Ligand survey results in identification of PNP pincer complexes of iridium as long-lived and chemoselective catalysts for dehydrogenative borylation of terminal alkynes

Article abstract of DOI:10.1039/c5sc02161h

Following the report on the successful use of SiNN pincer complexes of iridium as catalysts for dehydrogenative borylation of terminal alkynes (DHBTA) to alkynylboronates, this work examined a wide variety of related pincer ligands in the supporting role in DHBTA. The ligand selection included both new and previously reported ligands and was developed to explore systematic changes to the SiNN framework (the 8-(2-diisopropylsilylphenyl)aminoquinoline). Surprisingly, only the diarylamido/bis(phosphine) PNP system showed any DHBTA reactivity. The specific PNP ligand (bearing two diisopropylphosphino side donors) used in the screen showed DHBTA activity inferior to SiNN. However, taking advantage of the ligand optimization opportunities presented by the PNP system via the changes in the substitution at phosphorus led to the discovery of a catalyst whose activity, longevity, and scope far exceeded that of the original SiNN archetype. Several Ir complexes were prepared in a model PNP system and evaluated as potential intermediates in the catalytic cycle. Among them, the (PNP)Ir diboryl complex and the borylvinylidene complex were shown to be less competent in catalysis and thus likely not part of the catalytic cycle.

Full text of DOI:10.1039/c5sc02161h

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Ligand survey results in identification of PNP pincer
complexes of iridium as long-lived and
chemoselective catalysts for dehydrogenative
borylation of terminal alkynes†
Cite this: DOI: 10.1039/c5sc02161h
Chun-I Lee,a Jessica C. DeMott,a Christopher J. Pell,a Alyson Christopher,b Jia Zhou,c
a
a
*
Nattamai Bhuvanesh and Oleg V. Ozerov
Following the report on the successful use of SiNN pincer complexes of iridium as catalysts for
dehydrogenative borylation of terminal alkynes (DHBTA) to alkynylboronates, this work examined a wide
variety of related pincer ligands in the supporting role in DHBTA. The ligand selection included both new
and previously reported ligands and was developed to explore systematic changes to the SiNN
framework (the 8-(2-diisopropylsilylphenyl)aminoquinoline). Surprisingly, only the diarylamido/
bis(phosphine) PNP system showed any DHBTA reactivity. The specific PNP ligand (bearing two
diisopropylphosphino side donors) used in the screen showed DHBTA activity inferior to SiNN. However,
taking advantage of the ligand optimization opportunities presented by the PNP system via the changes
in the substitution at phosphorus led to the discovery of a catalyst whose activity, longevity, and scope
far exceeded that of the original SiNN archetype. Several Ir complexes were prepared in a model PNP
system and evaluated as potential intermediates in the catalytic cycle. Among them, the (PNP)Ir diboryl
complex and the borylvinylidene complex were shown to be less competent in catalysis and thus likely
not part of the catalytic cycle.
Received 15th June 2015
Accepted 3rd August 2015
DOI: 10.1039/c5sc02161h
www.rsc.org/chemicalscience
The development of C–H borylation methods did not include
the C(sp)–H bonds of terminal alkynes. The products of dehy-
Introduction
drogenative borylation of terminal alkynes (DHBTA), alkynyl-
boronates, are versatile building blocks in synthetic chemistry.
Their synthetic value derives not just from the direct use in
C–Calkynyl coupling,23 but more so from pursuing the reactions of
the triple bond. Cyclotrimerization,24 [3 + 2] cycloaddition,25
cyclopentenone synthesis,26 hydrozirconation,27 enyne metath-
esis,28 and others29–32 have been reported; these reactions yield
more complex molecules that contain C–B bonds in positions
that would be difficult to borylate by alternative means.
The classical synthesis of alkynylboronates was developed by
Brown et al.: deprotonation of alkyne by n-BuLi, followed by
reaction with a boric ester and quench with anhydrous acid.33
An Ag-catalyzed variation was reported in 2014 by Hu et al.34
Ingleson et al. also recently demonstrated that certain
Selective conversion of C–H bonds into C–B bonds (Fig. 1) has
attracted broad attention over the last two decades.1,2 The
resulting organoboron compounds are relatively stable, non-
toxic, and can be easily transformed into C–O or C–C bonds via
oxidation3–6 or Suzuki–Miyaura coupling.7–9 Many examples of
alkane,10–12 arene,13,14 and alkene15,16 dehydrogenative boryl-
ation, as well as benzylic17 and allylic18 C–H borylation have
been reported. The dehydrogenative arene borylation has been
proven especially fruitful with very impressive advances by
Hartwig et al.19,20 and Smith and Maleczka et al.21,22 already
nding applications.2
aDepartment of Chemistry, Texas A&M University, College Station, TX 77842, USA.
E-mail: ozerov@chem.tamu.edu
bDepartment of Chemistry, Brandeis University, MS 015, 415 South Street, Waltham,
MA 02454, USA
cDepartment of Chemistry, Harbin Institute of Technology, Harbin 150001, China
† Electronic supplementary information (ESI) available: Supporting information
in the form of descriptions of experiments, characterization data,
crystallographic information in the form of CIF les, and a video recording of
hydrogen evolution from the mixture of 3-methyl-3-trimethylsiloxy-1-butyne,
17-Ir-COE and HBpin in toluene. CCDC 1014977–1014979 and 1015119. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5sc02161h
Fig. 1 C–H borylation (dehydrogenative borylation).
