Selective catalytic transfer dehydrogenation of alkanes and heterocycles by an iridium pincer complex

Article abstract of DOI:10.1002/anie.201306559

Catalytic alkane dehydrogenation is a reaction with tremendous potential for application. We describe a highly active PSCOP-pincer iridium catalyst for transfer dehydrogenation of cyclic and linear alkanes. The dehydrogenation of linear alkanes occurs under relatively mild conditions with high regioselectivity for a-olefin formation. In addition, the catalyst system is very effective in the dehydrogenation of heterocycles to form heteroarenes and olefinic products.

Full text of DOI:10.1002/anie.201306559

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DOI: 10.1002/anie.201306559
Iridium Catalysis
Selective Catalytic Transfer Dehydrogenation of Alkanes and
Heterocycles by an Iridium Pincer Complex**
Wubing Yao, Yuxuan Zhang, Xiangqing Jia, and Zheng Huang*
Abstract: Catalytic alkane dehydrogenation is a reaction with
tremendous potential for application. We describe a highly
active PSCOP-pincer iridium catalyst for transfer dehydrogen-
ation of cyclic and linear alkanes. The dehydrogenation of
linear alkanes occurs under relatively mild conditions with
high regioselectivity for a-olefin formation. In addition, the
catalyst system is very effective in the dehydrogenation of
heterocycles to form heteroarenes and olefinic products.
Figure 1. Structures of PCP, POCOP, and PSCOP iridium complexes.
T
he selective functionalization of abundant alkanes to form
metathesis,[3a,9] alkane dehydroaromatization,[10] and the syn-
thesis of p-xylene from ethylene.[11]
value-added products is an attractive, but highly challenging,
task. Among the few reported transformations of alkanes,
catalytic alkane dehydrogenation (AD) has received signifi-
cant attention, because it converts low-value hydrocarbon
feedstocks into olefins and arenes, which are important
intermediates in organic synthesis and widely used as raw
materials in many industrial processes. Heterogeneous AD is
carried out on a large-scale in the petrochemical industry;
however, such processes typically operate at very high
temperatures (> 5008C) and afford low product selectivi-
ties.[1]
Despite tremendous progress in the field of homogenous
AD, the reported catalytic systems suffer from harsh reaction
conditions (150–2508C) and the lack of regioselectivity in the
dehydrogenation of acyclic alkanes.[3b] Furthermore, poor
functional-group compatibility of these homogenous catalysts
restricts the application of catalytic dehydrogenation in the
synthesis of fine chemicals. Herein, we report the preparation
of a PSCOP-type Ir pincer complex supported by a hybrid
phosphinothious/phosphinite pincer ligand (Figure 1). The
(iPr4PSCOP)Ir complex exhibits very high catalytic activity
for transfer dehydrogenation of cyclic alkanes.[12] It effects
dehydrogenation of linear alkanes under mild conditions and
affords excellent regioselectivity for a-olefin formation. This
new system can be applied to the selective dehydrogenation
of a variety of heterocycles to access heteroatom-containing
aromatic or olefinic compounds.
Pioneered by Crabtree and Felkin,[2] numerous homoge-
nous AD catalysts, mainly the group 9 metal complexes, have
been developed in the past three decades.[3] A major break-
through in homogenous AD was the report of bis(phosphine)
(PCP)Ir pincer catalysts for dehydrogenation of cyclic and
linear alkanes by Kaska, Jensen, and Goldman.[4] Following
that, the Brookhart group reported the isostructural bis(phos-
phinite) (POCOP)Ir pincer complexes (Figure 1).[5] The
subtle change of the linker from CH2 group to an O atom
leads to Ir catalysts that are somewhat more efficient than the
PCP analogues in transfer dehydrogenation of cyclooctane.
More recently, related (PCP)Ir complexes with metallocene
backbones,[6] (PCP)Ir and (PCP)Ru complexes containing
fluorinated phosphino substitutes,[7] and CCC pincer car-
bene–Ir complexes[8] have been developed for homogenous
AD. In addition, Goldman and Brookhart have developed
several Ir-pincer AD-based tandem reactions, such as alkane
The synthesis of (iPr4PSCOP)IrHCl complex 3 is outlined
in Scheme 1. Deprotonation of meta-mercaptophenol with
NaH, followed by diphosphorylation with chlorodiisopropyl-
Scheme 1. Synthesis of (iPr4PSCOP)IrHCl 3.
phosphine afforded the PSCOP ligand 4 in 66% yield. The
iridium complex 3 was obtained in 50% yield through
cyclometalation of the ligand with [{Ir(cod)Cl}2] (cod = 1,5-
cyclooctadiene) in toluene under reflux. The 31P{1H} NMR
spectrum of complex 3 exhibits an AB splitting pattern, which
is consistent with two nonequivalent metal-bridged P nuclei.
