Trialkylborane-Mediated Multicomponent Reaction for the Diastereoselective Synthesis of Anti-δ,δ-Disubstituted Homoallylic Alcohols

Article abstract of DOI:10.1021/acs.orglett.8b03761

The trialkylborane/O₂-mediated reaction of propargyl acetates having a tributylstannyl group at an alkyne terminus with aldehydes in a THF-H₂O solvent system gave anti-δ,δ-disubstituted homoallylic alcohols with good to high diastereoselectivity. Intriguingly, two alkyl groups derived from trialkylborane were embedded into the reaction product. The trialkylborane plays a key role not only as a radical initiator but also as a source of alkyl radicals.

Full text of DOI:10.1021/acs.orglett.8b03761

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Trialkylborane-Mediated Multicomponent Reaction for the
Diastereoselective Synthesis of Anti-δ,δ-Disubstituted Homoallylic
Alcohols
Yoshikazu Horino,*,† Miki Murakami,† Ataru Aimono,† Jun Hee Lee,‡ and Hitoshi Abe†
†Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
‡Department of Advanced Materials Chemistry, Dongguk University, Gyeongju 780-714, Republic of Korea
S
* Supporting Information
ABSTRACT: The trialkylborane/O2-mediated reaction of
propargyl acetates having a tributylstannyl group at an alkyne
terminus with aldehydes in a THF−H2O solvent system gave
anti-δ,δ-disubstituted homoallylic alcohols with good to high
diastereoselectivity. Intriguingly, two alkyl groups derived from
trialkylborane were embedded into the reaction product. The trialkylborane plays a key role not only as a radical initiator but
also as a source of alkyl radicals.
Scheme 1. Diverse Reactivities of Alkynylstannanes
evising elaborate synthetic strategies for the ever-
increasing complexity of organic molecules from simple
D
precursors is an important subject in organic synthesis.1
Alkynyl-metalloid/metal compounds, such as alkynylboronates
and alkynylstannanes, serve as versatile building blocks for such
purposes because sequential stereospecific carbon−carbon and
carbon−heteroatom bond formation can be achieved.2,3
Among these important classes of reagents, alkynylstannanes
are particularly attractive because practical and functional-
group-tolerant synthesis of alkynylstannanes has been estab-
lished.4,5 Moreover, these compounds exhibit distinct reactivity
that can lead to several synthetically useful intermediates
depending on the choice of reaction conditions. For example,
the Migita−Kosugi−Stille coupling reaction using alkynyl-
stannanes is still a powerful tool for the construction of
C(sp)−C(sp2) and C(sp)−C(sp) bonds.6 In addition,
palladium-catalyzed carbostannylation of alkynes has demon-
strated the significant synthetic potential of this reaction to
provide 1-stannylated-1,3-enynes regio- and stereoselectively
(Scheme 1a).7 Furthermore, the stannyl group can undergo
1,2-migration upon alkyne−vinylidene isomerization of
alkynylstannanes (Scheme 1b).8,9 Interestingly, trialkylboranes
readily react with alkynylstannanes by cleavage of the Sn−C
bond in inert solvent to give vinylstannanes containing a vicinal
boryl group (Wrackmeyer reaction) (Scheme 1c).10 However,
to the best of our knowledge, no examples of the addition of
alkyl radical to alkynylstannanes have ever been reported.11 In
addition, the reactivities of alkynylstannanes possessing a
leaving group at the propargylic position remain largely
unexplored, even though these reagents offer a great degree
of synthetic flexibility. After many screenings of free-radical
reactions using 1-phenyl-3-(tributylstannyl)propargyl acetates
(1a), we found that the reaction of 1a and triethylborane in the
THF/H2O solvent system gave the unexpected formation of 1-
phenyl-3-ethylpent-2-ene, where two ethyl groups derived
from triethylborane were incorporated in the product (Scheme
1d, right). Inspired by this interesting transformation, we
carried out the reaction in the presence of deuterium oxide
instead of H2O (Scheme 1d, left). As a result, deuterium was
Received: November 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03761
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
incorporated exclusively at two specific positions. We
hypothesized that such formation might involve both vinyl
radical and allylstannane intermediates. We were pleased to
find that anti-δ,δ-disubstituted homoallylic alcohol 4aaa was
obtained in 79% with excellent diastereoselectivity if the
reaction of 1a and triethylborane (3a) was performed in the
presence of benzaldehyde (2a) (Scheme 2). Trialkylboranes
homoallylic alcohols 4 from readily available 3-
(tributylstannyl)propargyl acetates 1,5 aldehydes 2, and
trialkylboranes 3 (Scheme 1e). The diastereoselective synthesis
of functional group tolerated δ,δ-disubstituted homoallylic
alcohols remains a formidable challenge.15
With optimal conditions in hand,16 we first sought to explore
the substrate scope with regard to 3-(tributylstannyl)propargyl