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Fig. 2 Synthesis of SiNN Ir complexes (top) and DHBTA catalyzed by
SiNN Ir complexes (bottom).
borenium cations can react with terminal alkynes to give alky-
nylboronates.35 Just as with borylation of sp2 and sp3 C–H
bonds, catalysis of direct coupling of a C–H bond with a B–H
bond (Fig. 1 and R ¼ alkynyl for DHBTA) carries signicant
advantages (if the catalysis is efficient enough): better atom
economy, as well as milder conditions allowing greater func-
tional group compatibility. In comparison to C(sp2)–H and
C(sp3)–H bonds, for the relatively acidic C(sp)–H bonds (pKa ꢀ
25) of terminal alkynes, the C–H activation itself is generally not
a difficult task and C–H bond selectivity would not typically be
an issue. On the other hand, in contrast to the non-olenic
C(sp2)–H and C(sp3)–H substrates, a combination of a triple
C^C bond, a B–H bond and a metal catalyst is very likely to lead
to hydroboration.36,37 In addition, hydrogenation38 of the alkyne
substrate or product with H2 (the by-product of DHBTA) may
also be a concern.
In 2013, we reported the rst example of catalytic DHBTA
performed by Ir complexes of a SiNN pincer39 ligand (1-Ir-COE
and 1-Ir-Bpin2, Fig. 2).40 The reaction was strictly chemo-
selective and could be performed under very mild conditions
(ambient temperature, ca. 10 turnovers per min) with a variety
of alkyl-, aryl- and silyl- terminal alkynes in high yield. However,
the catalyst longevity was limited to ca. 100 turnovers. Very
recently, Tsuchimoto et al. described DHBTA catalysis by
Scheme 1 Synthesis of ligands used in screening for DHBTA.
Zn(OTf)2/pyridine using 1,8-nathpthalenediamidoborane.41 The
Tsuchimoto process displayed a wide scope similar to (SiNN)Ir,
but operated much slower (ca. 1 turnover per hour) even at
100 ꢁC. Our group also reported that (POCOP)Pd complexes are
modest DHBTA catalysts for some substrates.42
The discovery of the prowess of the SiNN ligand in DHBTA
was rather serendipitous, and we sought to explore the associ-
ated ligand space in a more systematic fashion. Here we report
the exploration of a series of Ir complexes of related ligands as
potential catalysts in DHBTA that has led to the discovery of a
new highly active, much more long-lived catalyst with a broader
scope, as well as to the insight into the role of possible inter-
mediates in DHBTA.
Results and discussion
Synthesis and screening of ligands for DHBTA
In light of the success of 1-Ir-COE in DHBTA, we decided to
examine a series of ligands that systematically explored varia-
tions of the SiNN ligand features (Fig. 3). From 2-H to 7-H, we
preserved the central amido donor and the quinoline fragment
of SiNN but removed the silane side arm (2-H) or replaced it
with hemilabile donors (3-H to 7-H). For 8-H and 9-H, the silane
segment and the central amido donor were maintained while
Fig. 3 Ligands selected for DHBTA screening.
Scheme 2 DHBTA catalyzed by 1-Ir-COE generated in situ from 1-Na.
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Scheme 3 Synthesis of 13-Ir-C2H4 and 14-Ir-COE.
Chart 2 DHBTA catalyzed by 10-Ir-H2.
the quinoline moiety was eliminated (8-H) or substituted with a
phosphine donor (9-H). We also included the PNP ligand (10-H)
and the PCP/POCOP ligands (11-H to 14-H) because these are
commonly used pincer ligands with a rich history of C–H acti-
vation chemistry with Ir.43–46
The syntheses of ligands used in the screening of DHBTA are
shown in Scheme 1. Quinoline derivatives (i.e. 2-H,47 3-H, 4-H, 5-
H, 6-H,48 7-H48) were readily synthesized via Buchwald–Hartwig
coupling of 8-bromoquinoline with various anilines or 8-ami-
noquinoline with various bromoarenes. 8-H was prepared via
the intermediate S1. The synthesis of S1 relied on the same
selective dilithiation of bis(2-bromo-4-methylphenyl)amine we
previously used in the synthesis of S2,49,50 followed by quench-
ing with water. Pure S1 was isolated in 86% yield by column
chromatography. Treatment of S1 with n-BuLi, followed by
i
addition of Pr2SiHCl and workup gave 8-H in 73% yield. The
new SiNP ligand 9-H was prepared from S2 through a similar
protocol.
In our original DHBTA report,40 we demonstrated that
generation of the 1-Ir-COE precatalyst in situ from 1-Na and
[(COE)2IrCl]2 (Scheme 2) produced results equivalent to those
obtained using isolated 1-Ir-COE. Therefore, we used a similar
synthetic approach here for testing catalysis using the series of
ligands with central amido donors (2-H to 10-H). They were
deprotonated with 1 equiv. of NaN(SiMe3)2 in situ, allowed to
react with 0.5 equiv. of [(COE)2IrCl]2 in C6D6 and the resultant
solutions were tested for catalytic DHBTA activity. In the case of
PCP/POCOP ligands 11–14, we isolated dihydride complexes
51
(11-Ir-H2 and 12-Ir-H252) or alkene complexes (13-Ir-C2H4 and
14-Ir-COE) to be used in DHBTA testing.
The precatalyst 13-Ir-C2H4 was obtained aer modication of
previously reported procedures for related compounds (Scheme
3, top).53–55 Thermolysis of 13-H with [(COE)2IrCl]2 at 80 ꢁC
overnight in toluene resulted in a dark red solution that con-
tained ca. 85% of the desired product (31P NMR evidence).
