Stereodivergent Hydroboration of Allenes

Article abstract of DOI:10.1002/asia.201800134

Full details of a stereodivergent hydroboration of allenes are reported. While hydroboration of an allene with 9-BBN provided a thermodynamically stable (E)-allylic alcohol after oxidative work-up, the reaction of an identical allene with HB(Sia)₂ (disiamylborane) formed a (Z)-allylic alcohol as the kinetic product. The developed conditions allowed for the synthesis of trisubstituted olefins in a highly stereoselective fashion, which is known to be challenging. The method was also applied to the stereodivergent synthesis of structural motifs such as skipped dienes and allylbenzenes, which are often embedded in biologically active natural products.

Full text of DOI:10.1002/asia.201800134

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Accepted Article
Title: Stereodivergent Hydroboration of Allenes
Authors: Yoshiyuki Nagashima, Keiji Sasaki, Takahiro Suto, Takaaki
Sato, and Noritaka Chida
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Stereodivergent Hydroboration of Allenes
Yoshiyuki Nagashima, Keiji Sasaki, Takahiro Suto, Takaaki Sato* and Noritaka Chida*[a]
Abstract: Full details of the stereodivergent hydroboration of allenes
substituents on boron (10-TMS-9-borabicyclo[3,3,2]decane)
could efficiently inhibit the isomerization of kinetically formed (Z)-
are reported. While hydroboration of an allene with 9-BBN provided
a thermodynamically stable (E)-allylic alcohol after oxidative work-up, 2, which was also used in the enantioselective stereodivergent
the reaction of an identical allene with HB(Sia)2 formed a (Z)-allylic
alcohol as the kinetic product. The developed conditions allowed for
the synthesis of trisubstituted olefins in a highly stereoselective
fashion, which is known to be challenging. The method was also
applied to the stereodivergent synthesis of structural motifs such as
skipped dienes and allylbenzenes, which are often embedded in
biologically active natural products.
allylation of aldehydes.[2j-l] Very recently, the Huang/Hong group
reported a stereodivergent allylation via hydroboration of allenes
in the synthesis of functionalized tetrahydropyrans.[2r] Soon after
their report, our research group independently disclosed a
stereodivergent hydroboration/oxidation of allenes and its
application to a unified total synthesis of the madangamine
alkaloids. [4] In this paper, we report the full details of systematic
studies on stereodivergent hydroboration/oxidation of allenes to
give allylic alcohols. The developed method was then applied to
the synthesis of skipped dienes and allylbenzenes embedded in
a variety of biologically active natural products.
Introduction
Allene is a unique functional group containing two contiguous
carbon-carbon double bonds.[1] Recent studies on allene have
made this unique functional group an attractive intermediate in
the synthesis of complex molecules due to its easy accessibility
and distinct reactivities derived from the two orthogonal -bonds.
However, reactions of allenes require precise control of the
selectivities compared with the reactions of the corresponding
alkenyl
and
alkynyl
groups.
For
example,
the
hydroboration/oxidation of allene 1 to give allylic alcohol 3 must
achieve three selectivities including the regioselectivity for two
double bonds. (Scheme 1).[2] The face-selectivity is another
challenge due to the two orthogonal double bonds. While
hydroboration of 1 from the same face as the R1 group would
provide (E)-allylic borane 2 (path A), hydroboration from the less
hindered side opposite to the R1 group would give (Z)-allylic
borane 2 as the kinetically favored intermediate (path B).
Furthermore, (Z)-allylic borane 2 is known to undergo two 1,3-
Scheme 1. Hydroboration/oxidation of allenes.
allylic rearrangements via
4 under equilibrium conditions,
leading to the formation of thermodynamically stable (E)-2.[2,3]
Although the simultaneous control of the three selectivities
(regio-, stereo-, and kinetic vs thermodynamic) is highly
challenging, the reaction has the potential to become a useful
stereodivergent method to give both allylic alcohols (E)-3 and
(Z)-3 from the identical allene 1.
