Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-Diborylalkanes from Terminal and Internal Alkenes

Article abstract of DOI:10.1002/anie.202002642

A general and atom-economical synthesis of 1,1-diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth-abundant zirconium-based catalyst, which allows a balance of self-contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long-chain aliphatic alkenes that did not react before also underwent 1,1-diboration in our system. Significantly, the unprecedented 1,1-diboration of internal alkenes enabled the preparation of 1,1-diborylalkanes.

Full text of DOI:10.1002/anie.202002642

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Accepted Article
Title: Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-
Diborylalkanes from Terminal and Internal Alkenes
Authors: Xianjin Wang, Xin Cui, Sida Li, Yue Wang, Chungu Xia,
Haijun Jiao, and Lipeng Wu
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10.1002/anie.202002642
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COMMUNICATION
Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-
Diborylalkanes from Terminal and Internal Alkenes
Xianjin Wang,[a,c] Xin Cui,[a] Sida Li,[a] Yue Wang,[a] Chungu Xia,[a,*] Haijun Jiao,[b,*] and Lipeng Wu[a,*]
[a]
X. Wang, X. Cui, S. Li, Dr. Y. Wang, Prof. C. Xia, Dr. L. Wu
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP)
Chinese Academy of Sciences, Lanzhou, 730000 (P. R. China)
E-mail: cgxia@licp.cas.cn; lipengwu@licp.cas.cn
Prof. H. Jiao
Leibniz-Institut für Katalyse e. V. Albert-Einstein-Straße 29a, Rostock, 18059 (Germany)
E-mail: haijun.jiao@catalysis.de
[b]
[c]
X. Wang
University of Chinese Academy of Sciences, Beijing, 100049 (P. R. China)
Supporting information for this article is given via a link at the end of the document.
Abstract:
A general and atom-economical synthesis of 1,1-
diborylalkanes from bulk alkenes with borane under H2-acceptorless
conditions is reported for the first time. The key to our success is the
use of an earth-abundant zirconium-based catalyst, which allows a
balance of self-contradictory reactivities (dehydrogenative boration
and hydroboration) to be achieved. Our methodology avoids using
excess amount of another alkene as H2-acceptor that was otherwise
required in the reported system. Furthermore, substrates such as
simple long-chain aliphatic alkenes that did not work before also
worked in our system. Significantly, the unprecedented 1,1-
diboration of internal alkenes for the preparation of 1,1-
diborylalkanes was also realized.
In recent years, 1,1-diborylalkanes have emerged as versatile
building blocks and fundamental intermediates.[1] Traditionally,
they were synthesized from bis(boryl)methane with alkylhalides[2]
or the boration of geminal dihalides.[3] Approaches such as the
dihydroboration of alkynes[4] and the hydroboration of vinyl
boronates were also known,[1a, 1c, 5] but they are relatively less
practical because of the accessibility of substrates. Hence,
several other approaches have recently been developed, for
instance, the insertion of diazoalkanes into diborons,[6] the Ir- or
Co- catalyzed benzylic C–H diboration[7] and boryl group direct
C–H borylation of alkyl boronic esters;[8] the deoxygenative
diboration of carbonyls,[9] expanding the synthetic routes to 1,1-
diborylalkanes.
≠
Scheme 1. Catalytic 1,1-diboration of alkenes with borane: a) challenges
and possible side pathways; b) strategy by using excess amount H2-acceptor;
c) our current H2-acceptorless methodology for the (remote) 1,1-diboration of
terminal and internal alkenes.
Despite the above progress, the synthesis of 1,1-
diborylalkanes from readily available and bulk alkene has
seldom been achieved. Actually, only two systems were
reported to our knowledge. In 2017, a nickel-catalyzed 1,1-
diboration of alkenes with B2pin2 as the boration reagent was
developed.[10] On the other hand, the direct 1,1-diboration of
alkenes using relatively inexpensive borane as the boration
reagent has proven to be challenging because of the robust
reduction ability of borane which, when reacting with alkenes,
would preferably afford the saturated mono-hydroboration
products and the reaction would be terminated.[11] To achieve
the selective 1,1-diboration of alkenes with borane, a catalyst
should fulfill the following criteria: 1) chemo-selective formation
of the vinyl boronate ester instead of alkyl boronate ester (DHB
over HB; DHB
=
dehydrogenative boration, HB
=
hydroboraiton);[11c, 12] 2) proceed chemo-selective HB over DHB
of vinyl boronate ester, to produce diborylalkane rather than
diborylalkene[1o, 13]; 3) regio-selective HB of vinyl boronate ester
at the α-position of the bulky -Bpin group instead of at the β-
position (1,1-diboration over 1,2-diboration).[14] Thus, apart from
selectivity issues, a catalyst must also possess self-contradictory
reactivities (DHB and HB), which is difficult to balance (Scheme
1a). Indeed, it was not until 2018 when a Co(acac)2/phosphine
system for the synthesis of 1,1-diborylalkane from alkenes and
HBpin was reported, but 1.2 equiv. of norbornene or cyclooctene
had to be added as H2-acceptor (Scheme 1b).[15] Besides, the
substrates were limited to 1,1-disubstituted alkenes. Moreover,
no system was known to be applicable to the bulkier and
industrially favored internal alkenes.[16] Thus, a general and
direct H2-acceptorless 1,1-diboration of bulk alkenes with borane
1
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Table 1. Zirconium-catalyzed 1,1-diboration of styrene: condition
optimization[a]
Yields [%][b]
Entry
Catal.