Column chromatography allowed for the collection of 99% pure
13-Ir-HCl in 22% yield. This portion of 13-Ir-HCl was then
treated with a slight excess of NaOtBu in toluene, degassed, and
then stirred under an atmosphere of ethylene for 30 min. Aer
ltration and removal of volatiles under vacuum, analytically
pure 13-Ir-C2H4 was obtained as a dark brown solid in 66%
isolated yield (based on 13-Ir-HCl). 14-Ir-COE was synthesized
by reacting the previously reported 14-Ir-HCl56,57 with a slight
excess of NaOtBu and COE in C6D6 (Scheme 3, bottom).
4-Ethynyltoluene (A1-H) was selected as the alkyne for
testing. We mimicked the conditions that were successful for
the SiNN ligand 1, with 1 mol% Ir loading and 2 equiv. of HBpin
Chart 1 Ligand screening in DHBTA. For 2-H to 10-H: in the following
order, the ligand (0.0010 mmol), NaN(TMS)2 (0.0010 mmol),
[(COE)2IrCl]2 (0.00050 mmol) and HBpin (0.20 mmol) were mixed in
C6D6 in a J. Young tube. 4-Ethynyltoluene (0.10 mmol) was then
added in 4 portions with 1 min intervals and the mixture was allowed to
stand at ambient temperature for 10 min (see ESI† for details). For 11-H
to 14-H: the iridium complex (0.0010 mmol) and HBpin (0.20 mmol)
were mixed in C6D6 in a J. Young tube. 4-ethynyltoluene (0.10 mmol)
was then added in 4 portions with 1 min intervals and the mixture was
allowed to stand at ambient temperature for 10 min (see ESI† for
details). The numbers for “% con” refer to the conversion of A1-H.
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1
Fig. 4 Partial H NMR spectra of DHBTA reaction mixtures catalyzed
by (a) 1 mol% 15-Ir-COE (entry 3 in Table 1) and (b) 1 mol% 1-Ir-COE
(entry 1 in Table 1).
Scheme 4 Synthesis of (PNP)Ir(COE) complexes.
systems ([(COD)Ir(OMe)]2 + 4,40-di-tert-butyl-bipyridine)58 but no
catalysis of any kind was observed aer 1 h at RT.59 It is difficult
to rationalize the results of the ligand screen other than to
cautiously note that a central amido donor may be crucial and
that selectivity for DHBTA is quite sensitive to the balance of
steric and electronic factors.
used at ambient temperature in C6D6 solvent. The results are
summarized in Chart 1. Surprisingly, of all the ligands tested,
only 10-H showed any DHBTA reactivity. In all other cases, no
evidence for the DHBTA product A1-Bpin was visible by 1H NMR
spectroscopy aer 1 h. In general, sluggish and nonselective
hydrogenation and hydroboration was observed for 2-H to 9-H,
and for the iso-propyl PCP/POCOP iridium complexes (13-Ir-
C2H4 and 14-Ir-COE). For tert-butyl PCP/POCOP iridium
complexes (11-Ir-H2 and 12-Ir-H2), a mixture of trans-alke-
nylboronate (A1-1) and cis-alkenylboronate (A1-2) were observed
as major products. The use of 10-H resulted in 76% A1-Bpin
aer 10 min and 90% (NMR evidence) aer 1 h, with about 3%
of 4-ethyltoluene (A1-3, from apparent hydrogenation of A1-H).
We also tested one of the most active arene borylation catalyst
59
With this lead in hand, we tested isolated 10-Ir-H2 as a
catalyst in reactions with 4-ethynyltoluene (A1-H), trimethylsi-
lylacetylene (A2-H), and 1-hexyne (A3-H) (Chart 2). The effec-
tiveness of 10-Ir-H2 in the DHBTA of 4-ethynyltoluene (A1-H)
was similar to the catalyst generated from 10-H in situ. DHBTA
of trimethylsilylacetylene (A2-H) was nished in 1 h and gave an
excellent yield of A2-Bpin. The catalytic activity of 10-Ir-H2
towards A3-H was signicantly lower than towards A1-H and A2-
H and only 50% yield was achieved aer 3 h. A small amount of
the hydrogenation product A1-3 was observed in DHBTA of A1-
H, but no hydrogenation products were detected in the reac-
tions of A2-H and A3-H.
Table 1 Catalytic results for DHBTA using various PNP Ir complexes
and 1-Ir-COEa
Testing of (PNP)Ir complexes with various phosphine
substituents
The effectiveness of 10-Ir-H2 fell somewhat short of the SiNN-
based catalysis, where >95% yield of A1/2/3-Bpin was obtained
in <10 min and without any hydrogenation side products.
Nonetheless, we were encouraged by the results because the
PNP framework offers facile opportunities for optimization of
the ligand via substituent variation. We selected previously
reported PNP ligands 15-H,50 16-H,60 and 17-H50,61 for further
testing in DHBTA. The syntheses of the corresponding Ir-COE
complexes are depicted in Scheme 4. 15-H is an oil that is
difficult to purify; however, the Li derivative (15-Li) could be
isolated in in 56% yield as a pure solid. 15-Li was then reacted
with 0.5 equiv. of [(COE)2IrCl]2 to yield 15-Ir-COE. 16-Ir-COE62
and 17-Ir-COE were synthesized via one-pot reactions by
deprotonation of the neutral ligands in situ and treatment with
[(COE)2IrCl]2.
