Carbenes

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In chemistry, carbenes are a group of extremely unstable compounds of divalent carbon with an electron sextet . As electron deficiency compounds with two non-binding electrons on carbon, they are highly reactive. In the past, they were only known as intermediate stages in chemical reactions , which usually continue to react immediately. That changed with the discovery of stable carbenes from the late 1980s (especially N -heterocyclic carbenes ), which led to a great boom in research on carbenes and applications in catalysis, for example.

history

Stabilization of the singlet state relative to the triplet state in fluorocarbene. The orbitals involved and the resonance structure of the HCF molecule are shown.

The history of carbenes began in the 1830s ( Jean-Baptiste Dumas , Eugène-Melchior Péligot ) when attempts were unsuccessfully to dehydrate methanol to methylene . At that time it was not yet clear that carbon was tetravalent. Between 1892 and 1904, John Ulric Nef of the University of Chicago pursued the idea that carbon was sometimes divalent. In 1897 he predicted the existence of methylene and claimed to be able to synthesize it in a few months, but could not provide experimental evidence for his hypothesis either here or for other carbon compounds. The high reactivity of short-lived carbenes from diazo compounds or ketenes was demonstrated in 1885 by Eduard Buchner and Theodor Curtius and in 1912 by Hermann Staudinger . Work on carbenes as transition states was popular in the 1940s and 1950s, for example in the work of William von Eggers Doering , who synthesized cyclopropanes by reacting carbenes with carbon double bonds, and Philip Skell . The name carbene also comes from Doering, Saul Winstein and Robert B. Woodward . Doering and Prinzbach also used carbenes for CH insertion. At the end of the 1950s, Ronald Breslow and Hans-Werner Wanzlick discovered that carbenes can be stabilized by amino groups without them successfully isolating them. In the 1960s, carbenes could finally be detected and characterized spectroscopically, which dispelled any remaining doubts about their existence. In 1959 Gerhard Herzberg succeeded in spectroscopic detection of methylene with flash photolysis of diazomethane. Herzberg also showed from the spectra that there are two isomers, which theoretically corresponds to the two quantum mechanical possibilities of a triplet and singlet state, depending on the mutual spin alignment of the two unpaired electrons. This was directly demonstrated in the 1960s by ESR spectroscopy on aryl and diaryl carbenes, which were generated by photolysis of their diazo derivatives at low temperatures (−190 degrees Celsius) in glass-like solidified solvents. In the 1960s, the analysis of carbene reactions in the gas phase was also successful. In 1988 Guy Bertrand succeeded in isolating a stable carbene, followed in 1991 by Anthony J. Arduengo with the N -heterocyclic carbenes (NHC), which were in crystalline form and which actually brought about the breakthrough with a wealth of publications and many new stable carbenes, which also were used as ligands of transition metal complexes in catalysis (first in 1995), as did the carbenes themselves in organic catalysis. Carbene complexes themselves had been known for a long time ( Lev Alexandrowitsch Tschugajew 1912, Ernst Otto Fischer 1964, the Fischer carbenes). Stable carbenes were also used, for example, in the stabilization of radicals and diradicaloids.

Structure and structure

Singlet and triplet carbenes

A distinction is made between singlet and triplet spin states , depending on whether the two free electrons are paired or unpaired (triplet, then it is a diradical ). In the case of the singlet carbene, all electrons are in hybrid orbitals, while an unoccupied z orbital remains on the C atom . The triplet carbene, on the other hand, carries an electron in an orbital and an orbital (the bond is established via an sp orbital).

The energy difference between triplet and singlet states is usually small. Certain substituents such as B. Donor substituents stabilize the singlet state. These donate electron density into the unoccupied p orbital on the carbene C atom and thus lower its electrophilicity. Combined strong - and - donor substituents can stabilize a carbene in the singlet state and provide nucleophilic reactivity. Conversely, strongly electron-withdrawing substituents additionally increase the electrophilicity of the carbene. Singlet carbenes can also be preferred due to steric influences, since the triplet sp hybridization (180 °), for example in a five-membered ring such as the Arduengo carbene, cannot be achieved.

The ground state of methylene is the triplet state, the singlet state represents the excited state. The energy difference is about 36 k J / mol . More recent studies suggest a substitution-dependent difference of 7.1–20 kJ / mol.

