Base-catalyzed diborylation of alkynes: synthesis and applications of cis-1,2-bis(boryl)alkenes

Article abstract of DOI:10.1007/s11426-018-9344-4

An efficient, transition-metal free, and practical approach to cis-bis(boryl)alkenes from various alkynes was disclosed in the presence of a catalytic amount of K₂CO₃ under mild conditions. Meanwhile, tetrasubstituted alkenes and phenanthrene derivatives were readily constructed from the target diborylalkenes via Suzuki-Miyaura cross coupling.

Full text of DOI:10.1007/s11426-018-9344-4

SCIENCE CHINA
Chemistry
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Base-catalyzed diborylation of alkynes: synthesis and applications
of cis-1,2-bis(boryl)alkenes
Zhijie Kuang1†, Guoliang Gao1† & Qiuling Song1,2*
1Institute of Next Generation Matter Transformation, College of Chemical Engineering, Huaqiao University, Xiamen 361021, China;
2State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
Received June 3, 2018; accepted August 1, 2018; published online September 14, 2018
An efficient, transition-metal free, and practical approach to cis-bis(boryl)alkenes from various alkynes was disclosed in the
presence of a catalytic amount of K2CO3 under mild conditions. Meanwhile, tetrasubstituted alkenes and phenanthrene deri-
vatives were readily constructed from the target diborylalkenes via Suzuki-Miyaura cross coupling.
base-catalyzed, diborylation of alkynes, cis-1,2-bis(boryl)alkenes
Citation:
Kuang Z, Gao G, Song Q. Base-catalyzed diborylation of alkynes: synthesis and applications of cis-1,2-bis(boryl)alkenes. Sci China Chem, 2018, 61,
https://doi.org/10.1007/s11426-018-9344-4
With the development of Suzuki-Miyaura cross-coupling
reactions, alkenylboron compounds have gained great in-
terests as key intermediates for the synthesis of diverse
functional molecules as electronic materials [1] and bioac-
tive natural products [2]. Diborylalkenes are particularly
attractive building blocks for multi-substituted alkenes [3]
and π-extended polyarenes [4], and their expedient synthesis
has attracted much attention from synthetic chemists. A
number of transition metals, such as cobalt, copper [5], ir-
idium [6], gold [7], iron [8], platinum [9], cobalt [10a] and
palladium [10b,10c] have been utilized in both homogenous
and heterogenous catalytic additions of B–B bonds (e.g.,
diboration) to alkynes rendering cis-bis(boryl)alkenes
(Scheme 1(a)). Among them, platinum is by far the most
effective and widely studied, which is attributed to the facile
cleavage of the B–B bond and the lability of the corre-
sponding bis(boryl)platinum complexes [9]. Although these
metal-catalyzed approaches to cis-bis(boryl)alkenes proceed
successfully, a metal-free diboration [11] process has long
been desired, partially because these boron compounds are
intermediates for the preparation of drugs and electronic
materials where a heavy metal can be a vital problem. In
recent years, organocatalytic protocols have also been de-
veloped, examples include PBu3 [12a], (PhS)2 [12b], and
PPh3 [12c], however, trans-bis(boryl)alkenes instead of the
cis ones were afforded (Scheme 1(b)). To our knowledge, a
transition-metal-free version of diboration (B2pin2) of al-
kynes leading to 1,2-diborylalkenes has not been well studies
and the rare elegant examples [13] for theses also covered the
trans-diboration of propargylic alcohols with stoichiometric
amount of n-BuLi (Scheme 1(c)), which is air- and moisture-
sensitive and has strong nucleophilicity, such caused a lim-
ited functional group tolerance. Herein, we report an efficient
and convenient approach to cis-bis(boryl)alkenes through a
catalytic amount of K2CO3 catalyzed reaction between var-
ious alkynes and B2pin2 (Scheme 1(d)). The simple, mild and
transition-metal-free reaction conditions as well as cheap and
readily available K2CO3 as catalyst are attractive features for
this protocol.
Based on our previous studies on domino-borylation-pro-
todeboronation (DBP) [14] strategy and base-promoted B–B
†These authors contributed equally to this work.
*Corresponding author (email: qsong@hqu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

