Regio- and Stereoselective 1,2-Carboboration of Ynamides with Aryldichloroboranes

Article abstract of DOI:10.1002/anie.202107647

Catalyst-free 1,2-carboboration of ynamides is presented. Readily available aryldichloroboranes react with alkyl- or aryl-substituted ynamides in high yields with complete regio- and stereoselectivity to valuable β-boryl-β-alkyl/aryl α-aryl substituted enamides which belong to the class of trisubstituted alkenylboronates. The 1,2-carboboration reaction is experimentally easy to conduct, shows high functional group tolerance and broad substrate scope. Gram-scale reactions and diverse synthetic transformations convincingly demonstrate the synthetic potential of this method. The reaction can also be used to access 1-boraphenalenes, a class of boron-doped polycyclic aromatic hydrocarbons.

Full text of DOI:10.1002/anie.202107647

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Accepted Article
Title: Regio- and Stereoselective 1,2-Carboboration of Ynamides with
Aryldichloroboranes
Authors: Cai You, Mika Sakai, Constantin Gabriel Daniliuc, Klaus
Bergander, Shigehiro Yamaguchi, and Armido Studer
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10.1002/anie.202107647
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Regio- and Stereoselective 1,2-Carboboration of Ynamides with
Aryldichloroboranes
Cai Youa, Mika Sakaib, Constantin G. Daniliuca, Klaus Bergandera, Shigehiro Yamaguchib,c,* and
Armido Studera,*
In memoriam Klaus Hafner
Abstract: Catalyst-free 1,2-carboboration of ynamides is presented.
Readily available aryldichloroboranes react with alkyl- or aryl-
substituted ynamides in high yields with complete regio- and
stereoselectivity to valuable -boryl--alkyl/aryl -aryl substituted
enamides which belong to the class of trisubstituted alkenylboronates.
The 1,2-carboboration reaction is experimentally easy to conduct,
shows high functional group tolerance and broad substrate scope.
Gram-scale reactions and diverse synthetic transformations
convincingly demonstrate the synthetic potential of this method. The
reaction can also be used to access 1-boraphenalenes, a class of
boron-doped polycyclic aromatic hydrocarbons.
electrophilic boranes can directly be used for alkyne
carboboration. However, in contrast to the well-studied 1,1-
carboboration, the “Wrackmeyer” reaction,[9] only a few reports on
catalyst-free direct 1,2-carboboration have appeared to date, and
these procedures suffer from specific boron species or low yields,
which limits their applicability in organic synthesis.[10,11] Therefore,
the development of an efficient and practical catalyst-free direct
1,2-carboboration with readily available boron reagents, which
would expand the scope and the synthetic utility of boron-
mediated organic transformations, is highly desirable.
Organoboron compounds are versatile reagents in organic
synthesis that also play an important role in materials science and
medicinal chemistry. Particularly, alkenylboronates are highly
useful building blocks in organic synthesis because of their broad
application as substrates in the Suzuki-Miyaura cross-coupling[1],
conjugate additions,[2] Zweifel olefination[3] and other interesting
transformations that proceed via their alkenylboronate
complexes.[4] To date, many methods for the preparation of
alkenylboronates have been developed; however, regio- and
stereoselective
construction
of
-trisubstituted
alkenylboronates, which can be used as precursors in the
stereoselective synthesis of tetrasubstituted alkenes, is
challenging.[5] The carboboration of internal alkynes with
organoboron compounds, in which a C−C and a C−B bond are
formed, offers
a straightforward approach towards -
trisubstituted alkenylboronates. Along these lines, metal-
catalyzed[6] (Scheme 1a) and phosphine-catalyzed[7] (Scheme 1b)
1,2-carboboration of internal alkynes have been disclosed.
Recently, atom economic environmentally benign catalyst-free
direct carboboration, which offers an alternative pathway to -
trisubstituted alkenylboronates, has received growing attention.
Scheme 1. Synthesis of -trisubstituted alkenylboronates via direct 1,2-
carboboration of internal alkynes (EWG = electron withdrawing group).
