Carbene

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Methylene is the simplest carbene. Carbene.svg
Methylene is the simplest carbene.

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.

Contents

The term "carbene" may also refer to the specific compound :CH2, also called methylene, the parent hydride from which all other carbene compounds are formally derived. [1] [2]

There are two types of carbenes: singlets or triplets, depending upon their electronic structure. [3] The different classes undergo different reactions.

Most carbenes are extremely reactive and short-lived. A small number (the dihalo carbenes, carbon monoxide, [4] and carbon monosulfide) can be isolated, and can stabilize as metal ligands, but otherwise cannot be stored in bulk. A rare exception are the persistent carbenes, [5] which have extensive application in modern organometallic chemistry.

Generation

There are two common methods for carbene generation.

In α elimination, two substituents eliminate from the same carbon atom. This occurs with reagents with no good leaving groups vicinal to an acidic proton are exposed to strong base; for example, phenyllithium will abstract HX from a haloform (CHX3). [6] Such reactions typically require phase-transfer conditions.[ citation needed ]

Molecules with no acidic proton can also form carbenes. A geminal dihalide exposed to organolithiums with undergo metal-halogen exchange and then eliminate a lithium salt to give a carbene, and zinc metal abstracts halogens similarly in the Simmons–Smith reaction. [7]

R2CBr2 + BuLi → R2CLi(Br) + BuBr
R2CLi(Br) → R2C + LiBr

It remains uncertain if these conditions form truly free carbenes or a metal-carbene complex. Nevertheless, metallocarbenes so formed give the expected organic products. [7] In a specialized but instructive case, α-halomercury compounds can be isolated and separately thermolyzed. The "Seyferth reagent" releases CCl2 upon heating:

C6H5HgCCl3 → CCl2 + C6H5HgCl

Separately, carbenes can be produced from an extrusion reaction with a large free energy change. Diazirines and epoxides photolyze with a tremendous release in ring strain to carbenes. The former extrude inert nitrogen gas, but epoxides typically give reactive carbonyl wastes, and asymmetric epoxides can potentially form two different carbenes. Typically, the C-O bond with lesser fractional bond order (fewer double-bond resonance structures) breaks. For example, when one substituent is alkyl and another aryl, the aryl-substituted carbon is usually released as a carbene fragment.

Ring strain is not necessary for a strong thermodynamic driving force. Photolysis, heat, or transition metal catalysts (typically rhodium and copper) decompose diazoalkanes to a carbene and gaseous nitrogen; this occurs in the Bamford-Stevens reaction and Wolff rearrangement. As with the case of metallocarbenes, some reactions of diazoalkanes that formally proceed via carbenes may instead form a [3+2] cycloadduct intermediate that extrudes nitrogen.

Alkylidene carbene Alkylidene carbene.svg
Alkylidene carbene

To generate an alkylidene carbene a ketone can be exposed to trimethylsilyl diazomethane and then a strong base.

Structures and bonding

Singlet and triplet carbenes Carbenes.svg
Singlet and triplet carbenes

The two classes of carbenes are singlet and triplet carbenes. Triplet carbenes are diradicals with two unpaired electrons, typically form from reactions that break two σ bonds (α elimination and some extrusion reactions), and do not rehybridize the carbene atom. Singlet carbenes have a single lone pair, typically form from diazo decompositions, and adopt an sp2 orbital structure. [8] Bond angles (as determined by EPR) are 125–140° for triplet methylene and 102° for singlet methylene.

Most carbenes have a nonlinear triplet ground state. For simple hydrocarbons, triplet carbenes are usually only 8 kcal/mol (33 kJ/mol) more stable than singlet carbenes, comparable to nitrogen inversion. The stabilization is in part attributed to Hund's rule of maximum multiplicity. However, strategies to stabilize triplet carbenes at room temperature are elusive. 9-Fluorenylidene has been shown to be a rapidly equilibrating mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference, although extensive electron delocalization into the rings complicates any conclusions drawn from diaryl carbenes. [9] Simulations suggest that electropositive heteroatoms can thermodynamically stabilize triplet carbenes, such as in silyl and silyloxy carbenes, especially trifluorosilyl carbenes. [10]

Lewis-basic nitrogen, oxygen, sulphur, or halide substituents bonded to the divalent carbon can delocalize an electron pair into an empty p orbital to stabilize the singlet state. This phenomenon underlies persistent carbenes' remarkable stability.

