Z-Selective Alkyne Functionalization Catalyzed by a trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Article abstract of DOI:10.1021/acs.inorgchem.0c02057

The Z-selective functionalization of terminal alkynes is a useful transformation in organic chemistry and mainly catalyzed by noble metals. Here, we present the Z-selective hydroboration of terminal alkynes catalyzed by a stable trans-dihydride iron compl

Full text of DOI:10.1021/acs.inorgchem.0c02057

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Z‑Selective Alkyne Functionalization Catalyzed by a trans-Dihydride
N‑Heterocyclic Carbene (NHC) Iron Complex
Subhash Garhwal, Natalia Fridman, and Graham de Ruiter*
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ABSTRACT: The Z-selective functionalization of terminal alkynes is a useful transformation in organic chemistry and mainly
catalyzed by noble metals. Here, we present the Z-selective hydroboration of terminal alkynes catalyzed by a stable trans-dihydride
iron complex [(PCNHCP)Fe(H)2N2)] (2). Overall, the reaction occurs at room temperature and provides near quantitative yields of
the Z-vinylboronate ester. Interestingly, the same catalyst could also provide the E-vinylboronate by heating the reaction mixture at
slightly elevated temperatures (50 °C). If, however, the reaction is performed in the absence of HBpin, rapid Z-selective alkyne
dimerization is observed, which is further discussed in this report.
he transition metal catalyzed hydroboration of unsatu-
rated C−C and C−X bonds (X = N or O) is a powerful
T
synthetic method in organic chemistry.1 To illustrate, the
hydroboration of terminal alkynes offers an atom economical
route toward vinylboronate esters that are important starting
materials in a diverse set of organic transformations.2 Because
the addition of the borane is typically syn with respect to the
CC bond, product selectivity commonly favors the more
stable (E)-vinylboronate.3 Accessing the (Z)-vinylboronate is
challenging and has mainly been reported with noble metal
catalysts like ruthenium,4 rhodium, or iridium.5 Recent studies,
however, have shown that earth-abundant metals can also
facilitate the Z-selective hydroboration of terminal alkynes.6
For instance, Chirik and co-workers reported the first example
of an earth-abundant cobalt catalyst capable of Z-selective
hydrobration.6b Kirchner and co-workers reported a non-
classical iron polyhydride catalyst [Fe(PNP)(H)2(η2-H2)] that
also showed excellent selectivity for the corresponding (Z)-
vinylboronate.6a Interestingly, a few years earlier, Leitner and
co-workers reported a similar ruthenium complex [Ru(PNP)-
(H)2(η2-H2)] that also showed comparable Z selectivity.4c
A common feature of these catalysts is that they all contain a
rigid pincer ligand with a central pyridine donor (Figure 1).
Intriguingly, pincer ligands with an XNX (X = NR, P(OR)2,
PR2) type geometry and a central amino or pyridine donor
have been fundamental to the development of earth-abundant
metal catalysis.7 Expanding on the versatility of the pincer
ligand design, we recently reported the first example of a
PCNHCP functionalized iron complex [(PCNHCP)FeCl2] (1),
which facilitated catalytic hydrogen isotope exchange (HIE) at
aromatic hydrocarbons upon formation of the stable trans-
dihydride iron complex [(PCNHCP)Fe(H)2N2)] (2).8 Given
the importance of transition-metal hydrides in alkyne
functionalization strategies,4c,6a we were intrigued if complex
2 could facilitate the Z-selective hydroboration of alkynes,
which still remains uncommon with earth-abundant metals.6
Here, we demonstrate that iron complex 2 facilitates the
hydroboration of alkynes with excellent selectivity. The
Figure 1. (Top) reported transition metal pincer complexes for Z-
selective alkyne hydroboration. (Bottom) Herein reported Z-selective
alkyne functionalization with iron PCNHCP pincer complex
[(PCNHCP)Fe(H)2(N2)] (2).
reaction occurs at room temperature and provides near
quantitative yields of the corresponding (Z)-vinylboronate
within 4 h. Interestingly, the corresponding (E)-vinylboronate
could also be exclusively accessed via isomerization of the
formed (Z)-vinylboronate at 50 °C. In addition, we found that
when the reaction is performed in the absence of borane, the
Z-selective homodimerization of alkynes was observed. Overall,
complex 2 presents a dual alkyne functionalization strategy
that, in particular for alkyne hydroboration, exhibits a good
Received: July 12, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c02057
© XXXX American Chemical Society
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substrate scope and functional group tolerance (e.g., esters,
ethers, halides).