Bases
3
0
4
0
5
6
6
0
1
2
Cp2ZrCl2
Cp2ZrHCl
Cp2ZrCl2
Cp2ZrCl2
Cp2ZrMe2
CpZrCl3
Cp2ZrHCl
Cp2ZrCl2
Cp2ZrCl2
-
-
-
0
73
47
2
0
0
3
MeONa
MeOLi
MeOLi
MeOLi
MeOLi
MeOLi
MeOLi
MeOLi
29
88
63
3
11
6
1
4
1
5
0
34
84
7
2
6
1
12
1
7
87
81
69
0
1
8[c]
9[d]
10
4
8
1
1
24
83
3
0
12
[a]Reaction conditions: 0.2 mmol 1, 0.6 mmol HBpin, 1 equiv. of bases, 5 mol%
catal., 1 mL of toluene in 15 mL pressure tube and heated at 100 C for 8 h;
[b]yields were determined by GC-MS using n-dodecane as internal standard;
[c]reaction was performed at 80 C; [d]reaction was performed with 2 mol% of
catalyst loading.
for
the
preparation
of
1,1-diborylalkanes
remains
underdeveloped. Herein, we report our application of
a
zirconium-based catalyst that allows the first example of H2-
acceptorless 1,1-diboration of alkenes with borane to be realized.
Our system is operationally simple, cost-efficient, and suitable
for a series of different alkenes ranging from aryl alkenes to
Scheme 2. Substrate generality for zirconium-catalyzed 1,1-diboration of
various alkenes. Reaction conditions: 0.2 mmol alkenes, 0.6 mmol HBpin, 5
mol% Cp2ZrCl2, 0.2 mmol MeOLi and 1 mL toluene in 15 mL pressure tube,
heated at 100 C for 8 - 16 h, isolated yields are given.
long-chain
aliphatic
alkenes.
Most
significantly,
the
unprecedented remote 1,1-diboration of internal alkenes was
also achieved (Scheme 1c).
Cp2ZrHCl in the presence of MeOLi gave almost similar results
to Cp2ZrCl2 (Table 1, entry 7). In addition, the reaction
temperature and catalyst loading could be further reduced, albeit
slightly lower yields of 3 were obtained (Table 1, entries 8-9). It
is noted that when the reaction was performed with MeOLi alone,
only the hydroboration products 5 and 6 were obtained (Table 1,
entry 10). It is also noted that product 5 was not the intermediate
of 3 as submitting it to the catalytic conditions with HBpin did not
lead any reactions.
Early transition metals often have different structures and
orthogonal reactivities compared with late ones.[17] Moreover,
several of them are earth-abundant, e.g., zirconium, which is
almost as abundant as carbon. In addition, zirconocene
complexes were reported to be able to catalyze the
hydroboration of alkynes[18] and alkenes.[19] We then became
interested in exploring the possibility of using zirconocene
complexes to achieve the unreported H2-acceptorless 1,1-
diboration of alkenes. In a model reaction, 0.2 mmol styrene (1)
and 0.6 mmol HBpin (2) in 1 mL toluene was reacted with 5
mol% of different zirconocene complexes at 100 C. First, the
reaction with Cp2ZrCl2 was carried out, but only 6% mono
hydroboration product 5 was obtained (Table 1, entry 1). Then,
the reaction with Cp2ZrHCl was performed and 73% of the
dehydrogenative boration product 4 was observed (Table 1,
entry 2). Interestingly, we found that the addition of MeONa
promoted the formation of 1,1-diborylalkane 3 in 29% yield
(Table 1, entry 3). Changing the base from MeONa to MeOLi
largely improved the yield of 3 to 88% (Table 1, entry 4). The
use of other zirconium-catalysts such as Cp2ZrMe2 and CpZrCl3
gave inferior results (Table 1, entries 5-6). While the use of
We then investigated the generality of different alkenes
under the optimized reaction conditions (Table 1, entry 4). We
were pleased to find that our methodology worked well with
numerous aryl alkenes with electronically and sterically varied
substituents (Scheme 2). Thus, substrates with -Me, -tBu, -Ph, -
OMe, -NMe2, -CF3 groups at various positions on the phenyl ring
reacted well in our system (65-82% isolated yields, 3, 7–12, 17–
18, 22–23). In addition, halogen-substituted aryl alkenes
selectively produced the corresponding 1,1-diborylalkanes
without side reactions from C–X bonds (X = F, Cl, Br), and yields
up to 80% were obtained (13–15, 19–21, 24–25). Notably, the -
NH2 group which was supposed to react with HBpin was
survived in our system (16). Moreover, the reaction also worked
2
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10.1002/anie.202002642
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Scheme 3. Proposed reaction mechanism.
