Well-Defined, Versatile and Recyclable Half-Sandwich Nickelacarborane Catalyst for Selective Carbene-Transfer Reactions

Article abstract of DOI:10.1002/chem.202005014

Catalytic carbene-transfer reactions constitute a class of highly useful transformations in organic synthesis. Although catalysts based on a range of transition-metals have been reported, the readily accessible nickel(II)-based complexes have been rarely used. Herein, an air-stable nickel(II)-carborane complex is reported as a well-defined, versatile and recyclable catalyst for selective carbene transfer reactions with low catalyst loading under mild conditions. This catalyst is effective for several types of reactions including diastereoselective cyclopropanation, epoxidation, selective X?H insertions (X = C, N, O, S, Si), particularly for the unprotected substrates. This represents a rare example of carborane ligands in base metal catalysis.

Full text of DOI:10.1002/chem.202005014

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Catalysis |Hot Paper|
Well-Defined, Versatile and Recyclable Half-Sandwich
Nickelacarborane Catalyst for Selective Carbene-Transfer
Reactions
Linghua Wang,[a] Saima Perveen,[b] Yizhao Ouyang,[a] Shuai Zhang,[a] Jiao Jiao,[b, e] Gang He,[a]
Yong Nie,*[d] and Pengfei Li*[a, c, e]
Abstract: Catalytic carbene-transfer reactions constitute a
class of highly useful transformations in organic synthesis.
Although catalysts based on a range of transition-metals
have been reported, the readily accessible nickel(II)-based
complexes have been rarely used. Herein, an air-stable nick-
el(II)-carborane complex is reported as a well-defined, versa-
tile and recyclable catalyst for selective carbene transfer re-
actions with low catalyst loading under mild conditions. This
catalyst is effective for several types of reactions including
diastereoselective cyclopropanation, epoxidation, selective
XÀH insertions (X = C, N, O, S, Si), particularly for the unpro-
tected substrates. This represents a rare example of carbo-
rane ligands in base metal catalysis.
catalysts in various carbene transfer reactions.[8] In view of the
broad utility of its all neighbors including palladium and plati-
num,[9] therefore, it is surprising that nickel has been rarely
used in the carbene transfer catalysis. Sporadic examples in-
volved donor type carbenes that were derived from unstable
diazo compounds and using air-sensitive Ni0 catalysts.[10,11] For
example, Hillhouse and co-workers prepared Ni0/bis-NHC com-
plex and used it as a catalyst (20 mol%) for cyclopropanation
of 1-hexene (Scheme 1a).[12] Besides, Uyeda’s group reported
the binuclear Ni0 complex and applications in carbene transfer
to tBuNC or Simmon–Smith-type cyclopropanation.[13] Howev-
er, effective transfer reactions of the more synthetically rele-
vant acceptor-containing carbenes using more available NiII
catalysts have been very rare.[14]
Introduction
Carbene transfer reactions, particularly of diazo compounds
with electron-withdrawing substituents, have been extensively
studied using transition-metal catalysts or Lewis acid catalysts,
and successfully applied in preparation of many functional
molecules.[1] Up to now, a number of transition-metals, ranging
from the middle (Cr group)[2] to the late groups (Cu and Zn
groups),[3] have been capable of catalyzing these reactions via
metal–carbene intermediates. Among them, rhodium[4] and
copper[5] are by far the most widely used ones, followed by iri-
dium and ruthenium.[6] In recent years, iron has received signif-
icant attention in this regard and particularly relevant to direct-
ed evolution of heme enzymes.[7] In parallel, cobalt-porphyrin
systems have also been employed as reactive and selective
In most cases, the rate-determining step of a carbene-trans-
fer reaction is denitrogenative formation of the highly electro-
philic metal-carbene intermediate (Scheme 1b).[15] Although
usually the formal valence of the metal center does not
change if the carbene is considered as a neutral ligand, this
step may well be seen as an oxidative addition step that
would lead to higher (formally +2) valence count, particularly
with an electron-withdrawing substituent.[16] Therefore, we rea-
soned that the poor performance of nickel(II) as catalysts in
carbene-transfer reactions was likely due to its lower propensi-
ty to be oxidized. Indeed, in comparison with the neighboring
elements, high-valence nickel complexes have presented sig-
nificant challenges in synthetic coordination chemistry.[17] Con-
sequently, we hypothesized that: 1) introduction of a highly
electron-donating ligand should support formation of high-va-
lence nickel species, and 2) an extensive conjugation system
might provide an electron sink to be compatible with other
steps of the desired catalytic cycle.
