Cope reaction

The Cope reaction or Cope elimination, developed by Arthur C. Cope, is an elimination reaction of the N-oxide to form an alkene and a hydroxylamine.[1][2][3][4]

Not to be confused with Cope rearrangement.
Cope reaction
Named after Arthur C. Cope
Reaction type Elimination reaction
Identifiers
Organic Chemistry Portal cope-elimination
RSC ontology ID RXNO:0000539

Mechanism and applications

The reaction mechanism involves an intramolecular 5-membered cyclic transition state, leading to a syn elimination product, an Ei pathway. This organic reaction is closely related to the Hofmann elimination, but the base is a part of the leaving group. The amine oxide is prepared by oxidation of the corresponding amine with an oxidant such as meta-chloroperoxybenzoic acid (mCPBA). The actual elimination just requires heat.

Cope reaction
Cope reaction

Illustrative of the Cope reaction is a synthesis of methylenecyclohexane:[5]

synthesis of methylenecyclohexane

Piperidines are resistant to an intramolecular Cope reaction[6][7][8] but with pyrrolidine and with rings of size 7 and larger, the reaction product is an unsaturated hydroxyl amine. This result is consistent with the 5-membered cyclic transition state.

intramolecular Cope reaction

Reverse reaction

The reverse or retro-Cope elimination has been reported, in which an N,N-disubstituted hydroxylamine reacts with an alkene to form a tertiary N-oxide.[9][10] The reaction is a form of hydroamination and can be extended to the use of unsubstituted hydroxylamine, in which case oximes are produced.[11]

Sulfoxides can undergo an essentially identical reaction to produce sulfenic acids which is important in the antioxidant chemistry of garlic and other plants of the genus Allium. Selenoxides likewise undergo selenoxide eliminations. Other Ei reactions proceed similarly.

References
  1. Cope, Arthur C.; Foster, Theodore T.; Towle, Philip H. (1949). "Thermal Decomposition of Amine Oxides to Olefins and Dialkylhydroxylamines". Journal of the American Chemical Society. 71 (12): 3932–3935. doi:10.1021/ja01180a014.
  2. Cope, Arthur C.; Towle, Philip H. (1949). "Rearrangement of Allyldialkylamine Oxides and Benzyldimethylamine Oxide". Journal of the American Chemical Society. 71 (10): 3423–3428. doi:10.1021/ja01178a048.
  3. Cope, Arthur C.; Pike, Roscoe A.; Spencer, Claude F. (1953). "Cyclic Polyolefins. XXVII. cis- and trans-Cycloöctene from N,N-Dimethylcycloöctylamine". Journal of the American Chemical Society. 75 (13): 3212–3215. doi:10.1021/ja01109a049.
  4. Peter C. Astles; Simon V. Mortlock; Eric J. Thomas (1991). "The Cope Elimination, Sulfoxide Elimination and Related Thermal Reactions". Comprehensive Organic Synthesis. Vol. 6. pp. 1011–1039. doi:10.1016/B978-0-08-052349-1.00178-5. ISBN 978-0-08-052349-1.
  5. Cope, Arthur C.; Ciganek, Engelbert (1963). "Methylenecyclohexane and N,N-Dimethylhydroxylamine Hydrochloride". Organic Syntheses. 4: 612. doi:10.15227/orgsyn.039.0040.
  6. March, Jerry; Smith, Michael B. (2007). March's advanced organic chemistry: reactions, mechanisms, and structure (6th. ed.). Wiley-Interscience. p. 1525. ISBN 978-0-471-72091-1.
  7. Amine Oxides. VIII. Medium-sized Cyclic Olefins from Amine Oxides and Quaternary Ammonium Hydroxides Arthur C. Cope, Engelbert Ciganek, Charles F. Howell, Edward E. Schweizer J. Am. Chem. Soc., 1960, 82 (17), pp 4663–4669 doi:10.1021/ja01502a053
  8. Amine Oxides. VII. The Thermal Decomposition of the N-Oxides of N-Methylazacycloalkanes Arthur C. Cope, Norman A. LeBel; J. Am. Chem. Soc.; 1960; 82(17); 4656-4662. doi:10.1021/ja01502a052
  9. Ciganek, Engelbert; Read, John M.; Calabrese, Joseph C. (September 1995). "Reverse Cope elimination reactions. 1. Mechanism and scope". The Journal of Organic Chemistry. 60 (18): 5795–5802. doi:10.1021/jo00123a013.
  10. Ciganek, Engelbert (September 1995). "Reverse Cope elimination reactions. 2. Application to synthesis". The Journal of Organic Chemistry. 60 (18): 5803–5807. doi:10.1021/jo00123a014.
  11. Beauchemin, André M.; Moran, Joseph; Lebrun, Marie-Eve; Séguin, Catherine; Dimitrijevic, Elena; Zhang, Lili; Gorelsky, Serge I. (8 February 2008). "Intermolecular Cope-Type Hydroamination of Alkenes and Alkynes". Angewandte Chemie. 120 (8): 1432–1435. Bibcode:2008AngCh.120.1432B. doi:10.1002/ange.200703495.
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