well with poly-substituted alkenes, for example, 1,4-dimethyl and
pentafluoro-substituted aryl alkenes afforded their products 26
and 27 in up to 80% yields. Vinyl naphthalene and α-methyl
styrene yielded 28 and 29 in 83% and 40% yields. Starting from
vinyl ferrocene, ferrocenyl-substituted 1,1-diborylalkane 30 was
also obtained. Further extending the substrates’ scope to
heteroatom-containing alkenes were also successful: 2-vinyl-
with HBpin and forms Cp2Zr(H)Bpin species (B) with release of
H2. Then, insertion of alkene into the Zr-B bond forms the
zirconium boryl alkyl species (C) which gives the vinyl boronate
ester intermediate
4
after β-H elimination. Addition of
Cp2Zr(H)Bpin (B) to vinyl boronate ester 4 affords zirconium
diboryl alkyl species (D), which then reacts with another HBpin
via σ-bond metathesis to give 1,1-diborylalkane 3 (Scheme 3).
thiophene,
5-vinylbenzofuran,
and
3-vinyl-9H-carbazole
Mixtures of internal alkenes, commonly obtained from
cracking processes, are substantially cheaper and more easily
accessible.[16, 21] However, the selective remote 1,1-diboration of
simple and bulk internal alkenes has not yet been reported. It
was well known that stoichiometric amount of zirconium-reagent
could mediate the isomerization of double bonds[22] and one
catalytic system has recently been reported.[23] Then, we
became interested to explore whether the isomerization of
internal alkenes to terminal alkenes and the subsequent
selective 1,1-diboration could be achieved using our system. To
our delight, a variety of internal alkenes with different chain
lengths and positions of the double bond worked well in our
system (Scheme 4).
underwent efficient 1,1-diboration in yields up to 85% (31–33).
Of note was that the -NH group on the carbazole moiety
remained intact (33).
Next, we turned our attention to validate the generality of
our methodology to long-chain aliphatic alkenes. We were
pleased to find that our protocol worked for a series of bulk
aliphatic alkenes with different carbon lengths albeit with
reduced yields due to the presence of alkane and mono-
hydroboration side-products: thus 34–36 were obtained in up to
65% yields. In addition, our diboration process was also
applicable to aliphatic alkenes with various substituents:
cyclohexyl-, cyclopentyl-, and phenyl-substituted aliphatic
alkenes all worked well to give the corresponding products in
yields up to 72% (37–43). N-Allylaniline was one exception, in
which case, only 14% of 44 was obtained. Fortunately, other N-
and O-containing moieties, such as methyl benzylamine and
benzyl ether, all survived the 1,1-diboration process, with yields
ranging from 61% to 73% (45–47).
Then, in order to gain insights into the reaction mechanism,
we carried out the following experiments: analyzing the
stoichiometric reaction of Cp2ZrCl2, MeOLi, and HBpin (molar
ratio 1:2:2) revealed the formation of MeO-Bpin, which was also
detected in the catalytic reaction with or without styrene (Figure
1
S1-S3). In addition, Zr-H was detected on H NMR spectrum by
trapping with acetone (Figure S4). Furthermore, H2 and vinyl
boronate ester 4 was observed on 1H NMR spectrum in the
catalytic reaction with styrene (Figure S5). Moreover, the
sequential addition of MeOLi followed by HBpin to the Cp2ZrCl2
solution in d8-toluene showed obvious color change from
colorless to dark brown and the chemical shift changes of the Cp
1
and MeO- peaks as detected by H NMR spectroscopy (Figure
S6). On the basis of those observations and literature
precedents,[20] we proposed the following reaction mechanism:
Cp2ZrCl2 first interacts with 2 equiv. MeOLi to yield Cp2Zr(OMe)2,
and then further reacts with HBpin to form Cp2ZrH2 (A) and
MeO-Bpin. Cp2ZrH2 (A) enters into the catalytic cycle by reaction
Scheme 4. Zirconium-catalyzed remote 1,1-diboration of various internal
alkenes. Reaction conditions: 0.2 mmol internal alkenes, 0.6 mmol HBpin, 5
mol% Cp2ZrCl2, 0.2 mmol MeOLi and 1 mL toluene in 15 mL pressure tube,
heated at 100 C for 8 - 16 h; isolated yields are given.
3
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Conflict of interest
The authors declare no conflict of interest
Keywords: zirconium • 1,1-diborylalkanes • H2-acceptorless •
alkenes • boration
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4
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10.1002/anie.202002642
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Entry for the Table of Contents
A general and atom-economical synthesis of 1,1-diborylalkanes from various alkenes with borane under H2-acceptorless conditions is
reported for the first time. Many aryl alkenes and bulk long-chain aliphatic alkenes that did not work in other systems also worked
here. Significantly, the unprecedented 1,1-diboration of internal alkenes for the preparation of 1,1-diborylalkanes is also realized.
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