Iron-Catalyzed Dehydrogenative Borylation of Terminal Alkynes

Article abstract of DOI:10.1002/adsc.201800588

The catalytic system based on Fe(OTf)₂ (2.5 mol%) and DABCO (1 mol%) selectively promotes the dehydrogenative borylation of both aromatic and aliphatic terminal alkynes to afford alkynylboronate derivatives in the presence of 1 equiv. of pinaco

Full text of DOI:10.1002/adsc.201800588

Title: Iron-Catalyzed Dehydrogenative Borylation of Terminal Alkynes
Authors: Duo WEI, Bertrand Carboni, Jean-Baptiste Sortais, and
Christophe DARCEL
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10.1002/adsc.201800588
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Iron-Catalyzed Dehydrogenative Borylation of Terminal
Alkynes
Duo Wei, a Bertrand Carboni, a Jean-Baptiste Sortais,a,b,c Christophe Darcela*
a
Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes), UMR 6226, F-35042 Rennes, France
[33 2 23 23 62 71 ; christophe.darcel@univ-rennes1.fr]
LCC-CNRS, Université de Toulouse, CNRS, UPS, Toulouse, France.
Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France.
b
c
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
More particularly, there has recently been intense
Abstract. The catalytic system based on Fe(OTf)2
(2.5 mol%) and DABCO (1 mol%) selectively promotes interest in developing first row transition metal
complexes for catalytic hydroboration of alkenes,[11]
the dehydrogenative borylation of both aromatic and
alkynes,[12] and enynes.[13] By contrast, the
aliphatic terminal alkynes to afford alkynylboronate
derivatives in the presence of 1 equiv. of pinacolborane at
100 °C in toluene. This methodology is applicable to a
variety of terminal alkynes (16 examples, yield: 62-93%).
dehydrogenative hydroboration of alkenes was more
scarcely reported.[14]
Here we describe the use of iron salt as catalyst for
the selective dehydrogenative borylation of terminal
Keywords: Iron; alkyne; dehydrogenative borylation;
pinacolborane; alkynylboronates
alkynes
leading
to
the
corresponding
alkynylboronates.
Our initial studies showed that the dehydrogenative
borylation of p-tolylacetylene a2 could be achieved in
toluene solution at 100 °C for 72 h with 1 equiv. of
HBpin (pin = pinacolate) in the presence of 10 mol%
Thanks to the impressive progress made in Suzuki-
Miyaura coupling reactions,[1] the selective
preparation of organoboron compounds has attracted
broad interest over the last two decades.[2] More
particularly, efficient accesses to these versatile
intermediates by C-H dehydrogenative borylation
have been described.[3] In the area of metal-catalyzed
hydroboration of alkenes and alkynes, a competing
side reaction, namely the dehydrogenative borylation,
can operate. However, an accurate design of the
catalytic system can be performed to favor such
pathway, then yielding alkenyl- or alkynyl-boronates
from terminal alkenes or alkynes, respectively.[4]
Alkynylboronates are useful building blocks and are
classically prepared by deprotonation of the
corresponding alkynes by n-BuLi, then reaction with
a boric ester and finally quench with anhydrous
acid.[5] Transition metal catalyzed dehydrogenative
borylation of terminal alkynes was only scarcely
reported: the known catalytic systems are SiNN and
PNP pincer iridium complexes,[6] silver[7] or NHC-
copper[8] well defined complexes.
[15-16]
of Fe(OTf)2
as precatalyst, and 10 mol% of
pyridine in 67% conversion. The borylated p-
tolylacetylene b2 was obtained as the major product
(87%) along with trace amounts of the hydroborated
derivative c2 (7%) and of 4-methylstyrene (5%)
(Table 1, entry 1). The chemoselectivity decreased
significantly when 2,6-lutidine, 2,2’-bipyridine or
Et3N (10 mol%) was used as the base (entries 2-4).
Upon screening various bases, DABCO was found to
lead to both high conversion (84%) and selectivity
towards the formation of the borylated p-
tolylacetylene b2 (87%) besides trace amounts of the
alkenyl derivative c2 (10%) and of 4-methylstyrene
(3%) (Entry 5).
