Synthesis of Triborylalkenes from Terminal Alkynes by Iridium-Catalyzed Tandem C-H Borylation and Diboration

Article abstract of DOI:10.1002/anie.201507372

A two-step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer l

Full text of DOI:10.1002/anie.201507372

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507372
German Edition: DOI: 10.1002/ange.201507372
Homogeneous Catalysis
Synthesis of Triborylalkenes from Terminal Alkynes by Iridium-
À
Catalyzed Tandem C H Borylation and Diboration
Chun-I Lee, Wei-Chun Shih, Jia Zhou, Joseph H. Reibenspies, and Oleg V. Ozerov*
Abstract: A two-step reaction to convert terminal alkynes into
triborylalkenes is reported. In the first step, the terminal alkyne
and pinacolborane (HBpin) are converted into an alkynylbor-
onate, which is catalyzed by an iridium complex supported by
a SiNN pincer ligand. In the second step, treatment of the
reaction mixture with CO generates a new catalyst which
mediates dehydrogenative diboration of alkynylboronate with
pinacolborane. The mechanism of the diboration remains
unclear but it does not proceed via intermediacy of hydro-
boration products or via B2pin2.
catalyzed by platinum[13–19] or copper[20–22] complexes. Only
three examples have been reported for the synthesis of
triborylalkenes and they employed platinum-catalyzed dibo-
ration of alkynylboronates (Figure 1c).[15,23] Interestingly, in
the example from Marder and co-workers,[15] Me3SiC CBpin
ꢀ
ꢀ
was formed by desilylative borylation of Me3SiC CSiMe3
with B2pin2.
Recently, we demonstrated the first example of dehydro-
genative borylation[24] of terminal alkynes (DHBTA) cata-
lyzed by the SiNN pincer cyclooctene iridium complex
1 (Figure 2)[25] Tsuchimoto et al. described DHBTA catalysis
P
olysubstituted olefins are important structural motifs in
drugs, natural products, and functional materials.[1,2] Among
the many methodologies[1,2] developed for stereocontrolled
synthesis of polysubstituted olefins, selective Suzuki–Miyaura
coupling and other reactions of polyborylated alkenes appear
to be an especially attractive route.[3–6] 1,1-Diborylalkenes can
be made from either 1,1-dichloro- or 1,1-dibromoalkenes
through lithium–halide exchange and subsequent reaction
with bis(pinacolato)diboron (B2pin2; Figure 1a).[7,8] Alterna-
tively, they can be accessed by hydroboration of alkynylbor-
onates,[9] or by dehydrogenative borylation of alkenes with
diboron reagents.[10] Metal-catalyzed addition of diboron
reagents to terminal or internal alkynes leads to 1,2-cis-
diborylalkenes (Figure 1b).[11,12] These reactions are usually
Figure 2. DHBTA catalyzed by 1. COE = cis-cyclooctene.
by Zn(OTf)2/pyridine using 1,8-nathpthalenediamidobor-
ane.[26] Very recently, we showed that iridium complexes
supported by various PNP ligands are very effective catalysts
for DHBTA.[27] The reaction was highly chemoselective, thus
leading to alkynylboronates in excellent yields. This result
motivated us to explore the potential to further borylate
alkynylboronates in a one-pot reaction. Herein we report the
discovery of a new selective dehydrogenative diboration of
alkynylboronates.
The DHBTA reaction catalyzed by [(SiNN)Ir] complexes
produces alkynylboronate in high yield and purity, and does
so in minutes at ambient temperature. Because our protocol
utilizes an excess of pinacolborane (HBpin), one or more
equivalents of HBpin per alkynylboronate remains in the
reaction mixture at the end of DHBTA catalysis. We became
interested in the potential of a one-pot synthesis that would
combine DHBTA and hydroboration in one sequence. It was
envisaged that ideally, hydroboration might be performed at
more forcing conditions by the same DHBTA catalyst or its
decomposition products, or by modifying the DHBTA
catalyst in the mixture by a simple additive. syn-Addition of
HBpin to alkynylboronates would yield 1,1-diborylalkenes or
1,2-trans-diborylalkenes.
Figure 1. Syntheses of 1,1-diborylalkenes (top), 1,2-cis-diborylalkenes
(middle), and triborylalkenes (bottom). TMP=2,2,6,6-tetramethylpiper-
idide.
[*] Dr. C.-I Lee, W.-C. Shih, Dr. J. H. Reibenspies, Prof. O. V. Ozerov
Department of Chemistry, Texas A&M University
TAMU - 3255, College Station, TX 77843 (USA)
E-mail: ozerov@mail.chem.tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/ozerov/
Prof. J. Zhou
The complex 1 cleanly catalyzed DHBTA of 4-ethynylto-
luene (A1-H) to A1-Bpin with 3.5 equivalents of HBpin.
