Fenestrane

Last updated
Fenestranes, from left to right: generic fenestrane, [4.4.4.4]fenestrane with carbon atoms shown and [5.5.5.5]fenestrane Fenestranes.png
Fenestranes, from left to right: generic fenestrane, [4.4.4.4]fenestrane with carbon atoms shown and [5.5.5.5]fenestrane

A fenestrane in organic chemistry is a type of chemical compound with a central quaternary carbon atom which serves as a common vertex for four fused carbocycles. [1] They can be regarded as spiro compounds twice over. Because of their inherent strain and instability, fenestranes are of theoretical interest to chemists. The name—proposed in 1972 by Vlasios Georgian and Martin Saltzman [2] —is derived from the Latin word for window, fenestra. Georgian had intended that "fenestrane" solely referred to [4.4.4.4]fenestrane, whose skeletal structure looks like windows, and Kenneth B. Wiberg called that specific structure "windowpane". [3] The term fenestrane has since become generalized to refer to the whole class of molecules that have various other ring-sizes. Georgian recommended rosettane for the class, based on the structural appearance as a rosette of flowers. [3]

Contents

Nomenclature and structure

Fenestranes can have various numbers of atoms in the rings. Fenestrane ring-sizes.png
Fenestranes can have various numbers of atoms in the rings.
[3.3.3.3]Fenestrane is the smallest possible structure. See the SS Pyramidanes section for discussion of its geometry. 3.3.3.3-fenestrane.png
[3.3.3.3]Fenestrane is the smallest possible structure. See the § Pyramidanes section for discussion of its geometry.

Structures within this class of chemicals can be named according to the number of atoms in each ring in addition to the systematic nomenclature of IUPAC naming rules. The smallest member of the family, consisting of four fused cyclopropane rings, is [3.3.3.3]fenestrane, which has systematic name tetracyclo[2.1.0.01,3.02,5]pentane and is also called pyramidane. The next symmetric member, [4.4.4.4]fenestrane, has four cyclobutane rings fused, and has systematic name tetracyclo[3.3.1.03,9.07,9]nonane. The rings need not all be the same size as each other, so [4.4.4.5]fenestrane has three cyclobutane rings and one cyclopentane ring. Other structural modifications vary the name as usual in systematic nomenclature, so a [4.6.4.6]fenestradiene has two cyclobutane rings and two cyclohexane rings in an alternating pattern and two alkene units in the ring structure.

c,t,c,c-[4.5.5.5]Fenestrane, with ring-sizes and the one trans ring-fusion highlighted. C,t,c,c-4.5.5.5-fenestrane.png
c,t,c,c-[4.5.5.5]Fenestrane, with ring-sizes and the one trans ring-fusion highlighted.

In addition to the ring sizes, fenestranes can have various combinations of cis and trans geometry at each ring fusion. These details are denoted by "c" and "t" prefixes to the structure name, listed in the same order as the ring-sizes. [4] For example, c,t,c,c-[4.5.5.5]fenestrane has a trans configuration at one of the cyclopentane/cyclopentane fusions, but cis configuration at the other cyclopentane/cyclopentane fusion and at both butanepentane/cyclopentane fusions.

In an extreme case[ clarification needed ] the central carbon atom, which would ordinarily have tetrahedral molecular geometry for its four bonds gets completely flattened. In the molecular orbital picture for the resulting square planar geometry of methane, two of a total of three sp2-hybridized carbon atomic orbitals form regular bonds with two of the hydrogen atoms as in a planar alkene. The third sp2 orbital interacts in a three-center two-electron bond with the two remaining hydrogen atoms utilizing only the hydrogen electrons. Two additional carbon valence electrons are situated in a p orbital perpendicular to the plane of the molecule. The four C–H bonds are equal due to resonance. In silico calculations show that it takes 95 to 250 kcal/mol (400 to 1,050 kJ/mol) for this process.[ citation needed ]

One of the most highly strained fenestranes to have been isolated is a [4.4.4.5]fenestrane with bond angles at the central carbon atom of around 130° (based on X-ray crystallography), as compared to the 109.45° standard for tetrahedral atoms. The carbon–carbon bond-lengths deviate from those of normal alkanes as well. Whereas the C–C bond in ethane is 155 pm, in this fenestrane, the bonds extending from the central carbon atom are shortened to 149  pm while those at the perimeter are lengthened to 159 pm. [5]

