TheInfoList

OR:

In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, propellane is any member of a class of polycyclic
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s, whose carbon skeleton consists of three rings of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalentâ€”its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up o ...
atoms sharing a common carbonâ€“carbon covalent bond. The concept was introduced in 1966 by D. Ginsburg Propellanes with small cycles are highly strained and unstable, and are easily turned into
polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s with interesting structures, such as staffanes. Partly for these reasons, they have been the object of much research.

# Nomenclature

The name derives from a supposed resemblance of the molecule to a
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
: namely, the rings would be the propeller's blades, and the shared Câ€“C bond would be its axis. The bond shared by the three cycles is usually called the "bridge"; the shared carbon atoms are then the "bridgeheads". The IUPAC nomenclature of the homologue series of all-carbon propellanes would be called tricyclo .y.z.01,(x+2)lkane. More common in literature is the notation means the member of the family whose rings have ''x'', ''y'', and ''z'' carbons, not counting the two bridgeheads; or ''x'' + 2, ''y'' + 2, and ''z'' + 2 carbons, counting them. The chemical formula is therefore . The minimum value for ''x'', ''y'', and ''z'' is 1, meaning three fused cyclopropyl-rings forming the .1.1ropellane. There is no structural ordering between the rings; for example, .3.2ropellane is the same substance as .2.1ropellane. Therefore, it is customary to sort the indices in decreasing order, . Further, heterosubstituted propellanes or structurally embedded propellane moieties exist and have been synthesised and follow a more complex nomenclature (see below).

# General properties

## Strain

Propellanes with small cycles, such as .1.1ropellane or .2.2ropellane, bear a high absolute strain energy. The two interbridgeheaded carbons and their bonds may even be described as an inverted tetrahedral geometry. The resulting steric strain causes such compounds to be unstable and highly reactive. The interbridgehead C-C bond is easily broken (even spontaneously) to yield less-strained bicyclic or even monocyclic hydrocarbons. This so-called strain-release chemistry is used in strategies to access otherwise hard-to-obtain structures. Surprisingly, the most strained member .1.1is far more stable than the other small ring members ( .1.1 .2.1 .2.2 .2.1 .1.1 and .1.1, which can be explained by special bonding situation of the interbridgehead bond.

## Bonding properties

The bonding situation of small-ring propellanes, such as .1.1ropellanes, is topic of debate. Recent computational studies explain the interbridgehead bond as a Charge-shift bond possessing an unusual positive Laplace operator $\nabla^2$ of the electron density $\rho$. Studies by Sterling et al. suggest delocalisation effects onto the three-membered bridges relaxing Pauli-repulsion and thus stabilising the propellane core.

## Reactivity

Propellanes, especially the synthetically studied .1.1ropellane, is known to possess omniphilic reactivity. Anions and radicals add towards the interbridgehead bond resulting in bicyclo .1.1entyl-units. In contrary, cations and metals decompose the tricyclic core towards monocyclic systems by opening of the bridged bonds forming ''exo''-methylene cyclobutanes. For .1.1ropellane only radical addition is reported. The reactivity of other propellanes is far less explored and their reactivity profile is less clear.

## Polymerization

In principle, any propellane can be
polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
ized by breaking the axial Câ€“C bond to yield a radical with two active centers, and then joining these radicals in a linear chain. For the propellanes with small cycles (such as .1.1 .2.1 or 1,3-dihydroadamantane), this process is easily achieved, yielding either simple polymers or alternating copolymers. For example, .1.1ropellane yields spontaneously an interesting rigid polymer called staffane; and .2.1ropellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units €“C8H12â€“alternate with €“Oâ€“Oâ€“groups.

## Synthesis

The smaller-cycle propellanes are difficult to synthesize because of their strain. Larger members are more easily obtained. Weber and Cook described in 1978 a general method which should yield 'n''.3.3ropellanes for any ''n'' â‰¥ 3.