For example, Uchiyama and Hirano reported
a
1,2-
alkynylboration of alkynes enabled by intramolecular activation of
alkynylboronates by propargylic alcohols (Scheme 1c).[8] Highly
Encouraged by our recent work on catalyst-free boration,[12] we
decided to study the electrophilic 1,2-carboboration of ynamides,
that are versatile N-containing alkyne synthons in organic
synthesis,[13] with readily available boron reagents for the
preparation of highly substituted -boryl-enamides. Such
enamides are valuable in organic chemistry as they can be readily
further transformed to various highly substituted enamides.
Moreover, they can also serve as substrates for the preparation
of boron-doped polycyclic aromatic hydrocarbons. On the basis of
the inherent nature of the polarized ynamide triple bond,[13] we
assumed that ionic 1,2-carboboration would be feasible with
readily available boron reagents in the absence of any catalyst
(Scheme 1d). Thus, reaction of an electrophilic boron reagent with
an ynamide should generate the corresponding keteniminium-
type zwitterion, which upon stereoselective intramolecular 1,3-
[a]
[b]
[c]
Dr. C. You, Dr. C. G. Daniliuc and Prof. Dr. A. Studer
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: studer@uni-muenster.de
M. Sakai and Prof. Dr. S. Yamaguchi
Department of Chemistry, Graduate School of Science and
Integrated Research Consortium on Chemical Sciences (IRCCS),
Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan)
Prof. Dr. S. Yamaguchi
Institute of Transformative Bio-Molecules (ITbM), Nagoya University,
Furo, Chikusa, Nagoya 464-8601 (Japan)
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202107647
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migration of the R3-substituent of the boron ate complex moiety
from boron to carbon would deliver the targeted -
trisubstituted alkenylboron derivative with complete regio- and
stereoselectivity.
bench-stable trimethyl(aryl)silanes as starting materials.
Dichloroboranes 2 were readily generated in situ through the
reaction of the corresponding trimethyl(aryl)silanes with BCl3 in
CH2Cl2 (volatiles were then removed in vacuo).[15] Then, a CHCl3
solution of 1a was added to the dichloroborane and the mixture
was stirred at room temperature for 16 h. Functional groups, such
as methyl (6a), phenyl (6b), phenoxy (6c), diphenylamino (6d)
and halide (6e) at the para position of the phenyl group were
compatible with this sequence and the corresponding alkenyl
boronates were isolated in 68-86% overall yields. The sterically
more hindered ortho-tolyldichloroborane also reacted well (6f).
Naphthyl- and thienyl-based boron reagents engaged in the 1,2-
carboboration of 1a (6g-6i). Interestingly, dichloro(naphthalen-1-
yl)borane (from 2e’) and dichloro(naphthalen-2-yl)borane (from
2f’) led to the same product 6g (see SI for detailed discussion).[16]
Notably, the reaction of the dichloroalkenylborane with 1a
proceeded efficiently and the -trisubstituted alkenylboronate
6i was isolated in high yield. However, reaction of
cyclohexyldichloroborane with 1a provided the carboboration
product in traces only (not shown).
We first explored the reaction of the ynamide 1a with various
readily available phenylboronic acid derivatives that differ in
electrophilicity at ambient temperature in CH2Cl2. To our delight,
commercially available dichlorophenylborane 2a (3 equiv) gave
after treatment of the crude product with pinacol the targeted
trisubstituted alkenylpinacol boronate 3a in good yield (76% NMR
yield; for the structure of 3a, see Scheme 2).[14] Other common
boron reagents, including phenyl boronic acid (2b), phenyl pinacol
boronic ester (2c), phenyl catechol boronic ester (2d), and
triphenylboroxine (2e) did not engage in the 1,2-carboboration,
because their boron Lewis acidity is too low for the initial
electrophilic borylation step (see the Supporting Information, SI).
Solvent screening revealed CHCl3 to be superior to all other
solvents tested. Decreasing the amount of boron reagent 2a from
3 to 1.5 equivalents did not influence reaction outcome to a large
extent and the best result was obtained with 2 equivalents of 2a
(74% isolated yield, see SI for details on the optimization study).