In 1979, Brown and co-workers reported the pioneering
hydroboration of allene 1 with 9-BBN that resulted in the
formation of thermodynamically stable allylic borane (E)-2, which
was subsequently used as an allylating reagent with acetone.[2d]
The Roush group disclosed that the proper choice of
Results and Discussion
The hydroboration of allenes was first evaluated using
commercially available 1-cyclohexylallene 1a with a variety of
borane reagents (Table 1). A solution of allene 1a in THF was
treated with 9-BBN at room temperature for 30 min, and then
quenched with H2O2 in aqueous NaOH to give allylic alcohols
(E)-3a and (Z)-3a in 86% combined yield (Table 1, Entry 1). The
reaction showed (E)-selectivity under thermodynamic control via
1,3-allylic rearrangement (E/Z = 13.3:1). The same reaction for a
longer time (24 h) provided allylic alcohols 3a in a lower
combined yield but with the identical E/Z-selectivity (Table 1,
Entry 2). The hydroboration with Cy2BH (dicyclohexylborane) at
0 °C for 5 min showed a lower (E)-selectivity than that with 9-
BBN (Table 1, Entry 3). Interestingly, the reaction for 24 h led to
a slightly higher (E)-selectivity (Table 1, Entries 3 and 4, E/Z =
5.4:1 vs 8.5:1).[5] These results indicated that while the allylic
boranes derived from 9-BBN quickly reached equilibrium, the
[a]
Y. Nagashima, K. Sasaki, T. Suto, Prof. Dr. T. Sato, Prof. Dr. N.
Chida
Department of Applied Chemistry
Faculty of Science and Technology, Keio University
3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
E-mail: takaakis@applc.keio.ac.jp
chida@applc.keio.ac.jp
allylic boranes derived from Cy2BH underwent
equilibrium reaction. The hydroboration with (Thx)BH2
a slower
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(thexylborane) for 5 min provided allylic alcohol 3a with E/Z =
3.8:1 ratio (Table 1, Entry 5). In contrast, the reaction for 24 h
did not provide the product, probably because of the second
hydroboration (Table 1, Entry 6). Gratifyingly, hydroboration of
allene 1a with sterically bulky HB(Sia)2 (disiamylborane) at 0 °C
for 5 min showed (Z)-selectivity (E/Z = 1:4.3) in 89% combined
yield (Table 1, Entry 7). The reaction time and the temperature
were critical to keep the high (Z)-selectivity. A longer reaction
time for 24 h at 0 °C resulted in a lower (Z)-selectivity probably
due to the partial 1,3-allylic rearrangement (Table 1, Entry 8).
The stereoselectivity was switched to the opposite (E)-
preference by the reaction for 24 h at room temperature (Table 1,
Entry 9, E/Z = 4.5:1).[6] Thus, we developed stereodivergent
conditions to give both stereoisomers of allylic alcohols 3a from
the identical allene 1a by simply changing the borane reagents.
Table 1. Optimization of hydroboration of allene 1a[a]
Figure 1. 1H NMR spectra (400 MHz) in the hydroboration of allene 1a with 9-
BBN in d8-THF. (A) allene 1a in d8-THF; (B) allene 1a, 9-BBN (2.0 equiv) in d8-
THF, RT, 10 min; (C) allene 1a, 9-BBN (2.0 equiv) in d8-THF, RT, 60 min.
Combined
Entry
HBR2
T (°C)
Reaction
time
E:Z
yields
(%)[b]
1
2
3
4
5
6
7
8
9
9-BBN
RT
RT
0
30 min
24 h
86
13.3:1
12.9:1
5.4:1
8.5:1
3.8:1
-
9-BBN
53
Cy2BH
5 min
24 h
60
Cy2BH
0
72
(Thx)BH2
(Thx)BH2
HB(Sia)2
HB(Sia)2
HB(Sia)2
0
5 min
24 h
72
0
trace
89
0
5 min
24 h
1:4.3
1:1.9
4.5:1
0
62
RT
24 h
65
[a] Reaction conditions: 1a (150 mol), HBR2 (1.5 equiv), THF (0.1 M), 0 °C, 5
min; then H2O2 (30% aq, 1 mL), NaOH aq. (3 M, 1 mL), 0 °C, 1 h. [b] Yields of
isolated products after purification by column chromatography are given.