Fe(OTf)2 and DABCO loadings can be efficiently
decreased to 2.5 mol% as full conversion was
obtained after 72 h at 100 °C, b2 being produced
selectively in 84% NMR yield (entry 6). Decreasing
the reaction time to 48 h led to lower conversion
(90%, entry 7). However, with only 1 mol% of
Fe(OTf)2 and DABCO, even with 90% conversion,
the selectivity dropped (b2/c2 = 45:47, entry 9).
Noticeably, the addition of hydrogen scavengers such
as norbornadiene or cyclooctene has a deleterious
effect on the chemoselectivity of the reaction (entries
10-11).
On the other hand, even if its catalytic ability has
been demonstrated for a long time in the Haber
process for ammonia production,[9] iron catalysis has
made a real breakthrough during the two last decades
and is now able to compete favorably with noble
metals.[10]
1
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10.1002/adsc.201800588
Advanced Synthesis & Catalysis
chemoselectivity of this transformation (entry 13 and
SI).
Table 1. Optimization of the reaction parameters for p-
The influence of the nature of the iron precursors
was also investigated. FeF2, FeBr2 and Fe(OAc)2
(2.5 mol%) in association with DABCO (2.5 mol%)
led to full conversion under standard conditions but
with a lower selectivity towards b2 (24-42%, entries
14-17), whereas FeCl2 was less active (entry 15). No
improvement was observed when 2-methyl-THF,
Bu2O and dimethylcarbonate were used as solvent
(see Table S2). Hence, the optimal conditions
selected to probe the substrate scope of the reaction
are 2.5 mol% of Fe(OTf)2, 1.0 mol% of DABCO, in
toluene (1M) at 100 °C for 72 h (Table 2).
tolylacetylene.[a]
Entry
1
[Fe]
Base
Conv. b2/c2
(%)
Yield
(%) b2
(mol%)
(mol%)
Fe(OTf)2
(10)
Fe(OTf)2
(10)
Fe(OTf)2
(10)
Fe(OTf)2
(10)
Fe(OTf)2
(10)
Fe(OTf)2
(2.5)
Fe(OTf)2
(2.5)
Fe(OTf)2
(2.5)
Pyridine
(10)
2,6-lutidine
(10)
2,2’-bipyr
(10)
Et3N
(10)
DABCO
(10)
DABCO
(2.5)
DABCO
(2.5)
DABCO
(1.0)
DABCO
(1.0)
DABCO
(1.0)
DABCO
(1.0)
DABCO
(5.0)
76 87/7
66
40
2
3
93
43/36
Phenylacetylene and arylacetylene derivatives
bearing para-electron-donating substituents, e.g. p-
methyl, p-tert-butyl or p-methoxy, led selectively to
85 44/43
57 54/20
84 87/10
99 84/8
90 85/9
99 81/11
90 45/47
88 59/12
91 76/17
37
the
corresponding
borylated
arylacetylene
4
31
compounds b1-b4 with isolated yields up to 85%
(Table 2, entries 1-4).
5
73
It is worth noting that electron-withdrawing
substituted arylacetylene derivatives such as p-
trifluoromethylphenylacetylene, required shorter
reaction times (24 h instead of 72 h at 100 °C, entry
5) to lead to the corresponding borylated acetylenic
derivative b5 specifically obtained with 87% isolated
yield. Interestingly, the extension of the reaction time
to 72 h permitted to only obtain specifically pinacol
(E)-styrylborane c5 in 92 % yield (entry 6). This
result suggests that the production of the
hydroborylated compounds c5 could occur through
the hydrogenation of the borylated acetylenic
derivative b5. Noteworthy, the bis(ethynyl)benzene
afforded selectively the bis(pinacolborylethynyl)-
benzene b6 in 93% yield (entry 7).
