Substrate-Assisted, Transition-Metal-Free Diboration of Alkynamides with Mixed Diboron: Regio- and Stereoselective Access to trans-1,2-Vinyldiboronates

Article abstract of DOI:10.1002/anie.201700946

A substrate-assisted diboration of alkynamides using the unsymmetrical pinacolato-1,8-diaminonaphthalenato diboron (pinBBdan) is described. The transition-metal-free reaction proceeds in a regio- and stereoselective fashion to exclusively afford trans-vinyldiboronates in good to excellent yields. Notably, Bdan and Bpin are installed on the α- and β-carbon atoms, respectively.

Full text of DOI:10.1002/anie.201700946

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700946
German Edition: DOI: 10.1002/ange.201700946
Alkenes
Substrate-Assisted, Transition-Metal-Free Diboration of Alkynamides
with Mixed Diboron: Regio- and Stereoselective Access to trans-1,2-
Vinyldiboronates
Astha Verma+, Russell F. Snead+, Yumin Dai, Carla Slebodnick, Yinuo Yang, Haizhu Yu, Fu Yao,
and Webster L. Santos*
Abstract: A substrate-assisted diboration of alkynamides using
the unsymmetrical pinacolato-1,8-diaminonaphthalenato
diboron (pinBBdan) is described. The transition-metal-free
reaction proceeds in a regio- and stereoselective fashion to
exclusively afford trans-vinyldiboronates in good to excellent
yields. Notably, Bdan and Bpin are installed on the a- and b-
carbon atoms, respectively.
Lin, Marder, Norman etal. reported the first example of
cobalt-catalyzed alkyne diboration wherein the trans product
was formed in a minor amount.[4] Uchiyama and co-workers
elegantly utilized a pseudo-intramolecular activation of
B2pin2, by treating propargylic alcohols with n-butyllithium
at elevated temperature, to generate the trans product
(Scheme 1a).[9] Surprisingly, the pinacol group of one of the
V
inylboronic acid derivatives play a pivotal role as key
building blocks in the construction of polysubstituted alkenes,
which are important structural motifs present in medicines,
natural products, and materials science. Control of the stereo-
and regiochemistry in these products is typically mediated by
cross-coupling reactions such as the Suzuki–Miyaura reaction.
As such, vinylboronic acid derivatives bearing multiple boron
moieties with chemoselective reactivity are highly desirable.
Diboration of alkynes with bis(pinacolato)diboron (B2pin2)
has been intensely investigated and provides access to
vinyldiboronates. However, synthetic approaches that
employ transition-metal complexes, such as Pt,[1] Cu,[2] Au,[3]
Co,[4] or Fe,[5] exclusively generate 1,2-cis-diborylalkenes.[6]
Recently, metal-free conditions have been reported. For
example, photocatalyzed diborations in the presence of
a catalytic amount of diphenyl disulfide under light irradi-
ation,[7] and diborations employing strongly Lewis acidic,
unsymmetrical diborane (pinBBMes2),[8] afford the 1,2-dibor-
ylalkenes as a mixture of E- and Z-configured products.
In contrast, diboration reactions of alkynes to generate
the trans-configured products are scarce. Seminal work by
Scheme 1. Approaches to trans diboration of alkynes.
[*] Dr. A. Verma,[+] R. F. Snead,[+] Dr. Y. Dai, Dr. C. Slebodnick,
Prof. Dr. W. L. Santos
boron moieties was deprotected to feature a cyclic oxabor-
olole and vinyl pinacolboronate. While a lithium counterion is
crucial for the reaction, the reaction is limited to secondary
and tertiary alcohols. Subsequently, Sawamura, Ohmiya, and
co-worker disclosed an organocatalytic protocol which
employed tributylphosphine and a series of alkynoates as
substrates (Scheme 1b).[10] Interestingly, the mechanism of
the reaction involves the formation of a phosphonium
allenolate, which facilitates boryl transfer to afford the trans
product exclusively. The limitation on the substrate scope as
exemplified by these key contributions, and the paucity of
protocols, highlight the necessity for the development of
alternative strategies for the trans-diboration of alkynes. Our
group has been interested in the installation of unsymmetrical
Department of Chemistry, Virginia Tech
900 West Campus Drive, Blacksburg, VA 24061 (USA)
E-mail: santosw@vt.edu
Homepage: http://santosgroup.chem.vt.edu
Y. Yang, Prof. Dr. H. Yu
Department of Chemistry and Center for Atomic Engineering of
Advanced Materials, Anhui University
Hefei, Anhui, 230601 (P.R. China)
Prof. Dr. F. Yao
Department of Chemistry and Materials Science, University of
Science and Technology of China
Hefei, Anhui, 230026 (P.R. China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700946.