Previously, we reported an efficient method for synthesizing
PCNHCP functionalized iron complexes.8 The catalyst of
interest, the trans-dihydride iron complex [(PCNHCP)Fe-
(H)2N2)], was readily obtained upon treating 1 with 2 equiv
of NaBHEt3 (Scheme 1). Having facile access to such a stable
tion of 2 to a solution of phenylacetylene and HBpin in THF
resulted in the quantitative formation of (Z)-phenylvinylbor-
onate ester (5a) with excellent stereoselectivity (E/Z; 2:98).
Given that complex 2 is active for the hydroboration of
phenylacetylene, other terminal alkynes were explored as well
(Table 1). For example, p-tolylacetylene yielded the corre-
sponding (Z)-tolylvinylboronate ester with excellent stereo-
selectivity and in excellent yields. Changing the substitution
pattern to either meta or ortho did not considerably change the
yield or stereoselectivity (Table 1; 5b−5d).
Scheme 1. Synthesis of Iron PCNHCP Pincer Complexes 1−4
Electronically differentiated phenylacetylenes were also
hydroborated efficiently. For example, para substituted phenyl-
acetylenes that bear both electron withdrawing (5f, −F; 5g,
−Br; 5h, −CF3) and electron donating (e.g., 5i, −OMe)
substituents are hydroborated in excellent yields (>95%) with
similar Z selectivities (Table 1). Other functional groups such
as esters (5j) or heteroaromatics (5k and 5l) are also tolerated,
although for the ester (5j) a diminished yield and erosion of
the Z selectivity was observed.
In contrast, for aliphatic acetylenes, higher catalyst loadings
and longer reaction times were necessary to obtain reasonable
conversion of the starting materials. Yet, under these
conditions, diminished Z selectivity was observed when
compared to their aromatic counterparts. For example, for 1-
octyne, only moderate stereo selectivity was obtained (Table 2;
E/Z ∼ 45:55). Using sterically more demanding alkynes did
not result in a significant improvement of the observed Z
selectivity (Table 1). The diminished selective could be the
result of a competing alkene isomerization side reaction that
might occur at prolonged reaction times and catalyst loadings
(vide infra). No hydroboration is observed for internal alkynes.
These results demonstrate that the herein reported iron
iron(II) trans-dihydride prompted us to investigate its
reactivity toward terminal alkynes in the presence of 4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (HBpin). Gratifyingly, addi-
a c
−
Table 1. Scope and Limitations of the Iron Catalyzed (2) Hydroboration of Alkynes
a
Reactions were performed with alkyne (0.5 mmol), HBpin (0.55 mmol, 1.1 equiv), and catalyst 2 (2 mol %) in THF (0.5 mL) at room
b
c
temperature (RT) for 4 h. Yields and selectivity were determined by 1H NMR spectroscopy with trimethoxybenzene as an internal standard. For
aliphatic alkynes 5o−5q, reactions were performed with 5 mol % catalyst for 12 h at RT.
B
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PCNHCP pincer complex 2 is a good catalyst for the Z-selective
hydroboration of terminal alkynes and one of the few earth-
abundant metal alternatives.6 However, for aliphatic alkynes
the observed stereoselectivity is lower than the state-of-the-art
as reported by Chirik and Kirchner and a limitation of the
catalyst.