With the optimized conditions in hand, we first tested the scope
with respect to the ynamide (Scheme 2). The relative
configuration in selected examples were unambiguously assigned
by NMR spectroscopy (see SI). Various N-tosyl ynamides bearing
different R1-N-substituents such as n-butyl (3b), benzyl (3c) and
allyl (3d) were subjected to the 1,2-carboboration. The desired
-trisubstituted alkenylboronates were isolated in good yields
(66-78%). Aromatic N-substituents are also tolerated, as shown
by the 1,2-carboboration of the N-phenyl-tosyl ynamide 1e which
gave 3e in a high yield. The N-sulfonyl protecting group can also
be varied. Electronic effects exerted by the aryl group are not that
pronounced, as arylsulfonyl groups bearing halides (4b-4d),
trifluoromethyl (4e) and nitro (4f) all gave good yields (64-84%).
Sterically more crowded ynamides also furnished the
corresponding products 4g and 4h in good yields and the thienyl
moiety was compatible with the electrophilic B-reagent (see 4i).
-Styrenylsulfonyl and methanesulfonyl protected ynamides
engaged in the carboboration, albeit with moderate yields (4j and
4k). Next, N-tosylated N-methyl-ynamides bearing different R2-
substituents were tested in the reaction with 2a (5a-5p).
Surprisingly, for aryl substituted ynamides with electron donating
groups at the para position, lower reactivity was noted as
compared to the less nucleophilic systems bearing electron-
withdrawing para-substituents (see 5a-5g). Aryl-substituted
ynamides with meta- and ortho-substitution at the aryl moiety
were tolerated as well (5h-5j). Again, for the electron-poorer halo-
substituted congeners, higher yields were obtained. Furthermore,
alkyl-substituted ynamides could be employed in this 1,2-
carboboration and the corresponding -trisubstituted
alkenylboronates 5l-5p were obtained in moderate to good yields.
Of note, the -branched alkyl substituted alkynes 1o and 1p
provided higher yields as compared to the primary alkyl
substituted ynamides (see 5l-5m).
We continued the studies by varying the R-substituent at the
dichloroborane 2 with the ynamide 1a as the substrate (Scheme
3). Considering the limited stability of dichloroboranes, we
developed a highly practical one-pot, two-step process utilizing
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Scheme 2. 1,2-Carboboration of various ynamides. Reaction conditions: 1 (0.20
mmol), 2a (0.40 mmol) in CHCl3 (2 mL), rt, 16 h, under Ar; pinacol (1.0 mmol),
NEt3 (1.0 mL), 1 h, isolated yields. [a] 2a (0.60 mmol), 50 °C. [b] 2a (0.24 mmol),
1 h.
To demonstrate the synthetic value of the carboboration, two
gram-scale reactions and four follow-up transformations were
conducted (Scheme 4). First, gram-scale reaction of 1a and 1ab
with PhBCl2 (2a) under standard conditions afforded the desired
products 3a and 5l in 72% and 57% yield, demonstrating the
practicality (Scheme 4a,b). The triaryl substituted enamide 7 was
obtained by Suzuki−Miyaura cross-coupling of 3a with 4-MeC6H4I
Scheme 4. Gram-scale reactions and follow-up chemistry. Reaction conditions:
(i) 3a, 4-MeC6H4I, Pd(dppf)Cl2, Cs2CO3, 1,4-dioxane, H2O, 70 C, 12 h; (ii) 5l,
AgNO3, NEt3, EtOH, H2O, 80 C, 3 h; (iii) 5l, NaBO3∙4H2O, THF, H2O, r.t. 4 h;
(iv) 5l, ZnEt2, CF3COOH, CH2I2, CH2Cl2, 0 C to r.t., overnight.
