Published on Web 10/28/2006
Uncatalyzed Zirconium-Mediated Biphenylation of o-Dihalobenzenes To Form
Triphenylenes
Cameron L. Hilton, Christopher R. Jamison, and Benjamin T. King*
Department of Chemistry, UniVersity of NeVada, Reno, NeVada 89577
Aryl-aryl bond forming reactions are ubiquitous in synthesis.1
Zirconium has, however, played a limited role in this field.
Buchwald reported a Pd-mediated reaction of a Zr-benzyne
complex with benzenes or biphenyls to give biphenyls or ter-
Scheme 1. Substitution of o-Dihalides by 2,2′-Biphenyldiyl
2
phenyls. Takahashi reported a Cu-mediated reaction of an alkyl
zirconocyclopentadiene with o-diiodobenzenes to give alkyl-
substituted naphthalenes.3
We report herein the uncatalyzed reaction of [Zr(biphe)
3
][(Li‚
4
(
THF) ) ] (1), where biphe is 2,2′-biphenyldiyl, with o-dihaloarenes
4 2
to give triphenylenes (Scheme 1). The reaction is noteworthy from
a synthetic viewpoint because it is a new disconnection for the
synthesis of polycyclic aromatic hydrocarbons (PAHs). The closest
reported analogue is the double nucleophilic aromatic substitution
involving the reductive generation of benzyne from the o-dihalo-
arene and its subsequent insertion into Zr-(biphe) bonds is also
possible. Trapping experiments, however, do not support this
mechanismsthe reaction of o-dibromobenzene with 1 in the
presence of a 10-fold excess of furan proceeded normally and did
not produce detectable (GC-MS) amounts of the trapping product,
reaction of dilithiobiphenyl with hexafluorobenzene to afford
5
1
,2,3,4-tetrafluorotriphenylene. The reaction is noteworthy from
an organometallic viewpoint because it is a rare, if not unprec-
edented, example of an uncatalyzed, Zr-mediated aryl-aryl bond
formation.
The scope is broad. The reaction works with all halogens
1,4-dihydro-1,4-epoxynaphthalene.
(Scheme 1). Dichloro-, dibromo-, and diiodobenzene give higher
A mechanism involving oxidative insertion of Zr into an aryl-
yields (14, 26, and 48%) than difluorobenzene (2%). Substituents
are tolerated: 4,5-dibromoveratrole or 4,5-dibromo-o-xylene react
with 1 to give the corresponding 2,3-disubstituted triphenylene.
This method provides rapid entry into 2,3-disubstituted tri-
0
halogen bond may operate. Since the Zr center is d , reduction
would have to precede oxidative insertion. The formation of
quaterphenyl suggests this may occur. σ-Bond metathesis, perhaps
involving electron transfer, is another possibility.
6
phenylenes, which may be useful as discotic liquid crystals. The
This transformation enables a new disconnection for the synthesis
of PAHs: the double substitution of two o-halogens with a
biphenyldiyl moiety to give a triphenylene fragment. The reaction
tolerates functionality and can generate strained molecules. This
reaction is a rare example of an uncatalyzed aryl-aryl bond
formation reaction involving Zr.
reaction of 1,2-diiodo-4,5-dihexylbenzene with 1 gave 2,3-dihexyl-
triphenylene in 8% yield (Scheme 1).
Multiple biphenyldiyl moieties can be introduced in a single step.
The reaction of 1,2,4,5-tetrabromobenzene with 1 afforded tetra-
benz[a,c,h,j]anthracene in 17% isolated yield (Scheme 1).
Strained molecules can be produced. The reaction of 1 with
tetrabromo-p-xylene gave 9,18-dimethyltetrabenz[a,c,h,j]anthracene
Acknowledgment. This work was supported by the National
Science Foundation (CHE-0449740).
(
2) in 20% yield. This molecule is slightly more strained (35
kcal/mol, B3LYP/6-31+G*) than Pascal’s analogous 9,18-diphen-
Supporting Information Available: Strain definitions, geometries,
synthetic procedures, and NMR spectra for 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
7
yltetrabenz[a,c,h,j]anthracene 3 (34 kcal/mol, B3LYP/6-31+G*).
8
Like 3, 2 is easily reducedsexcess 1, which is presumably a
reductant, gives the corresponding 9,18-dihydro product as a major
component.
References
This reaction works in a variety of conditions: in toluene,
dioxane, or THF solution, or with neat liquid reagents. The best
procedure utilizes toluene at 25 °C with sufficient solvent to dissolve
all reactants. Most reactions are complete after 20 min.
This unoptimized reaction is not ideal. Significant amounts of
quaterphenyl are produced, which may arise from the oxidation of
(
(
(
1) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. ReV.
2002, 102, 1359.
2) Frid, M.; Perez, D.; Peat, A. J.; Buchwald, S. L. J. Am. Chem. Soc. 1999,
121, 9469.
3) Takahashi, T.; Hara, R.; Nishihara, Y.; Kotora, M. J. Am. Chem. Soc.
1996, 118, 5154.
(
4) Hilton, C. L.; King, B. T. Organometallics 2006, 25, 4058.
(
5) Cho, D. M.; Parkin, S. R.; Watson, M. D. Org. Lett. 2005, 7, 1067.
1
or zirconates derived from 1 upon workup or from a reductive
(6) Kumar, S. Liq. Cryst. 2004, 31, 1037.
(7) Pascal, R. A. J.; McMillan, W. D.; Van Engen, D. J. Am. Chem. Soc.
elimination step. The yields vary from poor to modest.
1986, 108, 5652.
We are uncertain of the mechanism. Double nucleophilic
aromatic substitution is unlikely since electron-rich dihalobenzenes,
such as 4,5-dibromoveratrole, are good substrates. A mechanism
(8) Eshdat, L.; Ayalon, A.; Beust, R.; Shenhar, R.; Rabinovitz, M. J. Am.
Chem. Soc. 2000, 122, 12637.
JA065602I
14824
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J. AM. CHEM. SOC. 2006, 128, 14824
10.1021/ja065602i CCC: $33.50 © 2006 American Chemical Society