Reactivity

Carbenes behave like very aggressive Lewis acids. They can attack lone pairs, but their primary synthetic utility arises from attacks on π bonds, which give cyclopropanes; and on σ bonds, which cause carbene insertion. Other reactions include rearrangements and dimerizations. A particular carbene's reactivity depends on the substituents, including any metals present.

Singlet-triplet effects

Carbene addition to alkenes Singlettriplet.svg
Carbene addition to alkenes

Singlet and triplet carbenes exhibit divergent reactivity. [11] [ page needed ] [12]

Triplet carbenes are diradicals, and participate in stepwise radical additions. Triplet carbene addition necessarily involves (at least one) intermediate with two unpaired electrons.

Singlet carbenes can (and do) react as electrophiles, nucleophiles, or ambiphiles. [4] Their reactions are typically concerted and often cheletropic.[ citation needed ] Singlet carbenes are typically electrophilic, [4] unless they have a filled p orbital, in which case they can react as Lewis bases. The Bamford-Stevens reaction gives carbenes in aprotic solvents and carbenium ions in protic ones.

The different mechanisms imply that singlet carbene additions are stereospecific but triplet carbene additions stereoselective. Methylene from diazomethane photolysis reacts with either cis- or trans-2-butene to give a single diastereomer of 1,2-dimethylcyclopropane: cis from cis and trans from trans. Thus methylene is a singlet carbene; if it were triplet, the product would not depend on the starting alkene geometry. [13]

Cyclopropanation

Carbene cyclopropanation Cyclopropanation.svg
Carbene cyclopropanation

Carbenes add to double bonds to form cyclopropanes, [14] and, in the presence of a copper catalyst, to alkynes to give cyclopropenes. Addition reactions are commonly very fast and exothermic, and carbene generation limits reaction rate.

In Simmons-Smith cyclopropanation, the iodomethylzinc iodide typically complexes to any allylic hydroxy groups such that addition is syn to the hydroxy group.

C—H insertion

Carbene insertion Carbene one-step-insertion.svg
Carbene insertion

Insertions are another common type of carbene reaction, [15] a form of oxidative addition. Insertions may or may not occur in single step (see above). The end result is that the carbene interposes itself into an existing bond, preferably X–H (X not carbon), else C–H or (failing that) a C–C bond. Alkyl carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.

Carbene intramolecular reaction Carbene intra.svg
Carbene intramolecular reaction
Carbene intermolecular reaction Carbene intermolecular insertion.svg
Carbene intermolecular reaction

The 1,2-rearrangement produced from intramolecular insertion into a bond adjacent to the carbene center is a nuisance in some reaction schemes, as it consumes the carbene to yield the same effect as a traditional elimination reaction. [16] Generally, rigid structures favor intramolecular insertions. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. When such insertions are possible, no intermolecular insertions are seen. Both inter- and intra-molecular insertions admit asymmetric induction from a chiral metal catalyst.

Electrophilic attack

Carbenes can form adducts with nucleophiles, and are a common precursor to various 1,3-dipoles. [16]

Carbene dimerization

Wanzlick equilibrium Wanzlick equilibrium lemal Hahn 1999.svg
Wanzlick equilibrium

Carbenes and carbenoid precursors can dimerize to alkenes. This is often, but not always, an unwanted side reaction; metal carbene dimerization has been used in the synthesis of polyalkynylethenes and is the major industrial route to Teflon (see Carbene § Industrial applications). Persistent carbenes equilibrate with their respective dimers, the Wanzlick equilibrium.

