Editing Allenes (section)

===Occurrence===
[[File:Fucoxanthin.svg|thumb|422 px|[[Fucoxanthin]], the most abundant of all carotinoids, is the light-absorbing [[pigment]] in the [[chloroplast]]s of [[brown algae]], giving them a brown or olive-green color.]]
Numerous natural products contain the allene functional group. Noteworthy are the pigments [[fucoxanthin]] and [[peridinin]]. Little is known about the biosynthesis, although it is conjectured that they are often generated from alkyne precursors.<ref>{{cite book |title= Modern Allene Chemistry |chapter= 18. Allenic Natural Products and Pharmaceuticals |first1= Norbert |last1= Krause |first2= Anja |last2= Hoffmann-Röder |editor1-first= Norbert |editor1-last= Krause |editor2-first= A. Stephen K. |editor2-last= Hashmi |year= 2004|pages= 997–1040 |doi= 10.1002/9783527619573.ch18 |isbn= 9783527619573 }}</ref>

Allenes serve as ligands in [[organometallic chemistry]]. A typical complex is Pt([[hapticity|η<sup>2</sup>]]-allene)(PPh<sub>3</sub>)<sub>2</sub>. Ni(0) reagents catalyze the cyclooligomerization of allene.<ref>Otsuka, Sei; Nakamura, Akira "Acetylene and allene complexes: their implication in homogeneous catalysis" Advances in Organometallic Chemistry 1976, volume 14, pp. 245-83. {{doi|10.1016/S0065-3055(08)60654-1}}.</ref> Using a suitable catalyst (e.g. [[Wilkinson's catalyst]]), it is possible to reduce just one of the double bonds of an allene.<ref>{{cite journal |last1 = Bhagwat|first1=M. M.|last2= Devaprabhakara|first2=D. |journal = [[Tetrahedron Letters]] |pages = 1391–1392 |year = 1972 |doi = 10.1016/S0040-4039(01)84636-0 |title = Selective hydrogenation of allenes with chlorotris-(triphenylphosphine) rhodium catalyst |volume = 13 |issue = 15}}</ref>