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Table 1 Optimization of reaction conditions
Conditions: 1a (0.2 mmol), 2 (x equiv.), K2CO3 (y equiv.), Et2O (1 mL),
MeOH (10 equiv.). a) GC yield; b) isolated yield.
Scheme 1 Synthesis of vinyl 1,2-bis(boronates) (color online).
Table 2 Synthesis of cis-bis(boryl)alkenes from terminal aliphatic al-
bond activation and C–B bond formation, we directly
screened base catalysts and found that a catalytic amount
kynesa)
K2CO3 (other bases such as Cs2CO3, KOtBu, K3PO4 were
also effective) can promote the diboration of alkynes. We
commenced our studies by examining the reaction between
5-phenyl-1-pentyne (1a) with B2pin2 (2) under catalytic
amount of K2CO3 in the presence of MeOH. Further
screening delivered 70% of the desired 1,2-diborylalkene
(3a) when 0.1 equiv. of K2CO3 was used as the catalyst with
3 equiv. of B2pin2 in Et2O at 50 °C for 12 h (Table 1, entry 1).
It was of note that further increasing (Table 1, entries 2–4) or
reducing (Table 1, entries 5, 6) the amount of K2CO3 only led
to the inferior yields. Of note, stoichiometric amount base
would lead to alkyl 1,2-bis(boronates) [11g, 14e] (entry 4).
Screening on the amount of B2pin2 suggested that 2 equiv. is
the best one (Table 1, entries 7–10). To further optimize this
condition, the impact of temperature was examined as well
(Table 1, entries 11, 12) and it turned out that 40 °C is the
a) Conditions: alkyne 1 (0.2 mmol), B2pin2 (2) (2 equiv.), K2CO3 (0.1
optimal one, since alkyl 1,1,2-tris(boronate) [14e] was pro-
duced at higher temperatures.
equiv.), Et2O (1 mL), MeOH (10 equiv.), 12 h, N2; isolated yield. b) B2pin2
(4 equiv.), K2CO3 (0.3 equiv.), Et2O (2 mL).
With the optimal conditions in hand (Table 1, entry 12), the
substrate scope of terminal aliphatic alkynes were first in-
vestigated and the results were shown in Table 2. In addition
to 3a, 3b and 3c in which a benzene ring was attached on the
end of the linear alkyl chain could be obtained in good yields,
we found that a more general terminal alkynes which contain
no benzene rings were compatible for this transformation as
well and when they were subjected to the standard condi-
tions, good yields were obtained (Table 2, 3d–3j). It is worth
mentioning that 1,7-octadiyne could acquire mono-alkyne
3k under the standard conditions in 80% yield, gratifyingly,
when the amount of B2pin2 was increased to 4 equiv., 3l,
which contained four boryl moieties was obtained in 70%
yield. We noticed the subtlety of the reaction conditions that
five times amount of base (0.5 equiv. NaOMe) gave the alkyl
1,2-bis(boronates) at the same temperature [11g], higher
temperature (60 °C) and six times amount of base (0.6 equiv.
K2CO3) yielded the alkyl 1,1,2-tris(boronates) [14e], and the
stronger, stoichiometric Cs2CO3 led to the alkyl 1,2-bis
(boronates) [14e] with the same substrates.
We next investigated the scope of internal alkynes under
the optimal conditions and the results were listed in Table 3.
As we can see, linear aliphatic internal alkynes were com-

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Table 3 Synthesis of cis-bis(boryl)alkenes from internal alkynesa)
Table 4 Synthesis of cis-bis(boryl)alkenes from terminal aromatic al-
kynesa)
a) Conditions: alkyne 6(0.2 mmol), B2pin2 (2 equiv.), K2CO3 (0.2
equiv.), Et2O (0.2 mL), MeOH (10 equiv.), 12 h, N2, isolated yield.
substrates suffered with the more (0.5 equiv.) and stronger
base (NaOMe) to render the mono(boryl)alkenes at same
temperature [11g], the alkyl 1,1,2-tris(boronates) [14e] (0.3
equiv. K2CO3 at 60 °C) and monoalkylboranes [14f] (4
equiv. Cs2CO3 at 70 °C).
This transformation has excellent chemoselectivity which
was demonstrated by employing our conditions to some
olefin-containing alkynes even conjugate enyne (8a) (Table
5), most remarkably, internal alkene with terminal alkyne 8b
was compatible in this transformation with excellent che-
moselectivity.
Based our DBP studied [14e–14g] and previous reports
[11], we proposed a path to for this transformation depicted
in Scheme 2. First, B2pin2 is activated in the present of base
and MeOH, resulting the [BaseH]+·[B2pin2·MeO]− A in situ.
Subsequently, the activated and nucleophilic A species at-
tacks the C–C triple bond of alkyne, leading to the adduct B.
Both Bpin moieties attach on the C–C triple bond along with
the conjugated acid of the base (BaseH+) and MeO− to form
cis-1,2-bis(boryl)alkenes and MeOH and regenerate the base
catalyst.
To demonstrate the utility of these transformations, gram-
scale reaction was performed under the standard conditions
for the formation of cis-bis(boryl)alkenes. Treatment of
5 mmol of 1-phenyl-1-butyne (4e) with B2pin2 afforded the
corresponding desired product 5e in 84% yield (1.62 g) un-
der the optimal conditions (Scheme 3(a)). As we can see,
even if the reaction scale was magnified up to 25 times,
comparable synthetic valuable yield was still obtained. The
utility of cis-bis(boryl)alkenes was demonstrated by further
extension of compound 5e into polyaryl olefins 10 [15] and
11b [4b] via a Suzuki-Miyaura crossing-coupling reaction
(Scheme 3(b, c)). Very interestingly yet not surprisingly,
when cis-bis(boryl)alkene 3a was exposed under basic
conditions (in the presence of 1.5 equiv. of Cs2CO3), corre-
sponding 1,2-bis(boronate) 12 [14e] was obtained at 80 °C,
which was consistent with our previous research as well as
precedent report. Further exposure of compound 3a to 0.6
a) Conditions: alkyne 4 (0.2 mmol), B2pin2 (2) (2 equiv.), K2CO3 (0.1
equiv.), Et2O (1 mL), MeOH (10 equiv.), 12 h, N2, isolated yields.
patible under the standard conditions, leading to the corre-
sponding cis-bis(boryl)alkenes with high chemoselectivity in
good to excellent yields (5a–5c, 80%–84%). The broad
substrate scope of alkynes was further demonstrated by the
successful reaction of aryl-(alkyl) internal alkynes (4d–4i) to
products 5d–5i (78%–90%), and also by the reaction of 4k–
4m, which were sterically hindered around the triple bond
yet were all well tolerated in this transformation. Moreover,
the structure of vicinal diborylalkene 5j was unambiguously
confirmed by X-ray single crystal analysis. Not surprisingly,
1-phenyl-1-octyne (4i) can acquire meritoriously yield
(89%), so we intend to change the substituents on the ben-
zene ring to explore the compatibility of the reaction, and the
corresponding cis-bis(boryl)alkenes were obtained in decent
yields (5n–5o, 74%−78%). Moreover, the heteroaromatic
ring was also good candidates and the target molecule was
generated in moderate yields (5p, 50%). But, when the
substrate with strong electron-deficient groups, such as cy-
ano, ketone and ester, the major products were geminal-di-
borylalkanes [14g].
The substrate scope was subsequently explored and broa-
dened by investigating the more specific terminal aryl al-
kynes (Table 4) (we screened the conditions again in order to
gain best results, see Supporting Information online for de-
tails). To our delight, this protocol was found to successfully
incorporate all above terminal aromatic alkynes into the
corresponding diborylalkenes in moderate yields with high
regio-selectivities (7a–7f, 33%–43%). However, the same