Stable carbenes

Mesomeric stabilized Arduengo carbene (R 1 , R 2 = organyl radical)

The group around the American chemist Anthony J. Arduengo III succeeded in 1991 for the first time in producing a carbene as a crystalline substance that can be stored for any length of time ( N-heterocyclic carbenes , NHC). Guy Bertrand (1988) synthesized the first stable carbenes three years earlier . The Arduengo carbenes have electronegative atoms with lone pairs of electrons as substituents on the carbene center. On the one hand, the free electron pairs donate into the unoccupied p orbital of the carbene carbon atom and thus reduce its electrophilicity (see above), on the other hand, the nucleophilicity of the free electron pair on the carbon atom is reduced by the electron pull of the electronegative substituents. Particularly stable carbenes are obtained by resonance stabilization when π systems are formed which, according to Hückel's rule, represent aromatics . In most Arduengo carbenes it is nitrogen atoms that form the π bond, but there are also examples with other atoms (e.g. S, O and P). In addition, two π donors are not absolutely necessary to stabilize carbenes, as Guy Bertrand's research group showed in 2005 through the synthesis of stable cyclic (alkyl) (amino) carbenes (CAACs).

Also, carbon monoxide (CO) and representatives of the class of compounds isonitriles (RNC) can at least carbene analog molecules are considered to be stable carbenes or. This is because they also contain divalent carbon, except that instead of two substituents, only one substituent (O or NC) is attached via a multiple bond. Here, too, there is an additional π bond, which leads to an overall bond order of 3.

Carbene complexes

Because of the lone pair of electrons, carbenes are suitable for use as ligands in organometallic complex compounds . In simplified terms, the bond to the metal is based on the Dewar-Chatt-Duncanson model. The special subclass of N -heterocyclic carbenes (e.g. Arduengo carbenes) are of great importance in inorganic chemistry because of their pronounced sigma donor character . They are widely used in catalysis, where they are often an advantageous alternative to phosphanes (e.g. in the metathesis of alkenes ).

Syntheses

Carbenes are typically too reactive to be isolated, so they are usually generated in situ. Under certain conditions, such as B. embedding in an inert matrix at low temperature, but methylene could also be stabilized and examined spectroscopically .

From diazo compounds

The thermo- or photolysis of diazo compounds offers good access to carbene compounds . These split off a nitrogen molecule and thus generate a carbene. For low molecular weight diazo compounds, the uncontrolled decomposition is a significant problem, which is why these are often in situ from suitable precursors such as. B. hydrazones are produced. Substituted hydrazones, such as, for example, tosylhydrazones, can also be converted to carbene with strong bases. This reaction is known as the Bamford-Stevens reaction .

From ketenes

Also ketenes can be used, which is a molecule of carbon monoxide secede. However, ketenes themselves are complex to synthesize and tend to polymerize, so that they are seldom used in practice to generate carbenes.

By α-elimination

The study of this reaction led to the development of carbene chemistry. First you remove the proton from chloroform with a strong base . The resulting trichloromethyl anion undergoes an α-elimination, so that dichlorocarbene is formed. The reaction is now carried out with chloroform and sodium hydroxide solution with the aid of a phase transfer catalyst . A quaternary ammonium salt generally functions as such .

Further

There are other options for carbene production such as B. from ylides , heterocycles or sterically hindered alkanes .

Reactions

Insertions

Carbenes can-X bonds C in C-H bonds and insert . When inserting into CH bonds, the reactivity of the different spin states of the carbene must again be taken into account. The singlet form adds to the C – H bond in an intermediate step, so that the H atom migrates to the carbene C atom. A new C – C bond is formed while the configuration of the substrate is retained. The triplet carbene, on the other hand, abstracts a hydrogen atom from the substrate and recombines with the resulting radical. Thus, the reaction is not stereospecific because the geometry of the intermediate radical is not fixed.

Bertrand and his co-workers were also able to show that cyclic (alkyl) (amino) carbenes can insert into H – H bonds. The reason for this lies in an energetically higher highest occupied molecular orbital (HOMO) compared to NHCs and a smaller singlet-triplet distance . This leads to a reduction in the activation energy from around 150 kJ / mol to around 100 kJ / mol.

