Lewis and Bronsted acid cocatalyzed reductive deoxyallenylation of propargylic alcohols with 2-nitrobenzenesulfonylhydrazide

Article abstract of DOI:10.1002/chem.201404692

Reductive deoxyallenylation of sterically hindered tertiary propargylic alcohols was realized on reaction with 2-nitrobenzenesulfonylhydrazide (NBSH) by the combined use of Lewis and Bronsted acid catalysts. This method features a broad substrate scope, m

Full text of DOI:10.1002/chem.201404692

DOI: 10.1002/chem.201404692
Communication
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Deoxyallenylation |Hot Paper|
Lewis and Brønsted Acid Cocatalyzed Reductive Deoxyallenylation
of Propargylic Alcohols with 2-Nitrobenzenesulfonylhydrazide
Zhaohong Liu,[a] Peiqiu Liao,[a] and Xihe Bi*[a, b]
the most practical method because of the wide applications in
Abstract: Reductive deoxyallenylation of sterically hin-
organic synthesis (Figure 1a).[10] However, this method has to
dered tertiary propargylic alcohols was realized on reac-
meet the requirements of the Mitsunobu reaction, which is
tion with 2-nitrobenzenesulfonylhydrazide (NBSH) by the
only compatible with less hindered primary and secondary
combined use of Lewis and Brønsted acid catalysts. This
propargylic alcohols.[11] Therefore, the search for a novel and
method features a broad substrate scope, mild reaction
efficient reductive deoxyallenylation reaction that is suitable
conditions, and good functional-group tolerance, and af-
for sterically hindered propargylic alcohols is certainly in
fords various mono-, di-, and trisubstituted allenes in
demand. Recently, Thomson and co-workers reported a Lewis
good-to-excellent yields. The synthetic utility of this
acid catalyzed one-pot synthesis of allenes by the NBSH-medi-
method was demonstrated by the synthesis of 2H-chro-
ated coupling reaction of alkynyl trifluoroborate salts with hy-
menes and 1,2-dihydroquinolines.
droxyaldehydes or ketones, and proposed that the reaction
proceeded via intermediate B (Figure 1b).[12] Inspired by this
Allenes are useful structural motifs that function as
important substrates or intermediates in organic syn-
thesis,[1] and are also found in many biologically
active natural products.[2] Thus, substantial advances
in the development of novel synthetic methods for
these compounds, either in a racemic or an enantio-
merically enriched form, have already been ach-
ieved.[3] Nonetheless, despite this impressive prog-
ress, the discovery and development of novel meth-
odologies for allene syntheses from easily available
starting materials remains an important objective.
Propargylic alcohols are readily available bifunctional
building blocks, and the exploration of their synthetic
potency has attracted much attention in recent
years.[4] Reductive deoxyallenylation of propargylic al-
cohols represents an extremely convenient approach
Figure 1. NBSH-mediated allene synthesis. DEAD=diethyl azodicarboxylate.
to allenes. So far, only four approaches are capable of
performing such reactions, including the use of
Schwartz reagent [Cp2Zr(H)Cl] (Cp=cyclopentadienyl),[5] lithium
aluminum hydride (LiAlH4),[6] triphenylphosphine (Ph3P),[7] and
2-nitrobenzenesulfonylhydrazide (NBSH).[8] However, applica-
tion of these methods to sterically hindered tertiary propargyl-
ic alcohols remains a formidable challenge.[9] On comparison of
these methods, the NBSH-mediated method via intermediate
A, initially developed by Myers and Zheng in 1996,[8] may be
study and our continued interest in the reactions of functional-
ized alkynes,[13] we envisaged that an acidic catalyst could pro-
mote the deoxyamination of propargylic alcohols with
NBSH,[14] thus generating the key intermediate C, which is es-
sential for subsequent allenation (Figure 1c). Pleasingly, this ex-
pectation was realized experimentally, and a practical reductive
deoxyallenylation reaction, which could be applied to the com-
plementary substrates to those in in the reaction developed by
Myers and Zheng,[8] was discovered. Herein, we report this new
strategy for the reductive deoxyallenylation of propargylic
alcohols.