diboron,
pinacolato-1,8-diaminonaphthalenato
diboron
(pinBBdan; 1),[11] to unsaturated carbon–carbon bonds.[12]
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Therefore, we sought to develop a novel diboration protocol
that not only provides trans-vinyldiboronates but also affords
the opportunity for chemoselective functionalization, for
example, in cross-coupling reactions (Scheme 1c). Inspired
by studies on Lewis base intramolecular diboron activation
and transfer,[13] we envisioned that the differential in Lewis
acidity of the two boron atoms in pinBBdan could facilitate
the selective complexation of a Lewis base to the Bpin moiety,
thereby selectively transferring the Bdan group[12,14] to the a-
carbon atom (Scheme 1d, 2).[15] Intramolecular trapping of
the resulting anion with Bpin affords the desired trans-
diborated product.
reaction yield (entry 18). Further, reaction conditions, which
employed triethylphosphine and were developed by Sawa-
mura and Ohmiya, were ineffective (entry 19).[10]
Characterization of 5a by X-ray crystallography unam-
biguously confirmed a stereoselective trans configuration of
the two boron moieties and regioselective installation of Bdan
on the a-carbon atom and Bpin on the b-carbon atom of the
product (Figure 1). Interestingly, the amide oxygen atom is
We initiated our studies by investigating suitable bases to
effect the diborylation of N-methyl-3-phenylpropiolamide
(4a) with pinBBdan (Table 1). Organic bases such as pyridine,
DBU, and TEA were inefficient (entries 1–3). We surmised
that chelation of the metal counterion with a crown ether is
key to generating a naked anion, thereby facilitating the
formation of a Lewis acid/base complex with 1 to form 2
(Scheme 1d). Therefore, a crown ether was utilized in
subsequent studies. Whereas weak bases were ineffective
(Table 1, entries 4–7), strong bases such as KOtBu, organo-
lithium, and hydride sources promoted the reaction
(entries 8–13). KOtBu was sluggish but NaH was more
efficient than BuLi (entries 13 and 14). A survey of solvents
revealed THF as the optimal reaction medium (entries 13–
17). The optimal reaction conditions afforded the product in
81% yield, as determined by NMR spectroscopy (entry 14),
within 1 hour at room temperature. A control reaction
without a crown ether as an additive showed diminished
Figure 1. X-ray crystal structure[16] and 11B NMR spectroscopy. Aniso-
tropic displacement ellipsoid drawing (30%) of 5a. Hydrogen atoms
have been omitted for clarity.
coordinated with Bpin to form a tetracoordinate boron
adduct. Indeed, 11B NMR spectroscopy supports the presence
of a tricoordinate (d = 27.6 ppm) and tetracoordinate boron
(d = 14.1 ppm; see the Supporting Information). Such an
intramolecular complexation between the carbonyl oxygen
atom and boron on the b-carbon atom is observed in solution
by 11B NMR studies of all vinyldiboronate products.
With the optimal reaction conditions in hand (Table 1,
entry 14), we investigated the scope and limitations of the
trans-diboration reaction by employing a variety of N-
substituted phenylpropiolamides (Scheme 2). The model
reaction with 4a afforded 5a in 67% yield upon isolation.
While the primary amide 4b yielded 5b in good yield, the
tertiary amide 4c was not transformed into the desired
product 5c. This result further supports our prediction that
activation of 1 by the alkynamide itself, enabled by increased
Lewis basicity upon deprotonation, is required for efficient
conversion into the diborylated product. Alkyl substitutions
Table 1: Optimization of the reaction conditions using pinB-Bdan.[a]
Entry
Base
Solvent
Crown ether
Yield [%][b]
1
2
3
4
5
6
7
8
pyridine
DBU
TEA
NaOH
NaOAc
Cs2CO3
CsOH
KOtBu
LiH
MeLi
BuLi
NaH
BuLi
NaH
THF
THF
THF
THF
–
–
–
trace
trace
trace
trace
trace
16
27
54
14
21
44
70
71
81
74
50
55
50
15-crown-5
15-crown-5
18-crown-6
18-crown-6
18-crown-6
12-crown-4
12-crown-4
12-crown-4
15-crown-5
12-crown-4
15-crown-5
15-crown-5
15-crown-5
15-crown-5
–
THF
toluene
toluene
THF
toluene
toluene
toluene
toluene
THF
9
10
11
12
13
14
15
16
17
18
19
THF
NaH
NaH
NaH
NaH
1,4-dioxane
CPME
CH3CN
THF
PEt3
THF
–
0
[a] General procedure: Base (1 equiv), alkynamide (1 equiv), and crown
ether (1 equiv) were added in THF (0.29m, 08C to RT) and stirred for
30 min. 1 (1.0 equiv) was added and the reaction was allowed to stir for
1 h. [b] Yields were determined by 1H NMR analysis of the crude reaction
mixture after aqueous workup.
