Xylylene
In organic chemistry, a xylylene is any of the constitutional isomers having the formula C6H42. These compounds are related to the corresponding quinones and quinone methides by replacement of the oxygen atoms by CH2 groups. Arene [substitution patterns|ortho- and para]-xylylene are best known, although neither is stable in solid or liquid form. The meta form is a diradical. Certain substituted derivatives of xylylenes are however highly stable, such as tetracyanoquinodimethane and the xylylene dichlorides.
p-Xylylene
p-Xylylene forms upon pyrolysis of p-xylene or, more readily, the α-substituted derivatives. p-Xylylene dimerizes with moderate efficiency to give p-cyclophane:[Image:2,2-paracyclophane.png|center|400px|Scheme 7. 2,2-paracyclophane synthesis]
Further heating of the p-cyclophane gives poly(para-xylylene).
o-Xylylenes
o-Xylylenes are often generated in situ, e.g., by the pyrolysis of the corresponding sulfone. Another method involves 1,4-elimination of ortho benzylic silanes. or stannanes,α,α'-ortho Xylene dibromides have been well developed for generating o-xylyenes. For example, reaction of tetrabromo-o-xylene with sodium iodide affords α,α'-dibromo-o-xylylene, which can be trapped to give naphthylene derivatives. In the absence of trapping agents, the xylylene relaxes to α,α'-dibromobenzocyclobutane:
Cycloadditions of these o-xylylenes provides a pathway to acenes.
The diene unit formed by the two exocyclic alkene units of the ortho isomer can serve as a ligand in coordination complexes. For example, reaction of α,α'-dibromo-o-xylene with iron carbonyls affords low yields of the xylylene complex Fe3. This product is structurally analogous to Fe3.
At high temperatures, benzocyclobutenes undergo electrocyclic ring-opening to form o-xylylenes. This and other syntheses of o-xylylenes, and their subsequent dimerization by cycloaddition to form cycloctyl structures, were used repeatedly in the synthesis of superphane.