in 80% yield (Scheme 4c).[17] AgNO3-catalyzed protodeboration of
5l gave stereospecifically the disubstituted enamide
8
(87%).[18Treatment of 5l with NaBO3 provided the ketone 9 in a
good yield and 5l was successfully converted into the fully
substituted cyclopropylboronate 10 using ZnEt2/CH2I2 (82%).[19]
Recently, boron-doped polycyclic aromatic hydrocarbons (B-
PAHs) have attracted increasing attention because of their
interesting materials properties.[20] Among them,
a
1-
boraphenalene scaffold is one of the minimal substructures of B-
PAHs, to which several synthetic accesses have been reported.[21]
We found that our ynamide 1,2-carboboration can also be applied
to the preparation of 1-boraphenalenes that furnish amino groups
at the 3-position (Scheme 5). Thus, a one-pot synthesis of
boraphenalene 11 was achieved via a sequence comprising the
1,2-carboboration of 1a with dichloro(naphthalen-1-yl)borane
followed by an intramolecular boron-Friedel-Crafts arylation and
subsequent hydrolysis. The stable hydroxy-substituted
boraphenalene 11 was obtained in 71% overall yield (Scheme 5a).
Upon treatment with BCl3 in CH2Cl2,[22] the boraphenalene 11 was
further converted into the corresponding chloride. Removal of the
solvent and reaction with mesityllithium in THF eventually
provided mesityl-substituted boraphenalene 12 (78%, Scheme
5b).
Scheme 3. 1,2-Carboboration with various dichloroboranes. Reaction
conditions: RSiMe3 (0.60 mmol), BCl3 (1.20 mmol, 1 M in CH2Cl2), 0 °C to rt or
to 60 C, 24 h, under Ar; then 1a (0.20 mmol) in CHCl3 (1 mL), rt, 16 h, under
Ar; pinacol (1.0 mmol), NEt3 (1.0 mL), 1 h, isolated yields. [a] The dichloroborane
was prepared in advance.
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mild conditions. The reaction is operationally easy to conduct and
features broad substrate scope and high functional group
tolerance. The value of current methodology was documented by
successful follow-up reactions and the synthesis of B-PAHs.
Acknowledgements
We thank the European Research Council (ERC Advanced Grant
Agreement no. 692640) for financial support. This work was
supported by KAKENHI grant 18H05261 from the Japan Society
for the Promotion of Science (JSPS). M. S. thanks the JSPS for a
Research Fellowship for Young Scientists and the "Graduate
Program of Transformative Chem-Bio Research" at Nagoya
University, supported by MEXT (WISE Program).
Keywords: 1,2-carboboration • catalyst-free reaction • synthetic
methods • trisubstituted alkenylboronates • ynamides
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phenyl groups, but also the amino plane are oriented in an
orthogonal fashion against the boraphenalenyl plane (Scheme
5b). In UV-vis absorption spectrum in CHCl3, boraphenalene 12
exhibited two broad absorption bands with the lowest energy
absorption maximum (abs) of 399 nm. In the same solvent, 12
exhibited a broad emission band with the maximum (em) of 524
nm with a large Stokes shift of 5979 cm–1 (Figure S1), where the
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time-dependent (TD)-DFT calculation at the B3LYP/6-31+G(d)
level of theory suggested that the lowest-energy absorption band
can be attributed to the mixture of two electronic transitions from
HOMO to LUMO and from HOMO–1 to LUMO (Table S2, Figure
S3). While LUMO is delocalized over the boraphenalene skeleton,
HOMO and HOMO–1 are located on the mesityl moiety or the
phenylboraphenalene skeleton without contribution of the amino
group. In the solid state, the fluorescence quantum yield was
increased to F = 0.41, while retaining the emission spectrum
almost identical to that in solution (Figure S2). Its nonplanar
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In summary, we have described an efficient method for the
preparation of -boryl--alkyl/aryl -aryl substituted enamides
from ynamides via catalyst-free direct 1,2-carboboration. Readily
available dichloroboranes react with ynamides to afford the
corresponding valuable -trisubstituted alkenylboronates in
good yields with complete regio- and stereoselectivity under very
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10.1002/anie.202107647
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C. You, M. Sakai, C. G. Daniliuc, K.
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Regio- and Stereoselective 1,2-
Carboboration of Ynamides with
Aryldichloroboranes
Catalyst-free direct 1,2-carboboration of ynamides with readily available
dichloroboranes is presented. Valuable -trisubstituted alkenylboronates are
obtained in high yields with complete regio- and stereoselectivity under very mild
conditions.
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