The characteristic hydridic (IrH) resonance appears at
[*] W. Yao, Y. Zhang, X. Jia, Prof. Dr. Z. Huang
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, 345 Lingling Road
Shanghai 200032 (China)
E-mail: huangzh@sioc.ac.cn
[**] We are very grateful to Prof. M. Brookhart for helpful suggestions
and comments. Financial support from the National Natural
Sciences Foundation of China (21272255, 21121062), the “1000
Youth Talents Plan”, and the Shanghai Rising Star Program
(13A404200) is gratefully acknowledged.
1
À37.06 ppm in the H NMR spectra, which is close to that
expected for a five-coordinate IrIII hydrido chloride species
with the hydride trans to a vacant coordination site.
Complex 3 was first tested as a precatalyst for transfer
dehydrogenation of cyclooctane (COA) with tert-butylethy-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306559.
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lene (TBE) as the hydrogen acceptor. The COA/TBE transfer
dehydrogenation is a benchmark reaction. A system contain-
ing Ir precatalyst 3 (1.3 mm), NaOtBu (1.95 mm), COA
(3.9m), and TBE (3.9m, 3000 equiv relative to Ir), was
heated at 2008C under argon in a sealed vessel. The results
are summarized in Table 1. Upon activation with NaOtBu, 3
containing 3 (1.0 mm), NaOtBu (1.5 mm), TBE (0.5m,
500 equiv relative to Ir) in an n-octane solution was heated
at 2008C under argon. The reaction gave 495 turnovers after
30 min at 2008C, and TBE was fully converted into TBA
within 60 min (500 turnovers) (Table 2, entry 1). As a compar-
ison, performing the catalysis with (tBu4PCP)IrH2 1 and
(tBu4POCOP)IrHCl 2 under identical reaction conditions
gave only 135 and 143 turnovers after 60 min, respectively
(entries 2 and 3). The initial rate of the n-octane/TBE transfer
dehydrogenation using 3 is also greater than that obtained
using complexes 1 and 2 (114 turnovers with 3 in 5 min vs. 69
with 1 and 19 turnovers with 2). Additionally, the reaction
conducted with a higher concentration of TBE (3.0m,
3000 equiv relative to 3) gave 930 after 1 h and 1211 turnovers
after 4 h (entry 4). On the basis of comparison with prece-
dents,[4j] we conclude that 3/NaOtBu is among the most active
systems for transfer dehydrogenation of linear alkanes.
The selective formation of valuable a-olefins by alkane
dehydrogenation is of fundamental interest. Goldman et al.
have shown that the (tBu4PCP)Ir complex 1 exhibits high
kinetic regioselectivity for a-olefin formation, but the termi-
nal olefin can be rapidly converted into internal olefins
through olefin isomerization.[4e] As shown in Table 2, after
5 min of thermolysis at 2008C, 1-octene constitutes 33%,
51%, and 26% of the total octenes for the run with complexes
3, 1, and 2, respectively (entries 1–3). At first glance, the
regioselectivity for a-olefin formation with 3 appears to be
significantly lower than that with 1. However, a comparison of
the product distributions after the same turnovers reveals
a similar regioselectivity between complexes 3 and 1. The
reaction with 3 afforded 33% of 1-octene after 114 turnovers
in 5 min; and the run with 1 gave 31% of 1-octene after 120
turnovers in 30 min.
Homogeneous catalysis under mild conditions is impor-
tant for broad synthetic applications of alkane dehydrogen-
ation. Given the high activity of 3 in transfer dehydrogenation
of alkanes at 2008C, we assessed its catalytic performance at
lower temperatures. To our delight, complex 3 exhibits
appreciable activity for the n-octane/TBE transfer dehydro-
genation at 1008C. Significantly, it shows exceptionally high
regioselectivity for a-olefin formation under these conditions
at low turnover numbers. As shown in Table 2, entry 5, 1-
octene is the sole dehydrogenation product within the first
hour at 1008C (14 turnovers). After heating for 8 h (35
turnovers), 1-octene still accounts for 48% of the total
octenes, and the combined percentage of 1-octene and 2-
octenes accounts for 100% of the total. The data imply that
the 3-catalyzed isomerization of 2-octenes to 3- or 4-octenes is
much slower relative to the isomerization of 1-octene to 2-
octenes.