[a] L. Wang, Y. Ouyang, S. Zhang, Prof. Dr. G. He, Prof. Dr. P. Li
Frontier Institute of Science and Technology, Xi’an Jiaotong University
Xi’an, Shaanxi 710054 (P. R. China)
E-mail: lipengfei@mail.xjtu.edu.cn
[b] Dr. S. Perveen, Prof. Dr. J. Jiao
Department of Applied Chemistry, School of Chemistry
Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (P. R. China)
[c] Prof. Dr. P. Li
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P. R. China)
[d] Prof. Dr. Y. Nie
Institute for Smart Materials & Engineering, University of Jinan
Jinan, Shandong 250022 (P. R. China)
E-mail: chm_niey@ujn.edu.cn
[e] Prof. Dr. J. Jiao, Prof. Dr. P. Li
Xi’an Key Laboratory of Sustainable Energy Materials Chemistry
Xi’an Jiaotong University, Xi’an, Shaanxi 710049 (P. R. China)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202005014.
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Results and Discussion
Synthesis and characterization of 3,3-(bis(diphenylphosphi-
no)ethane)-3,1,2-closo-NiC2B9H11 (1)
To test our hypothesis, we first synthesized the half-sandwich
nickelacarborane 3,3-dppe-3,1,2-closo-NiC2B9H11 (1) (dppe =
1,2-bis(diphenylphosphino)ethane) and then explored its usage
in catalytic carbene transfer reactions. Compound 1 was pre-
pared by first deboronation of commercial closo-o-carborane
with ethanolic potassium fluoride to form K+[nido-7,8-
C2B9H12]À, followed by coordinative metalation with Ni-
(dppe)Cl2 in refluxing benzene. After removal of potassium
chloride by simply redissolving the crude product in dichloro-
methane and filtration, the desired product could be easily iso-
lated as 1:1 co-crystal with one equivalent of dichloromethane,
1
as indicated in H NMR spectrum, in 87% yield over two steps
(Scheme 2). The co-crystalized dichloromethane could not be
removed under high-vacuum for at least 3 hours.
Scheme 2. Synthesis of 1·CH2Cl2 and crystal structure of thermal ellipsoids
are drawn at 30% probability level. Deposition Number 1948028 contains
the supplementary crystallographic data for this paper. These data are pro-
vided free of charge by the joint Cambridge Crystallographic Data Centre
and Fachinformationszentrum Karlsruhe Access Structures service
Scheme 1. Half-sandwich nickelacarborane for carbene transfer reaction.
closo-Carboranes, particularly o-carborane, featuring three-di-
mensional aromaticity, are known as thermally stable sphere
boron clusters. A partially deboronated derivative of o-carbor-
ane, i.e., nido-dicarbollide, could serve as an h5, p6 5-ligand for
many transition-metals (Scheme 1c).[18] Due to the similarity to
cyclopentadienyl ligands, good thermal stability and structural
tunability, it has long been proposed that nido-dicarbollides
might be used as ligands for catalysis.[19] However, up to date,
metallacarboranes have been scarcely employed as catalysts in
synthetic organic chemistry.[20] Relevant to this work, the early
reports from Hawthorne had shown that nido-dicarbollide was
very strong electron-donor and could support formation of a
stable sandwich NiIV complex.[21] Therefore, during our recent
investigations of boron-based ligands and catalytic cyclopropa-
nation reactions,[22] we wondered if a suitable nickelacarborane
complex could serve as an effective base-metal catalyst for car-
bene-transfer reactions. Herein we wish to report our results
that has led to a well-defined, stable half-sandwich nickelacar-
borane 1 as robust, generally useful, selective, and recyclable
catalyst (Scheme 1d).
www.ccdc.cam.ac.uk/structures. Hydrogen atoms are omitted for clarity.
