Activation of cyclopropanes by transition metals explained

In organometallic chemistry, the activation of cyclopropanes by transition metals is a research theme with implications for organic synthesis and homogeneous catalysis.[1] Being highly strained, cyclopropanes are prone to oxidative addition to transition metal complexes. The resulting metallacycles are susceptible to a variety of reactions. These reactions are rare examples of C-C bond activation. The rarity of C-C activation processes has been attributed to Steric effects that protect C-C bonds. Furthermore, the directionality of C-C bonds as compared to C-H bonds makes orbital interaction with transition metals less favorable.[2] Thermodynamically, C-C bond activation is more favored than C-H bond activation as the strength of a typical C-C bond is around 90 kcal per mole while the strength of a typical unactivated C-H bond is around 104 kcal per mole.Two main approaches achieve C-C bond activation using a transition metal. One strategy is to increase the ring strain and the other is to stabilize the resulting cleaved C-C bond complex (e.g. through aromatization or chelation). Because of the large ring strain energy of cyclopropanes (29.0 kcal per mole), they are often used as substrates for C-C activation through oxidative addition of a transition metal into one of the three C-C bonds leading to a metallacyclobutane intermediate.

Substituents on the cyclopropane affect the course of its activation.[3]

Reaction scope

Cyclopropane

The first example of cyclopropane being activated by a metal complex was reported in 1955, involving the reaction of cyclopropane and hexachloroplatinic acid. This reaction produces the polymeric platinacyclobutane complex Pt(C3H6)Cl2.[4] [5] The bis(pyridine) adduct of this complex was characterized by X-ray crystallography.[6]

The electrophile Cp*Ir(PMe3)(Me)OTf reacts with cyclopropane to give the allyl complex:[7]

Cp*Ir(PMe3)(Me)OTf + C3H6 → [Cp*Ir(PMe<sub>3</sub>)(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)]OTf + CH4

Fused and spiro-cyclopropanes

Rhodium-catalyzed C-C bondactivation of strained spiropentanes leads to a cyclopentenones.[8] In terms of mechanism, the reaction proceeds by apparent oxidative addition of the 4-5 carbon-carbon bond, leading to a rhodacyclobutane intermediate. In the presence of carbon monoxide, migratory insertion of CO into one of the carbon-rhodium bonds gives a rhodacyclopentanone intermediate. Beta-carbon elimination to form an alkene from the other carbon-rhodium bond leads to a rhodacyclohexanone intermediate with an exocyclic double bond. Reductive elimination of the two carbon-rhodium bonds followed by isomerization of the exocyclic double bond leads to the desired beta-substituted cyclopentenone product. This reaction was applied to the total synthesis of (±)-β-cuparenone.Using the same rhodium(I) catalyst and C-C bond activation strategy one can access compounds with fused rings.[9] Once again the reaction involves oxidative addition to give a rhodacyclobutane eventually affording a rhodacycloheptene intermediate. Insertion of carbon monoxide into one of the carbon-rhodium bonds form a rhodacyclooctenone intermediate that can reductively eliminate to yield a 6,7-fused ring system. The authors propose that the regioselectivity of the initial oxidative addition is controlled by coordination of the endocyclic double bond to the rhodium catalyst.

Cyclopropyl halides

Nickel(0) complexes oxidatively cleave halocyclopropanes to give allyl)Ni(II) halides.[10]

Cyclopropylketones

With cyclopropylketones, transition metal can coordinate to the ketone to direct oxidative addition into the proximal C-C bond. The resulting metallacyclobutane intermediate can be in equilibrium with the six-membered alkyl metal enolate depending on presence of a Lewis acid (e.g. dimethylaluminum chloride[11]).With the metallacyclobutane intermediate, 1,2-migratory insertion into an alkyne followed by reductive elimination yields a substituted cyclopentene product. Examples of intramolecular reactions with a tethered alkyne and intermolecular reactions with a nontethered alkyne[12] both exist with use of a nickel or rhodium catalyst. With the six-membered alkyl metal enolate intermediate, dimerization[13] [14] or reaction with an added alpha-beta unsaturated ketone[15] yields a 1,3-substituted cyclopentane product.