Table 1: TONs for transfer dehydrogenation of COA and TBE catalyzed
by complexes 3 and 2 upon activation with NaOtBu.[a]
Entry
t [min]
3[b]
485
1399
2649
2905
2985
–
2[b]
3[c]
3[d]
1
2
3
4
5
6
10
30
60
240
480
900
1186
1423
1514
1794
1876
–
937
999
–
–
–
–
–
1401
3805
5728
5901
–
[a] Average of three runs. TONs were calculated based on conversion of
TBE determined by GC analysis. [b] Using a 3000:3000:1 ratio of COA/
TBE/catalyst with 3.9m TBE. [c] Using a 4840:1000:1 ratio of COA/TBE/3
with 1.3m TBE. [d] Using a 6000:6000:1 ratio of COA/TBE/3.
was highly active for the COA/TBE transfer dehydrogen-
ation. Catalysis with 3 gave 2649 turnovers after 1 h, and 2905
turnovers after 4 h. The process converted > 99% of TBE to
TBA (2985 turnovers) after 8 h. Analysis of the dehydrogen-
ation products by NMR spectroscopy revealed the formation
of 1,3-cyclooctadiene (1,3-COD) besides the major product
COE (COE/1,3-COD = 5.4:1).[13]
For comparison, we carried out catalysis in parallel with
the Brookhart complex (tBu4POCOP)IrHCl 2, which has
been reported for effective COA/TBE transfer dehydrogen-
ation.[5b] Although the initial rate with 3 was lower than
catalysis with 2 (485 turnovers with 3 in 10 min vs. 1186
turnovers with 2), the productivity of the former is signifi-
cantly higher than the latter (2985 turnovers with 3 in 8 h vs.
1876 turnovers with 2). The initial lower turnover frequencies
(TOFs) for the reaction with 3 compared to that with 2 is
likely due to catalysis inhibition by TBE at high concen-
trations, as found for the PCP-Ir system.[4a] Indeed, we found
that the concentration of the hydrogen acceptor affects the
reaction rate. A run using TBE (1.3m, 1000 equiv relative to
3) gave 937 turnovers after 10 min, compared to 485 turnovers
in the reaction with TBE (3.9m; Table 1).
To further evaluate the catalytic efficiency of 3, we
decreased the loading of the catalyst in the COA/TBE
transfer dehydrogenation. A ferrocene-based iridium pincer
catalyst has been previously reported to be most active for the
transfer dehydrogenation of COA; a TON of up to 3300 was
obtained after 8 h at 1808C with a COA/TBE/catalyst ratio of
21420:21420:1.[6,14] When a 6000:6000:1 molar ratio of COA/
TBE/3 was used, the reaction gave 5728 turnovers after 8 h
and 5901 turnovers after 15 h at 2008C (Table 1); more than
98% of TBE was converted to TBA in 15 h.
The high activity and mild reaction conditions in AD
encouraged us to examine the viability of the new Ir complex
3 in the dehydrogenation of heterocycles. Compared to well-
documented catalytic dehydrogenation reactions of alka-
nes,[3b,15] alcohols,[15] and amines,[15] examples of homogeneous
catalytic dehydrogenation of heterocycles are rare. In 1997,
Jensen and Kaska reported the complex 1-catalyzed transfer
dehydrogenation of tetrahydrofuan (THF) with TBE to give
a mixture of dihydrofurans and furan at 200 or 1508C.[4c] In
Next, we investigated the catalytic activity of 3 in the
transfer dehydrogenation of a linear alkane. A system
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Table 2: Ir-catalyzed transfer dehydrogenation of n-octane and TBE at 200 or 1008C.[a]
(1 mol%) gave 2-methylbenzo-
furan 5c in 67% yield after 24 h.
