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Xylylene

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(Redirected from Quinodimethane)
ortho-Xylylene
meta-Xylylene
para-Xylylene

In organic chemistry, a xylylene (sometimes quinone-dimethide) is any of the constitutional isomers having the formula C6H4(CH2)2. These compounds are related to the corresponding quinones and quinone methides by replacement of the oxygen atoms by CH2 groups. 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:

Scheme 7. 2,2-paracyclophane synthesis
Scheme 7. 2,2-paracyclophane synthesis

Further heating of the p-cyclophane gives poly(para-xylylene).

o-Xylylenes

o-Xylylenes (o-quinodimethanes) 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,

in situ generation of o-quinodomethanes
in situ generation of o-quinodomethanes

α,α'-ortho Xylene dibromides have been well developed for generating o-xylyenes. For example, reaction of tetrabromo-o-xylene (C6H4(CHBr2)2) 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:

C6H4(CHBr2)2 + 2 NaI → C6H4(=CHBr)2 + 2 NaBr + I2
C6H4(=CHBr)2 → C6H4(CHBr)2

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 Fe(CO)3. This product is structurally analogous to Fe(CO)3.

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.

Electronic structure

Despite the observed chemistry of para-xylylene (i.e. its rapid polymerization to poly-p-xylylene), which suggests the compound exists as a diradical, physical evidence unanimously concludes that the lowest electronic state of p-xylylene is a closed shell singlet. Additionally, several computational methods confirm this assignment. Conversely, meta-xylylene is a non-Kekulé molecule that has a triplet ground-state.

References

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  13. Montgomery, L. K., Huffman, J. C., Jurczak, E. A. & Grendze, M. P. The molecular structures of Thiele’s and Chichibabin’s hydrocarbons. J. Am. Chem. Soc. 108, 6004–6011 (1986) doi:10.1021/ja00279a056
  14. Steglich, Mathias; Custodis, Victoria B. F.; Trevitt, Adam J.; daSilva, Gabriel; Bodi, Andras; Hemberger, Patrick (2017). "Photoelectron Spectrum and Energetics of the meta-Xylylene Diradical". J. Am. Chem. Soc. 139 (41): 14348–14351. doi:10.1021/jacs.7b06714. PMID 28965395.
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