A clear advantage of the herein reported method is, however,
that by controlling the temperature, catalyst selectivity could
be selectively switched from the (Z)-vinylboronate to the (E)-
vinylboronate (Figure 2). To illustrate, products 5a, 5b, 5f, 5i,
tional isotope labeling experiments with phenylacetylene-d1 did
not provide additional information in order to discern between
the different types of 1,1-trans hydroboration pathways; e.g., a
metal vinylidene or syn-hydrometalation based pathway.11
To gain further mechanistic insight into the herein reported
hydroboration, we performed a series of stoichiometric
experiments. The addition of 1 equiv of HBpin to complex 2
led to the exclusive formation of [(PCNHCP)Fe(H)(μ-
H)2Bpin)] (Scheme 1; 3), which might be an important
catalytic intermediate. Such a proposal would certainly be
consistent with the results reported by Kirchner and Leitner
who have shown that their analogous iron and ruthenium
complexes ([(PNP)Fe(H)(μ-H)2Bpin)]) and ([(PNP)Ru-
(H)(μ-H)2Bpin)]) are competent precatalyst for the Z-
selective hydroboration of alkynes.4c,6a However, under the
herein reported reaction conditions, complex 3 gave 15% yield
of the (Z)-vinylboronate after 24 h, suggesting that 3 is an off-
cycle species rather than an active (pre)catalyst. It is possible
that the poor activity might be related to the reactivity of the
Fe−H bond. Recent studies have shown that for a related iron
species [(PNP)Fe(H)(μ-H)2BH2], the Fe−H bond is quite
stable and unreactive toward alkynes.6a To verify for our
system, we have synthesized the corresponding complex
[(PCNHCP)Fe(H)(μ-H)2BH2] (4) and found out that 4 is
also catalytically inactive. These data imply that the Fe−H
bonds in 3 and 4 are not reactive enough to initiate the
hydroboration of terminal alkynes.
Because neither the isotope labeling experiments nor the
stoichiometric experiments provided clear evidence for a metal
vinylidene or syn-hydrometalation pathway, we attempted to
isolate an iron(II) bis(acetylide) species that might be formed
upon the addition of the alkyne to iron complex 2. Metal-
acetylides have been shown by both Chirik and Kirchner to be
relevant intermediates in the syn-hydrometalation pathway.6b,9
Unfortunately, adding 2 equiv of alkyne to complex 2 resulted
in rapid alkyne dimerization, even when trimethylsilylacetylene
was used. Not dismayed by the additional reactivity of 2 (vide
infra), we realized that such intermediates might also be
generated by the addition of phenylacetylene to complex 1 in
the presence of base. Although the bis(acetylide)iron(II)
species could still not be isolated, the addition of phenyl-
acetylene to complex 1 in the presence of HBpin and base
(NaHMDS) resulted in the formation of the corresponding
(Z)-vinylboronate ester, which is consistent with the
observations by Kirchner and co-workers (Figure S80).9
Because the reactivities of complexes 1 and 2 in these
experiments are identical to that observed by Kirchner and co-
workers, and other mechanistic studies have proven to be
inconclusive (vide supra), we believe that it is plausible that a
similar mechanism might be operating akin to that proposed
by Chirik and Kirchner (Figure S81).6b,9
Because in the above-mentioned experiments alkyne
dimerization was observed, we explored this reaction in more
detail.12 The addition of phenylacetylene to a solution complex
2 (0.5 mol %) in THF gave the corresponding enyne in
quantitative yield and with exclusive Z selectivity. At present,
the substrate scope is limited to aromatic acetylenes such as,
tolyl-, biaryl-, arylhalide-, anisole-, and thiophene-substituted
acetylenes (Table 2). All of these substrates give quantitative
conversion within 2 h to the corresponding (Z)-enynes.
Aliphatic alkynes, such as 1-octyne, only produced an
intractable mixture of products. Compared to the state-of-the-
art for earth-abundant metals,12a the obtained Z selectivity in
Figure 2. (Top) Temperature dependent stereoselective synthesis of
E- and Z-vinylboronate esters. (Bottom) Selective formation of 6a,
6b, 6f, 6i, 6n, and 6l with catalyst 2 under elevated temperatures (50
°C).
5n, and 5l are selectively converted to the (E)-vinylboronate
esters (6) after stirring the reaction mixture for an additional 4
h at slightly elevated temperatures (50 °C). The corresponding
(E)-vinylboronate esters were isolated with excellent stereo-
selectivity (E/Z ≥ 99:1) and in near quantitative yields.
(Figure 2). In the absence of a catalyst, the isolated Z-
vinylboronate esters did not convert to their corresponding
(E)-isomers.