1H NMR experiments were performed to elucidate the
mechanistic details of the hydroboration (Figure 1). Upon
treatment of allene 1a with 9-BBN (2.0 equiv) in d8-THF at room
temperature, two newly generated intermediates were observed
(spectra A-C). The major compound contained a doublet-triplet-
Figure 2. 1H NMR spectra (400 MHz) in the hydroboration of allene 1a with
HB(Sia)2 in d8-THF. (A) allene 1a in d8-THF; (B) allene 1a, HB(Sia)2 (2.0 equiv)
in d8-THF, RT, 10 min; (C) allene 1a, HB(Sia)2 (2.0 equiv) in d8-THF, RT, 24 h.
1,3-allylic rearrangement of (Z)-allylic borane 5a was much
doublet peak at
doublet-triplet peak at
indicated the formation of (
hand, the minor compound corresponded to (
containing a doublet-triplet-doublet peak at
0.9 Hz, H2) and a doublet-doublet-triplet peak at

5.65 (
J
= 15.1, 7.8, 1.1 Hz, H2) and a doublet-
5.16 ( = 15.1, 6.9, 1.4 Hz, H3), which
)-allylic borane 5a. On the other
)-allylic borane 5a
5.54 ( = 11.0, 8.0,
5.03 (
faster than the hydroboration itself, and the rearrangement of
two allylic boranes 5a quickly reached equilibrium. A different
mechanistic tendency was observed by 1H NMR experiments
with HB(Sia)2, which was prepared with BH3·SMe2 (2.0 equiv)
and 2-methyl-2-butene (6 equiv) (Figure 2). After 10 min at room
temperature, the 1H NMR spectrum showed the complete
consumption of allene 1a, and the formation of two new

J
E
Z

J

J =
11.0, 9.2, 1.8 Hz, H3). Comparison between spectra B and C
showed that the starting material 1a still remained after 10 min,
intermediates (Spectra B,
intermediate was assigned as (
coupling constant of the double bond (J2,3 = 10.7 Hz), the minor
E/
Z
= 1:4.2). While the major
but the
E/Z ratio of the generated allylic boranes 5a was
Z
)-allylic borane 6a by the
identical to the ratio after 1 h. These results revealed that the
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isomer was identified as (
E
)-allylic borane 6a by the resonances
selectivity, Bn-protected allenyl alcohol 1h did not provide
allylic alcohol (E)-3h at all. As shown in Scheme 3, primary
alcohol 9h was obtained in 32% yield instead, probably
of (
spectrum after 24 h indicated that (
the major isomer (Spectra C,
E
)-olefin peaks (J2,3 = 15.3 Hz). However, the 1H NMR
E)-allylic borane 6a became
E/
Z
= 3.8:1). These results
through the -elimination of 4h and the subsequent
suggested that the 1,3-allylic rearrangement took place even if
sterically bulky HB(Sia)2 was used, but the rate of the
rearrangement was much slower than that with 9-BBN, allowing
hydroboration of diene 7h. In contrast to 9-BBN, the
hydroboration with HB(Sia)2 was relatively independent of
the nature of the protecting groups, leading to high (Z)-
stereoselectivities (1g vs 1h). Interestingly, free allenyl
alcohol was tolerated with both (E)- and (Z)-selective
hydroborations, although 2.5 equivalents of borane
reagents were needed (1i). The exo allene of cyclohexane
derivative 1j underwent the stereoselective hydroborations
under both conditions.
for the (Z)-selective hydroboration.