The diamagnetic nickelacarborane 1 was air-stable at room
temperature for at least one month without observable de-
1
composition, as determined by H and 31P NMR spectroscopy
of its solution structure. The singlet at d = 64.2 ppm in its
31P NMR spectrum indicated a symmetric structure. The solid
state structure of 1 was confirmed by X-ray diffraction of a
single crystal grown from its solution in dichloromethane/n-
hexane. The asymmetric unit contains a molecule of 1 and a
co-crystallized molecule of dichloromethane. The central Ni
atom is surrounded almost “symmetrically” by the dicarbollide
ligand (NiÀC 2.124(3) and 2.116(3) Š; NiÀB 2.065(3)–2.171(3) Š)
and the dppe ligand (NiÀP 2.173(8) and 2.139(8) Š) in a distort-
ed icosahedral framework. Multiple intermolecular non-classical
hydrogen bonds[23] (CÀH···Cl, CÀH···p, BÀH···p, CÀH···HÀB, CÀ
H···HÀC) and CÀCl···p interaction in crystal were found, as
shown in Figures S1–3 and Table S2. The dichloromethane and
the adjacent four nickelacarborane molecules are connected
by two types of C55ÀH55A···H4AÀB4, C55ÀH55B···H1ÀC1 dihy-
drogen bonds with the corresponding H···H distance being
2.20 Š, 2.34 Š, two kinds of CÀH···Cl (H11···Cl1 2.93 Š,
H16A···Cl2 2.85 Š) hydrogen bonds, and CÀCl···p interaction
(Cl2···Cg2 3.57 Š, Figure S3). In addition, the intermolecular CÀ
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H···p (H15B···Cg1 2.97 Š, Figure S1), and BÀH···p interactions
(H9···Cg3 3.19 Š, H7A···Cg4 3.57 Š, Figure S2) further link the
carborane molecules. These data also reveal the +2-oxidation
state of nickel in complex 1, filled with 18-electron configura-
tion, in agreement with its diamagnetism and thermal stability.
from both the carborane ligand and the chelating diphosphine
ligand on the catalytic activity of cyclopropanation. Lastly, the
reaction at 258C was inferior, leading to lower yields but the
diastereoselectivity could be maintained (Entry 8).
Stimulated by the above results, we briefly tested Darzens-
type epoxidation with diazo compounds catalyzed by nickela-
carborane 1 using benzaldehyde as the substrate. Pleasingly,
the reaction afforded the trans-epoxide product in 79% isolat-
ed yield as a single diastereomer (Scheme 3).
Nickelacarborane 1-catalyzed diastereoselective cyclopropa-
nation and epoxidation
We began our investigation in nickelacarborane 1-catalyzed cy-
clopropanation and the diastereoselectivity using styrene
(0.6 mmol) as the substrate and methyl 2-diazo-2-phenylace-
tate (0.2 mmol) as
a donor-acceptor carbene precursor
(Table 1). After screening various reaction parameters, we
found that a combination of 1·CH2Cl2 co-crystal as the catalyst
and NaPF6 as the additive under argon atmosphere at 358C
provided the best results (Tables S3–5). NMR analysis of the
crude product indicated formation of the expected cyclopro-
pane in 88% yield, 90% conversion and its d.r. ratio was
beyond 20:1 (Entry 1). After purification by flash column chro-
matography, the cyclopropane (4a) was obtained in 85% yield.
Under the same condition, we tested the different classes of
nickel catalysts and found that NiCl2, Ni(dppe)Cl2 with chelating
diphosphine ligand, Ni(dtbpy)Cl2 with bipyridine ligand gave
only trace amount of the expected product while no reaction
occurred in the presence of Ni(PPh3)2Cl2 (Entries 2–5). Interest-
ingly, with a different nickelacarborane complex 3,3-(PPh3)2-
3,1,2-closo-NiC2B9H11 as the catalyst,[24] the cyclopropanation
gave 21% yield and over 20:1 d.r. (Entry 6). While only trace
amount of product was produced in the absence of the cata-
lyst (Entry 7). Remarkably, these results revealed the key effects
Scheme 3. Nickelacarborane 1-catalyzed epoxidation.