Ligands in organometallic chemistry

In organometallic species, metal complexes with the formulae LnMCRR' are often described as carbene complexes. [17] Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.[ citation needed ] [18] The transition metal carbene complexes can be classified according to their reactivity, with the first two classes being the most clearly defined:

Industrial applications

A large-scale application of carbenes is the industrial production of tetrafluoroethylene, the precursor to Teflon. Tetrafluoroethylene is generated via the intermediacy of difluorocarbene: [22]

CHClF2 → CF2 + HCl
2 CF2 → F2C=CF2

The insertion of carbenes into C–H bonds has been exploited widely, e.g. the functionalization of polymeric materials [23] and electro-curing of adhesives. [24] Many applications rely on synthetic 3-aryl-3-trifluoromethyldiazirines [25] [26] (a carbene precursor that can be activated by heat, [27] light, [26] [27] or voltage) [28] [24] but there is a whole family of carbene dyes.

History

Carbenes had first been postulated by Eduard Buchner in 1903 in cyclopropanation studies of ethyl diazoacetate with toluene. [29] In 1912 Hermann Staudinger [30] also converted alkenes to cyclopropanes with diazomethane and CH2 as an intermediate. Doering in 1954 demonstrated their synthetic utility with dichlorocarbene. [31]

See also

Related Research Articles

An ylide or ylid is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. The result can be viewed as a structure in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X+–Y−. Ylides are thus 1,2-dipolar compounds, and a subclass of zwitterions. They appear in organic chemistry as reagents or reactive intermediates.

In chemistry, a nitrene or imene is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and univalent, so it has only 6 electrons in its valence level—two covalent bonded and four non-bonded electrons. It is therefore considered an electrophile due to the unsatisfied octet. A nitrene is a reactive intermediate and is involved in many chemical reactions. The simplest nitrene, HN, is called imidogen, and that term is sometimes used as a synonym for the nitrene class.

<span class="mw-page-title-main">Bamford–Stevens reaction</span> Synthesis of alkenes by base-catalysed decomposition of tosylhydrazones

The Bamford–Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens (1900–2000). The usage of aprotic solvents gives predominantly Z-alkenes, while protic solvent gives a mixture of E- and Z-alkenes. As an alkene-generating transformation, the Bamford–Stevens reaction has broad utility in synthetic methodology and complex molecule synthesis.

A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.

<span class="mw-page-title-main">Atomic carbon</span> Chemical compound

Atomic carbon, systematically named carbon and λ0-methane, is a colourless gaseous inorganic chemical with the chemical formula C. It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation.

A nitrenium ion in organic chemistry is a reactive intermediate based on nitrogen with both an electron lone pair and a positive charge and with two substituents. Nitrenium ions are isoelectronic with carbenes, and can exist in either a singlet or a triplet state. The parent nitrenium ion, NH+2, is a ground state triplet species with a gap of 30 kcal/mol (130 kJ/mol) to the lowest energy singlet state. Conversely, most arylnitrenium ions are ground state singlets. Certain substituted arylnitrenium ions can be ground state triplets, however. Nitrenium ions can have microsecond or longer lifetimes in water.

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

In organic chemistry, diazirines are a class of organic molecules consisting of a carbon bound to two nitrogen atoms, which are double-bonded to each other, forming a cyclopropene-like ring, 3H-diazirene. They are isomeric with diazocarbon groups, and like them can serve as precursors for carbenes by loss of a molecule of dinitrogen. For example, irradiation of diazirines with ultraviolet light leads to carbene insertion into various C−H, N−H, and O−H bonds. Hence, diazirines have grown in popularity as small, photo-reactive, crosslinking reagents. They are often used in photoaffinity labeling studies to observe a variety of interactions, including ligand-receptor, ligand-enzyme, protein-protein, and protein-nucleic acid interactions.

<span class="mw-page-title-main">Carbene C−H insertion</span>

Carbene C−H insertion in organic chemistry concerns the insertion reaction of a carbene into a carbon–hydrogen bond. This organic reaction is of some importance in the synthesis of new organic compounds.

Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.

<span class="mw-page-title-main">Germylene</span> Class of germanium (II) compounds

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.

<span class="mw-page-title-main">Phosphinidene</span> Type of compound

Phosphinidenes are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The "free" form of these compounds is conventionally described as having a singly-coordinated phosphorus atom containing only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties. In the last few decades, several strategies have been employed to stabilize phosphinidenes, and researchers have developed a number of reagents and systems that can generate and transfer phosphinidenes as reactive intermediates in the synthesis of various organophosphorus compounds.