Scheme 3. Formation of primary alcohol 9h via -elimination.
Our research group has been pursuing the utility of allenes
as productive intermediates in the synthesis of complex natural
products. For example, a skipped diene is an important
structural motif widely distributed in biologically active natural
products such as madangamines,[4,7] corallopyronin A8 and
ripostatin A[9] (Scheme 4A).[10] The skipped diene natural
products consist of a variety of stereoisomers derived from two
olefins of the skipped dienes. Another structural feature is that
one of two olefins embedded in most of these natural products is
Scheme 2. Substrate scope in the hydroboration/oxidation of allenes. [a] 2.5
equivalents of 9-BBN and HB(Sia)2 were used.
a
challenging trisubstituted olefin. Our stereodivergent
hydroboration has the potential to provide quick access to all
four possible stereoisomers of skipped dienes by combination
with the well-known Migita-Kosugi-Stille coupling[11] (Scheme
4B). The sequence commenced with stereodivergent
hydroboration of allene 1g with 9-BBN and HB(Sia)2, providing
allylic alcohols (E)-3g and (Z)-3g, respectively. After
transformation of allylic alcohol (E)-3g to methyl carbonate (E)-
10, the Migita-Kosugi-Stille coupling of the resulting (E)-10 with
both vinyl stannanes (E)- and (Z)-11 provided skipped dienes
(E,E)- and (E,Z)-12 in high yields. Allylic alcohol (Z)-3g was also
converted to skipped dienes (Z,E)- and (Z,Z)-12 with retention of
the stereochemistry. It is noteworthy that the resulting
geometry of (Z,E)-12 corresponds to that of corallopyronin A.
Thus, we demonstrated that the developed method would be
useful for the synthesis of skipped diene natural products
including challenging trisubstituted olefins with a high level of
stereocontrol.
With the stereodivergent conditions with 9-BBN [method
A] and HB(Sia)2 [method B] in hand, we then surveyed the
substrate scope of the stereodivergent hydroboration
(Scheme 2). When allenes
1 possess a branched
substituent, both E- and Z-selective hydroborations showed
higher diastereoselectivity (1b vs 1c). The reactions of 1,1-
disubstituted allenes under both conditions gave
trisubstituted olefins with comparable or even higher
stereoselectivities than the corresponding 1-substituted
allenes (1a vs 1d, 1b vs 1e, 1c vs 1g). It is noteworthy that
the high level of stereocontrol for trisubstituted olefins is
known to be challenging even in modern organic synthesis.
However, our stereodivergent hydroboration can provide
both (E)- and (Z)-trisubstituted olefins from an identical
allene. The stereoselectivity depended highly on the steric
bulkiness of the two substituents R1 and R2 of allenes
1.
The method was then applied to construction of an
allylbenzene motif embedded in biologically active natural
products including cylindrol A[12] and lobatamide C[13] (Scheme
5A). These natural products also contain a trisubstituted olefin
with either (E)- or (Z)-arrangements. Allylic carbamate (E)-10,
which was prepared through E-selective hydroboration of allene
1g with 9-BBN, was subjected to the Migita-Kosugi-Stille
coupling with PhSnBu3 in the presence of Pd2(dba)3·CHCl3 (5
mol%) and LiCl (Scheme 5B). Although slight scrambling of
While the hydroborations of 1f with a less hindered
substituent than 1e showed lower stereoselectivities under
both conditions, the reactions of 1g bearing a bulky
branched substituent resulted in higher stereoselectivities.