Nickelacarborane 1-catalyzed XÀH (X = C, N, O, S, Si) inser-
tion and their regio- or chemoselectivity
Encouraged by these results, we further examined the scope
of nickelacarborane 1 as a catalyst for other types of carbene
transfer reactions, such as (hetero)aromatic CÀH functionaliza-
tion or XÀH insertion reactions (X = C, N, O, S, Si) (Scheme 4).
Whenever possible, the results were compared with literature-
reported results using other metal-based catalytic systems.
First, directly using the Ni catalytic system analogous to the
above cyclopropanation (1.7 mol% of 1, CDCl3 as the solvent
at 35–458C), various electron-rich arenes, such as N, N-dime-
thylaniline, phenylpyrrolidine, 1,3,5-trimethoxybenzene gave
CÀH insertion products in good yields (6a 74%, 6b 65%, 6c
75%) and azine as the only byproduct. The catalytic activity
was comparable to the known catalytic systems.[25–30] Further-
more, we found that both substituted and unprotected in-
doles, pyrroles were well tolerated and high regioselectivity,
chemoselectivity and yields were realized (6d–6h). Generally,
unprotected indoles and pyrroles have been problematic due
to regioselectivity between C-2 and C-3 CÀH insertion and che-
moselectivity among cyclopropanation, CÀH insertion and NÀ
H insertion.[31] Pleasingly, in our conditions, the reaction could
selectively afford CÀH insertion products 6e, 6 f and 6h in
high yields. Thus, the Ni catalyst showed better regioselectivity
and chemoselectivity than the previously reported ones based
on Ru,[34] Rh,[35] Cu[33,35] or Pd.[36] Different from indoles, benzo-
furan smoothly underwent cyclopropanation instead of CÀH
insertion, affording the product 6i in 62% yield and comparing
favorably with previous Rh-based system.[37] In contrast, when
2-ethylfuran was used as the substrate, carbene transfer/ring
opening product 6j was isolated in 73% yield.
Table 1. Variations of the nickelacarborane 1-catalyzed cyclo-
propanation.[a]
Entry
Catalyst
Yield (%)[b]
d. r.[b]
1
2
3
4
5
6
7
8[d]
[Ni 1]·CH2Cl2
NiCl2
Ni(dppe)Cl2
Ni(dtbpy)Cl2
Ni(PPh3)2Cl2
[Ni 2]
88 (85[c])
10
8
>20:1
–
–
–
–
>20:1
–
>20:1
3
Next, we investigated that if the Ni catalyst could be also
used in NÀH or OÀH insertion reactions. We particularly fo-
cused on the chemoselectivity in cases where multiple sites
were potentially reactive. First, anilines were found as compe-
tent substrates and the desired NÀH insertion products (6k,
6l–6n, crystal structure of 6l Figure S4) could be isolated in
good yields. The yield from simple aniline was comparable
with reported systems using Rh, Cu, etc.[39,40] Remarkably, ani-
–
21
5
–
[Ni 1]·CH2Cl2
60
[a] Conditions:
2 (0.6 mmol), 3 (0.2 mmol), [Ni] (0.0034 mmol), NaPF6
(0.036 mmol) in 1 mL of CDCl3, 358C, under Ar for 12 h. [b] Yields and d.r.
ratio based on 1H NMR analysis with 1,3,5-trimethoxybenzene as the in-
ternal standard. N.R: no reaction. [c] Yield of isolated product. [d] 258C.
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Scheme 4. Substrate scope and selectivity of nickelacarborane 1-catalyzed carbene transfer reaction.[a] [a] Conditions: 5 (0.6 mmol), 3 (0.2 mmol), [Ni 1]
(0.0034 mmol), NaPF6 (0.036 mmol) in 1 mL of CDCl3, 35–458C, 5–24 h and yield of isolated product. [b] NaBArF (0.006 mmol) was used, instead of NaPF6.