A diterpene called laurenene containing a [5.5.5.7]fenestrane ring system was the first natural fenestrane to be discovered. [6] [7] The first fenestrane ever synthesized was a [4.5.5.6]fenestrane: [2] [8]

Fenestrane synthesis 1972 FenstraneSynthesis1972.png
Fenestrane synthesis 1972

Pyramidanes

Pyramidane ([3.3.3.3]fenestrane) is the smallest possible fenestrane, and has never been synthesised. If the central carbon were to be tetrahedral, it would have the form of spiropentadiene, but with additional bonds between the two cyclopropyl rings rather than double-bonds within them. The analogous germa- and stannapyramidanes, with trimethylsilyl groups bonded to the corners, Ge[C4(SiMe3)4] and Sn[C4(SiMe3)4] on the other hand have been synthesised. [9] These adopt a square pyramidal geometry analogous to the trigonal pyramid of tetrahedrane, with the germanium or tin atom at the vertex. That atom has an inverted tetrahedral geometry. According to nuclear magnetic resonance analysis, the four carbons of the base of the pyramid behave as an aromatic ring.

Synthetic approaches

In one study, a [4.5.5.5]fenestrane was synthesized with one carbon atom replaced by nitrogen because aza- compounds and their salts are more likely to form crystalline compounds suitable for X-ray analysis than low-molecular-weight alkanes. [4] In step 1 the alkyl halide 1-iodo-3-butene 1 is converted to a cyanozinc cuprate 2 (by transmetalation of the organozinc iodide with copper cyanide) which reacts in the next step with 1-nitrocyclopentene 3 in a nucleophilic addition whereby the nitronate 4 is captured by phenylselenenyl bromide to the selenium intermediate 5. Hydrogen peroxide oxidation of 5 yields the nitroalkene 6 as a mixture of syn and anti isomers. A [4+2]cycloaddition with n-butylenol ether in presence of trimethylaluminium gives the nitronate 7 and a second [3+2]cycloaddition by heating in presence of potassium carbonate gives the nitroso acetal 8. Hydrogenation with Raney nickel gives the diol 9 which on a double Mitsunobu reaction (with an amine proton donor) gives the azafenestrane 10 as the borane salt.

Azafenestrane synthesis 2006 AzaFenestraneSynthesis.png
Azafenestrane synthesis 2006

In the borane salt the N–C–C bond angle is 126°.

One study describes an unusual 8π disrotatory – 6π conrotatory electrocyclic cascade reaction aiming to minimise the number of steps required to synthesise a fenestrane. [10] [11]

Fenestrane Hulot 2008 FenestraneHulot2008.svg
Fenestrane Hulot 2008

See also

Related Research Articles

Aromatic compounds are organic compounds also known as "mono- and polycyclic aromatic hydrocarbons". The parent member is benzene. Heteroarenes are closely related, since at least one carbon atom of CH group is replaced by one of the heteroatoms oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.

Alkene Type of chemical compound

In chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.

Cycloalkane

In organic chemistry, the cycloalkanes are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing a single ring, and all of the carbon-carbon bonds are single. The larger cycloalkanes, with more than 20 carbon atoms are typically called cycloparaffins. All cycloalkanes are isomer of Alkene.

Cyclopropane Chemical compound

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 mainly of theoretical interest but many of its derivatives are of commercial or biological significance.

Unsaturated hydrocarbon

Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. The term "unsaturated" means more hydrogen atoms may be added to the hydrocarbon to make it saturated. The configuration of an unsaturated carbons include straight chain, such as alkenes and alkynes, as well as branched chains and aromatic compounds.

Tetrahedrane Chemical compound

Tetrahedrane is a hypothetical platonic hydrocarbon with chemical formula C4H4 and a tetrahedral structure. The molecule would be subject to considerable angle strain and has not been synthesized as of 2021. However, a number of derivatives have been prepared. In a more general sense, the term tetrahedranes is used to describe a class of molecules and ions with related structure, e.g. white phosphorus.

In chemistry, orbital hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds in valence bond theory. For example, in a carbon atom which forms four single bonds the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp3 mixtures in a tetrahedral arrangement around the carbon to bond to four different atoms. Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties and are symmetrically disposed in space. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies.