acetates 1 by carrying out reactions with benzaldehyde (2a)
and triethylborane (3a) (Scheme 2). We found that various 1-
aryl- and 1-heteroaryl-substituted 3-(tributylstannyl)propargyl
acetates 1a−j afforded the desired homoallylic alcohols 4aaa−
jaa in good yields with excellent anti-selectivities. The
stereochemistry of 4aaa was carefully confirmed by its
derivatization to a known compound.16 In contrast, the
reaction of β-styryl-substituted propargyl acetate 1k gave the
desired product 4kaa in 64% yield with a diastereoselectivity of
6:1. We also found that alkyl- and alkoxy-substituted substrates
were compatible under these conditions, enabling the
preparation of homoallylic alcohols 4laa−paa in moderate to
good yields with excellent diastereoselectivities. Next, we
investigated the generality of aldehyde scope. We were pleased
to find that the MCR displayed wide functional-group
compatibility. For example, the reactions proceeded cleanly
regardless of the electronic nature of the substituents on
aromatic aldehydes. Thus, a diverse range of homoallylic
alcohols 4aba−ala were prepared from the corresponding
aromatic aldehydes with either an electron-donating or an
electron-withdrawing group on each position of the aromatic
ring. To our delight, without the prior protection of the
hydroxyl group, 4-hydroxybenzaldehyde was smoothly engaged
in the reaction to furnish 4aka in an excellent yield of 93%.
Furthermore, heterocyclic aldehydes such as furfural and 2-
thiophenecarboxaldehyde could be effectively transformed into
the desired homoallylic alcohols 4ama and 4ana in 93% and
73% yields, respectively, with excellent diastereoselectivity. An
α,β-unsaturated aldehyde also participated in the current
MCR, providing a homoallylic alcohol 4aoa as a sole product
in 47%, which indicates that a competing reaction pathway of
the conjugate addition of ethyl radical to the aldehyde is
efficiently suppressed.11,17 Several substantially less reactive
aliphatic aldehydes also took part in the reaction to provide
access to both 4apa and 4aqa in good yields. The present
MCR with α-chiral alkoxy-substituted aldehydes proceeded
with a good level of diastereoselectivity to give 4ara, as in the
case of Pd-catalyzed umpolung allylation,18 which provides a
different diastereoselectivity from that obtained in the
crotylation of α-chiral alkoxy-substituted aldehydes with (E)-
crotylboronates.19
a
Scheme 2. Scope of the Et3B-Mediated MCR
a
Unless noted otherwise, the reaction of 1 (0.5 mmol) with 2 (1.2
mmol) and 3a (1 M in hexane, 0.6 mmol) was carried out in THF/
b
H2O (2.5 mL, 4/1) at 50 °C under Ar. The ratio of anti:syn
determined by 1H NMR analysis of the crude reaction mixture.
c
(1R,2R,3S)-4ara:(1R,2S,3R)-4ara = 5:1. TBDPS = tert-butyldiphe-
nylsilyl.
Finally, we explored the scope of this MCR with other
organoboranes (Scheme 3). We observed that the rate of a
reaction decreased as the size of the alkyl substituent on the
borane was increased.20 Commercially available tri-n-butylbor-
ane took part in this reaction to furnish 4aab in 70% yield. The
practicality of the present method was demonstrated by a
scale-up experiment at a gram scale of 1a, where 4aab was
produced in 66% yield. Additionally, it was found that
unpurified trialkylboranes derived from the hydroboration of
alkenes could be used. For example, reactions with several
functionalized organoboranes prepared from TBS-protected
allylic and homoallylic alcohols with borane dimethyl sulfide
complex also gave the desired products 4aac and 4aad,
respectively, with excellent diastereoselectivity, and two alkyl
groups derived from a trialkylborane were nicely incorporated
generally participate in free-radical reactions as an initiator
only;11 little is known about the use of these compounds as
both radical initiators and sources of alkyl radicals.12 In
general, diastereoselectivity in free-radical reactions can be
controlled by several strategies, namely, substrate-controlled,
Lewis acid controlled, and chiral auxiliary-controlled strat-
egies.13 Therefore, the development of a new strategy for
diastereoselective multicomponent reactions (MCRs) based
on a R3B/O2-mediated radical chain process is challenging and
an important subject in synthetic organic chemistry.14 Herein,
we report a diastereoselective free-radical-mediated three-
component assembly reaction with an Et3B/O2 system that
provides access to a diverse range of anti-δ,δ-disubstituted
B
DOI: 10.1021/acs.orglett.8b03761
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Scheme 3. Scope with Respect to Triorganoboranes
Scheme 5. Mechanistic Studies
rearrangement, affording 4qaa in 92% yield with good
diastereoselectivity (Scheme 5c). This represents the rare
example of diastereoselective carbonyl α-(cyclopropyl)-
allylation.24 In addition, an attempt to use chiral substrate
(R)-1a gave a racemic form of 4aaa (Scheme 5d).