4
[c] 458C, 24 h. [d] Adding 50 equiv 4 Š MS. [e] No sodium additives were added. [f] Reaction solvent: acetonitrile. [g] 10 mmol scale, salicyclic acid (1.5 equiv),
2.6 g 6x was prepared.
line NÀH group could be selectively reacted in the presence of
unprotected hydroxyl even sulfhydryl groups, and the elec-
tron-rich multiple arene CÀH sites. Carbazole, a relatively hin-
dered secondary aniline, still preferred NÀH insertion and gen-
erated 6o in 75% yield. The reaction of benzamide also
formed NÀH insertion product 6p albeit in lower yield.
addition, SÀH insertion reaction of p-methoxythiophenol was
tested and the desired product 6ab could also be isolated in
47% yield.
Furthermore, SiÀH insertion product 6ac was smoothly
formed in high yield using simple triethylsilane as the coupling
partner. Besides the representative push–pull diazoester 3,
simple and commercially available ethyl diazoacetate could
also be successfully used. For example, using catalyst 1, reac-
tion of ethyl diazoacetate with salicyclic acid cleanly formed
the OÀH insertion product 6y in 99% yield. Finally, to exempli-
fy the scalability, 2.6 g of 6x was prepared with the catalytic
system in 10 mmol scale in 91% yield.
This catalytic system worked also well for OÀH insertion re-
actions of alcohols, phenols and carboxylic acids. Thus, using
1.7 mol% of nickelacarborane 1 as the catalyst, reaction of di-
azoester 3 with primary alcohols such as n-butanol and etha-
nol gave the desired products 6q and 6r in excellent yield.
Secondary and tertiary alcohols were also viable substrates al-
though the yields were somehow lower (6s and 6t). Using
phenol and 2-hydroxypyridine as the reaction partner also gen-
erated the desired OÀH insertion products in high yields (6u
and 6v). Insertion to acetic acid was also possible, forming the
product in 82% yield (6w). When salicyclic acid (for 6x, see
Figure S5) or osalmid (for 6z) was used as the substrate, OÀH
insertion selectively took place at the carboxylic acid site and
the phenolic hydroxyl group remained intact. When 3-butenol
was reacted with 3, selective OÀH insertion product 6aa was
isolated in 82% yield; no cyclopropanation was observed. In
The stability and recycling of catalyst 1
Most organometallic catalysts are susceptible to decomposi-
tion when, if possible, isolated from the reaction mixture. In
this work, however, we found that the nickelacarborane cata-
lyst 1 was stable enough that after completion of the catalytic
reactions it could be readily recovered in high yield by usual
silica gel flash chromatography. NMR spectra indicated its
structural homogeneity. Therefore, we attempted to reuse the
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recovered catalyst for further transformations. Pleasingly, we
found that when salicylic acid was used as the substrate, at
least for continuous 5 runs, catalyst 1 could be recovered with
little loss (>87%) and showed nearly constant catalytic activity
(>93% yield) (Figures 1, S18, Table S14 for detail). That a ho-
mogeneous catalyst exhibits both high activity and recyclabili-
ty is remarkable considering that most catalysts need in situ
activation and are not amenable to recycling.[54] We attribute
the robustness of catalyst 1 to its neutrality, stable valence,
and coordinative saturation of nickel, and its unique three di-
mensional aromaticity.
Scheme 5. Mechanistic study of nickelacarborane 1-catalyzed OÀH insertion.
Figure 1. Recycling experiments of 1 in OÀH insertion.