Cyclopropylimines

Oxidative addition into cyclopropylimines gives a metalloenamine intermediate similar to oxidative addition to cyclopropylketones giving alkylmetalloenolates. These intermediates can also reaction with alpha-beta unsaturated ketones to give disubstituted cyclopentane products following reductive elimination.[16]

With rhodium, the intermediate metalloenamine reacts with tethered alkynes.[17] and alkenes[18] to give cyclized products such as pyrroles and cyclohexenones, respectively.

Alylidenecyclopropanes

Alkylidenecyclopropanes more readily undergo C-C bond oxidative addition than cyclopropanes.

Following oxidative addition, 1,2-insertion mechanisms are common and reductive elimination yields the desired product. The 1,2-insertion step usually occurs with an alkyne,[19] alkene,[20] or allene[21] and the final product is often a 5 or 7 membered ring. Six-membered rings may be formed after dimerization of the metallocyclobutane intermediate with another alkylidenecyclopropane substrate and subsequent reductive elimination.[22] Common transition metals utilized with alkylidenecyclopropanes are nickel, rhodium, and palladium. It has been shown that the metallacyclobutane intermediate following oxidative addition to the distal C-C bond can isomerize.[23]

Vinylcyclopropanes

Oxidative addition of vinylcyclopropanes primarily occurs at the proximal position, giving pi-allyl intermediates. Through subsequent insertion reactions (e.g. with alkynes,[24] alkenes,[25] and carbon monoxide[26]), rings of various sizes and fused ring systems[27] can be formed.

Cyclopropenes

Oxidative addition into cyclopropenes normally occurs at the less hindered position to yield the metallacyclobutane. This reaction can result in formation of cyclopentadienones,[28] cyclohexenones,[29] and phenols.