# Members

## True propellanes

* .1.1ropellane, C5H6,
CAS number A CAS Registry Number (also referred to as CAS RN or informally CAS Number) is a unique identification number assigned by the Chemical Abstracts Service (CAS), US to every chemical substance described in the open scientific literature. It inclu ...
( K. Wiberg and F. Walker, 1982). It is a highly strained molecule: the two central carbons have an inverted tetrahedron geometry, and each of the three cycles is the notoriously strained
cyclopropane Cyclopropane is the cycloalkane with the molecular formula (CH2)3, consisting of three methylene groups (CH2) linked to each other to form a ring. The small size of the ring creates substantial ring strain in the structure. Cyclopropane itself is ...
ring. The length of the central bond is only 160 pm. It is an unstable product that undergoes thermal
isomerization In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomeri ...
to 3-methylene cyclobutene at 114 Â°C,and spontaneously reacts with
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
to form a methylenecyclobutane
ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides are f ...
. Several synthetic procedures are established making it accessible on scales useful for synthesis to derive bicyclo .1.1entane which are used a bioisosteres for ''para''-substituted arene systems. * .1.1ropellane, C6H8, CAS number (K. Wiberg, F. Walker, W. Pratt, and J. Michl). This compound was detected by
infrared spectroscopy Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...
at 30  K but has not been isolated as a stable molecule at room temperature (as of 2003). It is believed to polymerize above 50 K. The bonds of the shared carbons have an inverted tetrahedral geometry; the compound's strain energy was estimated as 106 kcal/mol. * .2.1ropellane, C7H10, CAS number (F. Walker, K. Wiberg, and J. Michl, 1982). Obtained gas-phase dehalogenation with
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the nam ...
atoms. Stable only in frozen gas matrix below 50 K; oligomerizes or polymerizes at higher temperatures. The strain energy released by breaking the axial bond was estimated as 75 kcal/mol. * .1.1ropellane, C7H10, CAS number ( Gassman, 1980; Szeimies, 1992;
Anderson Anderson or Andersson may refer to: Companies * Anderson (Carriage), a company that manufactured automobiles from 1907 to 1910 * Anderson Electric, an early 20th-century electric car * Anderson Greenwood, an industrial manufacturer * Anderso ...
, 2022). Several synthetic procedures are established making it accessible on scales useful for synthesis to derive bicyclo .1.1eptanes which are proposed as isosteres for ''meta''-substituted arene systems. * .2.1ropellane or tricyclo .2.1.01,5ctane, C8H12, CAS number (K. Wiberg and G. Burgmaier, 1969). Isolable. Has inverted tetrahedral geometry at the shared carbons. Estimated strain energy of 60 kcal/mol. Remarkably resistant to thermolysis; polymerizes in
diphenyl ether Diphenyl ether is the organic compound with the formula ( C6 H5)2 O. It is a colorless solid. This, the simplest diaryl ether, has a variety of niche applications. Synthesis and reactions Diphenyl ether and many of its properties were first rep ...
solution with halflife of about 20 hours at 195 Â°C. It reacts spontaneously with oxygen at room temperature to give a copolymer with â€“Oâ€“Oâ€“ bridges. * .1.1ropellane, C8H12, CAS number (D. Hamon, V. Trennery, 1981) Isolable. * .2.2ropellane or tricyclo .2.2.01,4ctane, C8H12, CAS number ( P. Eaton and G. Temme, 1973). This propellane is unstable, too, due to the three
cyclobutane Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commercia ...
-like rings and the highly distorted bond angles (three of them nearly 90Â°, the other three nearly 120Â°) at the axial carbons. Its strain energy is estimated to be 93 kcal/mol (390 kJ/mol). * .3.3ropellane, C11H18, CAS number . It is a stable solid that melts at 130 Â°C. It was synthesized in 1978 by Robert W. Weber and James M. Cook who developed a general synthetic route for all , 3, 3ropellanes, with n â‰¥ 3: : * .3.3ropellane, C12H20, CAS number (R. Weber and J. Cook, 1978). A stable solid that melts at 100â€“101 Â°C. * .3.3ropellane, C14H24, CAS number (R. Weber and J. Cook, 1978). An oily liquid that boils at 275â€“277 Â°C. * 0.3.3ropellane, C18H32, CAS number (S. Yang and J. Cook, 1976). A stable solid that sublimes at 33â€“34 Â°C.

## Propellane derivatives

* 1,3-Dehydroadamantane, C10H14 (Pincock and Torupka, 1969). This compound is formally derived from
adamantane Adamantane is an organic compound with a formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the ...
by removing two hydrogens and adding an internal bond. It can be viewed as .3.1ropellane (whose axis would be the new bond), with an extra
methylene bridge In organic chemistry, a methylene bridge, methylene spacer, or methanediyl group is any part of a molecule with formula ; namely, a carbon atom bound to two hydrogen atoms and connected by single bonds to two other distinct atoms in the rest of ...
between its two larger "propeller blades". It is unstable and reactive and can be polymerized. *2,4-Methano-2,4-dehydroadamantane: C11H14 (Majerski, 1980) It can be interpreted as a adamantyl-caged .1.1ropellane derivative. A general reactivity profile was investigated showing similarities to the omniphilic behaviour of .1.1ropellane.

## Propellane natural products

*Dichrocephone B, a sesquiterpenoid with a .3.3ropellane core was isolated in 2008 from '' Dichrocephala benthamii''. It was first synthesized in 2018 using a general strategy for the synthesis of carbocyclic propellanes from 1,3-cycloalkanediones.