#
Catalyst
[Ir] mol% Time
Con (%) Yield (%) A1-3 (%)
1
2
3
4
5
6
7
8
9
1-Ir-COE
10-Ir-H2
1
1
1
1
10 min 100
1 h 100
10 min 100
10 min 27
10 min 100
99
0
5
2
95b
97
15-Ir-COE
16-Ir-COE
17-Ir-COE
1-Ir-COE
10-Ir-H2
21
2
1
97
2
0
6
5
0.25
0.25
1 h
4 h
2 h
44
100
100
43c
90d
82e
92
15-Ir-COE 0.25
17-Ir-COE 0.25
10 min 100
2 h
2
10 17-Ir-COE 0.05
11 17-Ir-COE 0.025
12 17-Ir-COE 0.025
13 17-Ir-COE 0.01
100
100
100
77
85
7
9
10
5
8 h
85f
84
1 hg
2 hg
65
The newly synthesized and isolated (PNP)Ir(COE) complexes
(15-Ir-COE, 16-Ir-COE, 17-Ir-COE), 10-Ir-H2, and the previously
a
The iridium complex and HBpin (0.20 mmol) were mixed in C6D6 in a
J. Young tube. A1-H (0.10 mmol) was then added in 4 portions with 1
min intervals and the mixture was allowed to stand at ambient reported 1-Ir-COE were all tested in DHBTA by using A1-H as the
b
c
temperature (see ESI for details). 10 min: 81% yield. 10 min: 37%
substrate with 2 equiv. of HBpin at ambient temperature in
C6D6 solvent. The results are summarized in Table 1. At 1 mol%
catalyst loading, 15-Ir-COE, 17-Ir-COE, 10-Ir-H2, as well as 1-Ir-
d
e
f
yield. 10 min: 34% yield. 10 min: 45% yield. 10 min: 13% yield.
g
Run at 60 ꢁC.
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Scheme 5 The synthesis of 10-Ir-HBpin and its equilibrium with 10-Ir-
H3Bpin (top). The synthesis of 10-Ir-Bpin2 (bottom).
was observed and accompanied with 10% hydrogenation
product A1-3; similar results were obtained with A6-H and A7-H.
96–99% NMR yields were obtained for A2-, A4-, A5, and A9-Bpin
with 0.025 to 0.1 mol% loading of 17-Ir-COE as the catalyst.
Borylation of A2-H and A6-H was also performed with reduced
Chart 3 DHBTA of representative terminal alkynes catalyzed by 17-Ir-
COE. 17-Ir-COE and HBpin (0.20 mmol) were mixed in C6D6 in a J.
a
Young tube. Alkyne (0.10 mmol) was then added in 4 portions with 1 amount of HBpin (1.1 eq.) and comparable yields/side product
min intervals at RT and the mixture was heated at 60 ꢁC (see ESI† for
details). NMR yield. Yields in parentheses are isolated yields in
preparative-scale (10 mmol alkyne) reactions that used toluene or
fluorobenzene as solvent instead of C6D6.
(A6-1 for A6-H) were obtained. No DHBTA products were
b
c
observed for phenyl propargyl sulde, 3-ethynylpyridine, 4-
cyano-1-butyne, 3,3-diethoxy-1-propyne, and methyl propiolate
with 0.1 mol% 17-Ir-COE. A1-Bpin, A5-Bpin and A8-Bpin could
be easily puried by recrystallization and were isolated in good
yields in preparative-scale reactions. In contrast, 1-Ir-COE
requires 1% loading for high yields of A1-, A2-, A4-, and A5-Bpin,
and is altogether ineffective for the synthesis of propargyl
derivatives A6- and A8-Bpin (<10% yield at 1% catalyst loading).
A mercury drop test63 was performed with 0.025 mol% 17-Ir-
COE loading and A1-H as substrate. No signicant yield
changes were observed for either A1-Bpin or the major side-
product A1-3 which suggested that the catalysis is
homogeneous.
COE gave excellent yields of A1-Bpin at ambient temperature,
whereas 16-Ir-COE did not (entry 4) and was eliminated from
further consideration. At 0.25% Ir, 17-Ir-COE showed superior
reactivity to 1-Ir-COE, 10-Ir-H2, and 15-Ir-COE by producing 92%
A1-Bpin in 10 min. 1-Ir-COE gave 43% yield aer 1 h (entry 6)
and the yield did not increase with longer reaction times, sug-
gesting faster catalyst decomposition for the SiNN-based cata-
lyst. 17-Ir-COE was able to effect 100% conversion of A-1H and
85% yield of A1-Bpin (NMR evidence) even at 0.025% loading in
only hours at ambient temperature. The reaction rate was
higher at 60 ꢁC (entry 12) without loss in yield. The 84% yield of
A1-Bpin at 0.025 mol% catalyst loading (entry 12) corresponds,
impressively, to 3400 turnovers. Under incomplete conversion
with 0.01 mol% loading (entry 13), a turnover number of 6500
was achieved aer 2 h at 60 ꢁC. In terms of chemoselectivity, 1-
Ir-COE is superior in DHBTA of A1-H as it gave A1-Bpin as the
product exclusively; 2–10% of hydrogenation product A1-3 was
observed in all reactions catalyzed by the (PNP)Ir complexes
(Fig. 4). Because 15 and 17 gave faster catalysis than 10, it is
possible that a less sterically encumbered ligand is advanta-
geous. The lower reactivity of 16 may in turn reect sensitivity to
the electronic factors.
Synthesis of plausible DHBTA intermediates
In order to gain new insight into the reaction mechanism, we
set out to examine conceivable intermediates in DHBTA.
Because of its NMR-friendly C2v-symmetric structure, and
because a number of its iridium complexes are already
known,44–46 we opted for ligand 10 for this study. We were
particularly interested in determining the possible products
To further explore the catalytic reactivity of 17-Ir-COE, A1-H,
A2-H, 5-chloro-1-pentyne (A4-H), as well as 3-methyl-3-trime-
thylsiloxy-1-butyne (A5-H), trimethylsilyl propargyl ether (A6-H),
4-dimethylamino-phenylacetylene (A7-H), N-tosylated allyl
propargyl amine (A8-H), and dimethyl 2-allyl-2-(prop-2-yn-1-yl)
malonate (A9-H) were chosen as representative substrates for
aromatic, silyl, aliphatic terminal alkynes, propargyl derivatives
and 1,6-enynes, respectively (Chart 3). For A1-H, 84% NMR yield
Fig. 5 The upfield region of 1H and 1H{11B} NMR spectrum (400 MHz,
C6D6) of 10-Ir-HBpin (left) and 10-Ir-H3Bpin (right).