Transfer dehydrogenation of 1,4-
dioxane with 3 (1 mol%) selec-
tively formed 1,4-dioxene 5d in
84% yield after 12 h; no 1,4-dioxin
from double dehydrogenation was
observed, even in the presence of
2 equiv of TBE. Dehydrogenation
of THF with TBE (2 equiv) using
a higher catalyst loading (5 mol%)
selectively formed furan 5e in 72%
Entry
1[c,d]
Cat.
t [min]
TON
1-Octene
1-Octene
trans-2-Octene
[mm]
cis-2-Octene
[mm]
[mm]
fraction [%][b]
3
5
10
30
60
114
192
495
500
38
37
13
10
33
19
3
44
77
137
110
20
37
65
50
2
2[c,d]
3[c,d]
4[c,e]
5[f,d]
1
2
3
3
5
10
30
60
69
92
120
135
35
39
37
27
51
42
31
20
20
32
48
64
10
16
23
27
yield after 24 h. Only
a trace
amount of 2,3-dihydrofuran (5e’;
2%) was observed under these
conditions.[20] Similarly, the reaction
of 2-methyltetrahydrofuan selec-
tively yielded the fully dehydrogen-
ated product 2-methylfuran (57%;
5 f) after 12 h at 1508C. In contrast
to the good conversion observed in
5
10
30
60
19
26
84
5
5
6
7
26
19
7
9
12
31
47
4
5
14
21
143
5
10
30
60
154
539
930
43
50
47
39
28
9
5
64
169
211
220
25
85
109 reactions with O-heterocycles,
240
1211
3
107
transfer dehydrogenation of tetra-
hydrothiophene (4g) resulted in
a low conversion. Because the neu-
tral S donors are softer and more
polarizable than the neutral O do-
nors, we tentatively attribute the
low reactivity of S-heterocycles to
the high binding affinity of S atom
to the Ir center, which inhibits the
dehydrogenation process.
30
60
240
480
10
14
26
35
10
14
17
17
100
100
68
0
0
9
0
0
0
7
48
11
[a] Average of three runs; TONs were calculated based on conversion of TBE, as determined by GC
analysis. [b] The fraction of 1-octene relative to the total octenes. [c] At 2008C. [d] Ir (1.0 mm), TBE
(0.5m). [e] Ir (1.0 mm), TBE (3.0m). [f] At 1008C.
2009, Fujita and Yamaguchi reported the first example of Ir-
catalyzed dehydrogenation of tetrahydroquinolines.[16] Very
recently, Xiao et al. demonstrated that a cyclometalated IrIII
imino complex is highly efficient for acceptorless dehydro-
genation of N-heterocycles.[17] To date, however, the scope of
heterocycles in dehydrogenation reactions is mainly limited to
benzofused N-heterocycles bearing NH functional groups.[18]
Catalytic dehydrogenation of O- and S-heterocycles, and N-
heterocycles without benzofusion remains to be explored.
Our preliminary results show that the (iPr4PSCOP)Ir
complex is capable of catalyzing dehydrogenation of a variety
of O- and N-containing heterocycles (Table 3). The reactions
were carried out at 1208C, unless otherwise noted.[19] In the
presence of only 0.1 mol% of 3, isochroman (4a) was readily
dehydrogenated to form benzopyran (5a) in 91% yield. The
dehydrogenation of 2,3-dihydrobenzofuran with TBE
(1 equiv) gave benzofuran (5b) in 80% yield after 12 h.
Notably, a minimal amount of 2,2’-bibenzofuran (5b’; 2%)
was also detected in this reaction. This biaryl product was
likely formed through dehydrogenative dimerization of 5b.
When TBE (2 equiv) and 3 (5 mol%) were used, the yield of
5b’ was increased to 37% and benzofuran 5b was obtained as
the minor product (11%). As expected, the introduction of
a methyl substituent at the 2-position of 2,3-dihydrobenzo-
furan prevented dimerization. However, the dehydrogenation
of 2,3-dihydro-2-methylbenzofuran 4c is slower than the
reaction with 4b. Treatment of 4c with TBE (2 equiv) using Ir
(iPr4PSCOP)Ir-catalyzed transfer dehydrogenation of N-
heterocycles occurred efficiently, although such substrates
require a relatively high catalyst loading (1–5 mol%). Reac-
tions of indolines, tetrahydroquinoline, and tetrahydroisoqui-
noline with TBE gave indoles 5h (80%) and 5I (96%),
quinoline (5j; 76%), and isoquinoline (5k; 50%) in moderate
to high yields. Dehydrogenations of N-heterocycles without
benzofusion also occurred, albeit at relatively high temper-
atures (150–2008C). The conversion of piperidines to pyr-
idines 5l (47%) and 5m (63%) is remarkable, given the fact
that the products are good ligands and the build-up of
pyridines may result in catalyst inhibition. N-methylpyrrole
(5n) was obtained in 83% yield from N-methylpyrrolidine.[18]
The reaction of N-(1-cyclohexenyl)pyrrolidine with TBE
(4 equiv) gave N-(1-cyclohexenyl)pyrrole 5o (54%) and 1-
phenylpyrrole 5o’ (16%) after 24 h at 1508C. The results
indicate Ir-catalyzed dehydrogenation of the pyrrolidine ring
is more efficient than the dehydrogenation of the cyclo-
hexenyl ring under these reaction conditions. Finally, the
reaction of N-methylmorpholine with TBE (2 equiv) cata-
lyzed by 3 (0.5 mol%) selectively formed 2,3-dehydro-N-
methylmorpholine (5p) in 82% yield; the synthesis of 5p is
very challenging by other means.[21]
In conclusion, we have prepared a new phosphinothious/
phosphinite (iPr4PSCOP)Ir pincer complex. Upon activation
with NaOtBu, this complex exhibits exceptionally high
activity for transfer dehydrogenation of alkanes with tert-
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khan, R. Holmes-Smith, L. Yingrui, Tetrahedron Lett. 1985, 26,
1999.
