A Novel Synthesis of (E)-2-Alkenylborane from Chiral Borane and Diazoalkene: Asymmetric Alkenylboration of Aldehydes

Full text of DOI:10.1002/bkcs.11909

Communication
DOI: 10.1002/bkcs.11909
BULLETIN OF THE
G. Pak and J. Kim
KOREAN CHEMICAL SOCIETY
A Novel Synthesis of (E)-2-Alkenylborane from Chiral Borane and
Diazoalkene: Asymmetric Alkenylboration of Aldehydes
*
Gyungah Pak and Jimin Kim
Department of Chemistry, Chonnam National University, Gwangju 61186, South Korea.
*E-mail: jiminkim@chonnam.ac.kr
Received September 10, 2019, Accepted October 23, 2019
Keywords: Allylation, Borane, Diastereoselectivity, Enantioselectivity, Diazoalkene
In view of the significant advances in asymmetric methods,
applications, it was envisaged that chiral (E)-2-alkenylborane
3 could be obtained from the reaction of chiral borane 1 and
(E)-1-diazo-2-alkene 2 under mild reaction conditions.4 Reali-
zation of this method could extend applicability and practical-
ity of carbonyl allylic transfer strategy.
allylation reactions of carbonyl substrates with chiral auxil-
iaries or using catalysts have contributed to important
advances in the field of asymmetric synthesis.1 Despite
numerous studies and reports on improving chemical pro-
cesses for practical use, crotylation reactions still remain lim-
ited to simple systems. Each of known methods allows an
easy access to the propionate subunit via crotylation mainly
due to a method dependent on the preparation of reagent
using symmetric 2-butene with n-BuOK,2 but only few
methods can yield homologated subunits.3 To extent scope of
With this issue in mind, chiral boranes 1a and 1b were
prepared by known procedures,5 and a new chiral borane 1c
was prepared from the (R,R)-bis-sulfonamide
4 with
BH3•SMe2 in toluene (Scheme 1). The reaction required
80 ꢀC for 18 h to complete the formation of 1c. The forma-
tion of highly moisture sensitive 1c was detected by 11B
NMR (δ 19.1, d, 12.6 Hz).6 As a starting point, 1c was
selected as a model chiral borane mainly because such system
has proven to be highly efficient in asymmetric induction.7
Initial attempts to prepare the (E)-1-diazobut-2-ene 2a
from 5a under various conditions turned out to be unprom-
ising for this purpose. The reaction was performed with
sodium salt of 5a in toluene at 50 ꢀC to form 2a, and then
chiral borane 1c was added at −78 ꢀC in toluene. After 2 h,
benzaldehyde was added at −78 ꢀC; however, the formation
of 6a was not realized (Scheme 2).
We speculated that the problem of reaction can be caused
by existing byproduct, sodium p-toluene sulfinate which
formed a borate with chiral borane to retard the desired
Scheme 1. Chiral (E)-2-alkenylborane 3, reagents and conditions:
a. BH3•SMe2, 80 ꢀC, toluene, 18 h.
Table 1. Orienting experiments.a
Entry
1
Solvent
drb
eec
Yieldd
1
2
3
4
5
6
7
1a
1b
1c
1c
1c
1c
1c
CH2Cl2
CH2Cl2
CH2Cl2
THF
Toluene
Et2O
—
—
—
91
93
88
90
—
NR
NR
64
78
58
—
89:11
93:7
87:13
90:10
—
65
Trace
EtCN
a Reaction conditions appeared in Scheme 2.
b Ratio of anti: syn of 6 determined by the analysis of 500 MHz
1H NMR.
c Ee values (enantiomeric excess) were determined by 1H NMR of
(+)-MTPA ester of 6 in comparison with authetic samples prepared
from (S,S)-1c.
Scheme 2. Preliminary investigations, reagents and conditions:
(a) i. NaH, 0~20 ꢀC, THF, ii. evaporation of volatile materials, iii.
80 ꢀC, 1 mmHg, trap at −78 ꢀC. (b) Solvent, −78 ꢀC. (c) PhCHO,
−78 ꢀC, 2 h.
d Isolated yield after purification by SiO2 chromatography.
Bull. Korean Chem. Soc. 2019
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Communication
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BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
Table 2. Asymmetric alkenyboration of aldehydes.
Scheme 3. Formation of the 5-pyrazole 7 from 2c, reagents and
conditions: (a) i. NaH, 0~20 ꢀC, THF, ii. evaporation of volatile
materials, iii. 80 ꢀC, 1 mmHg, trap at −78 ꢀC.
Entry
R1
R2
6
dra
93:7
93:7
93:7
eeb
Yieldc
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Et
Et
Et
Et
Ph
6a
6b
6c
6d
6e
6f
93
91
94
95
86
90
87
91
78
74
77
81
68
78
81
75
not been successful mainly due to a rapid formation of
5-pyrazole from 2c (Scheme 3) as a side reaction.10
n-C5H11
c-C6H11
PhCH=CH
Ph
n-C5H11
c-C6H11
PhCH=CH
In summary, this paper describes an alkenylboration pre-
pared from newly developed chiral borane 1c with (E)-
1-diazo-2-alkene 2 resulting homoallylic alcohol 6 in high
levels of enantio- and diastereoselectivity. Studies are in
progress to extend the scope of reagents, substrates, and
syntheses of biologically active natural products.
93:7
>99:1
>99:1
>99:1
>99:1
6g
6h
a Ratio of anti: syn of 6 determined by the analysis of 500 MHz
1H NMR.
Acknowledgments. The authors are grateful to Chonnam
National University (2015-3040 and 2014-2038) and the
National Research Foundation (NRF-2018R1D1A1B070
44008), and Korea for supporting this work.
b Ee values (enantiomeric excess) were determined by 1H NMR of
(+)-MTPA ester of 6 in comparison with authetic samples prepared
from (S,S)-1c.
c Isolated yield after purification by SiO2 chromatography.
Supporting Information. Additional supporting informa-
tion is Appendix S1 (Supporting Information).
reaction pathway. Therefore, it was required to develop a
method for the preparation of salt-free 2a. After surveying
several conditions, we were delighted to find a method that
the (E)-1-diazobut-2-ene 2a was trapped at −78 ꢀC from
the reaction of sodium salt of 5a at 80 ꢀC under vacuo at
1 mmHg in about 50~60% yield.8 We subsequently
observed that the use of salt free 2a in the presence of 1c at
−78 ꢀC for 2 h resulted in the formation of 3a smoothly.
Upon surveying numerous conditions as appeared in
Table 1, the following observations emerged: (1) the use of
chiral boranes 1a and 1b turned out to be unsuccessful
(entries 1–2); (2) chiral borane 1c was superior to others
and chosen for systematic studies; (3) the use of tetrahydro-
furan (THF) resulted in optimum chemical yields and
stereoselectivity in comparison to other solvents (entry 4).
With the idea that this approach could lead to a general
and effective method, we began to examine the extent of the
reaction (Table 2). In fact, this method successfully provided
6 in good yields when introducing various aldehydes. It is
noteworthy to mention that the reaction can be extend to
homologated system (R1 = Et) without problems. We
observed higher diastereoselectivity (>99:1) than crotyl sys-
tem (R1 = Me), but slightly decreased enantioselectivities
were observed as shown in Table 2. In addition, this protocol
could be a suitable tool for the synthesis of (−)-bitungolide-
A.9 However, extension to cinnamyl system (R1 = Ph) has
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Bull. Korean Chem. Soc. 2019
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