5716 Organometallics, Vol. 28, No. 19, 2009
Yang et al.
studies further extended C-C coupling to C-N,8 C-O,9 and
other types of coupling reactions.10 Nowadays, coupling
reactions have become practical synthetic techniques appli-
cable to organic synthesis and to the synthesis of materials
or bioactive compounds.11 Palladium complexes of bulky
and electron-rich phosphines12 or N-heterocyclic carbenes
(NHC’s)13 are among the most effective catalysts. Recently,
many catalysts with ligands other than the phosphine- or
NHC-based compounds and catalytic systems with nickel,14
copper,15 or even iron16 metal centers have been established.
Although many catalytic systems achieve high yields of cou-
pling products, the specially tailored ligands, which are in
general not commercially available, limit those systems for
practical usage. In addition to the aforementioned coupling
reagents, a few examples of coupling reactions between aryl
halides and alkylaluminum reagents have been reported.17
However, conditions at high reaction temperatures and with
high equivalents of alkylaluminum reagents were required to
afford coupling products in satisfactory yields. Our recent
paper demonstrated that the easily prepared AlAr3(THF)
compounds are efficient reagents for coupling reactions with
aryl bromides or chlorides.18 One advantage of AlAr3(THF)
reagents is that an inorganic base, which is used as a promoter
in many types of coupling reactions, is not required. Another
advantage is that the economic palladium catalyst of the
commercially available and relatively cheap PCy3 ligand is
good enough to induce coupling reactions of aryl chlorides or
bulky aryl bromides under mild reaction conditions at room
temperature or e60 °C. However, a drawback of the AlAr3-
(THF) reagents is that only one aryl group out of the three
aryls is consumed in the coupling reactions. Hayashi et al. first
established that organotitanium compounds in refluxing THF
could be used as coupling reagents while employing palladium
catalysts of chiral ferrocene-based phosphine-amine ligands
to afford coupling products in good to excellent yields.19
Couplings of ArTi(OEt)3 with aryl bromides or chlorides were
later demonstrated by Knochel and co-workers using nickel
complexes of NHC’s at room temperature, giving coupling
products in moderate to excellent yields.20 ArTi(O-i-Pr)3 re-
agents were also used as aryl nucleophiles in a few examples of
asymmetric reactions reported by Seebach and a co-worker21
and by Hayashi et al.22 In comparison with AlAr3(THF)
reagents, ArTi(OR)3 compounds are aryl-economic reagents,
since the sole aryl nucleophile is consumed in the reactions.
Despite the scarce application of aryltitanium compounds, the
higher reactivity of the titanium metal center compared to the
boron or the zinc metal center makes ArTi(OR)3 compounds
attractive nucleophilic reagents in catalytic reactions.21
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Although ArTi(O-i-Pr)3 were prepared and used in cata-
lytic reactions, there has been no detailed information on the
synthesis and on the solution and solid-state structures of
aryltitanium complexes. To explore applications of aryltita-
nium compounds as practical reagents in catalysis, we herein
report the synthesis, characterizations, and structures of
ArTi(O-i-Pr)3 compounds. In solution, a binuclear [ArTi-
(O-i-Pr)2(μ-O-i-Pr)]2 species was suggested by a variable-
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1
temperature H NMR study and was further confirmed by
solid-state structural studies. Coupling reactions of [ArTi(O-
i-Pr)2(μ-O-i-Pr)]2 with a wide variety of aromatic bromides
were examined by employing the economic Pd(OAc)2/PCy3
catalytic system. Aryl-aryl coupling products were obtained
at room temperature in a short reaction time of 0.5 h, except
for products derived from bulky aryl bromides or pyridyl
bromides, for which longer reaction times of 3-7 h were
required to afford products in satisfactory yields. It is worth
noting that this study also demonstrates efficient syntheses of
3,30-diaryl-2,20-dimethoxy-1,10-binaphthylenes compared to
the typical synthesis of 3,30-diaryl-substituted BINOLs,
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€
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