The mechanism for Z-selective hydroboration has been
explored for both earth-abundant and precious metal catalysts.
For earth-abundant metals, Chirik and Kirchner proposed a
mechanism centered on the syn-hydrometalation of an
alkynylboronate intermediate.6b,9 In contrast, for precious
metal catalysts such as rhodium, iridium, and ruthenium,
Miyaura and Leitner proposed a metal vinylidene pathway.4c,5
Both mechanisms feature an overall 1,1-trans addition of the
borane to the alkyne. Recently, a more rare 1,2-trans addition
was also described.4a,10 To distinguish between the 1,1-trans or
1,2-trans hydroboration mechanism, we performed a series of
isotope labeling studies. Under the optimized reaction
conditions, deuterium labeling experiments with DBpin and
phenylacetylene resulted in almost exclusive incorporation of
the deuterium label at the β-carbon (Figures S1−S3).
Incorporation of the deuterium label at the β-carbon rules
out a 1,2-trans-hydroboration mechanistic pathway that has
been recently reported for copper10 and ruthenium.4a Addi-
C
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Table 2. Dimerization of Variously Substituted Alkynes by
Iron Complex 2
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02057.
■
a b
,
sı
Synthetic procedures, characterization data, and catalytic
procedures (PDF)
Accession Codes
CCDC 1994925−1994926 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Graham de Ruiter − Schulich Faculty of Chemistry, Technion −
Israel Institute of Technology, 3200008 Haifa, Israel;
orcid.org/0000-0001-6008-286X; Email: graham@
technion.ac.il
Authors
Subhash Garhwal − Schulich Faculty of Chemistry, Technion −
Israel Institute of Technology, 3200008 Haifa, Israel;
orcid.org/0000-0002-0093-9224
Natalia Fridman − Schulich Faculty of Chemistry, Technion −
Israel Institute of Technology, 3200008 Haifa, Israel
a
Reactions were performed with alkyne (0.5 mmol) and catalyst 2
(0.5 mol %) in THF (0.5 mL) at room temperature for 2 h (Table 1).
b
1
Yields and selectivity were determined by H NMR spectroscopy
with trimethoxybenzene as an internal standard.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02057
our studies are comparable to those reported by Milstein and
Kirchner,6a13 although the herein reported reaction times are
generally shorter. However, the scope of Milstein’s catalyst is
much larger and also features cross-dimerization. It is
interesting to note that Mashima and co-workers observed
the opposite E selectivity for cross-dimerization with a cobalt
bipyridine catalyst,14 while Song and co-workers have shown
geminal specific dimerization with an FeCp* catalyst
containing a bidentate NHC ligand.15 None of these
dimerization patterns have been observed in the herein
reported method under the herein used standard catalytic
conditions (RT, N2).
In summary, we have shown that the stable trans-dihydride
iron complex [(PCNHCP)Fe(H)2N2)] (2) is highly active for
the Z-selective hydroboration of terminal alkynes. The reaction
occurs at room temperature and exhibits a reasonable
functional group tolerance that includes halides, ethers, esters,
and heteroaromatics. The corresponding (E)-vinylboronates
could also be accessed by the same catalyst, by increasing the
reaction temperature to 50 °C. Isotope labeling experiments
suggest a mechanism most-likely based on a formal syn-
hydrometalation of an alkynylboronate intermediate. Addi-
tional experiments established that complex 2 was not only
active for alkyne hydroboration but also for Z-selective alkyne
dimerization. Although the substrate scope was not as
extensive as for the hydroboration, these experiments do
demonstrate that earth-abundant metal complexes based on
our newly reported PCNHCP pincer platform can exhibit
diverse reactivity. Current efforts are directed toward
synthesizing the corresponding manganese, cobalt, and nickel
complexes and exploring their reactivity in a variety of organic
transformations
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Research was supported by the Azrieli Foundations (Israel)
and the Technion EVPR Fund − Mallat Family Research
Fund. G.de.R. also acknowledges the Israel Science Foundation
for an equipment grant (579/20). G.de.R is an Azrieli young
faculty fellow and a Horev Fellow supported by the Taub
Foundation.
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