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

Methylene is an organic compound with the chemical formula CH
2. It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as an adduct.

The Buchner ring expansion is a two-step organic C-C bond forming reaction used to access 7-membered rings. The first step involves formation of a carbene from ethyl diazoacetate, which cyclopropanates an aromatic ring. The ring expansion occurs in the second step, with an electrocyclic reaction opening the cyclopropane ring to form the 7-membered ring.

<span class="mw-page-title-main">Fluorenylidene</span> Chemical compound

9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.

<span class="mw-page-title-main">Borylene</span>

A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.

<span class="mw-page-title-main">Plumbylene</span> Divalent organolead(II) analogues of carbenes

Plumbylenes (or plumbylidenes) are divalent organolead(II) analogues of carbenes, with the general chemical formula, R2Pb, where R denotes a substituent. Plumbylenes possess 6 electrons in their valence shell, and are considered open shell species.

A Fischer carbene is a type of transition metal carbene complex, which is an organometallic compound containing a divalent organic ligand. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-backdonation from the metal centre is generally weak, the carbene carbon is electrophilic.

References

  1. ↑ Hoffmann, Roald (2005). Molecular Orbitals of Transition Metal Complexes. Oxford. p. 7. ISBN   978-0-19-853093-0.
  2. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " carbenes ". doi : 10.1351/goldbook.C00806
  3. ↑ Grossman, Robert B. (2003). The Art of Writing Reasonable Organic Reaction Mechanisms (2nd ed.). New York: Springer. p. 84. ISBN   0-387-95468-6.
  4. 1 2 3 Grossman 2003, p. 35.
  5. ↑ For detailed reviews on stable carbenes, see: (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. (2000). "Stable Carbenes". Chem. Rev. 100 (1): 39–91. doi:10.1021/cr940472u. PMID   11749234. (b) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. (2010). "Stable cyclic carbenes and related species beyond diaminocarbenes". Angew. Chem. Int. Ed. 49 (47): 8810–8849. doi:10.1002/anie.201000165. PMC   3130005 . PMID   20836099.
  6. ↑ Grossman 2003, pp. 84–85.
  7. 1 2 Grossman 2003, p. 85.
  8. ↑ Grossman 2003, p. 84.
  9. ↑ Grasse, P. B.; Brauer, B. E.; Zupancic, J. J.; Kaufmann, K. J.; Schuster, G. B. (1983). "Chemical and physical properties of fluorenylidene: equilibration of the singlet and triplet carbenes". Journal of the American Chemical Society. 105 (23): 6833. doi:10.1021/ja00361a014.
  10. ↑ Nemirowski, A.; Schreiner, P. R. (November 2007). "Electronic Stabilization of Ground State Triplet Carbenes". J. Org. Chem. 72 (25): 9533–9540. doi:10.1021/jo701615x. PMID   17994760.
  11. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN   978-0-471-72091-1
  12. ↑ Contrariwise, Grossman 2003 , p. 85 states: "The reactivities of carbenes and carbenoids are the same no matter how they are generated." Grossman's analysis is not supported by modern physical organic chemistry texts, and likely refers to rapid equilibration between carbene states following most carbene generation methods.
  13. ↑ Skell, P. S.; Woodworth, R. C. (1956). "Structure of Carbene, Ch2". Journal of the American Chemical Society. 78 (17): 4496. doi:10.1021/ja01598a087.
  14. ↑ Grossman 2003, pp. 85–86.
  15. ↑ Grossman 2003, pp. 86–87.
  16. 1 2 Grossman 2003, p. 87.
