Allenes

For many years, allenes were viewed as curiosities but thought to be synthetically useless and difficult to prepare and to work with.

[2][3] Reportedly,[4] the first synthesis of an allene, glutinic acid, was performed in an attempt to prove the non-existence of this class of compounds.

[7] These compounds are not just interesting intermediates but synthetically valuable targets themselves; for example, over 150 natural products are known with an allene or cumulene fragment.

The structure can also be viewed as an "extended tetrahedral" with a similar shape to methane, an analogy that is continued into the stereochemical analysis of certain derivative molecules.

[8] For allenes with four identical substituents, there exist two twofold axes of rotation through the central carbon atom, inclined at 45° to the CH2 planes at either end of the molecule.

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.

Chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties.

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 1,3-dimethylallene [de].

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.

[46] Arguably, the most convenient modern method of allene synthesis is by sigmatropic rearrangement of propargylic substrates.

[47][48][49] Johnson–Claisen[49] and Ireland–Claisen[55] rearrangements of ketene acetals 4 have been used a number of times to prepare allenic esters and acids.

[47][48] Allene itself is the most commonly used member of this family; it exists in equilibrium with propyne as a component of MAPP gas.

With an appropriate substitution pattern, allenes exhibit axial chirality as predicted by van’t Hoff as early as 1875.

[66] Transition-metal-catalysed reactions proceed via allylic intermediates 15 and have attracted significant interest in recent years.