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the basis of the 1H{11B} and VT 1H NMR spectroscopic data, 10-
Ir-H3Bpin is best described as an exo-s-borane dihydride
complex, similarly to 12-Ir-H3Bpin in the study by Goldberg and
Heinekey.64
Reactions of 10-Ir-H2 or 10-Ir-HMes with one equivalent or
excess of A1-H under various conditions all led to mixtures of
unidentied products that have resisted our attempts at isola-
tion and separation. The phenomenon might be related to the
fact that Rh analog 10-Rh-H2 has been shown to be an alkyne
dimerization catalyst65 and PCP/POCOP iridium complexes
reacted with alkynes to form a variety of allene or enyne
complexes.66,67
For the 1 : 1 combination of 10-Ir with A1-Bpin, we used DFT
calculations (M06/SDD/6-311G(d,p) level of theory, see details in
ESI†) to evaluate the relative thermodynamic stability of the
three conceivable isomeric structures: the alkynylboronate p-
complex 10-Ir-p-tol; the vinylidene complex 10-Ir-v-tol; and the
alkynyl boryl complex 10-Ir-ynlBpin-tol (Fig. 7). 10-Ir-v-tol was
calculated to be the lowest energy isomer, with 10-Ir-p-tol and
10-Ir-ynlBpin-tol lying 3.4 and 7.7 kcal molꢂ1 higher in energy,
respectively.
Mixing 10-Ir-HMes with one equivalent of A1-Bpin at
ambient temperature overnight led to two products which
appeared at 43.9 (5%) and 25.6 (11%) ppm respectively in the
31P{1H} NMR spectrum (Scheme 6). Further heating the mixture
1
Fig. 6 Partial (upfield region) H NMR spectrum (500 MHz, toluene-
d8) of 10-Ir-H3Bpin as a function of temperature. Small amount of
unidentified impurity (marked with asterisks) was shown near ꢂ9.2
ppm.
arising from combining the 10-Ir fragment with HBpin,
terminal alkynes, and alkynylboronates and their catalytic
competence.
In our report on the DHBTA activity of SiNN-based catalysts,
we showed that the iridium diboryl complex 1-Ir-Bpin2 can be
synthesized by reacting 1-Ir-COE with 5 equivalents of HBpin,40
and that isolated 1-Ir-Bpin2 exhibited the same catalytic activity
as 1-Ir-COE. Treating 10-Ir-H2 with 5 equivalents of HBpin,
however, led to a mixture of 10-Ir-H3Bpin and 10-Ir-HBpin in
equilibrium with free H2 (top, Scheme 5). To access 10-Ir-Bpin2,
we employed an alternative route of heating 10-Ir-HMes, a good
synthon for 10-Ir,45 with 1 equiv. of B2pin2. This permitted
isolation of 10-Ir-Bpin2 in 83% yield (bottom, Scheme 5).
10-Ir-HBpin exhibited an upeld signal at ꢂ19.8 ppm (t, JP-H
¼ 8.4 Hz, 1H) in its 1H NMR spectrum, and the peak sharpened
upon 11B decoupling (Fig. 5, le) which suggested that this
proton interacted with a boron atom of the boryl. 10-Ir-H3Bpin
displayed two broad upeld signals at ꢂ5.3 (1H, u1/2 ¼ 60 Hz)
and ꢂ12.4 (2H, u1/2 ¼ 64 Hz) ppm in the 1H NMR spectrum at
ambient temperature. The resonance at ꢂ5.3 ppm (u1/2 ¼ 35
Hz) sharpened upon 11B decoupling (Fig. 5, right), but the width
of the peak at ꢂ12.4 ppm remained unchanged, indicating that
only the proton associated with the resonance at ꢂ5.3 ppm
displayed substantial coupling to the boron nucleus. The ꢂ12.4
ppm signal of 10-Ir-H3Bpin resolved into two distinct reso-
nances (ꢂ9.43, ꢂ15.35 ppm, Fig. 6) upon cooling to 213 K. On
Fig. 7 Relative free energies (DFT calculation) of three possible Scheme 6 Synthesis of vinylidene complexes 10-Ir-v-tol and 10-Ir-v-
isomers of [10-Ir + A1-Bpin].
TMS, and p-alkyne complexes 10-Ir-p-tol and 10-Ir-p-F3tol.
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ꢁ
at 100 C for 1 h cleanly converted all iridium complexes to a
single product that resonated at 43.9 ppm in the 31P NMR
spectrum, which was isolated and identied as 10-Ir-v-tol. The
Me3Si-substituted vinylidene analog 10-Ir-v-TMS was also char-
acterized by NMR spectroscopy in solution by using A2-Bpin as
the reactant (Scheme 6). The vinylidene resonances were
observed at 282.8 and 269.3 ppm in the 13C NMR spectrum as
expected for 10-Ir-v-tol and 10-Ir-v-TMS, respectively.68 These
results are consistent with the DFT prediction of 10-Ir-v-tol as
the thermodynamically favored isomer.
69
´
Esteruelas and Lopez were able to monitor the conversion
of osmium boryl alkynyl complexes to vinylideneboronate
esters, and we envisaged that other isomers of 10-Ir-v-tol might
be obtained if a suitable (MePNPiPr)Ir precursor was reacted with
A1-Bpin under milder conditions. We rst attempted to mix 10-
Ir-H2 with A1-Bpin with subsequent rapid removal of volatiles.