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Table 5 Synthesis of cis-bis(boryl)alkenes from special C–C double
Table 6 The construction of phenanthrene derivatives with diaryliodo-
nium salts and diborylalkenes
bond-containing alkynesa)
a) Conditions: alkyne 8(0.2 mmol), B2pin2 (2) (2 equiv.), K2CO3 (0.1
equiv.), Et2O (1 mL), MeOH (10 equiv.), 12 h, N2, isolated yield.
We envisaged that the target diborylalkenes would react
with diaryliodonium salts, which could be considered as a
diiodo compounds, by Suzuki-Miyaura cross coupling to
give phenanthrene derivatives. Gratifyingly, by brief condi-
tion screening, we can get the desired product with excellent
yields (see Supporting Information online for details). Sub-
sequent substrate scope for this protocol was carried out.
When the substituents R were Me, Et, Ph [16], the trans-
formation could be achieved smoothly to yield target pro-
ducts (11a–11c) with good yields (Table 6). However, when
the diborylalkenes were prepared from terminal alkynes and
aliphatic alkynes, the transformation was failed to afford
desired product (for example 11d).
In conclusion, we have developed an efficient and con-
venient approach for the synthesis of cis-bis(boryl)alkenes
through a transition-metal-free and base-catalyzed reaction
between various alkynes with B2pin2 under mild and op-
erational simple strategy. This method furnishes structurally
diverse cis-bis(boryl)alkenes in good yields and with high
chemoselectivity. Thus, the method allows the synthesis of
cis-bis(boryl)alkenes in a convergent and stereoselective
manner from simple precursors under mild conditions and
without the use of transition-metal. And the differences be-
tween this work and previous work [14e–14g] are the the
substrates and the temperature. Importantly, the installed
boron substituents of the cis-bis(boryl)alkenes could be
transformed effectively with Suzuki coupling and rendered
tetra-substituted olefins and phenanthrene derivatives which
are key scaffolds in many bioactive molecules. At the same
time, 1,2-diborylalkanes and 1,1,2-trisborylalkanes could
also be readily accessible under our standard basic condi-
tions.
Scheme 2 Proposed mechanism to cis-1,2-bis(boryl)alkenes.
Scheme 3 Gram-scale synthesis and transformations of cis-bis(boryl)al-
kenes (color online).
Acknowledgements This work was supported by the National Natural
Science Foundation (21772046), Program of Innovative Research Team of
Huaqiao University (Z14X0047), the Recruitment Program of Global Ex-
perts (1000 Talents Plan), the Natural Science Foundation of Fujian Pro-
vince (2016J01064) and Postgraduates’ Innovative Fund in Scientific
Research of Huaqiao University for K. Z. We also thank Instrumental
Analysis Center of Huaqiao University.
equiv. of K2CO3 in Et2O with MeOH led to alkyl 1,1,2-tris
(boronates) 13 [14e] in 79% yield (Scheme 3(d)), which
further validates our previous report that cis-bis(boryl)al-
kenes is not stable and Domino-Borylation process will oc-
cur under basic conditions under suitable temperature.
Conflict of interest The authors declare that they have no conflict of
interest.

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Supporting information The supporting information is available online
at http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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