Next, to examine whether the OÀH insertion occurred in a
concerted or stepwise manner, we conducted an aldol-type ex-
periment using 4-nitrobenzaldehyde 8 to trap the possible
enolate intermediate that would form during stepwise process
(Scheme 6a).[56] Thus, the nickelacarborane 1-catalyzed reaction
of (4-methoxyphenyl)methanol 7, methyl 2-diazo-2-(4-
methoxyphenyl)acetate 3’ and 4-nitrobenzaldehyde 8 generat-
ed a mixture of mainly three products. Together with the OÀH
insertion product 9, two diastereomeric three-component ad-
Mechanistic investigation
In order to gain more insights into the mechanism, we chose
the OÀH insertion as a model reaction that was relatively less
studied compared to the well-established concerted processes
for CÀH insertion.[55] First, we did kinetic experiments on OÀH
insertion of salicylic acid and measured the initial rates of the
reaction by in situ 1H NMR spectroscopy (Figures S7–S13, Ta-
bles S10–12, for details). The reaction appeared to have a
nearly first-order dependence of nickel catalyst (1) and diazo
compound (3) while a zeroth-order dependence on salicylic
acid. To investigate the electronic nature of the rate-determin-
ing step, a Hammett plot with a series of para-substituted aryl
methyl diazoesters (Figures S14–S17, Table S13) and a designed
competition experiment were performed (Scheme 5). Thus,
using salicyclic acid as the substrate, simultaneously adding
1.0 equivalent of electron rich 3’ and 1.0 equivalent of electron
poor 3’’, after full conversion, led to corresponding OÀH prod-
ucts 6x’ and 6x’’ in 97:3 ratio, and adding 3 and 3’’ led to 6x
and 6x’’ in 91:9 ratio. Similarly, adding 3’ and 3 led to 6x’ and
6x in 60:40 ratio. The fact that more electron rich diazoester
showed higher reaction rate and a Hammett plot of log (kX/kH)
+
as a function of the characteristic spara value for each sub-
stituent affords a better linear correlation (1 = À0.98, R2 =
0.98) than spara or sparaÀ, suggesting incoming positive charge
on the carbenoid carbon in the transition state.
Scheme 6. Evidence of stepwise process in OÀH insertion.
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ducts 10 and 11 were formed. Besides, controlling experiments
indicated that the formation of 10/11 was neither from a ring
opening reaction of the potential epoxide 12 with benzyl alco-
hol 7 (Scheme 6b) nor from the OÀH insertion product (9)
with aldehyde 8 (Scheme 6c). These results provided evidence
for a stepwise mechanism involving oxonium ylide and/or eno-
late-like intermediates.
lyst system might be extended to other relevant catalytic
transformations such as nitrene-involved reactions and to their
chiral versions.
Acknowledgements
Financial support was provided by the National Natural Science
Foundation of China (Nos. 21672168, 21971202, 21702157), the
Natural Science Basic Research Plan in Shaanxi Province of
China (2020JC-08), the Natural Science Foundation of Shan-
dong Province (No. ZR2017LB008) and assistance by Gang
Chang and Axin Lu at the Instrument Analysis Center of Xi’an
Jiaotong University for HRMS and NMR spectra. P.L. is a Chung
Ying Scholar of XJTU.
On the above basis, we proposed possible catalytic cycles
for nickelacarborane 1-catalyzed carbene transfer reactions as
shown in Figure 2. Thus, unlike many other in situ-activated
catalyst,[57] 1 serves as the genuine catalyst and slowly reacts
with a diazoester to generate nickel-carbene complex Int-1.
The electrophilic Int-1 then reacts with a hydroxyl group
donor ROH to form an oxonium ylide Int-2. With or without
nickel dissociation, Int-2 may behave as an ester enolate and
be trapped by an aldehyde through aldol reaction. A formal
1,2-proton shift of Int-2 then affords the OÀH insertion prod-
uct and regenerate catalyst 1. Reactions including cyclopropa-
nation, epoxidation, CÀH and XÀH insertion reactions might
take place from Int-1 via processes analogous to previously re-
ported other transition metal-carbene intermediates.[58]
Conflict of interest
The authors declare no conflict of interest.
Keywords: carbenes · carboranes · homogeneous catalysis ·
nickel · selectivity
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Conclusions
In summary, we have demonstrated that the readily accessible
yet previously incapable base metal nickel(II) species could be
enabled with a dicarbollide ligand for carbene transfer reac-
tions. In particular, the stable nickelacarborane complex 1
acted as a well-defined, highly active and recyclable catalyst
for a broad range of carbene transfer reactions under opera-
tionally simple conditions. In most cases, high chemoselectivity
and diastereoselectivity if applicable were achieved. With cata-
lyst 1, chemoselective or site-selective XÀH insertion (X = C, N,
O, S, Si) reactions of unprotected substrates are possible. Con-
sidering the structural modularity of 1 and the unique elec-
tronic property of dicarbollide ligand, this new mode of cata-
Chem. Eur. J. 2021, 27, 5754 –5760
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Full Paper
doi.org/10.1002/chem.202005014
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Manuscript received: November 18, 2020
Revised manuscript received: December 30, 2020
Accepted manuscript online: January 17, 2021
Version of record online: March 3, 2021
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