[a] Z. Liu, Dr. P. Liao, Prof. X. Bi
Department of Chemistry, Northeast Normal University
Changchun 130024 (P. R. China)
E-mail: bixh507@nenu.edu.cn
Initially, the reaction conditions were optimized; the reaction
of propargylic alcohol 1a with 1.2 equivalents of NBSH was
used as the model reaction. As shown in Table 1, different
Lewis acids, combinations with Brønsted acids, and solvents
were screened. Firstly, the Lewis acids were investigated by
[b] Prof. X. Bi
State Key Laboratory of Fine Chemicals
Dalian University of Technology, Dalian 116023 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404692.
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Table 1. Screening of reaction conditions.[a]
Entry
LA
BA
Solvent
Yield
[%][d]
[20 mol%][b]
[10 mol%][c]
1
2
3
4
5
6
7
8
La(OTf)3
Sc(OTf)3
AgOTf
Bi(OTf)3
FeCl3
FeF3
FeF3
FeF3
FeF3
FeF3
–
–
–
–
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3CN
DMF
58
72
84
78
86
TfOH
TfOH
AcOH
TfOH
TfOH
TfOH
TfOH
TfOH
90
trace
0[e]
9
85
0
0
10
11
12
FeF3
FeF3
EtOH
toluene
trace
[a] Reactions were performed with 1a (0.5 mmol) and NBSH (0.60 mmol)
in the presence of a catalyst (20 mol%) in solvent (3 mL) under ambient
atmosphere for 0.5 h. [b] LA=Lewis acid. [c] BA=Brønsted acid. [d] Isolat-
ed yields. [e] With p-tosylhydrazide instead of NBSH.
using CH3NO2 as the solvent. La(OTf)3 (OTf=triflate), previously
used by Thomson and co-workers,[12] afforded a promising
result (58% yield of 2a, Table 1, entry 1). Next, several Lewis
acids, including Sc(OTf)3, AgOTf, and Bi(OTf)3, were examined;
all afforded 2a in good yields (Table 1, entries 2–4). Previously,
we discovered the superior catalytic activity of a superacid cat-
alyst system comprising a Lewis acid and Brønsted acid conju-
gate such as FeCl3 and HOTf.[15] Therefore, the catalytic effect
of the combined FeCl3 and HOTf system was examined. Pleas-
ingly, the yield of 2a increased to 86% (Table 1, entry 5). Fur-
thermore, a much higher yield (90%) of 2a was obtained by
using FeF3 instead of FeCl3, whereas a trace amount of 2a was
generated when a weak protonic acid, such as HOAc was used
(Table 1, entries 6 and 7). Notably, when p-tosylhydrazide was
used instead of NBSH, the desired product was not obtained
(Table 1, entry 8). The solvent used significantly affected the re-
action outcome. For example, the use of acetonitrile afforded
2a in 85% yield (Table 1, entry 9). Dimethylformamide (DMF)
and ethanol were ineffective (Table 1, entries 10 and 11),
whereas the use of toluene resulted in a trace amount of prod-
uct (Table 1, entry 12). Therefore, the reaction conditions listed
in Table 1, entry 6 were found to be optimal and were selected
for further investigations.
Scheme 1. Synthesis of tri- and disubstituted internal allenes. [a] The reac-
tions were carried out in the presence of AgOTf (20 mol%) and TfOH
(10 mol%) at 358C. Tol=p-methylphenyl; PMP=4-methoxyphenyl.
of the reactions proceeded smoothly and completed within
0.5–1.0 h. Compared with previous reductive deoxyallenylation
methods, the synthesis of allene 2e, in an excellent yield
(85%), demonstrated the superior performance of this method
for sterically hindered propargylic alcohols (1e).[5–8] The prepa-
ration of allene 2 f was especially interesting because such an
allene structural motif is found in several biologically active
natural products.[2] Moreover, the functional-group tolerance of
the reaction is reasonable, for example, in addition to the
common electron-rich and electron-deficient (hetero)aryl and
alkyl groups, several sensitive functional groups, such as phe-
nolic hydroxyl (2c), cyclopropyl (2d, 4e, and 4l), alkenyl (4j),
alkynyl (2l), trimethylsilyl (4h), ether (2k), ethoxycarbonyl (4i),
halogen (4a, and 4g), and free amino (4 f) groups were found
to be well-tolerated. Some of these functionalized allenes have
proven to be useful intermediates in organic synthesis.[17] Thus,
a convenient and practical synthetic method was developed
for their preparation.