6
83
7
77[b]
80
8
9
Fe(OTf)2
(1.0)
Fe(OTf)2
(2.5)
Fe(OTf)2
(2.5)
41
10
11
12
13
14
15
16
17
52[c]
69[d]
<1
27
None
<1
-
In addition, the reaction can be also efficiently
performed with 1-dodecyne or terminal akynes
Fe(OTf)2
(5.0)
FeF2
(2.5)
FeCl2
(2.5)
FeBr2
(2.5)
Fe(OAc)2
(2.5)
None
80 34/29
99 24/58
59 11/9
99 42/24
98 33/44
bearing
a
benzyloxy group, leading to the
DABCO
(2.5)
DABCO
(2.5)
DABCO
(2.5)
DABCO
(2.5)
24
corresponding borylated alkynes b7-b9 in 78-93%
isolated yields (entries 8-10). Trimethylsilylacetylene
is also
6
a
suitable starting material as the
corresponding borylated compound b10 was isolated
in 89% yield (entry 11).
42
Using alkadiynes such as 1,7-octadiyne or 1,6-
heptadiyne, the monofunctionalization was only
observed in the presence of 2 equiv. of HBpin and the
corresponding monoborylated derivatives b11 and
b12 were obtained selectively in 70-72% isolated
yields (entries 12 and 13). Noticeably, no trace of
diborylated compounds was detected, the only by-
products observed in the crude mixture being the
corresponding alkenyl borylated compounds resulting
from the hydroborylation of one terminal triple bond.
With more steric demanding terminal alkynes such
as tert-butylacetylene and cyclopropylacetylene, both
dehydroborylation and borylated products were
selectively obtained depending upon the reaction time.
Pinacol tert-butylethynylborane b13 and pinacol
cyclopropylethynylborane b14 were isolated in 62
and 63% yields, respectively, after 60 h and 36 h
(entries 14 and 16).
33
[a]
Reaction conditions: Fe(OTf)2 (2.5-10 mol%), toluene
(0.5 mL), alkyne (0.5 mmol), HBpin (0.5 mmol) and base
(1-10 mol%) at 100 °C for 72 h. Conversion and yield were
measured by 1H NMR analysis of the crude product, based
on a2, and the identity of the products b2 and c2 was
[b]
[c]
confirmed by GC–MS.
norbornadiene.
48 h.
with 2 equiv. of
[d]
with 2 equiv. of cyclooctene. Bipyr:
bipyridine; DABCO: 1,4-diazabicyclo[2.2.2]octane
Notably, using DABCO, without iron precursor,
resulted in no activity (entry 12). By contrast, a low
yield and selectivity was obtained using Fe(OTf)2
(2.5 mol%) without base, even if the conversion can
reach 80%, thus showing the crucial role of the
DABCO catalytic additive on the efficiency and
2
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10.1002/adsc.201800588
Advanced Synthesis & Catalysis
Table 2. Scope of the reaction.[a]
A prolonged 72 h of reaction permitted to switch
the chemoselectivity as pinacol (E)-2-tert-butylvinyl-
boranate c13 and pinacol (E)-2-cyclopropylvinyl-
boranate c14 were selectively isolated in 85 and 90%
yields, respectively (entries 15 and 17). Notably,
cyclopropylacetylene furnished b14 and c14 in
quantitative yield, which seem to indicate that the
reaction did not proceed via stable radical
intermediates.
Starting from methyl hex-6-ynoate or 3-bromo-1-
propyne, only the hydroborated derivative c15 and
c16 were obtained in high yields, 95 and 92%,
respectively, whatever the reaction time, 9 or 72 h
(entries 18 and 19). Additionally, under the optimized
reaction conditions, no reaction was observed with
terminal alkynes bearing primary amine, alcohol or
carboxylic acid substituents (see Table S4).
Figure 1: 11B NMR spectra recorded at 96 MHz of the
reaction of p-tolylacetylene a2 with HBpin in C6D6 at
100 °C leading to the compounds b2 and c2.
Preliminary experiments aimed at gaining an
insight into the reaction course were then performed.