The residue was redissolved in C6D6 and analyzed by 1H and 31
P
NMR spectroscopy. The major product was assigned as the
alkynylboronate p-complex 10-Ir-p-tol (Scheme 6), and its
resonance in the 31P NMR spectrum appeared at 25.6 ppm,
which was identical to the observed intermediate in the
synthesis of 10-Ir-v-tol. We also observed 9% of 10-Ir-HBpin
formation indicating that Csp–B bond cleavage is facile; the
amount of 10-Ir-HBpin increased over time. The assignment of
10-Ir-p-tol was supported by the considerable downeld shi
(8.28 ppm) of the 1H NMR resonances of the ortho-hydrogens of
the p-tolyl group in the coordinated A1-Bpin. Such downeld
chemical shi is characteristic of internal aromatic alkyne p-
complexes.67,70,71 However, the isomerization from 10-Ir-p-tol to
10-Ir-v-tol proceeded at an appreciable rate at ambient
temperature (about 50% aer 15 h) and precluded the isolation
Fig. 9 ORTEP drawing75 (50% probability ellipsoids) of 10-Ir-Bpin2 (top
left) showing selected atom labeling and depiction of 1-Ir-Bpin2 (top
right). Hydrogen atoms are omitted for clarity in the ORTEP drawing.
The selected bond distances (A˚) and angles (deg) for 10-Ir-Bpin2 and
1-Ir-Bpin2 are summarized in the table at bottom.
of pure 10-Ir-p-tol. We surmised that a more electron-poor
alkyne should be thermodynamically less predisposed to form a
vinylidene,68 and that the alkyne isomer may also be kinetically
more long-lived. To this end, we replaced A1-Bpin with A10-Bpin
as the reactant (Scheme 6) and mixed it with pre-cooled 10-Ir-H2
ꢁ
at ꢂ35 C. We were able to isolate 10-Ir-p-F3tol as a pure red-
orange solid in 74% yield. To the best of our knowledge, this is
the rst alkynylboronate p-complex that has been isolated and
characterized. 31P NMR spectroscopic analysis showed a singlet
at 26.1 ppm, which was similar to that of 10-Ir-p-tol. Consistent
with our proposal, the conversion of 10-Ir-p-F3tol to the vinyli-
dene complex 10-Ir-v-F3tol is signicantly slower (about 5%
aer 15 h at ambient temperature) than the analogous trans-
formation of 10-Ir-p-tol. Similarly to 10-Ir-p-tol, the aromatic
proton signals of the 2,6-positions on A10-Bpin in 10-Ir-p-F3tol
were shied downeld to 8.22 ppm in the 1H NMR spectrum. In
the 13C NMR spectrum, the carbon signal of alkynyl–C (C^C–B)
in 10-Ir-p-F3tol (d 105.7) was slightly downeld of that in free
A10-Bpin, as expected72 for a two-electron donor alkyne.
Select X-ray and computational structural studies
We were able to determine molecular structures of 10-Ir-HBpin,
10-Ir-Bpin2, 10-Ir-v-tol, and 10-Ir-p-F3tol in the solid state by X-
ray diffractometry on corresponding single crystals. The struc-
ture of 10-Ir-HBpin (Fig. 8, top le) can be compared against the
analogous POCOP complex 12-Ir-HBpin64 (Fig. 8, top right)
Fig. 8 ORTEP drawing75 (50% probability ellipsoids) of 10-Ir-HBpin
(top left) showing selected atom labeling, and depiction of 12-Ir-HBpin reported by Heinekey et al. 10-Ir-HBpin is Y-shaped ve-coor-
(top right). Hydrogen atoms are omitted for clarity in the ORTEP
drawing, except for the hydride on the Ir atom. Metric parameters in
dinate if viewed as a hydride boryl complex. To reinforce the X-
ray studies, especially with respect to the location of the Ir–H,
density functional theory (DFT) analysis of 10-Ir-HBpin and 12-
Ir-HBpin in the gas phase using the M06 functional was also
performed. DFT calculations show 10-Ir-HBpin possesses
the Ir/B/H triangles in compounds 10-Ir-HBpin (bottom left) and 12-Ir-
HBpin64 (bottom right): DFT calculated distances (A˚) in blue, B–Ir–H
angles (ꢁ) in red, XRD-determined distances (A˚) and B–Ir–H angles (ꢁ)
in black.
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The coordination environment about Ir in the structures of
10-Ir-v-tol and 10-Ir-p-F3tol (Fig. 10) can be described as distorted
square planar, with the greatest deviation corresponding to the
P–Ir–P angles constrained by the pincer ligand. The C2–C1–Ir
bond angle (178.2(3)ꢁ) in 10-Ir-v-tol is very close to 180ꢁ which is
typical for a vinylidene complex.68 The Ir–C1 and C1–C2 bond
˚
lengths of 1.807(4) and 1.334(5) A, respectively, are similar to the
analogous distances in the [(Ph2PCH2SiMe2)2N]Ir]C]CH2
vinylidene complex reported by Fryzuk.76 In the structure of 10-
Ir-p-F3tol, A10-Bpin is bound to iridium in an h2 fashion (Ir–C1:
Fig. 10 ORTEP drawings75 (50% probability ellipsoids) of 10-Ir-v-tol
(left) and 10-Ir-p-F3tol (right) showing selected atom labeling and
hydrogen atoms are omitted for clarity. For 10-Ir-v-tol, one of two
molecules in the asymmetric unit is shown, and a non-coordinated
fluorobenzene molecule is omitted for 10-Ir-p-F3tol. Selected bond
distances (A˚) and angles (deg) for 10-Ir-v-tol: Ir–C1, 1.807(4); C1–C2,
1.334(5); P1–Ir–P2, 164.84(3); C2–C1–Ir, 178.2(3); C1–C2–C3,
120.1(3); C1–C2–B 113.1(3); C3–C2–B, 126.8(3). Selected bond
distances (A˚) and angles (deg) for 10-Ir-p-F3tol: Ir–C1, 2.165(11); Ir–C2,
2.101(12); C1–C2, 1.301(15); P2–Ir–P1, 163.81(10); C1–C2–C3,
147.5(11); B–C1–C2, 161.9(11).