A cycloaddition is a chemical reaction in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction. Many but not all cycloadditions are concerted and thus pericyclic. Nonconcerted cycloadditions are not pericyclic. As a class of addition reaction, cycloadditions permit carbon–carbon bond formation without the use of a nucleophile or electrophile.

Ene reaction

The ene reaction is a chemical reaction between an alkene with an allylic hydrogen and a compound containing a multiple bond, in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position.

The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles. Mechanistic investigation and synthetic application were established in the 1960s, primarily through the work of Rolf Huisgen. Hence, the reaction is sometimes referred to as the Huisgen cycloaddition. 1,3-dipolar cycloaddition is an important route to the regio- and stereoselective synthesis of five-membered heterocycles and their ring-opened acyclic derivatives. The dipolarophile is typically an alkene or alkyne, but can be other pi systems. When the dipolarophile is an alkyne, aromatic rings are generally produced.

Ring strain

In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are substantially smaller than the idealized value of approximately 109°. Because of their high strain, the heat of combustion for these small rings is elevated.

In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of the molecule.

Bent bond Type of covalent bond in organic chemistry

In organic chemistry, a bent bond, also known as a banana bond, is a type of covalent chemical bond with a geometry somewhat reminiscent of a banana. The term itself is a general representation of electron density or configuration resembling a similar "bent" structure within small ring molecules, such as cyclopropane (C3H6) or as a representation of double or triple bonds within a compound that is an alternative to the sigma and pi bond model.

Pyramidal alkenes are alkenes in which the two carbon atoms making up the double bond are not coplanar with their four substituents. This deformation results from geometric constraints. Pyramidal alkenes only are of interest because much can be learned from them about the nature of chemical bonding.

Tetrahedral molecular geometry Central atom with four substituents located at the corners of a tetrahedron

In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1(−1⁄3) = 109.4712206...° ≈ 109.5° when all four substituents are the same, as in methane as well as its heavier analogues. Methane and other perfectly symmetrical tetrahedral molecules belong to point group Td, but most tetrahedral molecules have lower symmetry. Tetrahedral molecules can be chiral.

In organic chemistry, propellane is any member of a class of polycyclic hydrocarbons, whose carbon skeleton consists of three rings of carbon atoms sharing a common carbon–carbon covalent bond. The name derives from a supposed resemblance of the molecule to a propeller: namely, the rings would be the propeller's blades, and the shared C–C bond would be its axis. The concept was introduced in 1966 by D. Ginsburg Propellanes with small cycles are highly strained and unstable, and are easily turned into polymers with interesting structures, such as staffanes. Partly for these reasons, they have been the object of much research. In the literature, the bond shared by the three cycles is usually called the "bridge"; the shared carbon atoms are then the "bridgeheads". The notation [x.y.z]propellane 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 C
2+x+y+zH
2(x+y+z). The minimum value for x, y, and z is 1, meaning a 3-carbon ring. There is no structural ordering between the rings, so, for example, [1.3.2]propellane is the same substance as [3.2.1]propellane. Therefore, it is customary to sort the indices in decreasing order, x ≥ y ≥ z.

Trimethylenemethane cycloaddition is the formal [3+2] annulation of trimethylenemethane (TMM) derivatives to two-atom pi systems. Although TMM itself is too reactive and unstable to be stored, reagents which can generate TMM or TMM synthons in situ can be used to effect cycloaddition reactions with appropriate electron acceptors. Generally, electron-deficient pi bonds undergo cyclization with TMMs more easily than electron-rich pi bonds.

In organic chemistry, enone–alkene cycloadditions are a version of the [2+2] cycloaddition This reaction involves an enone and alkene as substrates. Although the concerted photochemical [2+2] cycloaddition is allowed, the reaction between enones and alkenes is stepwise and involves discrete diradical intermediates.

Activation of cyclopropanes by transition metals

In organometallic chemistry, the activation of cyclopropanes by transition metals is a research theme with implications for organic synthesis and homogeneous catalysis. Being highly strained, cyclopropanes are prone to oxidative addition to transition metal complexes. The resulting metallacycles are susceptible to a variety of reactions. These reactions are rare examples of C-C bond activation. The rarity of C-C activation processes has been attributed to Steric effects that protect C-C bonds. Furthermore, the directionality of C-C bonds as compared to C-H bonds makes orbital interaction with transition metals less favorable. Thermodynamically, C-C bond activation is more favored than C-H bond activation as the strength of a typical C-C bond is around 90 kcal per mole while the strength of a typical unactivated C-H bond is around 104 kcal per mole.