Table 3: Transfer dehydrogenation of heterocycles with TBE. The product
yield was determined by H NMR spectroscopy with 1,3,5-trimethox-
1
ybenzene as an internal standard, unless otherwise noted.
[3] For reviews, see a) M. C. Haibach, S. Kundu, M. Brookhart, A. S.
Goldman, Acc. Chem. Res. 2012, 45, 947; b) J. Choi, A. H. R.
MacArthur, M. Brookhart, A. S. Goldman, Chem. Rev. 2011,
111, 1761; c) J. Choi, A. Goldman in Iridium Catalysis, Vol. 34
(Ed.: P. G. Andersson), Springer, Berlin, 2011, pp. 139 – 167;
d) F. Kakiuchi, N. Chatani, Adv. Synth. Catal. 2003, 345, 1077;
e) J. A. Labinger, J. E. Bercaw, Nature 2002, 417, 507; f) “Acti-
À
vation and Functionalization of C H Bonds”: ACS Symp. Ser.
Vol. 885 (Ed.: K. I. Goldberg, A. S. Goldman), American
Chemical Society, Washington, DC, 2004; g) R. G. Bergman,
Nature 2007, 446, 391; h) R. H. Crabtree, J. Chem. Soc. Dalton
Trans. 2001, 2437; i) C. M. Jensen, Chem. Commun. 1999, 2443.
[4] a) M. Gupta, C. Hagen, R. J. Flesher, W. C. Kaska, C. M. Jensen,
Chem. Commun. 1996, 2083; b) W.-w. Xu, G. P. Rosini, K.
Krogh-Jespersen, A. S. Goldman, M. Gupta, C. M. Jensen, W. C.
Kaska, Chem. Commun. 1997, 2273; c) M. Gupta, W. C. Kaska,
C. M. Jensen, Chem. Commun. 1997, 461; d) D. W. Lee, W. C.
Kaska, C. M. Jensen, Organometallics 1998, 17, 1; e) F. Liu, E. B.
Pak, B. Singh, C. M. Jensen, A. S. Goldman, J. Am. Chem. Soc.
1999, 121, 4086; f) M. W. Haenel, S. Oevers, K. Angermund,
W. C. Kaska, H.-J. Fan, M. B. Hall, Angew. Chem. 2001, 113,
3708; Angew. Chem. Int. Ed. 2001, 40, 3596; g) K. Krogh-
Jespersen, M. Czerw, K. Zhu, B. Singh, M. Kanzelberger, N.
Darji, P. D. Achord, K. B. Renkema, A. S. Goldman, J. Am.
Chem. Soc. 2002, 124, 10797; h) K. Zhu, P. D. Achord, X. Zhang,
K. Krogh-Jespersen, A. S. Goldman, J. Am. Chem. Soc. 2004,
126, 13044 – 13053; i) A. Ray, K. Zhu, Y. V. Kissin, A. E.
Cherian, G. W. Coates, A. S. Goldman, Chem. Commun. 2005,
3388; j) S. Kundu, Y. Choliy, G. Zhuo, R. Ahuja, T. J. Emge, R.
Warmuth, M. Brookhart, K. Krogh-Jespersen, A. S. Goldman,
Organometallics 2009, 28, 5432 – 5444; k) B. Punji, T. J. Emge,
A. S. Goldman, Organometallics 2010, 29, 2702.
[5] a) I. Gçttker-Schnetmann, P. White, M. Brookhart, J. Am. Chem.
Soc. 2004, 126, 1804; b) I. Gçttker-Schnetmann, M. Brookhart, J.
Am. Chem. Soc. 2004, 126, 9330; c) I. Gçttker-Schnetmann, P. S.