The reaction outlined in Table 2, entry 2, was
achieved in a Young NMR tube, charged under argon
atmosphere with 2.5 mol% of Fe(OTf)2 in C6D6
(1.0 mol/L), 0.5 mmol of a2, 0.5 mmol of HBpin and
o
DABCO (1 mol%) at 100 C for indicated time.
Analysis of 11B NMR spectra showed that the
dehydroborylated and the borylated compound b2
and c2 were formed simultaneously, b2 being always
the major product (Figure 1). Additionally, the results
described in Table 2, entries 6, 15 and 17 indicated
that the formation of the alkenyl boronates results
from the reduction of the corresponding
alkynylboronates. On the other hand, the evolution of
1
the H2 gas was also identified in H NMR at 4.47
ppm (see Figure S2).
From a mechanistic point of view, as a Lewis acid,
Fe(OTf)2 should be able to activate the B-H bond,
thus enhancing the electrophilic capacity of the boron
center to react with acetylenic derivative. This
process would be accelerated by the presence of
DABCO which increases the nucleophilicity of the
terminal acetylenic carbon.[17,18]
[a]
General conditions: alkyne (0.5 mmol), HBpin
(0.5 mmol), Fe(OTf)2 (2.5 mol%), DABCO (1.0 mol%),
toluene (0.5 mL), 100 °C; Measured by 1H NMR of the
[b]
[c]
crude mixture.
Isolated yields. In parentheses, isolated
yield on gram scale reaction. [d] Reaction in a Young NMR
tube in C6D6. [e] 2 equiv. of HBpin.
3
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10.1002/adsc.201800588
Advanced Synthesis & Catalysis
In summary, we have reported the first example of
highly selective catalytic dehydrogenative
borylation of terminal alkynes with pinacolborane,
using iron as an inexpensive earth abundant metal
and DABCO as a co-catalyst. Further studies on the
mechanism and synthetic applications are in progress
in our laboratory.
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a
Experimental Section
General procedure for Fe(OTf)2 catalyzed
dehydrogenative borylation of terminal alkynes: in an
argon filled glove box, a 20 mL Schlenk tube was
charged with Fe(OTf)2 (2.5 mol%), toluene (1.0
mol/L), alkyne (0.5 mmol), HBpin (0.5 mmol) and
DABCO (1 mol%, stock solution in toluene) in this
order. Then the reaction mixture was stirred at 100 oC
for 72 h. After cooling the mixture to room
temperature, the solution was diluted with pentane (2
mL) and filtered through a small pad of celite (2 cm
in a Pasteur pipette). The celite was washed with
pentane (2 mL×2). The filtrate was evaporated and
the crude residue was then purified by
recrystallization (slow evaporation form pentane) or
bulb to bulb distillation.
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We thank the Centre National de la Recherche Scientifique
(CNRS), the Université de Rennes 1, the Institut Universitaire de
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manganese, see: x) G.-Q. Zhang, H.-S. Zeng, J. Wu, Z.-
W. Yin, S.-P. Zheng, J. C. Fettinger, Angew. Chem.,
Int. Ed. 2016, 55, 14369; Angew. Chem.2016, 128,
1414581.
[18] In 1H- and 11B-NMR data collected during the
mechanism study, it was sometimes possible to observe
very small peaks around the baseline at -5.00 and +62.7
ppm, respectively, which may correspond to Fe-H and
Fe-B species. Unfortunately, we were not able to
isolate and fully characterize them in stoichiometric
reaction. See references 3l and 11e.
[12] With iron catalysts, see: a) J. V. Obligacion, P. J.
Chirik, Org. Lett. 2013, 15, 2680; b) M. D. Greenhalgh,
S. P. Thomas, Chem. Commun. 2013, 49, 11230; with
cobalt see: c) J. V. Obligacion, J. M. Neely, A. N.
5
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10.1002/adsc.201800588
Advanced Synthesis & Catalysis
COMMUNICATION
Iron-catalyzed Dehydrogenative Borylation of
Terminal Alkynes
Adv. Synth. Catal. Year, Volume, Page – Page
D. Wei, B. Carboni, J.-B. Sortais, C. Darcel*
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