˚
˚
2.165(11) A, Ir–C2: 2.101(12) A) and the Ir–C distances are within
the range of other square planar Ir(I) alkyne complexes.67,77,78
˚
Both the elongation of C^C bond (1.304 A) and the bending of
C^C–Cipso (147.5(11)ꢁ) and C^C–B (161.9(11)ꢁ) away from 180ꢁ
indicate back-donation from the iridium center to the p*
orbitals of C^C bond.72,79
Stoichiometric reactions of (MePNPiPr)Ir complexes
To examine the possible roles that four new isolated (MePNPiPr
)
Ir complexes played in DHBTA, these compounds were exam-
ined in reactions with the three components in DHBTA: a
terminal alkyne (substrate), HBpin (substrate), and H2 (by-
product). 10-Ir-HBpin was reacted with three different terminal
alkynes to study the boryl transfer ability: A1-H, A2-H and A10-H
(Scheme 7, top). Aer 10 min at ambient temperature, approx-
imately 50% yield of the corresponding alkynylboronate was
observed in the 1H NMR spectrum for each of the three
substrates. The amount of alkynylboronate did not increase
with longer reaction times; however, multiple side reactions
including hydrogenation occurred. By 31P NMR spectroscopic
analysis, different degrees of unreacted 10-Ir-HBpin were
observed along with multiple phosphorus-containing species
formed, but they could not be assigned at this stage. Surpris-
ingly, 10-Ir-Bpin2 was inert to all three major components in
shorter Ir–B and Ir–H bond distances and a B–H bond distance
˚
0.2 A longer than 12-Ir-HBpin which was judged to be a s-
borane complex, suggesting greater degree of B–H bond acti-
vation in 10-Ir-HBpin. The structure of 10-Ir-Bpin2 (Fig. 9, top
le) can be described as Y-shaped ve-coordinate where the Y is
dened by N(amido) and the two boryls with an acute B–Ir–B
angle (68.2ꢁ). The Y-shaped geometry is expected for a ve-
coordinate d6 complex73,74 when the equatorial plane contains a
single good p-donor (N(amido)) and two strong s-donors (two
boryls). The two Ir-bound Bpin fragments display essentially the
same metrics, and the associated Ir–B distances are similar to
the analogous Ir–Bpin distances reported in the literature (2.02–
13,40
˚
2.07 A).
In general, all parameters of bond distances and
bond angles in the NIrB2 plane are very close to the previously
reported 1-Ir-Bpin2 (Fig. 9, top right).40 The B/B distance of
˚
2.29 A is too long for boron–boron interaction, thus 10-Ir-Bpin2
should be unambiguously viewed as an Ir(III) diboryl complex.
Scheme 7 Boryl transfer from 10-Ir-HBpin to terminal alkynes (top)
and reactivity of 10-Ir-Bpin2.
Scheme 8 Reactivity of 10-Ir-v-tol (top) and 10-Ir-p-F3tol (bottom).
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DHBTA: HBpin, terminal alkyne and H2 (Scheme 7, bottom) in
stoichiometric reactions at ambient temperature. 31P NMR
spectroscopic analysis showed 10-Ir-Bpin2 was the only observ-
able phosphorus-containing compound in each reaction
mixture. Even at 100 ꢁC, 10-Ir-Bpin2 remained ostensibly intact
in reactions with A1-H and HBpin. Only heating of 10-Ir-Bpin2
under 1 atm H2 at 100 ꢁC overnight led to 41% 10-Ir-H3Bpin
formation.
10-Ir-v-tol was stable toward both HBpin and A1-H at
ambient temperature (Scheme 8, top). On the other hand,
treating 10-Ir-v-tol with H2 quickly resulted in 80% conversion
and the formation of 60% gem-alkenylboronate (A1-4) and
unidentied iridium compounds in 1 h at ambient tempera-
ture. Aer overnight, 10-Ir-H2 was the only observable species by
31P NMR spectroscopic analysis. Lack of observation of A1-4 in
catalytic reaction mixtures suggested that 10-Ir-v-tol is not
present in signicant concentrations during catalysis. Treating
10-Ir-p-F3tol with 1 equivalent of HBpin at ambient temperature
cleanly led to 83% 10-Ir-HBpin formation aer 3 h (Scheme 8,
bottom); meanwhile, equal amount of free A10-Bpin was
observed in the 1H NMR spectrum. The reaction between 10-Ir-
p-F3tol and A10-H was relatively sluggish with no noticeable
change aer 10 min, and only resulted in 8% conversion aer 2
h at ambient temperature based on analysis by 31P NMR spec-
troscopy. Exposing 10-Ir-p-F3tol to 1 atm H2 quickly yielded 27%
10-Ir-H3Bpin and 24% unknown iridium species in 10 min, and
the formation of 10-Ir-H3Bpin proved the Csp–B bond cleavage is
facile. cis-Alkenylboronate (A10-1) was also observed.
Chart 4 Catalytic results for DHBTA using various 10-Ir complexes.
intermediate or if it is merely an off-cycle precursor that can
access the catalytic cycle rapidly enough.
