Xylylene Explained

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:[1]

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

o-Xylylenes

o-Xylylenes (o-quinodimethanes) are often generated in situ,[2] e.g., by the pyrolysis of the corresponding sulfone.[3] Another method involves 1,4-elimination of ortho benzylic silanes.[4] or stannanes,[5] [6] [7]

α,α'-ortho Xylene dibromides have been well developed for generating o-xylyenes.[8] 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:[9]

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

C6H4(=CHBr)2 → C6H4(CHBr)2Cycloadditions of these o-xylylenes provides a pathway to acenes.[10]

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[η<sup>4</sup>-C<sub>6</sub>H<sub>4</sub>(CH<sub>2</sub>)<sub>2</sub>]. This product is structurally analogous to Fe(CO)3[η<sup>4</sup>-[[1,3-butadiene]]].[11]

At high temperatures, benzocyclobutenes undergo electrocyclic ring-opening to form o-xylylenes. This and other syntheses of o-xylylenes, and their subsequent dimerization by [4+4] cycloaddition to form cycloctyl structures, were used repeatedly in the synthesis of superphane.[12]

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.[13] Conversely, meta-xylylene is a non-Kekulé molecule that has a triplet ground-state.[14]

Notes and References

  1. H. E. Winberg, F. S. Fawcett "[2.2]Paracyclophane" Organic Syntheses, Coll. Vol. 5, p.883 (1973); Vol. 42, p.83 (1962) Link.
  2. Klundt . I. L. . 1970 . Benzocyclobutene and its derivatives . Chemical Reviews . 70 . 4 . 471–487 . 10.1021/cr60266a002.
  3. Nicolaou . K. C. . Snyder . S. A. . Montagnon . T. . Vassilikogiannakis . G. . 2002 . The Diels-Alder Reaction in Total Synthesis . Angewandte Chemie International Edition . 41 . 10 . 1668–1698 . 10.1002/1521-3773(20020517)41:10<1668::AID-ANIE1668>3.0.CO;2-Z. 19750686 .
  4. Ito . Y. . Nakatsuka . M. . Saegusa . T. . 1982 . Syntheses of polycyclic ring systems based on the new generation of o-quinodimethanes . Journal of the American Chemical Society . 104 . 26 . 7609–7622 . 10.1021/ja00390a036.
  5. Sano . H. . Ohtsuka . H. . Migita . T. . 1988 . A convenient method for the generation of o-quinodimethanes by proton induced 1,4-elimination of o-(1-hydroxyalkyl)benzyltributylstannanes . Journal of the American Chemical Society . 110 . 6 . 2014–2015 . 10.1021/ja00214a083.
  6. Soon . H. W. . 1993 . Tetrahedron Letters . A novel method for the generation of o-quinodimethane by selenium-induced fragmentation of o-vinyl benzyltributylstannane . 34 . 47 . 7587–7590 . 10.1016/S0040-4039(00)60407-0.
  7. Soon . H. W. . 1994 . Lewis acid-promoted generation of α-oxy-o-quinodimethanes and cycloaddition reactions . Tetrahedron Letters . 35 . 23 . 3975–3978 . 10.1016/S0040-4039(00)76717-7.
  8. Rubottom . G. M. . Way . J. E. . 1984 . An Improved Method for the Preparation of o-Quinodimethanes . Synthetic Communications . 14 . 6 . 507–514 . 10.1080/00397918408059572.
  9. 10.1021/ja01533a032. Condensed Cyclobutane Aromatic Compounds. VIII. The Mechanism of Formation of 1,2-Dibromobenzocyclobutene; A New Diels-Alder Synthesis. 1959. Cava. M. P.. Deana. A. A.. Muth. K.. Journal of the American Chemical Society. 81. 24. 6458–6460.
  10. 10.1055/s-1986-31603. An Efficient Synthetic Strategy for Naphthalene Annellation of Norbornenylogous Systems. 1986. Paddon-Row. Michael N.. Patney. Harish K.. Synthesis. 1986. 4. 328–330.
  11. 10.1021/om00054a059. Formation of Iron Carbonyl Complexes of Reactive Polyenes from Dihalides involving the Free Polyene. 1991. Kerber. Robert C.. Ribakove. Everett C.. Organometallics. 10. 8. 2848–2853.
  12. Sekine. Y.. Brown. M.. Boekelheide. V.. [2.2.2.2.2.2](1,2,3,4,5,6)Cyclophane: superphane. Journal of the American Chemical Society. 101. 11. 3126–3127. 1979. 10.1021/ja00505a053.
  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)
  14. Photoelectron Spectrum and Energetics of the meta-Xylylene Diradical . Mathias . Steglich . Victoria B. F. . Custodis . Adam J. . Trevitt . Gabriel . daSilva . Andras . Bodi . Patrick . Hemberger . J. Am. Chem. Soc. . 2017 . 139 . 41 . 14348–14351 . 10.1021/jacs.7b06714 . 28965395 .

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