A persistent carbene (also known as stable carbene) is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet (a carbene), but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), in which nitrogen atoms flank the formal carbene. Modern theoretical analysis suggests that the term "persistent carbene" is in fact a misnomer. Persistent carbenes do not in fact have a carbene electronic structure in their ground state, but instead an ylide stabilized by aromatic resonance or steric shielding. Acid catalyzes the carbene-like dimerization that some persistent carbenes undergo over the course of days. Persistent carbenes in general, and Arduengo carbenes in particular, are popular ligands in organometallic chemistry.
History
Early evidence In 1957, Ronald Breslow proposed that a relatively stable nucleophilic carbene derivied from vitamin B1 (thiamine), was the catalyst involved in the benzoin condensation that yields furoin from furfural. In this cycle, the vitamin's thiazolium ring exchanges a hydrogen atom (attached to carbon 2 of the ring) for a furfural residue. In deuterated water, the C2-proton was found to rapidly exchange for a deuteron in a statistical equilibrium:
This exchange was proposed to proceed via intermediacy of a thiazol-2-ylidene. In 2012 the isolation of the so-called Breslow intermediate was reported. In 1960, Hans-Werner Wanzlick and coworkers conjectured that carbenes derived from dihydroimidazol-2-ylidene were produced by vacuum pyrolysis of the corresponding 2-trichloromethyl dihydroimidazole compounds with the loss of chloroform. They conjectured that the carbene existed in equilibrium with its dimer, a tetraaminoethylene derivative, the so-called Wanzlick equilibrium. This conjecture was challenged by Lemal and coworkers in 1964, who presented evidence that the dimer did not dissociate; and by Winberg in 1965. However, subsequent experiments by Denk, Herrmann and others have confirmed this equilibrium, albeit in specific circumstances.
Isolation In 1970, Wanzlick's group generated imidazol-2-ylidene carbenes by the deprotonation of an imidazolium salt. Wanzlick as well as Roald Hoffmann, proposed that these imidazole-based carbenes should be more stable than their 4,5-dihydro analogues, due to Hückel-type aromaticity. Wanzlick did not however isolate imidazol-2-ylidenes, but instead their coordination compounds with mercury and isothiocyanate:
In 1988, Guy Bertrand and others isolated a phosphinocarbene. These species can be represented as either a λ3-phosphinocarbene or λ5-phosphaacetylene:
These compounds were called "push-pull carbenes" in reference to the contrasting electron affinities of the phosphorus and silicon atoms, and exhibited both carbenic and alkynic reactivity; their electronic structure was (and would remain!) unclear. In 2000, Bertrand would obtain additional carbenes of the phosphanyl type, including (phosphanyl)(trifluoromethyl)carbene, stable in solution at -30 °C. In 1991, Arduengo and coworkers obtained the first crystalline diaminocarbene by deprotonation of an imidazolium cation:
This carbene, heralding a large family of carbenes with the imidazol-2-ylidene core, is indefinitely stable at room temperature in the absence of oxygen and moisture, and melts at 240–241 °C without decomposition. The first air-stable Arduengo carbene, a chlorinated member of the imidazol-2-ylidene family, was obtained in 1997.
New examples and new theory
In the modern understanding, the superficially unoccupied p-orbital on a stable carbene is not, in fact, fully empty. Instead, the carbene Lewis structures are in resonance with dative bonds toward adjacent lone-pair or π bond orbitals. That persistent carbenes have ylidic character is hardly obvious, and indeed was initially contradicted. The X-ray structure of N,N′-diadamantyl-imidazol-2-ylidene revealed longer N–C bond lengths in the ring of the carbene than in the parent imidazolium compound, suggesting very little double bond character to these bonds. Hence early workers attributed the stability of Arduengo carbenes to the bulky N-adamantyl substituents, which prevent reaction with other molecules. However, replacement of the N-adamantyl groups with methyl groups also affords 1,3,4,5-tetramethylimidazol-2‑ylidene (Me4ImC:), a thermodynamically stable unhindered NHC (3D):
In 1995, Arduengo's group obtained a carbene derivative of dihydroimidazol-2-ylidene, proving that stability did not arise from the aromaticity of the conjugated imidazole backbone. The following year, the first acyclic persistent carbene demonstrated that stability did not require even cyclicity. Unhindered derivatives of the hydrogenated and acyclic carbenes dimerize over time, but proved key to resolving the electronic structure. Acyclic carbenes are flexible and bonds to the carbenic atom admit rotation. But bond rotation in the compound appeared hindered, suggesting that they did indeed have a double bond character. Subsequent research has focused on expanding the array of heteroatoms stabilizing the ylide.
Most persistent carbenes are stabilized by two flanking nitrogen centers. The outliers include an aminothiocarbene and an aminooxycarbene (3D)... ...and room-temperature-stable bis(diisopropylamino)cyclopropenylidene, in which the amines are connected through vinylogy. In 2000, Bertrand obtained a moderately stable (amino)(aryl)carbene with only one heteroatom adjacent to the carbenic atom.
Classes of stable carbenes Stable carbenes rely on adjacent heteroatoms to stabilize the "carbenic" carbon. Stable carbenes can be usefully categorized by the number of such atoms that are nitrogen. Carbenes that formally derive from imidazol-2-ylidenes by substitution of sulfur, oxygen, or other chalcogens for both α-nitrogens are expected to be unstable, as they have the potential to dissociate into an alkyne (R1C≡CR2) and a carbon dichalcogenide (X1=C=X2). Evidence for the reverse process exists: carbon disulfide (CS2) reacts with electron-deficient acetylene derivatives to conjecturally give transient 1,3-dithiolium carbenes (i.e. where X1 = X2 = S), which then dimerise to tetrathiafulvene derivatives.
Diaminocarbenes A wide variety of bisazomethine ylides are known, both cyclic and acylic:
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