Thermolysis of the resultant mixture at 558C for 24 hours
yielded multiple hydrogenation and hydroboration products
Department of Chemistry, Harbin Institute of Technology
Harbin 150001 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507372.
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or THF as the solvent at 558C (entries 1–3), or when the
reaction was carried out in C6D6 at 808C (entry 4), but
practical selectivity for either product was not attained.
Increasing the amount of HBpin to 5 equivalents and
reducing the solvent volume led to substantial improvement
in the selectivity for A1-Bpin3 (88%, entry 5). The in situ
fraction of A1-Bpin3 was only slightly increased when the
reaction was carried out in neat HBpin (entry 6), so we
focused on the reaction conditions of entry 5 for the
subsequent examination of the substrate scope.
Several aryl- and alkyl-substituted terminal alkynes were
subjected to a two-stage procedure whereby treatment with
5 equivalents of HBpin and 1 mol% 1 at ambient temper-
ature was followed by degassing of the reaction mixture,
introduction of CO (1 atm), and thermolysis at 558C for
18 hours (Scheme 2). The triborylalkenes derived from aryl
alkynes (A1-H to A4-H) were easily isolated as pure solids in
70–80% yields by removing the volatiles at the end of the
reaction, suspending the residue in n-pentane at ambient
temperature, and filtering off the products. For alkyl-sub-
stituted triborylalkenes (A5-Bpin3 to A7-Bpin3), 60–80%
yields (NMR) were observed in situ. However, the yields of
the isolated products were significantly lower since the
solubility of alkyl-substituted triborylalkenes in n-pentane
proved to be much higher. The yield of the isolated product
was significantly improved by using cold isooctane in the work
Scheme 1. Attempts at additional borylation of alkynylboronates.
(Scheme 1), including 34% yield of the 1,1-diborylalkene A1-
Bpin2, thus indicating that iridium compounds present in the
mixture do not give rise to effective hydroboration catalysis.
Degassing the reaction mixture after DHBTA and refilling
with 1 atm Ar before heating resulted in a similar product
distribution. However, treatment of the post-DHBTA mix-
ture with 1 atm of CO prior to heating at 558C led to
a different and unexpected outcome. After 8 hours, only two
products were observed: 34% of A1-Bpin2 and 55% yield of
a new triborylalkene product, A1-Bpin3 (Scheme 1). The
product ratio did not change after additional heating over-
night at 558C. To the best of our knowledge, this is the first
example of a dehydrogenative diboration of an alkyne using
a dialkoxyboron reagent, as opposed to the additive dibora-
tion with diboron reagents.[12]
After this fortuitous discovery, we attempted to optimize
the yield of triborylalkene and the results are summarized in
Table 1. Some variation in the ratio of the A1-Bpin2 and A1-
Bpin3 products was observed in either C6D6, fluorobenzene,
Table 1: Summary of the optimization of alkynylboronates diboration.
Entry
HBpin
(equiv)
Solvent
Yield [%]
A1-Bpin2
A1-Bpin3
Scheme 2. Substrate scope of one-pot diboration of alkynylboronates.
Reaction conditions: 1 (0.020 mmol) and HBpin (10 mmol) were
dissolved in PhF in a PTFE-valved gas-tight flask. Alkyne (2.00 mmol
for monoynes [1.00 mmol for 1,7-octadiyne]) was then added in 4
portions with 1 min intervals at RT. After 10 min, the mixture was
degassed, refilled with 1 atm of CO, and heated at 558C for 18 h (see
experimental for details). Yields are those of isolated products. Values
within parentheses are yields determined by 1H NMR spectroscopy.
[a] 87% A8-Bpin.
1[a]
2[a]
3[ab]
4[cd]
5[e]
3.5
3.5
3.5
3.5
5.0
10
C6D6
PhF
THF
C6D6
PhF
39
45
26
52
7
55
50
43
32
88
91
6[e]
neat
5
[a] Thermolysis stage: 558C, 8 h. [b] 26% A1-Bpin. [c] 3rd stage: 808C,
3 h. [d] 5% A1–1. [e] Thermolysis stage: 558C, 18 h.
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up instead.[28] The reaction with Me3SiC CH (A8-H) stopped
ꢀ
at the alkynylboronate stage, with no di- or triborylalkene
products observed, perhaps owing to steric hindrance. The
hexaborylated diene A9-Bpin6 and the homoallyl ether A10-
Bpin3 were obtained from 1,7-octadiyne and trimethylsilyl
homopropargyl ether, respectively, albeit in modest yields.
