In organic chemistry, allenes are organic compounds in which one carbon atom has with each of its two adjacent carbon atoms (, where R is hydrogen or some organyl group). Allenes are classified as cumulated dienes. The parent compound of this class is propadiene (), which is itself also called allene. A group of the structure is called allenyl, while a substituent attached to an allene is referred to as an allenic substituent (R is H or some alkyl group). In analogy to Allyl group and Propargyl group, a substituent attached to a saturated carbon α (i.e., directly adjacent) to an allene is referred to as an allenylic substituent. While allenes have two consecutive ('cumulated') double bonds, compounds with three or more cumulated double bonds are called .
An allene with two different substituents on each of the two carbon atoms will be chiral because there will no longer be any mirror planes. The chirality of these types of allenes was first predicted in 1875 by Jacobus Henricus van 't Hoff, but not proven experimentally until 1935. Where A has a greater priority than B according to the Cahn–Ingold–Prelog priority rules, the configuration of the axial chirality can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the back atom, only the group of higher priority need be considered.
Chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties. There are a few examples of drug molecule having an allene system in their structure. Mycomycin, an antibiotic with tuberculostatic properties, is a typical example. This drug exhibits enantiomerism due to the presence of a suitably substituted allene system.
Although the semi-localized textbook σ-π separation model describes the bonding of allene using a pair of localized orthogonal π orbitals, the full molecular orbital description of the bonding is more subtle. The symmetry-correct doubly-degenerate HOMOs of allene (adapted to the D2d point group) can either be represented by a pair of orthogonal MOs or as twisted helical linear combinations of these orthogonal MOs. The symmetry of the system and the degeneracy of these orbitals imply that both descriptions are correct (in the same way that there are infinitely many ways to depict the doubly-degenerate HOMOs and LUMOs of benzene that correspond to different choices of eigenfunctions in a two-dimensional eigenspace). However, this degeneracy is lifted in substituted allenes, and the helical picture becomes the only symmetry-correct description for the HOMO and HOMO–1 of the C2-symmetric . This qualitative MO description extends to higher odd-carbon cumulenes (e.g., 1,2,3,4-pentatetraene).
The C–H bonds of allenes are considerably weaker and more acidic compared to typical vinylic C–H bonds: the bond dissociation energy is 87.7 kcal/mol (compared to 111 kcal/mol in ethylene), while the Proton affinity is 381 kcal/mol (compared to 409 kcal/mol for ethylene), making it slightly more acidic than the propargylic C–H bond of propyne (382 kcal/mol).
The 13C NMR spectrum of allenes is characterized by the signal of the sp-hybridized carbon atom, resonating at a characteristic 200-220 ppm. In contrast, the sp2-hybridized carbon atoms resonate around 80 ppm in a region typical for alkyne and nitrile carbon atoms, while the protons of a CH2 group of a terminal allene resonate at around 4.5 ppm — somewhat upfield of a typical vinylic proton.
Allenes possess a rich cycloaddition chemistry, including both 4+2 and 2+2 modes of addition, as well as undergoing formal cycloaddition processes catalyzed by transition metals. Allenes also serve as substrates for transition metal catalyzed hydrofunctionalization reactions.
Much like acetylenes, electron-poor allenes are unstable. Tetrachloroallene polymerizes quantitatively to perchloro(1,2-dimethylenecyclobutane) at −50 °C.
Cyclic allenes with fewer than 10 ring atoms are ring strain. Those with fewer than 8 atoms generally only form unstable aryne-like intermediates. The latter are sometimes stabilized by diradical or ylidic resonance structures.
The first allene to be synthesized was penta-2,3-dienedioic acid, which was prepared by Burton and Pechmann in 1887. However, the structure was only correctly identified in 1954.
Laboratory methods for the formation of allenes include:
One of the older methods is the Skattebøl rearrangement (also called the Doering–Moore–Skattebøl or Doering–LaFlamme rearrangement), in which a gem-dihalocyclopropane 3 is treated with an organolithium compound (or dissolving metal) and the presumed intermediate rearranges into an allene either directly or via carbene-like species. Notably, even strained allenes can be generated by this procedure. Modifications involving leaving groups of different nature are also known. Arguably, the most convenient modern method of allene synthesis is by sigmatropic rearrangement of Propargyl group. Johnson–Claisen and Ireland–Claisen rearrangements of ketene acetals 4 have been used a number of times to prepare allenic esters and acids. Reactions of vinyl ethers 5 (the Saucy–Marbet rearrangement) give allene aldehydes, while propargylic sulfenates 6 give allene sulfoxides. Allenes can also be prepared by nucleophilic substitution in 9 and 10 (nucleophile Nu− can be a hydride anion), 1,2-elimination from 8, proton transfer in 7, and other, less general, methods.
The two Pi bond are located at the 90° angle to each other, and thus require a reagent to approach from somewhat different directions. With an appropriate substitution pattern, allenes exhibit axial chirality as predicted by Van 't Hoff as early as 1875.Van ’t Hoff, J. H. La Chimie dans l’Espace; P.M. Bazendijk, 1875; p. 43. Protonation of allenes gives cations 11 that undergo further transformations. Reactions with soft electrophiles (e.g. Br+) deliver positively charged 13. Transition-metal-catalysed reactions proceed via allylic intermediates 15 and have attracted significant interest in recent years. Numerous cycloadditions are also known, including 4+2-, (2+1)-, and 2+2-variants, which deliver, e.g., 12, 14, and 16, respectively.
Allenes serve as ligands in organometallic chemistry. A typical complex is Pt(hapticity-allene)(PPh3)2. Ni(0) reagents catalyze the cyclooligomerization of allene.Otsuka, Sei; Nakamura, Akira "Acetylene and allene complexes: their implication in homogeneous catalysis" Advances in Organometallic Chemistry 1976, volume 14, pp. 245-83. . Using a suitable catalyst (e.g. Wilkinson's catalyst), it is possible to reduce just one of the double bonds of an allene.
Synthesis
This mixture, known as MAPP gas, is commercially available. At 298 K, the Δ G° of this reaction is –1.9 kcal/mol, corresponding to Keq = 24.7.
The chemistry of allenes has been reviewed in a number of books and journal articles. Some key approaches towards allenes are outlined in the following scheme:
Use and occurrence
Research
Occurrence
Delta convention
See also
Further reading
External links
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