Cycloadditions

Of alkenes

One of the most famous reactions is the addition of carbenes to alkenes . It leads to cyclopropanes . The addition of a singlet carbene is always stereospecific and takes place while maintaining the stereochemistry of the alkene. A triplet carbene, on the other hand, can lead to stereospecific products, but does not have to. This is explained by a concerted mechanism that the singlet carbene enters into. The triplet carbene, on the other hand, initially adds to a carbon atom in the alkene. The ring closure to cyclopropane is spin forbidden. First there has to be a spin inversion, which takes time. During this period, rotation about a CC bond axis is therefore possible. Whether a triplet carbene adds stereospecifically depends on whether one of the rotamers is energetically preferred.
It turns out that a carbene can not attack the double bond directly. This process would be symmetry forbidden with regard to the orbital symmetry. No orbitals of the reactants with the same symmetry can be brought into congruence. Instead, the carbene has to attack laterally, with a flap of the orbital first. There is a risk of using solvents which deactivate the singlet state to the triplet state. Then the reaction may no longer be stereospecific.

Of alkynes

This cycloaddition leads to cyclopropenes instead of cyclopropanes .

Of aromatics

These additions serve, among other things, to expand the ring of cyclic aromatics . Carbonyl-substituted carbenes are mostly used because of their greater electrophilicity. First, the carbene adds to a formal double bond of the aromatic. A 6-3 bicycle is formed. A rearrangement occurs , whereby the bond between the two rings is broken. What remains is a seven-membered ring with a C group added.

Rearrangements

Like many electron-deficient compounds, carbenes are also capable of rearrangement . Alkenes are formed by 1,2-shifts of substituents of the carbene.

Organocatalysis

In addition to being used as ligands in metal-catalyzed reactions, carbenes themselves can be used as organocatalysts. Mostly N -heterocyclic carbenes are used.

See also

literature

  • Guy Bertrand (Ed.): Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents. Marcel Dekker / Fontis Media, New York 2002, ISBN 0-8247-0831-8 .

