Organometallics
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energy and its common use in related experiments,3 prompted
investigation of other alkenes for the coupling reaction.
respect to cyclohexene) and o-dichlorobenzene. In each case, the
conjugate acid of the pyridine was observed by 1H NMR
spectroscopy as resonances at 10.7-10.9 ppm for the N-H
proton.17 In the presence of 1 equiv of added base only 29% of
the product 2d was produced, and with 2 equiv of base, only 2%
of 2d was generated. These results establish that under the
coupling conditions, a protic acid was available in solution, and
this acid was essential to product formation.
Manipulation of the experimental conditions in the presence
of the base additive demonstrated that the combination of
cyclohexene and the platinum precursor was the source of the
protic acid. Pyridinium ion was not observed under the reaction
conditions in the absence of cyclohexene, even at extended
reaction times (18 h). When the reaction was run using
perdeuterated benzene in place of the mesitylene/o-dichloro-
benzene solvent mixture, with 10 mol % of 1a and 1 equiv of
cyclohexene, the protonated base was observed after 18 h at
100 °C. These results strongly suggest that the source of the
protons in the reaction mixture is cyclohexene. Pt-mediated
coupling of cyclohexene could generate 1 equiv of HOTf, as
shown by Szuromi and Sharp (eq 7).10
Cyclic alkenes are appealing substrates, since isomerization of these
alkenes does not affect the efficiency or selectivity of product forma-
tion. When cyclohexene was heated to 100 °C for 24 h in the presence
of 9 equiv of benzene and 10 mol % 1a, cyclohexylbenzene (2b) was
produced in moderate yield (36%), but dicyclohexylbenzene (2c)
was also generated in 21% yield as a mixture of isomers (eq 4).
The formation of 2c suggested that more electron-rich arenes
such as 2b might be more susceptible to the coupling reaction
than benzene. Indeed, the electron-poor arene o-dichloroben-
zene did not undergo observable hydroarylation, and the relative
reactivity of four different arenes was found to increase with their
nucleophilicity: o-dichlorobenzene <chlorobenzene<benzene<
mesitylene.
The catalytic coupling reaction of cyclohexene and mesitylene
with 10 mol % 1a gave a quantitative yield of 1-cyclohexyl-2,4,6-
trimethylbenzene (2d) after 4 h at 100 °C, and no multiply
coupled products were observed (eq 5a). This result provided a
suitable subject for mechanistic investigations; therefore, experi-
ments were undertaken to determine whether the reaction
mechanism for this process included C-H activation2,5 or
Friedel-Crafts pathways.11-14
The observation of olefin dimers as side products during the
catalytic runs provided additional evidence for the generation of
HOTf via alkene coupling. For example, when a solution of
norbornene, 9 equiv of benzene, and 10 mol % 1a in o-
dichlorobenzene was heated for 5 h (eq 2), a side product was
observed by GC-MS (Mþ at m/z 188) with a mass equivalent to
C14H20, a norbornene dimer. Such a dimer is consistent with Pt-
mediated production of HOTf.10
To eliminate the possibility that coupling of the COD ligand at
Pt is the major source of protons, the related catalyst 1b was
studied. Complex 1b exhibited hydroarylation reactivity nearly
identical with that of complex 1a, and the reaction was also
inhibited by the addition of base.
To show that the catalyst was a soluble species, rather than
heterogeneous Pt(0), the optimized reaction shown in eq 5a was
run in the presence of Hg(0) with vigorous stirring. The catalysis
was not inhibited.15,16
As a final test for a mechanism that requires proton release by
cyclohexene coupling, 1,2-diphenylacetylene (4) was used as a
hydroarylation substrate.
Substrate 4 does not possess vinylic protons and therefore is
unable to produce HOTf by the proposed coupling mechanism.
Treatment of substrate 4 with 10 mol % of 1b and 6 equiv of
mesitylene in o-dichlorobenzene for 4 h at 100 °C gave
hydroarylation products in only 14% yield (Table 1, entry 1).
The presence of a small amount of hydroarylation products can
be explained by the Pt species acting as a Lewis acid. For
comparison, the simple Lewis acid AgOTf, at 20 mol % catalyst
loading, gave 37% yield of product 2d under the same conditions.
In contrast to the low-yielding Pt-catalyzed reaction, hydroaryla-
tion products were observed in quantitative yield after 4 h at
100 °C (Table 1, entry 2) when HOTf was used in place of 1b as
the catalyst. In order to elucidate the role of cyclohexene, the
hydroarylation of 4 with 1b as catalyst was carried out in the
presence of 20 mol % cyclohexene as a cocatalyst, and the yield
Experiments were carried out to determine the role of protons
in the observed catalysis. The hydroarylation reaction shown in
eq 5 was run with 20 mol % triflic acid in place of 1a as the catalyst
(eq 5b). Triflic acid was observed to be a good catalyst for the
coupling reaction: a quantitative yield of 2d was obtained in
less than 1 h at 100 °C. This result confirmed that a Brønsted
acid can catalyze the effective hydroarylation of cyclohexene by
mesitylene.
To determine whether such a triflic acid catalyzed mecha-
nism was responsible for the coupling observed with 1a as a catalyst
precursor, the catalytic reaction was run in the presence of the
noncoordinating base 2,6-di-tert-butyl-4-methylpyridine (eq 6).
Base (1 or 2 equiv with respect to 1a) was added to a solution
of cyclohexene and 10 mol % 1a in mesitylene (9 equiv with
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dx.doi.org/10.1021/om2000458 |Organometallics 2011, 30, 1295–1298