigmatropic rearrangement  to afford an allylic sulfoxide which undergoes thermal  syn  elimination to yield the 1,3-diene.

11. Xanthate pyrolysis ( Chugaev elimination)

12. Selenoxide Elimination

13. The   selenoxide elimination has been used in converting ketones, esters, and  aldehydes  to their α,β-unsaturated derivatives . The mechanism for this reaction is analogous to the sulfoxide elimination, which is a thermal  syn  elimination through a 5-membered cyclic transition state. Selenoxides are preferred for this type of transformation over sulfoxides due to their increased reactivity toward β-elimination, in some cases allowing the elimination to take place at room temperature. The   areneselenic acid  generated after the elimination step is in equilibrium with the  diphenyl diselenide  which can react with olefins to yield β- hydroxy selenides under acidic or neutral conditions. Under basic conditions, this side reaction is suppressed .

14. Thank you