After determining the optimum reaction conditions, the sub-
strate scope of the reaction was investigated for different inter-
nal propargylic alcohols (i.e., R3 ¼
6
H) (Scheme 1). In general, the
substrate scope was found to be quite broad, and diverse sec-
ondary and tertiary propargylic alcohols with varying function-
al groups could be applied to this Lewis and Brønsted acid co-
catalyzed reductive deoxyallenylation with NBSH, affording the
corresponding functionalized trisubstituted (2b–2l) and 1,3-
disubstituted allenes (4a–4l) in good-to-excellent yields.[16] All
Terminal allenes are useful synthetic intermediates; general
methods for the synthesis of terminal allenes include the ho-
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mologation reaction of terminal alkynes reported by Rona and
Crabbꢁ[18] and the palladium-catalyzed hydrogen-transfer reac-
tion of propargylic amines reported by Nakamura and co-work-
ers.[19] Terminal propargylic alcohols (e.g., ethynyl carbinols) are
readily available from the addition of an ethynyl metal reagent
to aldehydes or ketones. Therefore, we were interested in the
application of our acid-catalyzed reductive deoxyallenylation
with NBSH to the synthesis of terminal allenes. As shown in
Scheme 2, we were pleased to find that the reactions of both
To demonstrate the synthetic utility of this new method, we
next investigated intramolecular cyclization by the in situ gen-
eration of ortho-allenyl phenols or anilines. To our delight, an
6-endo cyclization was observed, affording the corresponding
2H-chromenes (10a–10h), including the parent 2H-chromene
(10a) and 1,2-dihydroquinolines (12a–12e) in moderate-to-ex-
cellent yields (Scheme 3). Notably, the formation of 2H-chro-
Scheme 2. Synthesis of mono- and disubstituted terminal allenes.
secondary and tertiary terminal propargylic alcohols with NBSH
under slightly modified reaction conditions (using AgOTf in-
stead of FeF3), smoothly afforded the desired products (6a–
6h) in moderate-to-excellent yields (64–89%). All of the sub-
strates containing substituted aryl, fused aryl, and ferrocenyl
groups were suitable for this transformation.[20] The efficient
synthesis of various functionalized mono-, and 1,2-/1,1-di-, and
trisubstituted allenes (2, 4, and 6) demonstrated that the Lewis
and Brønsted acid cocatalyzed reductive deoxyallenylation of
propargylic alcohols with NBSH is a powerful method for
allene synthesis. This method has a completely complementary
substrate scope to that reported by Myers and Zheng.[8]
The sensitivity of the new method to steric effects has one
advantage, as exemplified in the reaction of compound 7a
with NBSH [Eq. (1)]. The sterically hindered tertiary propargylic
alcohol was reductively deoxyallenylated selectively, thus af-
fording the allenyne derivative, 8a, in 72% yield. Additionally,
this method can be applied to prepare synthetic diallenes,
such as compound 8b [Eq. (2)].[21]
Scheme 3. Synthesis of 2H-chromenes and 1,2-dihydroquinolines. The syn-
thesis of compounds 10h and 12 required a slightly increased temperature
to 35–408C.
menes and 1,2-dihydroquinolines in high yield required the
separation of allenes and part-cyclized products by chromatog-
raphy, followed by heating the reaction mixture at 40–608C
without a metal catalyst. Poor yields were obtained when the
reaction mixture was directly heated (without any separation)
after the first allene-forming step. The 2H-chromenes[22] and
1,2-dihydroquinolines[23] are two types of privileged heterocy-
clic scaffolds found in numerous natural products and medi-
cines that possess interesting biological activities. Considerable
efforts towards the synthesis of these scaffolds have resulted
in a number of efficient synthetic methods.[24] However, a path-
way involving the thermal 6-endo heterocyclization of ortho-al-
lenyl phenols or anilines under metal-free conditions remains
elusive.[25] Herein, we provided a novel and efficient method
for the construction of these two classes of heterocycles that is
of interest to synthetic and medicinal chemists.