Scheme 2. Substrate scope of the trans diborylation of N-substituted
phenylpropiolamides. [a] 2 equiv NaH and 15-crown-5 was used.
[b] 4 equiv NaH and 15-crown-5 was used. Heated to 608C for 3 h after
addition of diboron.
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ranging from a small ethyl group (5d) to a much larger butyl
aromatic ring could also be switched to a heteroatom-
group (5 f) provided the desired products in excellent yields.
In general, bulkier substituents on the nitrogen atom had
a slightly negative impact on the reaction yield and required
higher temperatures for efficient conversion. While a cyclo-
hexyl group (5h) was efficient (68% yield), cyclopentyl (5g),
phenyl (5i), and benzyl (5j) groups were formed in moderate
yields.
Having established the substrate scope with respect to the
amide functionality, we investigated the effect of aryl ring
substitution of the alkyne using the reaction conditions. As
shown in Scheme 3, aryl rings with para-, meta-, and ortho-
containing thiophene (7p) or naphthalene unit (7q). Alky-
namides containing commonly employed protecting groups
[methoxymethylether (7r) and benzyl ether (7s)] also served
as efficient substrates. Overall, in all cases, the presence of the
cis diastereomer was undetectable.
To further test the substrate scope of the developed
reaction conditions, we reacted the alkynamide 8a, bearing an
allyl group, with pinBBdan (Scheme 4). The results demon-
Scheme 4. Substrate scope of the diboration reaction.
strated a chemoselective diboration of the triple bond in the
presence of an alkene, thus affording 9a in 51% yield. To
show that the reaction was not limited to alkynamides
containing aryl rings, an alkynamide bearing a pentyl group
(8b) was synthesized and subjected to the same reaction
conditions. To our delight, 8b was efficiently diborated in
67% yield to provide the vinyldiboronate 9b. These inves-
tigations demonstrate the diverse functional-group tolerance
of the reaction.
A key advantage of utilizing the unsymmetrical diboron
pinBBdan is the orthogonal reactivity of the boron protecting
groups. The carbon atom (sp2) bearing Bpin is significantly
more reactive than that of Bdan and can be used to
strategically install various groups to generate exhaustively
functionalized alkenes. For example, selective estrogen-
receptor modulators which are structural derivatives of
tamoxifen can be rapidly synthesized. To demonstrate this
strategy, trans a,b-diborylacrylamide (5a) underwent facile
Suzuki–Miyaura cross-coupling with 4-iodoanisole using
tetrakis(triphenylphosphine)palladium and cesium carbonate
under microwave irradiation to afford the 1,1-diarylvinylbor-
onate 10 in 80% yield (Scheme 5). This reaction proceeded
Scheme 3. Substrate scope of trans diborylation of aryl-substituted
amide substrates. [a] 1.1 equiv NaH and 15-crown-5 was used.
methyl groups were efficiently diborated to the corresponding
trans products (7a–c) in excellent yields. While a 4-tert-
butylphenyl group at the 4-position afforded 7d in 56% yield,
diborylation rings with alkyl substituents, such as n-propyl
(7e) and n-butyl (7 f), proceeded in excellent yields. A
sterically encumbered mesityl group (6g) was also efficiently
transformed into the vinyldiboronate 7g. It is noteworthy that
alkynamides containing strongly electron-donating groups,
such as ethoxy and methoxy substituents, at various positions
of the aryl ring (7h–k) underwent the trans diboration in high
yields. Furthermore, phenyl rings with electron-withdrawing
groups (trifluoromethyl, chlorine, and fluorine) also per-
formed well under our reaction conditions. The nature of the
Scheme 5. Chemoselective cross-coupling application.
