A foiled carbene in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
is a special type of stabilized
carbene
In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a Valence (chemistry), valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms.
Th ...
due to the proximity of a
double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
. This type of
reactive intermediate
In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...
is implicated in certain
organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, mechanistic organ ...
s. The positive interaction between carbene and double bond is only present in the singlet type and based on
through-space electron transfer between the empty carbene
p-orbital
In quantum mechanics, an atomic orbital () is a function describing the location and wave-like behavior of an electron in an atom. This function describes an electron's charge distribution around the atom's nucleus, and can be used to calc ...
(
LUMO
In chemistry, HOMO and LUMO are types of molecular orbitals. The acronyms stand for ''highest occupied molecular orbital'' and ''lowest unoccupied molecular orbital'', respectively. HOMO and LUMO are sometimes collectively called the ''frontie ...
) and filled alkene double bond p-orbitals (
HOMO
''Homo'' () is a genus of great ape (family Hominidae) that emerged from the genus ''Australopithecus'' and encompasses only a single extant species, ''Homo sapiens'' (modern humans), along with a number of extinct species (collectively called ...
). The result is a
three-center two-electron bond
A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one ''non''-bonding, and one ''anti''- ...
akin to certain
non-classical ions.
The increased stabilization blocks certain otherwise ordinary reaction modes for the carbene, hence the term ''foiled''. An example is ''norbornen-7-ylidene'', which is
norbornene
Norbornene or norbornylene or norcamphene is a highly strained bridged cyclic hydrocarbon. It is a white solid with a pungent sour odor. The molecule consists of a cyclohexene ring with a methylene bridge between carbons 1 and 4. The molecule carr ...
with a carbenic carbon atom at the bridge position.
In silico
In biology and other experimental sciences, an ''in silico'' experiment is one performed on a computer or via computer simulation software. The phrase is pseudo-Latin for 'in silicon' (correct ), referring to silicon in computer chips. It was c ...
experiments reveal that the bridge in this molecule is actually bending towards the double bond with an optimum angle of around 90° vs 130° for norbornene.
[''Foiled Carbenes Revisited: When -Stabilization Surpasses -Stabilization'' Jean-Luc Mieusset and Udo H. Brinker J. Org. Chem.; 2007; 72(1) pp 263 - 268; (Article) ]
References
{{Reflist
Carbenes