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presence of ester or ketone functionalities was perfectly toler-
ated. Selective reduction of the alkene moiety occurred with
benzylidenemalonates (entries 4 and 5) or phenylbutenoates
(entries 6 and 7). Trans-chalcone was also selectively reduced
into 1,3-diphenylpropanone (entry 8). In the absence of an aryl
substituent, only traces of product were observed (entry 9).
The reduction of endocyclic alkenes was performed at 258C
(Table 3). Indenes substituted at the 3-position (entries 1–3), as
well as 4-ethyl-1,2-dihydronaphthalene (entry 4), gave rise to
the expected products in good to excellent yields. In contrast,
the use of indene and dihydronaphthalene themselves (R=H)
led to polymers.
Table 1. Catalytic transfer hydrogenation of cyclohexenylbenzene.
Entry Cat.
H Donor
Conv.
[%][a]
1
2
3
4
5
6
GaCl3
GaCl3
AlCl3
InCl3
iPrOH
0
87
0
1,4-CHD[b]
1,4-CHD
1,4-CHD
1,4-CHD
1,4-CHD
9
[IPrGaCl2][SbF6][c]
[IPrGaCl2·(2,4,6-
trifluorobenzonitrile)][SbF6]
[IPrGaCl2][AlCl4][d]
[IPrGaCl2][SbF6][c]
99
87
Owing to the ability of Ga(III) complexes to catalyze hydroar-
ylation of alkynes to give phenyl-substituted alkenes,[28]
a formal three-component coupling between 4-ethynylbiphen-
yl, anisole, and H2 was attempted (Scheme 2). Gratifyingly, the
expected product 7 was isolated in 78% yield.
7
8
1,4-CHD
87
30
2,4,6-trimethyl-
phenol
BHT[e]
[f]
9
[IPrGaCl2][SbF6][c]
[IPrGaCl2][SbF6][c]
[IPrGaCl2][SbF6][c]
[IPrGaCl2][SbF6][c]
[IPrGaCl2][SbF6][c]
–
10
11
12
13
iPrOH
0
0
Hantzsch ester[g]
Et3SiH
The dihydronaphthalene derivative
8 transformed very
<5
0
cleanly into 9 in 1 h at 808C with 91% yield (Scheme 3). Again,
the formation of 9 can be envisaged by hydroarylation of are-
nyne 10.[29] The reaction of the latter with 1,4-cyclohexadiene
eventually furnished 9 in 85% yield.
The generality of this tandem process was validated further
by using arenynes 11 a–f (Table 4). The desired bicyclic prod-
ucts were isolated in good to excellent yields even in the case
[h]
H2
[a] Conversion was determined by GLC analysis. [b] 1,4-Cyclohexadiene.
[c] Generated in situ by using IPrGaCl3 (5 mol%) and AgSbF6 (7 mol%);
see ref. [18d] for the solid-state structure of [IPrGaCl2][SbF6]. [d] Generated
in situ by using IPrGaCl3 (5 mol%) and AlCl3 (50 mol%). [e] Butylhydroxyto-
luene. [f] Complex mixture. [g] Diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridi-
nedicarboxylate. [h] 1 atm.
[25]
tageously replaced by the bench-stable IPrGaCl3
used jointly with AgSbF6 to generate [IPrGaCl2]-
[SbF6].[18a,d] With this catalyst, the conversion reached
99% (entry 5).[26] The use of silver could be avoided
by employing either the isolated cationic gallium
complex
[IPrGaCl2(2,4,6-trifluorobenzonitrile)]-
[SbF6][18a] (entry 6) or a catalytic mixture of IPrGaCl3
and AlCl3 (entry 7). With [IPrGaCl2][SbF6], a low con-
version was monitored with 2,4,6-trimethylphenol
(entry 8). The use of butylhydroxytoluene (BHT) and
iPrOH as H-donor proved inefficient (entries 9 and
10). Although oxidation of Hantzsch ester occurred
to give Hantzsch pyridine in 70% yield, no hydrogen
transfer was observed (entry 11). It is possible that
Hantzsch pyridine traps the catalyst in the form of
a stable donor–acceptor adduct,[27] or that it quench-
es an elementary step of the catalytic cycle for which
a proton is required (see mechanistic discussion
below). With Et3SiH, the product was observed in
trace amounts (entry 12). Lastly, with molecular hy-
drogen, no reaction took place (entry 13).
Scheme 2. Trimolecular hydroarylation/transfer hydrogenation tandem reaction.
Table 2. Catalytic transfer hydrogenation of alkenes.
Entry
R1
R2
R3 [b]
R4 [b]
T [8C]
t [h]
Yield [%][c]
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Me
Ph
H
H
H
H
Ph
E1
E2
H
E2
COPh
H
20
40
80
80
40
80
80
80
80
1
2
4
16
4
4
4
4
16
67
70
Ph
Ph
E1
E2
E2
H
0
72
By using 1,4-cyclohexadiene as hydrogen donor,
the reduction of other simple or activated alkenes
was then carried out (Table 2). The gem-disubstituted
alkene 1,1-diphenylethylene was fully converted
within one hour at 208C to give 1,1-diphenylethane
(entry 1). With the trisubstituted alkene (Z)-1,2-di-
phenyl-1-methylethene, the reaction took place at
408C (entry 2). On the other hand, tetraphenylethene
remained unchanged, even at 808C (entry 3). The
4-MeOC6H4
H
75[d]
50
Ph
Ph
Ph
Me
Ph
Me
H
50
56
<5
H
E2
Me
[a] Generated in situ by using IPrGaCl3 (5 mol%) and AgSbF6 (7 mol%); see ref. [18d] for
the solid-state structure of [IPrGaCl2][SbF6]. [b] E1 =CO2Et; E2 =CO2Me. [c] Yield deter-
1
mined by H NMR spectroscopic analysis by using p-anisaldehyde as internal standard.
[d] Isolated yield.
&
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Chem. Eur. J. 2014, 20, 1 – 6
2
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