  17. ↑ For a concise tutorial on the applications of carbene ligands also beyond diaminocarbenes, see Munz, D (2018). "Pushing Electrons—Which Carbene Ligand for Which Application?". Organometallics . 37 (3): 275–289. doi:10.1021/acs.organomet.7b00720.
  18. ↑ Contrariwise, Grossman 2003 : "Diazo compounds are converted to singlet carbenes upon gentle warming and to carbenoids by treatment with a Rh(II) or Cu(II) salt such as Rh2(OAc)4 or CuCl2. The transition-metal-derived carbenoids, which have a metal –– C double bond, undergo the reactions typical of singlet carbenes. At this point you can think of them as free singlet carbenes, even though they’re not."
  19. ↑ For a general review with a focus on applications with diaminocarbenes, see: Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. (2014). "An overview of N-heterocyclic carbenes". Nature . 510 (7506): 485–496. Bibcode:2014Natur.510..485H. doi:10.1038/nature13384. PMID   24965649. S2CID   672379.
  20. ↑ S. P. Nolan "N-Heterocyclic Carbenes in Synthesis" 2006, Wiley-VCH, Weinheim. Print ISBN   9783527314003. Online ISBN   9783527609451. doi : 10.1002/9783527609451
  21. ↑ Marion, N.; Diez-Gonzalez, S.; Nolan, S. P. (2007). "N-heterocyclic carbenes as organocatalysts". Angew. Chem. Int. Ed. 46 (17): 2988–3000. doi:10.1002/anie.200603380. PMID   17348057.
  22. ↑ Bajzer, W. X. (2004). "Fluorine Compounds, Organic". Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons. doi:10.1002/0471238961.0914201802011026.a01.pub2. ISBN   978-0471238966.
  23. ↑ Yang, Peng; Yang, Wantai (2013-07-10). "Surface Chemoselective Phototransformation of C–H Bonds on Organic Polymeric Materials and Related High-Tech Applications". Chemical Reviews. 113 (7): 5547–5594. doi:10.1021/cr300246p. ISSN   0009-2665. PMID   23614481.
  24. 1 2 Ping, Jianfeng; Gao, Feng; Chen, Jian Lin; Webster, Richard D.; Steele, Terry W. J. (2015-08-18). "Adhesive curing through low-voltage activation". Nature Communications. 6: 8050. Bibcode:2015NatCo...6.8050P. doi:10.1038/ncomms9050. ISSN   2041-1723. PMC   4557340 . PMID   26282730.
  25. ↑ Nakashima, Hiroyuki; Hashimoto, Makoto; Sadakane, Yutaka; Tomohiro, Takenori; Hatanaka, Yasumaru (2006-11-01). "Simple and Versatile Method for Tagging Phenyldiazirine Photophores". Journal of the American Chemical Society. 128 (47): 15092–15093. doi:10.1021/ja066479y. ISSN   0002-7863. PMID   17117852.
  26. 1 2 Blencowe, Anton; Hayes, Wayne (2005-08-05). "Development and application of diazirines in biological and synthetic macromolecular systems". Soft Matter. 1 (3): 178–205. Bibcode:2005SMat....1..178B. doi:10.1039/b501989c. ISSN   1744-6848. PMID   32646075.
  27. 1 2 Liu, Michael T. H. (1982-01-01). "The thermolysis and photolysis of diazirines". Chemical Society Reviews. 11 (2): 127. doi:10.1039/cs9821100127. ISSN   1460-4744.
  28. ↑ Elson, Clive M.; Liu, Michael T. H. (1982-01-01). "Electrochemical behaviour of diazirines". Journal of the Chemical Society, Chemical Communications (7): 415. doi:10.1039/c39820000415. ISSN   0022-4936.
  29. ↑ Buchner, E.; Feldmann, L. (1903). "Diazoessigester und Toluol". Berichte der Deutschen Chemischen Gesellschaft. 36 (3): 3509. doi:10.1002/cber.190303603139.
  30. ↑ Staudinger, H.; Kupfer, O. (1912). "Über Reaktionen des Methylens. III. Diazomethan". Berichte der Deutschen Chemischen Gesellschaft. 45: 501–509. doi:10.1002/cber.19120450174.
  31. ↑ Von E. Doering, W.; Hoffmann, A. K. (1954). "The Addition of Dichlorocarbene to Olefins". Journal of the American Chemical Society. 76 (23): 6162. doi:10.1021/ja01652a087.
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