Cyclopentyne is a cycloalkyne containing five carbon atoms in the ring. Due to the ideal bond angle of 180° at each atom of the alkyne but the structural requirement that the bonds form a ring, this chemical is a highly strained structure, and the triple bond is highly reactive. The triple bond easily undergoes both [2+2] and [4+2] cycloaddition reactions. Unlike benzyne, which undergoes a [2+2] addition with loss of stereochemistry at the alkene partner, cyclopentyne reacts with alkenes with retention of geometry of the partner, an example of the relevance of orbital symmetry even for highly reactive structures. The structure can also form a π complex with lithium cations, which affects the cycloaddition reactivity. It can even interact strongly enough with copper species to form a novel type of metallacycle.

References

  1. ↑ Venepalli, Bhaskar Rao; Agosta, William C. (1987). "Fenestranes and the flattening of tetrahedral carbon". Chem. Rev. 87 (2): 399–410. doi:10.1021/cr00078a007.
  2. 1 2 Georgian, Vlasios; Saltzman, Martin (1972). "Syntheses directed toward saturated "flat" carbon". Tetrahedron Letters . 13 (42): 4315–4317. doi:10.1016/S0040-4039(01)94304-7.
  3. 1 2 Nickon, Alex; Silversmith, Ernest F. (2013). Organic Chemistry: The Name Game: Modern Coined Terms and Their Origins. Elsevier. pp. 55–56. ISBN   9781483145235.
  4. 1 2 Denmark, Scott E.; Montgomery, Justin I.; Kramps, Laurenz A. (2006). "Synthesis, X-ray Crystallography, and Computational Analysis of 1-Azafenestranes". J. Am. Chem. Soc. 128 (35): 11620–11630. doi:10.1021/ja0632759.
  5. ↑ Rao, V. Bhaskar; George, Clifford F.; Wolff, Steven; Agosta, William C. (1985-10-01). "Synthetic and structural studies in the [4.4.4.5]fenestrane series". Journal of the American Chemical Society. 107 (20): 5732–5739. doi:10.1021/ja00306a022.
  6. ↑ Boudhar, Aicha; Charpenay, Mélanie; Blond, Gaëlle; Suffert, Jean (2 December 2013). "Fenestranes in Synthesis: Unique and Highly Inspiring Scaffolds". Angewandte Chemie International Edition. 52 (49): 12786–12798. doi:10.1002/anie.201304555.
  7. ↑ Corbett, R. Edward; Lauren, Denis R.; Weavers, Rex T. (1979). "The structure of laurenene, a new diterpene from the essential oil of Dacrydium cupressinum. Part 1". Journal of the Chemical Society, Perkin Transactions 1: 1774. doi:10.1039/P19790001774.
  8. ↑ The first step in this reaction sequence is an adaptation of the Stork enamine alkylation reacting cyclopentanone with 3-bromo-1-butene through an imine derivative with pyrrolidine and forming a magnesium salt with ethyl magnesium bromide. The next step is a regular Stork enamine reaction followed by an aldol condensation forming the cyclohexenone ring. The final step is a photolytic [2+2]cycloaddition.
  9. ↑ Lee, Vladimir Ya.; Ito, Yuki; Sekiguchi, Akira; Gornitzka, Heinz; Gapurenko, Olga A.; Minkin, Vladimir I.; Minyaev, Ruslan M. (2013). "Pyramidanes". J. Am. Chem. Soc. 135 (24): 8794–8797. doi:10.1021/ja403173e.
  10. ↑ Hulot, C.; Blond, G.; Suffert, J. (2008). "Synthesis of [4.6.4.6]Fenestradienes and [4.6.4.6]Fenestrenes Based on an 8π−6π-Cyclization-Oxidation Cascade". J. Am. Chem. Soc. 130 (15): 5046–5047. doi:10.1021/ja800691c.
  11. ↑ Reagents: P-2 Ni (Ni(OAc)2·4H2O) / hydrogen gas. Reaction initiated by organic reduction of alkyne to alkene
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.