Notes and References

  1. Book: Dong, Guangbin. C-C Bond Activation. Springer. 2014. 978-3-642-55054-6. London. 195–232.
  2. Souillart. Laetitia. Cramer. Nicolai. 2015-09-09. Catalytic C–C Bond Activations via Oxidative Addition to Transition Metals. Chemical Reviews. 115. 17. 9410–9464. 10.1021/acs.chemrev.5b00138. 26044343. 0009-2665.
  3. Bart. Suzanne C.. Chirik. Paul J.. 2003-01-01. Selective, Catalytic Carbon−Carbon Bond Activation and Functionalization Promoted by Late Transition Metal Catalysts. Journal of the American Chemical Society. 125. 4. 886–887. 10.1021/ja028912j. 12537484. 0002-7863.
  4. Osdene. T. S.. Timmis. G. M.. Maguire. M. H.. Shaw. G.. Goldwhite. H.. Saunders. B. C.. Clark. Edward R.. Epstein. P. F.. Lamchen. M.. 1955-01-01. Notes. Journal of the Chemical Society (Resumed). en. 2038–2056. 10.1039/jr9550002038. 0368-1769.
  5. Adams. D. M.. Chatt. J.. Guy. R. G.. Sheppard. N.. 1961-01-01. 149. The Structure of "Cyclopropane Platinous Chloride". Journal of the Chemical Society (Resumed). 10.1039/JR9610000738.
  6. Journal of Organometallic Chemistry. 33. 2. 1971. 247–258. Cyclopropane Complexes of Platinum: Some Synthetic Studies and the Reactivity and Crystal Structure of 1,6-Dichloro-2,3-trimethylene-4,5-bis(pyridine)platinum(IV) . R.D. Gillard . M Keeton . R. Mason . M.F. Pilbrow . D.R. Russell. 10.1016/S0022-328X(00)88414-4.
  7. Facile intermolecular activation of carbon-hydrogen bonds in methane and other hydrocarbons and silicon-hydrogen bonds in silanes with the iridium(III) complex Cp*(PMe3)Ir(CH3)(OTf) . Burger, Peter . Bergman, Robert G. . Journal of the American Chemical Society . 115 . 22 . 1993 . 10462–3 . 10.1021/ja00075a113.
  8. Matsuda. Takanori. Tsuboi. Tomoya. Murakami. Masahiro. 2007-10-01. Rhodium-Catalyzed Carbonylation of Spiropentanes. Journal of the American Chemical Society. 129. 42. 12596–12597. 10.1021/ja0732779. 17914819. 0002-7863.
  9. Kim. Sun Young. Lee. Sang Ick. Choi. Soo Young. Chung. Young Keun. 2008-06-16. Rhodium-Catalyzed Carbonylative [3+3+1] Cycloaddition of Biscyclopropanes with a Vinyl Substituent To Form Seven-Membered Rings. Angewandte Chemie International Edition. en. 47. 26. 4914–4917. 10.1002/anie.200800432. 18496802. 1521-3773.
  10. On the interaction of a nickel(0) complex with mono- and dibromo derivatives of cyclopropane. Novel η3-allylnickel complexes. Peganova, T. A. . Isaeva, L. S. . Petrovskii, P. V. . Kravtsov, D. N.. Journal of Organometallic Chemistry. 1990. 384. 3. 397–403. 10.1016/0022-328X(90)87131-V.
  11. Koga. Yuji. Narasaka. Koichi. 1999-07-01. Rhodium Catalyzed Transformation of 4-Pentynyl Cyclopropanes to Bicyclo[4.3.0]nonenones via Cleavage of Cyclopropane Ring. Chemistry Letters. 28. 7. 705–706. 10.1246/cl.1999.705. 0366-7022.
  12. Tamaki. Takashi. Ohashi. Masato. Ogoshi. Sensuke. 2011-12-09. [3+2] Cycloaddition Reaction of Cyclopropyl Ketones with Alkynes Catalyzed by Nickel/Dimethylaluminum Chloride. Angewandte Chemie International Edition. en. 50. 50. 12067–12070. 10.1002/anie.201106174. 22006658. 1521-3773.
  13. Ogoshi. Sensuke. Nagata. Midue. Kurosawa. Hideo. 2006-04-01. Formation of Nickeladihydropyran by Oxidative Addition of Cyclopropyl Ketone. Key Intermediate in Nickel-Catalyzed Cycloaddition. Journal of the American Chemical Society. 128. 16. 5350–5351. 10.1021/ja060220y. 16620100. 0002-7863.
  14. Tamaki. Takashi. Nagata. Midue. Ohashi. Masato. Ogoshi. Sensuke. 2009-10-05. Synthesis and Reactivity of Six-Membered Oxa-Nickelacycles: A Ring-Opening Reaction of Cyclopropyl Ketones. Chemistry – A European Journal. en. 15. 39. 10083–10091. 10.1002/chem.200900929. 19718721. 1521-3765.
  15. Liu. Lei. Montgomery. John. 2006-04-01. Dimerization of Cyclopropyl Ketones and Crossed Reactions of Cyclopropyl Ketones with Enones as an Entry to Five-Membered Rings. Journal of the American Chemical Society. 128. 16. 5348–5349. 10.1021/ja0602187. 16620099. 0002-7863.
  16. Liu. Lei. Montgomery. John. 2007-09-01. [3+2] Cycloaddition Reactions of Cyclopropyl Imines with Enones. Organic Letters. 9. 20. 3885–3887. 10.1021/ol071376l. 17760449. 1523-7060.
  17. Chen. Gen-Qiang. Zhang. Xiao-Nan. Wei. Yin. Tang. Xiang-Ying. Shi. Min. 2014-08-04. Catalyst-Dependent Divergent Synthesis of Pyrroles from 3-Alkynyl Imine Derivatives: A Noncarbonylative and Carbonylative Approach. Angewandte Chemie International Edition. en. 53. 32. 8492–8497. 10.1002/anie.201405215. 24964965. 1521-3773. free.
  18. Shaw. Megan H.. McCreanor. Niall G.. Whittingham. William G.. Bower. John F.. 2015-01-14. Reversible C–C Bond Activation Enables Stereocontrol in Rh-Catalyzed Carbonylative Cycloadditions of Aminocyclopropanes. Journal of the American Chemical Society. 137. 1. 463–468. 10.1021/ja511335v. 25539136. 0002-7863. free.
  19. Delgado. Alejandro. Rodríguez. J. Ramón. Castedo. Luis. Mascareñas. José L.. 2003-08-01. Palladium-Catalyzed [3+2] Intramolecular Cycloaddition of Alk-5-ynylidenecyclopropanes: A Rapid, Practical Approach to Bicyclo[3.3.0]octenes. Journal of the American Chemical Society. 125. 31. 9282–9283. 10.1021/ja0356333. 12889943. 0002-7863.
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