To examine the nature of possible SiNN-ligated iridium
species under CO-rich conditions, we treated the in situ
generated 1 with 1 atm CO at ambient temperature. This
treatment led to an equilibrium mixture of the monocarbonyl
adduct 2 and dicarbonyl adduct 3 (Scheme 3). We were able
Figure 3. ORTEP drawing[32] (thermal ellipsoids shown at 50% proba-
Scheme 3. Synthesis of the carbonyl adducts 2 and 3.
bility) of 2 (top) showing selected atom labeling, and drawing of the
DFT-calculated structures of 2 (bottom). Hydrogen atoms are omitted
for clarity, except for the hydride on the Ir atom. Selected bond
distances (Š) and angles (deg) for 2, with DFT-derived metrics in
square brackets: Ir1-Si1, 2.3366(15) [2.376]; Si1-H1, [2.183]; Ir1-H1,
[1.592]; Ir2-H1, [3.117]; Ir1-Ir2, 2.9074(8) [3.046]; Si1-Ir1-N2,
124.89(11) [127.51]; Si1-Ir1-H1, [63.17]; Ir1-C1-O1, 176.1(5) [177.75].
to isolate pure 2 as a brown solid in 92% yield. The 1H NMR
resonances corresponding to Ir-H in 2 (dÀ15.52 ppm) and 3
(dÀ6.10 ppm) showed little to no coupling to 29Si.[29] The more
downfield chemical shift for 3 is consistent with a hydride
trans to CO and not a nitrogenous ligand. The identification
of 2 and 3 as mono- and dicarbonyl complexes, respectively, is
supported by the corresponding single band at 1977 cmÀ1 for
2, and two bands at 2057 and 2007 cmÀ1, in about a 1:1
intensity ratio (indicating a cis-dicarbonyl), for 3 in the IR
spectra.
The result of the determination of solid-state structure of
2 by single-crystal X-ray diffractometry is shown in Figure 3
(top). In addition, density functional theory (DFT) calcula-
tions were carried out on 2 in the gas phase using the M06
functional (Figure 3, bottom). The positions of nonhydrogen
atoms from the experimental XRD determination were
closely reproduced in the DFT-calculated structure. The
longer calculated Si-H distance in 2 (2.183 Š) versus that in
1 (2.007 Š) and the diboryl complex [(SiNN)Ir(Bpin)2] (4;
1.889 Š), is in agreement with the lack of observable Si-H
coupling in 2, and the JSi-H values of 8 and 32 Hz for 1 and 4,
respectively.[25] Both the calculated Si-H distance and the
JSiÀH value of 2 are outside the range for a Si–H bonding
interaction, hence 2 should be viewed as a d6-iridium(III) silyl
hydride complex.[30,31] The coordination environment about
iridium in 2 can be viewed as derived from square-pyramidal
where the ligand trans to the empty site (silyl) is displaced
towards the base plane, presumably in part because of the
inability of the SiNN ligand to adopt an idealized facial
geometry.
bridged dimer. This arrangement appears to be a rare case of
a d6-d6 metallophilic interaction. Metallophilic interactions
are quite common for d10–d10 (Ag, Au) and d8–d8 (square-
planar Pd, Pt), with M–M distances around 2.8–3.1 Š being
rather typical for unbridged dimers.[33–36] The lack of steric
bulk projecting from the base of the square pyramid in 2 is
atypical for five-coordinate d6 complexes and likely contrib-
utes to permitting close contact.
To examine the possible role of 2 in the diboration of
alkynylboronates, we first tested 2 in the reaction between
isolated A1-Bpin and HBpin under 1 atm CO (Scheme 4).
The product ratio was consistent with the results observed
from the one-pot synthesis reactions, but with a slightly slower
reaction rate. This result suggested 2 acts as the actual entry
point into the catalytic cycle of the diboration. Performing the
reaction under the same reaction conditions except under
1 atm Ar instead of CO, interestingly, led to a much slower
rate (36% conversion) and higher product ratio of A1-Bpin2
to A1-Bpin3 (16%:12%). Catalysis of the diboration step can
utilize less iridium than DHBTA, as evidenced by a 61% yield
of A1-Bpin3, within 28 hours, when 0.1% of 2 was used (NMR
evidence in situ, ca. 600 TON) under reaction conditions
Interestingly, two molecules of 2 approach each other
closely in the solid state, with about a 2.91 Š distance between
two iridium centers. The gas-phase DFT calculation results in
a similar approach with about a 3.05 Š distance between
iridium atoms. No p–p stacking is observed and the hydrides
are clearly terminal (calculated values: Ir1-H1, 1.592 Š; Ir2-
H1, 3.117 Š), thus ruling out the possibility of a hydride-
Scheme 4. Borylation of A1-Bpin catalyzed by 2.
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analogous to those in Scheme 4. For the tandem process,
DHBTA is thus limiting with respect to the required iridium
catalyst loading.
