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:H. E. Winberg, F. S. Fawcett "[2.2]Paracyclophane" Organic Syntheses, Coll. Vol. 5, p.883 (1973); Vol. 42, p.83 (1962) [http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5p0883 Link].

Image:2,2-paracyclophane.png

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

o-Xylylenes (o-quinodimethanes) are often generated in situ,{{cite journal |last1=Klundt |first1=I. L. |year=1970 |title=Benzocyclobutene and its derivatives |journal=Chemical Reviews |volume=70 |issue=4 |pages=471–487 |doi=10.1021/cr60266a002}} e.g., by the pyrolysis of the corresponding sulfone.{{cite journal |last1=Nicolaou |first1=K. C. |last2=Snyder |first2=S. A. |last3=Montagnon |first3=T. |last4=Vassilikogiannakis |first4=G. |year=2002 |title=The Diels-Alder Reaction in Total Synthesis |journal=Angewandte Chemie International Edition |volume=41 |issue=10 |pages=1668–1698 |doi=10.1002/1521-3773(20020517)41:10<1668::AID-ANIE1668>3.0.CO;2-Z|pmid=19750686 }} Another method involves 1,4-elimination of ortho benzylic silanes.{{cite journal |last1=Ito |first1=Y. |last2=Nakatsuka |first2=M. |last3=Saegusa |first3=T. |year=1982 |title=Syntheses of polycyclic ring systems based on the new generation of o-quinodimethanes |journal=Journal of the American Chemical Society |volume=104 |issue=26 |pages=7609–7622 |doi=10.1021/ja00390a036}} or stannanes,{{cite journal |last1=Sano |first1=H. |last2=Ohtsuka |first2=H. |last3=Migita |first3=T. |year=1988 |title=A convenient method for the generation of o-quinodimethanes by proton induced 1,4-elimination of o-(1-hydroxyalkyl)benzyltributylstannanes |journal=Journal of the American Chemical Society |volume=110 |issue=6 |pages=2014–2015 |doi=10.1021/ja00214a083}}{{cite journal |last1=Soon |first1=H. W. |year=1993 |journal=Tetrahedron Letters |title=A novel method for the generation of o-quinodimethane by selenium-induced fragmentation of o-vinyl benzyltributylstannane |volume=34 |issue=47 |pages=7587–7590 |doi=10.1016/S0040-4039(00)60407-0}}{{cite journal |last1=Soon |first1=H. W. |year=1994 |title=Lewis acid-promoted generation of α-oxy-o-quinodimethanes and cycloaddition reactions |journal=Tetrahedron Letters |volume=35 |issue=23 |pages=3975–3978 |doi=10.1016/S0040-4039(00)76717-7}}

File:Quinodomethanes2.png

α,α'-ortho Xylene dibromides have been well developed for generating o-xylyenes.{{cite journal |last1=Rubottom |first1=G. M. |last2=Way |first2=J. E. |year=1984 |title=An Improved Method for the Preparation of o-Quinodimethanes |journal=Synthetic Communications |volume=14 |issue=6 |pages=507–514 |doi=10.1080/00397918408059572}} 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:{{cite journal |doi=10.1021/ja01533a032|title=Condensed Cyclobutane Aromatic Compounds. VIII. The Mechanism of Formation of 1,2-Dibromobenzocyclobutene; A New Diels-Alder Synthesis|year=1959|last1=Cava|first1=M. P.|last2=Deana|first2=A. A.|last3=Muth|first3=K.|journal=Journal of the American Chemical Society|volume=81|issue=24|pages=6458–6460}}

: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.{{cite journal |doi=10.1055/s-1986-31603|title=An Efficient Synthetic Strategy for Naphthalene Annellation of Norbornenylogous Systems|year=1986|last1=Paddon-Row|first1=Michael N.|last2=Patney|first2=Harish K.|journal=Synthesis|volume=1986|issue=4|pages=328–330}}

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[η⁴-C₆H₄(CH₂)₂]. This product is structurally analogous to Fe(CO)₃[η⁴-1,3-butadiene].{{cite journal|doi=10.1021/om00054a059|title=Formation of Iron Carbonyl Complexes of Reactive Polyenes from Dihalides involving the Free Polyene|year=1991|last1=Kerber|first1=Robert C.|last2=Ribakove|first2=Everett C.|journal=Organometallics|volume=10|issue=8|pages=2848–2853}}

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.{{cite journal|last1=Sekine|first1=Y.|last2=Brown|first2=M.|last3=Boekelheide|first3=V.|title=[2.2.2.2.2.2](1,2,3,4,5,6)Cyclophane: superphane|journal=Journal of the American Chemical Society|volume=101|issue=11|pages=3126–3127|year=1979|doi=10.1021/ja00505a053}}

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.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 Conversely, meta-xylylene is a non-Kekulé molecule that has a triplet ground-state.{{cite journal |title= Photoelectron Spectrum and Energetics of the meta-Xylylene Diradical |first1= Mathias |last1= Steglich |first2= Victoria B. F. |last2= Custodis |first3= Adam J. |last3= Trevitt |first4= Gabriel |last4= daSilva |first5= Andras |last5= Bodi |first6= Patrick |last6= Hemberger |journal= J. Am. Chem. Soc. |year= 2017 |volume= 139 |issue= 41 |pages= 14348–14351 |doi= 10.1021/jacs.7b06714 |pmid= 28965395 |url= https://ro.uow.edu.au/cgi/viewcontent.cgi?article=6078&context=smhpapers }}

{{Reflist}}

{{DEFAULTSORT:Xylylene}}

Category:Cyclohexadienes

Category:Hydrocarbons

Category:Vinylidene compounds