When using 9-BBN, the reaction depended highly on the
nature of the protecting groups on the allenyl alcohols. For
example, while the reaction of TBDPS-protected allenyl
alcohol 1g with 9-BBN showed both a high yield and (E)-
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stereochemistry was observed, allylbenzene (E)-13 was
obtained in 91% yield (E/Z = 10.6:1). On the other hand, the
cross-coupling of allylic carbonate (Z)-10 proceeded in 76%
yield with the retention of the stereochemistry in the absence of
LiCl.[14] The resulting (E)- and (Z)-allylbenzenes 13 correspond
(Z)-trisubstituted olefins. Combination of our hydroboration with
the Migita-Kosugi-Stille coupling achieved a stereodivergent
sequence, successfully giving all four possible stereoisomers of
skipped dienes embedded in biologically active natural products.
The method was also applicable to the stereodivergent
synthesis of allylbenzene motifs.
to the substructures of cylindrol
respectively.
A and lobatamide C,
Scheme 5. (A) Representative natural products containing the allyl benzene
motif. (B) Stereodivergent synthesis of allylbenzenes 13 derived from 1,1-
disubstituted allene 1g. Reagents and conditions: a) PhBu3Sn (1.5 equiv),
Pd2(dba)3·CHCl3 (5 mol%), LiCl (10 equiv), DMF (50 mM), RT. b) PhBu3Sn
(1.5 equiv), Pd2(dba)3·CHCl3 (5 mol%), DMF (50 mM), RT.
Keywords: allene • hydroboration • natural product • skipped
diene • stereoselectivity
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Scheme 4. (A) Representative natural products containing skipped dienes. (B)
Application of 1,1-disubstituted allene 1g to stereodivergent synthesis of
skipped dienes 12. Reagents and conditions: a) 9-BBN (1.5 equiv), THF (0.1
M), RT; H2O2, NaOH aq; b) HB(Sia)2 (1.5 equiv), THF (0.1 M), 0 °C; H2O2,
NaOH aq; c) ClCO2Me (1.5 equiv), py (1.5 equiv), CH2Cl2 (0.1 M), 0 °C; d) 9
(1.5 equiv), Pd2(dba)3·CHCl3 (5 mol%), LiCl (10 equiv), DMF (8 mM), RT.
Conclusions
We have developed a stereodivergent hydroboration of allenes,
which can provide quick access to both (E)- and (Z)-allylic
alcohols from an identical allene by simply changing borane
reagents. While use of 9-BBN provided a thermodynamically
stable (E)-allylic alcohol, the reaction of HB(Sia)2 resulted in the
formation of (Z)-allylic alcohol as the kinetic product. The
synthetic utility was demonstrated when the stereodivergent
method was applied to 1,1-disubstituted allenes, giving (E)- and
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[14] The Migita-Kosugi-Stille coupling of (Z)-10 with PhSnBu3 in the
presence of LiCl caused partial isomerization of the stereochemistry,
providing a mixture of two isomers 13 in 53% yield with E/Z = 1:5.5.
[4]
Part of this work was published as a preliminary communication, see: T.
Suto, Y. Yanagita, Y. Nagashima, S. Takikawa, Y. Kurosu, N. Matsuo,
T. Sato, N. Chida, J. Am. Chem. Soc. 2017, 139, 2952–2955.
[5]
[6]
[7]
The hydroboration of 1a with Cy2BH for prolonged reaction time (36 h)
provided 3a in 29% yield with E/Z = 9.8:1.
Hydroboration of allene 1a with HB(Sia)2 at room temperature for 48 h
resulted in the complete decomposition.
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10.1002/asia.201800134
Chemistry - An Asian Journal
FULL PAPER
FULL PAPER
The full details of stereodivergent
hydroboration of allenes were
Yoshiyuki Nagashima, Keiji Sasaki,
Takahiro Suto, Takaaki Sato*, Noritaka
Chida*
described. The hydroboration with 9-
BBN provided thermodynamically
stable E-allylic alcohols. On the other
hand, the reaction with HB(Sia)2
gave Z-allylic alcohols as a kinetic
product. The method was then
applied to stereodivergent synthesis
of skipped dienes and allylbenzenes,
which are embedded in biologically
active natural products.
Page No. – Page No.
Stereodivergent Hydroboration of
Allenes
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.