Organometallics
Article
readily to furan with 66% conversion and a TON of 660 using 4
thermodynamic benefit of ethylene is counterbalanced by its
strong coordination to iridium; thus use of low pressures of
ethylene give higher turnover frequencies. In this study, we
found that the dehydrogenation of ethers using 1 atm (2−3
equiv) of ethylene resulted in high TONs, comparable to those
obtained with TBE.
(entry 1). Increasing the catalyst loading to 1% of 5 provided
near-complete conversion (entry 3).
These results compare favorably to previous reports of THF
tBu4
dehydrogenation using complexes ( PCP)-Ir, 1, or
iPr4
(
PSCOP)-Ir, 3, each of which used higher catalyst loadings
6,9
with corresponding lower TONs.
Under the standard
We began our studies by using 2,3-dihydrobenzofuran as a
model substrate under 1 atm of ethylene (Table 3). Initial
conditions of 0.2 mol % catalyst, we achieved higher conversion
with 1,4-dioxane. Both 4 and 5 provide high conversion (89%
and 80%, respectively, entries 5, 6) and selectivity for the
monodehydrogenated ether. The Ir-PCP catalyst 6 provided
slightly lower conversions (65%, entry 7). No bis-dehydro-
genated ether was observed under these reaction conditions
using 4−6. In an effort to synthesize the bis-dehydrogenated
ether, we screened a range of conditions with higher TBE
loadings (up to 10 equiv), but no evidence for a diene product
was observed. Similar results were obtained using norbornene
Table 3. Dehydrogenation of 2,3-Dihydrobenzofuran with
a
Ethylene as Hydrogen Acceptor
b
entry
loading (mol %)
time (h)
TON
yield (%)
1
2
3
4
5
6
0.2
0.2
0.2
0.5
0.5
0.5
12
24
48
12
24
48
18
24
9
12
18
80
96
98
11
as the hydrogen acceptor in place of TBE. N-Methylmorpho-
line was also successfully dehydrogenated under the standard
conditions. With 0.2 mol % catalyst loading, 4 was superior,
giving 65% conversion (entry 8) vs 55% with 5 (entry 9) and
36
160
192
196
53% with 6 (entry 11). An increased catalyst loading of 5
a
Reaction conditions: 2,3-dihydrobenzofuran (1.7 mmol), ethylene (1
resulted in higher conversion (entry 10). The dehydrogenation
of 2,3-dihydrobenzofuran proceeded with excellent conversion
with 0.2 mol % of catalysts 4−6 to give benzofuran in high yield
atm), mesitylene (200 μL), [Ir] (0.2−0.5 mol %), NaOtBu (0.4−1
mol %), 120 °C, 12−48 h in a 100 mL Kontes flask. Flask recharged
b
with ethylene after each sampling. Yield determined by GC.
(
89−99%, entries 12−14). As previously noted, isochroman
was readily dehydrogenated to benzofuran in good yields with
low catalyst loadings (entries 15−18).
experiments were performed in a sealed Kontes flask (100 mL)
with sufficient head space to provide an excess of ethylene (∼3
equiv) at 120 °C using mesitylene as a cosolvent and catalyst 4.
Our initial catalyst loading of 0.2 mol % provided promising
results with 18% yield after 48 h (entry 3). An optimal catalyst
loading of 0.5 mol % led to near-quantitative conversion of 2,3-
dihydrobenzofuran to benzofuran (entry 6).
Acyclic ethers represent a more challenging class of
substrates. The small, electron-rich alkene products are strong
ligands for Ir(I) complexes and thus readily inhibit catalysis.
While stoichiometric dehydrogenation of diethyl ether by
iridium pincer complexes has been reported and a stable vinyl
12
ether complex was isolated as product, this report appears to
be the first case of catalytic dehydrogenation of an acyclic ether.
Diethyl ether was dehydrogenated with all three catalysts,
although 5 gave the best result, providing a TON of 90 with 0.2
mol % loading (entry 20). Higher catalyst loading provided
higher conversion (47%, entry 21). Dibutyl ether was
dehydrogenated with similar efficiency to give a mixture of
alkene isomers (entries 23−25). The more sterically bulky tert-
butyl ethyl ether was dehydrogenated with a maximum TON of
With these promising results in hand, we pursued the
dehydrogenation of a series of cyclic and acyclic ethers using
ethylene as the hydrogen acceptor, again using catalysts 4−6. In
these cases reactions were done in mesitylene at 120 °C by
employing a closed system with a reflux condenser, a procedure
similar to that reported by Goldman for keeping lower boiling
13
substrates in solution. The results are summarized in Table 4.
All catalysts surveyed provided near-quantitative dehydro-
genation of 2,3-dihydrobenzofuran (entries 1−3). Similar
results were also obtained with isochroman, giving between
68% and 85% yield (entries 4−6). Tetrahydrofuran could also
be dehydrogenated using ethylene as the hydrogen acceptor.
Surprisingly, 5 performed notably better than catalysts 4 and 6,
giving 71% yield (entry 8 vs 7 and 9). 1,4-Dioxane was
successfully dehydrogenated using 5 in 97% yield (entry 11).
The catalyst loading with this substrate could be lowered to 0.2
mol % with only a modest decrease in yield (75%, entry 12). N-
Methylmorpholine was dehydrogenated in good yield using 5
as the catalyst (57%, entry 15). Finally, a yield of 44% was
obtained when using diethyl ether with 2 mol % of catalyst 5
(entry 18). The differences in conversions for these substrates
are unexpected, and work is ongoing to understand these
results.
8
7 using 5 (entry 27). Ethoxytrimethylsilane was dehydro-
iPr4
3
genated with the ethylene derivative of complex 5, PC(sp )-
P−Ir(ethylene) (0.2 mol %), giving a TON of 60 (entry 30).
Dehydrogenation with Ethylene as a Hydrogen
Acceptor. Alkane and heterocycle dehydrogenations with Ir-
pincer catalysts have utilized a sterically bulky sacrificial alkene
as the hydrogen acceptor, most commonly TBE or NBE. While
effective as a hydrogen acceptor, these specialized olefins are
costly and may require distillation to separate the acceptor and
hydrogenated acceptor from the product alkene. Ethylene is a
more desirable hydrogen acceptor because it is inexpensive and
readily available, and the saturated product, ethane, can be
easily separated from liquid phase reactions. Previously, our
group has successfully used ethylene as a hydrogen acceptor in
4b
alkane dehydrogenation for the synthesis of para-xylene and
4
c
toluene. Thus, we sought to explore ethylene as a hydrogen
acceptor in the dehydrogenation of ethers.
CONCLUSION
■
The transfer dehydrogenation reactions using ethylene as
We have shown three previously reported alkane dehydrogen-
iPr4
iPr4
3
hydrogen acceptor (heat of hydrogenation (ΔH°(H ) = −136
ation catalysts, ( Anthraphos)-Ir(H)(Cl), 4,
PC(sp )P−
2
iPr4
kJ/mol) are even more thermodynamically favored than those
Ir(H)(Cl), 5, and PCP-Ir(H)(Cl), 6, to be effective, after
activation with NaOtBu, for the dehydrogenation of acyclic
10
using TBE (ΔH°(H ) = −126 kJ/mol). However, the
2
C
Organometallics XXXX, XXX, XXX−XXX