White, M. Brookhart, Organometallics 2004, 23, 1766.
[6] S. A. Kuklin, A. M. Sheloumov, F. M. Dolgushin, M. G. Ezernit-
skaya, A. S. Peregudov, P. V. Petrovskii, A. A. Koridze, Organo-
metallics 2006, 25, 5466.
[7] a) J. J. Adams, N. Arulsamy, D. M. Roddick, Organometallics
2012, 31, 1439; b) B. C. Gruver, J. J. Adams, S. J. Warner, N.
Arulsamy, D. M. Roddick, Organometallics 2011, 30, 5133.
[8] a) A. R. Chianese, A. Mo, N. L. Lampland, R. L. Swartz, P. T.
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[9] a) A. S. Goldman, A. H. Roy, Z. Huang, R. Ahuja, W. Schinski,
M. Brookhart, Science 2006, 312, 257; b) Z. Huang, E. Rolfe,
E. C. Carson, M. Brookhart, A. S. Goldman, S. H. El-Khalafy,
A. H. R. MacArthur, Adv. Synth. Catal. 2010, 352, 125.
[10] R. Ahuja, B. Punji, M. Findlater, C. Supplee, W. Schinski, M.
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[a] With 3 (0.1 mol%) and TBE (1 equiv). [b] With 3 (5 mol%) and TBE
(2 equiv). [c] With 3 (1 mol%) and TBE (2 equiv). [d] With 3 (5 mol%)
and TBE (4 equiv). [e] With 3 (0.5 mol%) and TBE (2 equiv). [f] The yield
of product was determined by GC analysis with mesitylene as an internal
standard.
butylethylene as the hydrogen acceptor. In addition, the
(iPr4PSCOP)Ir complex gives high kinetic selectivity for
dehydrogenation of linear alkanes to a-olefins under rela-
tively mild reactions conditions. The new catalytic system can
be applied to the selective dehydrogenation of a wide variety
of heterocycles to produce heteroarenes and olefinic products
by transfer dehydrogenation.
[11] T. W. Lyons, D. Guironnet, M. Findlater, M. Brookhart, J. Am.
Chem. Soc. 2012, 134, 15708.
Received: July 27, 2013
Revised: October 7, 2013
Published online: January 2, 2014
[12] A (PSCOP)Ir complex containing tBu phosphino substituents
has also been prepared. Initial experiments on this system were
carried out at the University of North Carolina and the Shanghai
Inisitute of Organic Chemistry, and the complex showed poor
dehydrogenation activity.
Keywords: alkanes · dehydrogenation · heterocycles ·
.
homogeneous catalysis · iridium
[13] Determined by 13C NMR spectroscopy, the sum of COE and
COD double bonds equals TON of TBE within 10% difference.
Trance amount of ethylbenzene (< 2%) and o-xylene (< 2%)
were detected by GC/MS.
[1] K. Weissermel, H.-J. Arpel, Industrial Organic Chemistry, Wiley-
VCH, Weinheim, 2003, pp. 59 – 89.
[14] During the preparation of this manuscript, Yamamoto et al.
reported Ir pincer complexes containing a “7-6-7” fused-ring
backbone, which gave a maxium TON of 4100 after 24 h at
[2] a) M. J. Burk, R. H. Crabtree, J. Am. Chem. Soc. 1987, 109,
8025 – 8032; b) M. J. Burk, R. H. Crabtree, C. P. Parnell, R. J.
Uriarte, Organometallics 1984, 3, 816; c) H. Felkin, T. Fillebeen-
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2008C with a COA/TBE/catalyst ratio of 10000:6750:1; see: Y.
Shi, T. Suguri, C. Dohi, H. Yamada, S. Kojima, Y. Yamamoto,
Chem. Eur. J. 2013, 19, 10672.
[18] Goldman et al. have reported that complex 1 catalyzes the
dehydrogenation of pyrrolidines to pyrroles; see: X. Zhang, A.
Fried, S. Knapp, A. S. Goldman, Chem. Commun. 2003, 2060.
[19] Two equivalents of NaOtBu relative to 3 were used in the
dehydrogenation reactions of heterocycles.
[20] In contrast, 1-catalyzed transfer dehydrogenation of THF with
TBE in a THF solution formed 2,3-dihydrofuran and furan as the
major products, and 3,4-dihydrofuran as the minor product. See
Ref. [4c].
[15] For reviews, see: a) G. E. Dobereiner, R. H. Crabtree, Chem.
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