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Scheme 6. DFT calculations for the trans diboration of N-methyl-3-phenylpropiolamide at the M06/(6-311+G(d,p),SMD)//B3LYP/(6-31+G-
(d),SMD) level of theory.
with excellent chemoselectivity. Subsequently, switching the
dan protecting group for pinacol occurred smoothly under
acidic conditions to generate 11. To facilitate the second
coupling reaction, we used a Pd(OAc)2/RuPhos/NaOtBu
catalyst system with 11 and 12 as coupling partners, and
installed the piperidinyl ethoxyphenyl group, thus providing
the densely functionalized, tetrasubstituted tamoxifen ana-
logue 13 in 66% yield.
probably a result of the ring strain of the six-membered ring
(compare bond angles in Scheme 6) and steric hindrance
between Bdan and phenyl ring. Because of the different
electronic properties in Bpin and Bdan, and the slightly larger
steric interaction, the energy barrier in TS2d is also slightly
higher than that of TS2. Therefore, the more stable structure
of TS2 can account for the regioselectivity. The stereoselec-
tivity for the trans product is due to a rapid carbon–carbon
bond rotation process (18!22), which is highly thermody-
namically favorable.
In summary, we have disclosed a transition-metal-free
method for the diboration of alkynamides using an unsym-
metrical diboron reagent, pinBBdan, which displays broad
functional-group tolerance. The corresponding products are
afforded in moderate to high yields and regioselectively
install Bdan and Bpin on the a- and b-carbon atoms,
respectively. Most notably, the diboration reaction proceeds
with exclusive formation of the trans-configured product. An
attractive feature of the orthogonally protected trans-1,2-
vinyldiboronate is the demonstration of a chemoselective
sequential Suzuki–Miyaura cross-coupling reaction to afford
densely functionalized alkenes.
To understand the regio- and stereoselectivity of the trans-
diboration reaction, we performed a detailed DFT investiga-
tion as summarized in Scheme 6. Deprotonation with sodium
hydride generates the charged Lewis base 14, which can
complex with pinBBdan. In agreement with the findings of
Fernandez and co-workers,[14b] preferential activation of the
more Lewis-acidic boron in Bpin (NBO charge + 0.885)
occurs in the presence of Bdan (NBO charge + 0.592;
compare 15 versus 16). Although Lewis-base activation of
diboron typically results in the addition of the boron moiety
on the b-carbon atom,[15] a substrate-assisted, pseudo-intra-
molecular Bdan transfer in TS1 has a high activation energy
barrier of + 44.3 kcalmolꢀ1. In contrast, a addition, such as in
TS2, requires less energy (+ 28.1 kcalmolꢀ1) and therefore
regioselectively installs Bdan on the a-carbon atom (18), and
is thermodynamically favorable (18 versus 17). Carbon–
carbon bond rotation (19/20) is associated with a small
activation energy barrier, which is rapidly proceeded by an
energetically favorable intramolecular trapping of Bpin to
generate the boronate complex 22, with a trans configuration.
Herein, the intramolecular Bdan-transfer process in TS2 is
the rate- and regioselectivity-determining step of the overall
transformation. The much higher energy barrier in TS1 is
Acknowledgements
We acknowledge financial support by the National Science
Foundation (CHE-1414458). We thank AllyChem for dona-
tion of B2(pin)2.
4
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The authors declare no conflict of interest.
Keywords: alkenes · boron · chemoselectivity · cross-coupling ·
synthetic methods
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[11] For authoritative reviews, see: a) E. C. Neeve, S. J. Geier, I. A.
Mkhalid, S. A. Westcott, T. B. Marder, Chem. Rev. 2016, 116,
9091 – 9161; b) R. D. Dewhurst, E. C. Neeve, H. Braunschweig,
T. B. Marder, Chem. Commun. 2015, 51, 9594 – 9607.
[12] X. Guo, A. K. Nelson, C. Slebodnick, W. L. Santos, ACS Catal.
2015, 5, 2172 – 2176.
Manuscript received: January 26, 2017
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Alkenes
A. Verma, R. F. Snead, Y. Dai,
C. Slebodnick, Y. Yang, H. Yu, F. Yao,
W. L. Santos*
&&&& — &&&&
Substrate-Assisted, Transition-Metal-Free
Diboration of Alkynamides with Mixed
Diboron: Regio- and Stereoselective
Access to trans-1,2-Vinyldiboronates
Who needs metals? Transition-metal-free
diboron activation with alkynamides
facilitates the intramolecular formation of
differentially protected trans-vinyldiboro-
nates. The reaction proceeds in a regio-
and stereoselective fashion with Bdan
and Bpin installed on the a- and b-carbon
atoms, respectively. dan=1,8-diamino-
naphthalene, pin=pinacol.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!