Recently, Huang demonstrated a much broader scope of heterocycle dehydrogenations using the hybrid phosphinothious/phosphinite (
iPr4 PSCOP)Ir(H)
(Cl) pincer complex 6 activated with NaO
t
Bu [40]. A large variety of O- and
N-containing heterocycles were successfully dehydrogenated at 120
C in the presence of TBE [Eq. (13)]. For example, 2,3-dihydrobenzofuran gives benzofuran in
high yield with low catalyst loading (0.1 mol%). Higher catalyst loading (5 mol%)
was required to dehydrogenate tetrahydrofuran to furan and piperidine to pyridine.
ð13Þ
Brookhart and coworkers recently showed that catalysts (
iPr4
Anthraphos)Ir(H)
(Cl), 3b,
iPr4 PC(sp
3 )PÀIr(H)(Cl), 8a, and (
iPr4 POCOP)Ir(H)(Cl), 2b, were effective
for the dehydrogenation of cyclic and acyclic ethers using TBE as a hydrogen
acceptor at 120
C after activation with NaO
t
Bu [Eq. (14)] [87]. For example, THF
and N-methylmorpholine were converted to furan and 2,3-dehydro-Nmethylmorpholine with TONs of 660 and 325, respectively, using the pre-catalyst
3b. 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. However, it was observed that diethyl ether was dehydrogenated
with all three catalysts, though pre-catalyst 8a gave the best result providing
90 TONs with 0.2 mol% loading. The dehydrogenation of cyclic and acyclic
ether substrates using ethylene as the hydrogen acceptor was demonstrated for the
first time. Under mild conditions at 120
C, a series of ether heterocycles can be
dehydrogenated with up to 375 TONs producing ethane as the hydrogenated
product. The pre-catalyst 8a was particularly active in this protocol, producing
good yields for all the substrates surveyed. For example, the reaction of Nmethylmorpholine with ethylene catalyzed by 8a (0.5 mol%)/NaO
t
Bu (1 mol%)
selectively formed 2,3-dehydro-N-methylmorpholine in 57 % yield, lower than the
78% yield obtained with TBE [Eq. (15)].
202
D. Be ´zier and M. Brookhart
iPr4 PSCOP)Ir(H)
(Cl) pincer complex 6 activated with NaO
t
Bu [40]. A large variety of O- and
N-containing heterocycles were successfully dehydrogenated at 120
C in the presence of TBE [Eq. (13)]. For example, 2,3-dihydrobenzofuran gives benzofuran in
high yield with low catalyst loading (0.1 mol%). Higher catalyst loading (5 mol%)
was required to dehydrogenate tetrahydrofuran to furan and piperidine to pyridine.
ð13Þ
Brookhart and coworkers recently showed that catalysts (
iPr4
Anthraphos)Ir(H)
(Cl), 3b,
iPr4 PC(sp
3 )PÀIr(H)(Cl), 8a, and (
iPr4 POCOP)Ir(H)(Cl), 2b, were effective
for the dehydrogenation of cyclic and acyclic ethers using TBE as a hydrogen
acceptor at 120
C after activation with NaO
t
Bu [Eq. (14)] [87]. For example, THF
and N-methylmorpholine were converted to furan and 2,3-dehydro-Nmethylmorpholine with TONs of 660 and 325, respectively, using the pre-catalyst
3b. 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. However, it was observed that diethyl ether was dehydrogenated
with all three catalysts, though pre-catalyst 8a gave the best result providing
90 TONs with 0.2 mol% loading. The dehydrogenation of cyclic and acyclic
ether substrates using ethylene as the hydrogen acceptor was demonstrated for the
first time. Under mild conditions at 120
C, a series of ether heterocycles can be
dehydrogenated with up to 375 TONs producing ethane as the hydrogenated
product. The pre-catalyst 8a was particularly active in this protocol, producing
good yields for all the substrates surveyed. For example, the reaction of Nmethylmorpholine with ethylene catalyzed by 8a (0.5 mol%)/NaO
t
Bu (1 mol%)
selectively formed 2,3-dehydro-N-methylmorpholine in 57 % yield, lower than the
78% yield obtained with TBE [Eq. (15)].
202
D. Be ´zier and M. Brookhart