Miyaura coupling with one equivalent of 4-iodoanisole to
afford B1-Bpin2 in 77% yield upon isolation (Scheme 6). The
E stereochemistry of B1-Bpin2 was confirmed by an X-ray
diffraction study of a suitable single crystal.[28] Only a few
examples have been demonstrated for trans-selective alkyne
diboration, but they required propargyl alcohols or alky-
noates as substrates.[38,39] Small quantities of the trans-
diboration product were observed by Lin, Marder, and co-
workers in the predominantly cis-selective catalysis of dibo-
ration of a diarylalkyne by [Co(PMe3)4].[40] The reaction in
Scheme 6 points to a potential complementary synthetic
strategy for trans-diaryldiborylalkenes.
During the optimization of diboration, we found that the
ratio of 1,1-diborylalkene to triborylalkene products
remained constant once the alkynylboronate had been fully
consumed. To expand on this observation, we treated A1-
[9]
Bpin2 with 5 equivalents of HBpin and 3 mol% 2 under
1 atm CO, but no change in the 1H NMR spectrum was
evident after 2 hours at 558C (Scheme 5). In contrast, when
Scheme 6. Selective monoarylation of A1-Bpin3 and ORTEP drawing[32]
of B1-Bpin2. The crystallographic disorder of the aryl groups and
hydrogen atoms is not shown. Thermal ellipsoids shown at 50%
probability.
Scheme 5. Borylation of A1-Bpin2 catalyzed by 2.
one equivalent of A2-Bpin was added to this mixture (with
restoration of CO atmosphere) and it was subjected to further
heating at 558C, A2-Bpin was fully converted into A2-Bpin2
and A2-Bpin3 after 3 hours, while A1-Bpin2 remained intact.
These results unambiguously show that a 1,1-diborylalkene is
not an intermediate en route to a triborylalkene. No signifi-
cant yield changes were observed for either A1-Bpin3 or the
major side-product A1-Bpin2 in the presence of mercury,[37]
whereas [Ir4(CO)12], iridium powder, and [{(COE)2IrCl}2],
with or without CO, were all unable to catalyze the diboration
of A1-Bpin to A1-Bpin3. This result suggests homogeneous
catalysis that relies on the SiNN ligated or at least derived
iridium species. A1-Bpin did not react with B2pin2 in the
presence of 2, and B2pin2 was not observed when HBpin was
thermolyzed in the presence of 2 under a CO atmosphere at
558C for 8 hours. Diboration via the intermediate B2pin2
formation can thus be ruled out.
We described the development of a convenient one-pot,
two-step synthesis of triborylalkenes directly from terminal
alkynes. The process combines DHBTA, which uses
a [(SiNN)Ir] catalyst, and a subsequent novel dehydrogen-
ative diboration initiated by the carbonylation of the DHBTA
catalyst. Good yields of the isolated products were obtained
for a variety of alkynes. The carbonyl complex 2 has been
independently synthesized from 1 and has been shown to
possess the ability to catalyze diboration. Preliminary mech-
anistic studies suggested that triborylalkenes are not obtained
from borylation of free diborylalkenes. Further work will be
aimed at the elucidation of the mechanism of dehydrogen-
ative diboration and the exploration of synthetic use of
triborylalkenes.
In a very generic sense, we anticipate that the tribor-
ylalkene is formed by insertion of alkynylboronate into an Experimental Section
À
À
Complete experimental details are available online as Supporting
Information.
Ir B bond and subsequent reductive C B elimination,
whereas the diborylalkene product is formed by insertion of
Supporting Information for this article in the form of descriptions
of experiments, characterization data, computational details is
available on the WWW under http://www.angewandte.org. CCDC
1417117 and 1417149 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
À
À
the alkynylboronate into Ir B and reductive C H elimination
(or insertion into Ir H and C B reductive elimination).
Increased triborylalkene formation would then be assisted by
À
À
À
À
conversion of Ir H bonds into Ir B bonds, which may explain
the beneficial role of increased HBpin content. Nonetheless,
the detailed mechanistic picture remains rather obscure. We
are especially challenged to explain the beneficial role of
excess CO in the diboration step and how a polydentate SiNN
ligand, CO, and two substrates all ostensibly coordinate to the
iridium center in some order.
In-depth exploration of the utility of triborylalkenes as
substrates for further transformation and functionalization is
outside the scope of this report. However, we were pleased to
find that A1-Bpin3 underwent stereoselective Suzuki–
Acknowledgements
We are grateful to the US National Science Foundation (grant
CHE-1310007 to O.V.O.) and the Welch Foundation (grant
A-1717 to O.V.O.) for the support of this research. W.-C.S. is
thankful for the Studying Abroad Scholarship from Ministry
of Education, Taiwan. J.Z. thanks the Fundamental Research
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Keywords: boranes · C C coupling ·
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Received: August 7, 2015
Published online: October 1, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 14007