To elucidate the reaction mechanism we, firstly, tried to iso-
late the deoxyaminated intermediate, which was not success-
fully isolated in the studies of Myers and Zheng, and Thomson
and co-workers.[8,12] Fortunately, the desired intermediate, 13,
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Keywords: allenes
·
2H-
chromenes · deoxyallenylation ·
propargylic alcohols
[1] For
a comprehensive review on
allene chemistry, see: N. Krause,
A. S. K. Hashmi, Modern Allene
Chemistry Vol 1 and 2, Wiley-VCH,
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Scheme 4. Mechanistic investigations. Ns=nosyl; Ts=tosyl.
[3] For leading reviews, see: a) R. K.
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was obtained in 68% yield from the reaction of pargylic alco-
hol 11 e with NBSH, only accompanied by small amounts of
1,2-dihydroquinoline 12e and eneyne 12e’ as byproducts.
Next, heating a solution of 13 in CHCl3, without a metal cata-
lyst, afforded 1,2-dihydroquinoline 12e in 80% yield. Instead
of 11 e, we isolated an allene intermediate, 14, in 95% yield by
using 11 a as the substrate and reducing the reaction time to
0.5 h. In the absence of a metal catalyst, the reaction of allene
14 smoothly proceeded to afford 1,2-dihydroquinoline 12a in
almost quantitative yield at 408C (6 h in CHCl3, see Scheme 4).
Based on these results and related precedents,[8,12] we con-
clude that the acid-catalyzed reductive deoxyallenylation of
propargylic alcohols with NBSH involves sequential deoxyami-
nation, in situ elimination of o-nitrobenzenesulfinic acid, and
hydride transfer accompanied by the release of nitrogen to
give allenes.[26] The formation of 2H-chromenes and 1,2-dihy-
droquinolines from allene precursors might proceed through
a regioselective 6-endo cyclization.[27]
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In conclusion, we have described a valuable methodology
for the reductive deoxyallenylation of sterically hindered prop-
argylic alcohols. This method is compatible with a broad scope
of tertiary and secondary propargylic alcohols and affords vari-
ous mono-, di-, and trisubstituted allenes in good-to-excellent
yields, This method is completely complementary in substrate
scope to the widely used reaction reported by Myers and
Zheng.[8] Further application to the synthesis of 2H-chromenes
and 1,2-dihydroquinolines demonstrates the synthetic utility of
this method. Considering the mild reaction conditions, broad
substrate scope, and good functional-group tolerance, this
new reaction for allene synthesis will undoubtedly find wide
applications in organic synthesis.
Acknowledgements
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This work was supported by the NNSFC (21172029, 21202016,
21372038), the Ministry of Education of the People’s Republic
of China (NCET-13-0714), and the Jilin Provincial Research
Foundation for Basic Research (20140519008 JH).
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Received: August 1, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Deoxyallenylation
Z. Liu, P. Liao, X. Bi*
&& – &&
Better together: A novel and highly ef-
ficient Lewis and Brønsted acid cocata-
lyzed reductive deoxyallenylation of
propargylic alcohols with 2-nitrobenze-
nesulfonylhydrazide (NBSH) has been
developed (see scheme). Diverse tertiary
and secondary propargylic alcohols par-
ticipated in the reaction, affording vari-
ous mono-, di-, and trisubstituted al-
lenes in good-to-excellent yields. Fur-
thermore, the synthesis of 2H-chro-
menes and 1,2-dihydroquinolines dem-
onstrated the synthetic utility of this
method.
Lewis and Brønsted Acid Cocatalyzed
Reductive Deoxyallenylation of
Propargylic Alcohols with 2-
Nitrobenzenesulfonylhydrazide
The reductive deoxyallenylation…
…of propargylic alcohols is an
attractive method for allene
synthesis due to the availability and
diversity of propargylic alcohols.
However, its applications to
sterically hindered tertiary
propargylic alcohols remain
a formidable challenge. In their
Communication on page && ff., X.
Bi et al. report a new strategy to
address this issue by using a Lewis
and Brønsted acids cocatalyzed
reductive deoxyallenylation with 2-
nitrobenzenesulfonylhydrazide
(NBSH).
&
&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!