a
b
Reaction conditions identical to those in Scheme 2. The ratio of
anti/syn determined by 1H NMR analysis of the crude reaction
mixture. 3 mmol scale reaction. TBS = tert-butyldimethylsilyl.
c
To further clarify the reaction mechanism, a control
experiment using 5aaa instead of 1a was examined. However,
5aaa was recovered quantitatively, suggesting that 5aaa can be
ruled out as an intermediate of the reaction. In this context,
while MCR occurred only with tributylstannyl-substituted
propargyl carboxylates such as acetate, pivalate, and carbonate
as a substrate,16 performing the reaction using 7−11 as a
substrate resulted in no reaction (Figure 1).
at the germinal position. In reactions with trialkylboranes
possessing benzylic protons, in contrast with the above results,
the efficiency decreased, resulting in the corresponding
alcohols 4aae and 4aaf in diminished yields, but high levels
of stereoselectivity persisted. In both cases, a significant
amount of propargyl acetate 6 was isolated, presumably via
protodestannylation of 1a under the reaction conditions. In
these examinations, homopropargyl alcohols 5aae and 5aag
were also observed. Further attempts to use triphenylborane or
sec-trialkylboranes resulted in no conversion into the desired
products (e.g., 4aag).
To gain insight into the mechanism of the present MCR, the
reaction of 1a, 2a, and 3a was conducted in THF/D2O as a
solvent (Scheme 4). Interestingly, the reaction afforded d-4aaa
Scheme 4. Kinetic Deuterium Isotope Effect
Figure 1. Unsuccessful substrates.
On the basis of the above results, a preliminary reaction
mechanism is described in Figure 2. Initially, Et3B reacts with
residual O2 in H2O by bimolecular homolytic substitution
reaction (SH2) to produce an ethyl radical (Et•) and a
diethylborylperoxy radical.11,25 Addition of Et• to 1 produces
vinyl radical I, which successively reacts with Et3B·OH2
complex to lead to II.21 A severe 1,3-allylic strain of II
would facilitate the addition of Et• to generate (E)-
allylstannane III along with EtB(OH)OAc and Et•. Given
the poor ability of acetoxy radical as a leaving group, the
organoborane presumably plays a crucial role as a Lewis acid in
this step. In addition, the observed difference in reactivity
between 11 and II might be explained by the greater 1,3-allylic
strain of the latter. Subsequently, the addition of allylstannane
III to an aldehyde proceeds via a chairlike cyclic transition
state IV to give the product 4 with anti-diastereoselectivity.26
In this context, the thermally promoted allylation of aldehydes
with α-alkoxyallylstannanes and α-alkylallylstannanes is known
to occur at 130−150 °C.27 In addition, allylation of aldehydes
with allylstannanes can take place at room temperature under
neutral conditions by using a high-pressure technique.28 In
sharp contrast to these reports, the developed reaction
with almost complete deuterium incorporation exclusively at a
specific position (Scheme 4a). In addition, a large kinetic
deuterium isotope effect for the overall reaction (kH/kD = 9.9)
was observed using THF/D2O/H2O as a solvent, suggesting
that O−H bond homolysis of the Et3B·OH2 complex is the
rate-limiting step (Scheme 4b).21
Next, we performed further mechanistic studies (Scheme 5).
Conducting the reaction in the presence of galvinoxyl free
radical22 (2.4 equiv to Et3B) as a radical scavenger led to no
reaction, suggesting that the present reaction proceeds through
a free-radical mechanism (Scheme 5a). Although no reaction
was observed in dry THF, subsequent addition of H2O
promoted the reaction to give 4aaa in 72% yield (Scheme 5b).
Interestingly, radical clock experiments23 using substrate 1q,
which contained a cyclopropyl ring, did not undergo
C
DOI: 10.1021/acs.orglett.8b03761
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Letter
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Hideki Ohtsu (University of Toyama) for his
assistance with ESR measurements. We also thank Prof. Syed
R. Hussaini (The University of Tulsa) for a fruitful discussion
about the reaction mechanism. This work was partially
supported by JSPS KAKENHI Grant No. JP18K05101.
J.H.L. acknowledges financial support from the Dongguk
University Research Fund of 2018.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03761.
■
S
Experimental procedures, analytical data, and copies of
1H and 13C NMR spectra of all newly synthesized
products (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: horino@eng.u-toyama.ac.jp.
ORCID
Yoshikazu Horino: 0000-0002-8916-6298
Jun Hee Lee: 0000-0003-4108-5074
D
DOI: 10.1021/acs.orglett.8b03761
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