Web links

Individual evidence

  1. ↑ For detailed reviews of stable carbenes, see: D. Bourissou, O. Guerret, FP Gabbai, G. Bertrand: Chem. Rev. 100, 2000, pp. 39–91.
    M. Melaimi, M. Soleilhavoup, G. Bertrand: Angew. Chem. Int. Ed. 49, 2010, pp. 8810-8849.
  2. ↑ Javier Izquierdo, Gerri E. Hutson, Daniel T. Cohen, Karl A. Scheidt: A Continuum of Progress: Applications of N-Hetereocyclic Carbene Catalysis in Total Synthesis . In: Angewandte Chemie International Edition . tape 51 , no. 47 , October 16, 2012, ISSN  1433-7851 , p. 11686–11698 , doi : 10.1002 / anie.201203704 , PMID 23074146 , PMC 3756610 (free full text).
  3. ↑ Illustration based on D. Martin, M. Melaimi, M. Soleilhavoup, G. Bertrand: Organometallics . 2011, 30, 5304-5313.
    For a general overview with a focus on applications with diaminocarbenes, see: MN Hopkinson, C. Richter, M. Schedler, F. Glorius: Nature . 510, 2014, pp. 485-496.
  4. ↑ Wolfgang Kirmse : Carbene, chemistry in our time. December 1969.
  5. ↑ E. Buchner, T. Curtius: Ber. German Chem. Ges. 18, 1885, pp. 2377-2379.
  6. ↑ H. Staudinger, O. Kupfer: Ber. German Chem. Ges. 45, 1912, pp. 501-509.
  7. ^ W. von E. Doering, AK Hoffmann: J. Am. Chem. Soc. 76, 1954, pp. 6162-6165.
  8. ↑ D. Wendisch: Cyclopropane. In: J. Houben, T. Weyl: Methods of Organic Chemistry. 4th edition. IV / 3, Thieme 1971, p. 100. The first use of the name afterwards in: W. von E. Doering, LH Knox: J. Am. Chem. Soc. 78, 1956, pp. 4947-4950. The three are said to have found the name on a night taxi ride in Chicago.
  9. ^ W. von E. Doering, H. Prinzbach: Tetrahedron . 6, 1959, pp. 24-30.
  10. ^ H. Wanzlick, E. Schikora. Angew. Chem. 72, 1960, p. 494.
  11. ↑ 1960 Work by M. Schmeisser on the isolation of dichlorocarbene with cold traps in the journal Angewandte Chemie, which attracted attention, turned out to be flawed.
  12. ↑ G. Herzberg, J. Shoosmith: Nature . 183, 1959, pp. 1801-1802.
  13. ↑ The chemistry of carbenes up to the end of the 1960s is presented in the article by Wolfgang Kirmse on carbenes in chemistry in our time, December 1969 and in Wolfgang Kirmse: Carbene, Carbenoide und Carbenanaloge. In: Chemical paperbacks. 7, Verlag Chemie, 1969. (English extended second edition: Carbene Chemistry. Academic Press, 1971).
  14. ↑ A. Igau, H. Grützmacher, A. Baceiredo, G. Bertrand: J. Am. Chem. Soc. 110, 1988, pp. 6463-6466.
  15. ↑ AJ Arduengo, RL Harlow, M. Kline: J. Am. Chem. Soc. 113, 1991, pp. 361-363.
  16. ↑ For a concise tutorial on the applications of carbene ligands beyond the diaminocarbene, see: D. Munz: Organometallics . 37, 2018, pp. 275-289.
  17. ↑ WA Herrmann, M. Elison, J. Fischer, C. Köcher, GR Artus: Angew. Chem. Int. Ed. 34, 1995, pp. 2371-2374.
  18. ↑ D. Enders, O. Niemeier, A. Henseler: Chem. Rev. 107, 2007, pp. 5606-5655.
  19. ^ CD Martin, M. Soleilhavoup, G. Bertrand: Chem. Sci. 4, 2013, pp. 3020-3030.
    MM Hansmann, M. Melaimi, D. Munz, G. Bertrand: J. Am. Chem. Soc. 140, 2018, pp. 2546-2554.
    J. Messelberger, A. Grünwald, P. Pinter, MM Hansmann, D. Munz: Chem. Sci. 9, 2018, pp. 6107-6117.
  20. ^ R. Bonneau et al.: J. Photochem. Photobiol. A . 116, 1998, p. 9.
  21. ↑ AJ Arduengo, RL Harlow, M. Kline: J. Am. Chem. Soc. 113, 1991, p. 361.
  22. ↑ Oliver Schuster, Liangru Yang, Helgard G. Raubenheimer, Martin Albrecht: Beyond Conventional N -Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization . In: Chemical Reviews . tape 109 , no. 8 , August 12, 2009, ISSN  0009-2665 , p. 3445-3478 , doi : 10.1021 / cr8005087 ( acs.org [accessed November 8, 2019]).
  23. ↑ David Martin, Antoine Baceiredo, Heinz Gornitzka, Wolfgang W. Schoeller, Guy Bertrand: A Stable P-Heterocyclic carbenes . In: Angewandte Chemie International Edition . tape 44 , no. 11 , March 4, 2005, ISSN  1433-7851 , p. 1700–1703 , doi : 10.1002 / anie.200462239 ( wiley.com [accessed November 8, 2019]).
  24. ↑ David Martin, Antoine Baceiredo, Heinz Gornitzka, Wolfgang W. Schoeller, Guy Bertrand: A Stable P-Heterocyclic carbenes . In: Angewandte Chemie . tape 117 , no. 11 , March 4, 2005, ISSN  0044-8249 , p. 1728–1731 , doi : 10.1002 / anie.200462239 ( wiley.com [accessed November 8, 2019]).
  25. ↑ Vincent Lavallo, Yves Canac, Carsten Presang, Bruno Donnadieu, Guy Bertrand: Stable Cyclic (Alkyl) (Amino) Carbenes as Rigid or Flexible, Bulky, Electron-Rich Ligands for Transition-Metal Catalysts: A Quaternary Carbon Atom Makes the Difference . In: Angewandte Chemie International Edition . tape 44 , no. 35 , September 5, 2005, ISSN  1433-7851 , p. 5705-5709 , doi : 10.1002 / anie.200501841 , PMID 16059961 , PMC 2427276 (free full text) - ( wiley.com [accessed November 8, 2019]).
  26. ^ SP Nolan: N-Heterocyclic Carbenes in Synthesis. Wiley-VCH, Weinheim 2006, ISBN 3-527-31400-8 . doi: 10.1002 / 9783527609451
  27. ↑ a b c G. D. Frey, V. Lavallo, B. Donnadieu, WW Schoeller, G. Bertrand: Facile Splitting of Hydrogen and Ammonia by Nucleophilic Activation at a Single Carbon Center . In: Science . tape 316 , no. 5823 , April 20, 2007, ISSN  0036-8075 , p. 439–441 , doi : 10.1126 / science.1141474 ( sciencemag.org [accessed November 8, 2019]).
  28. ↑ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry. Springer-Verlag, 1972, ISBN 3-211-81060-9 , p. 343.
  29. ^ N. Marion, S. Díez-González, SP Nolan: Angew. Chem. Int. Ed. 46, 2007, p. 2988.