Conclusions
Building on a recent report of successful dehydrogenative
borylation of terminal alkynes (DHBTA) with a pincer iridium
catalyst,40 we examined a series of ligands structurally related to
Competence of isolated compounds in catalytic DHBTA
Chart 4 summarizes the results of catalytic DHBTA experiments the successful SiNN ligand. Although most of the tested ligands
that utilized various isolated (MePNPiPr)Ir compounds as pre- failed to produce DHBTA products, we discovered that various
catalysts. The catalytic reactions were carried out in the fashion PNP pincer ligands do result in active iridium DHBTA catalysts.
consistent with our other studies – the Ir compound was treated Using the unsymmetric PNP-supported iridium complex 17-Ir-
with 200 equiv. of HBpin, followed by 100 equiv. of the terminal COE, useful yields were obtained with 0.025% loading of cata-
alkyne (i.e., 1 mol% Ir). When 10-Ir-H2 was treated with excess lyst, corresponding to thousands of turnovers. Good to excellent
HBpin, a yellow mixture of 10-Ir-HBpin and 10-Ir-H3Bpin yields were obtained under mild conditions for aryl-, silyl-, and
immediately formed before the addition of alkyne. Not alkyl-substituted terminal alkynes, even propargyl derivatives
surprisingly, essentially identical yields of A1-Bpin and hydro- and 1,6-enynes. Unlike the strict chemoselectivity in the (SiNN)
genation side-product A1-3 was observed when using 10-Ir- Ir system, <10% hydrogenation products were observed as the
HBpin and 10-Ir-H2 as pre-catalysts. The use of 10-Ir-v-tol did main side-products in all the (PNP)Ir systems with arylacetylene
lead to the formation of A1-Bpin, but in a signicantly smaller substrates. This has not precluded isolation of alkynylboronate
yield than with 10-Ir-HBpin and 10-Ir-H2. In contrast, 10-Ir- products in 70–82% yields on preparative scale.
Bpin2 showed no DHBTA at all aer the rst 10 min and only
Several iridium complexes of the symmetric PNP ligand 10
gave 37% A1-Bpin aer 3 h. The inertness of 10-Ir-Bpin2 in the were synthesized and examined as potential intermediates in
DHBTA correlated with its lack of reactivity in the stoichio- the catalytic cycle via testing in stoichiometric and catalytic
metric reactions described above. In the two reactions with A10- reactions. The vinylidene (10-Ir-v-tol) and diboryl (10-Ir-Bpin2)
H, 10-Ir-H2 and 10-Ir-p-F3tol led to the same yield of A10-Bpin complexes reacted too slowly with either terminal alkynes or
and the hydrogenation side-product A10-1 (4-CF3-C6H4-C2H5) at HBpin under the conditions of catalysis, which ruled them out
the 10 min and the 1 h mark.
of the catalytic cycle of the 10-Ir system. The inactivity of 10-Ir-
On the basis of the stoichiometric and catalytic experiments Bpin2 is in contrast to the analogous 1-Ir-Bpin240 which suggests
the diboryl complexes analogous to 10-Ir-Bpin2 and the vinyli- that a diboryl intermediate is not essential for successful
dene complexes analogous to 10-Ir-v-tol can be rmly ruled out DHBTA. On the other hand, the hydride boryl complex (10-Ir-
as intermediates in the DHBTA catalysis by (PNP)Ir complexes. HBpin) and the alkynylboronate p-complex (10-Ir-p-F3tol)
Interestingly, this raises the question of whether the previously showed nearly identical performance to 10-Ir-H2 indicating that
reported (SiNN)Ir catalysis actually requires 1-Ir-Bpin2 as an they either are intermediates in the catalytic cycle or are
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connected to such via a low-barrier pathway. Although the full 21 B. Ghaffari, S. M. Preshlock, D. L. Plattner, R. J. Staples,
mechanistic picture remains uncertain, the presently reported
results strongly suggest that a pincer ligand containing an
P. E. Maligres, S. W. Krska, R. E. Maleczka Jr and
M. R. Smith III, J. Am. Chem. Soc., 2014, 136, 14345.
amido donor is key to an active DHBTA catalyst with iridium. An 22 S. M. Preshlock, B. Ghaffari, P. E. Maligres, S. W. Krska,
intriguing possibility is that this is related to the facile migra-
tion of boryl from the metal to the amido nitrogen we recently
discovered for 1-Ir-Bpin2 and 1-Rh-Bpin2.80
R. E. Maleczka Jr and M. R. Smith III, J. Am. Chem. Soc.,
2013, 135, 7572.
23 D. Ogawa, J. Li, M. Suetsugu, J. Jiao, M. Iwasaki and
Y. Nishihara, Tetrahedron Lett., 2013, 54, 518.
24 V. Gandon, D. Leca, T. Aechtner, K. P. C. Vollhardt,
M. Malacria and C. Aubert, Org. Lett., 2004, 6, 3405.
25 J. Huang, S. J. F. Macdonald and J. P. A. Harrity, Chem.
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Acknowledgements
We are grateful for the support of this research by the US
´
´
National Science Foundation (grant CHE-1310007 to O. V. O.), 26 J. Barluenga, P. Barrio, L. Riesgo, L. A. Lopez and M. Tomas,
the Welch Foundation (grant A-1717 to O. V. O.), and Funda- J. Am. Chem. Soc., 2007, 129, 14422.
mental Research Funds for the Central Universities of China 27 L. Deloux, E. Skrzypczak-Jankun, B. V. Cheesman,
(Grant No. AUGA5710013115 to J. Z.). We are also grateful to Ms.
Linda Redd for editorial assistance.
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