Enantioselective iodolactonization of allenoic acids

Article abstract of DOI:10.1039/c4cc05414h

An enantioselective iodolactonization reaction of allenoic acids has been developed using a trisimidazoline catalyst and I₂. Our mechanistic study suggests the involvement of a π-allyl cation intermediate in this reaction system. This journal is

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Enantioselective Iodolactonization of Allenoic Acids
Cite this: DOI: 10.1039/x0xx00000x
Kenichi Murai, * Nozomi Shimizu and Hiromichi Fujioka*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
An enantioselective iodolactonization reaction of allenoic
acids has been developed using trisimidazoline catalyst and
I2. Our mechanistic study suggests the involvement of a πꢀ
allyl cation intermediate in this reaction system.
Halocyclization reactions between substrates bearing C–C multiple
bonds and nucleophilic functionalities are powerful transformations
in organic synthesis. Recently, extensive efforts have been devoted
to the development of enantioselective halocyclization reactions and
significant progress has been made in this field.1–3 We have also
developed asymmetric bromolactonization of 5ꢀhexenoic acid
derivatives to produce δꢀlactones with trisimidazoline 1a (Figure
1).2i,4 In these transformations, alkene moieties are typically used in
enantioselective halocyclization reactions. In addition to alkenes,
several examples of asymmetric halolactonization using alkynes are
reported. 2f, 5 On the other hand, substrates with allenes as multiple
C–C bond systems have been less well explored.
Figure 2 Electrophilic halogenation of alkene and allene and
possible issues for enantioselective halocyclization.
The first enantioselective bromocyclization of allenes has recently
been reported by Toste.9 This demonstration used a chiral dinuclear
gold complex and/or chiral phosphate anions with Nꢀbromolactams
to give a heterocyclic vinyl bromide having an allylic stereocenter,
with high enantioselectivity. The reaction was proposed to proceed
through a vinyl gold intermediate followed by bromodeauration with
an electrophilic bromine source, namely stepwise halogenation
process. On the other hand, enantioselective halocyclization via
direct halogenation of unsaturated allene bonds had not been
reported through the use of either organoꢀ or metal catalysts, when
we started this study. During preparing this manuscript, the first
organocatalytic enantioselective bromolactonization of allenoic acids
was appeared.10,11 In this communication, we report the first
enantioselective iodolactonization of allenoic acids using the
organocatalyst, trisimidazoline 1b (DMPꢀtris, Ar = 2,3ꢀdimethylꢀ
C6H3) and I2, which proceeds via direct iodination of the allene. In
this reaction, we propose the involvement of a πꢀallyl cation
intermediate in the enantioꢀdetermining step, based on a mechanistic
investigation using trisubstituted allenoic acid.
We began by screening halogenating agents for the reaction of
allenoic acid 2a in the presence of trisimidazoline catalyst 1a in
CHCl3 (Table 1). We found that iodine rather than bromine was the
preferred halogen source to give the desired lactone 3a. Reactions
with bromine sources, such as DBDMH and NBS produced complex
mixtures, while NIS gave 3a in moderate yield (62%, entry 2) with
an enantiomeric ratio (er) of 64:36 indicating a definite but low
enantioselectivity. Changing the reaction media to toluene or
CH3CN, and lowering the reaction temperature to −40°C did not
improve the selectivity (entries 3ꢀ5). The use of I(collidine)2PF612 or
ICl also did not give any improvements (entries 6 and 7). However,
Figure 1 Structure of trisimidazoline 1a and 1b
In the case of halocyclization of alkenes, the reactions of one C–C
double and one intermediate, such as bridged 3ꢀmembered ring
halonium ion, should be considered. (Figure 2, i). On the other hand,
the unique reactivity of allenes6 presents different challenges for
using these compounds in enantioselective halocyclization. (1)
Allenes have two C–C double bonds, therefore regioselective
halogen addition would be important. (2) It would also be a matter
that the reaction of allenes with electrophilic halogen source could
generate two different species, such as bridged 3ꢀmembered ring
halonium ion intermediates and πꢀallyl cation intermediates7 (Figure
2, ii).
These factors could make it difficult to control
enantioselectivity.8
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I2 was found to give the best results (73:27 er, entry 8). Although the
reaction with I2 was slow in CHCl3, the reaction could be accelerated
using toluene while maintaining similar yield and selectivity (entry
Table 2 Optimization study of iodolactonization of allenoic
acid with 2a and 2ba,b
9). In these conditions, 8ꢀmembered lactone,
a
possible
regioisomeric adduct, was not obtained. Based on these preliminary
studies, we tested further optimizations using I2 and toluene as a
solvent. It should be noteworthy that the difference in selectivity
between NIS and I2 gives some indication about the mechanism of
the reaction as will be discussed later in more detail.
Entry
1
2
Additive, temp. Time (h) Yield (%)
erc
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1b
1b
1b
1b
2a
2a
2a
K2CO3, rt
Li2CO3, rt
DTBP, rt
5
24
7
86
76
85
69
77
83
89
18
61:39
61:39
74:26
67:33
81:19
83:17
91:9
Table 1 Initial screening of halolactonization of allenoci acid
2aa,b
2a 2,4,6ꢀcollidine, rt
1
2a
2a
2b
2b
DTBP, rt
DTBP, 0 oC
DTBP, 0 oC
DTBP, ꢀ20 oC
9
48
18
48
76:24
Solvent,
temp.
Time Yield
X+ source
erc
Entry
c
(h)
(%)
a DTBP: 2,6ꢀdiꢀtertꢀbutylpyridine b 0.05ꢀ0.1 mmol of 2a and 2b was used.
Er was determined by HPLC.
DBDMH or NBS
complex mixtures
1
2
3
4
5
6
7
8
9
CHCl3, rt
CHCl3, rt
NIS
0.5
8
62
44
67
37
34
25
65
68
64:36
60:40
51:49
60:40
51:49
61:39
73:27
75:25
The generality of iodolactonization reaction using trisimidazoline
1b and I2 with the series of lipophilic allenoic acids 2c–k was
investigated (Table 3). For reference the results of 2a and 2b are
shown as entries 1 and 2. When the reactions were conducted with
the substrate 2c (R= 4ꢀTBSOCH2C6H4) and 2d (R= 4ꢀTMSC6H4),
similarly good selectivities were obtained (entries 3 and 4).
Changing the electron density of the aromatic ring revealed the
sensitivity of the reaction to electronic effects. The efficiency was
poor for allenoic acids having an electron rich aromatic ring such as
2e (R= 4ꢀtBuOC6H4), leading to racemic products (entry 5).
Conversely, the allenoic acid 2f (R= 4ꢀFC6H4) showed similar
selectivity to 2a (entry 6). The substrate 2g (R= 4ꢀCF3C6H4) also
gave comparable selectivity, although with decreased yield (entry 7).
Several allenoic acids 2h–j with meta or ortho substituents were also
used in the reaction (entries 8–10). These conditions were found to
work even with aliphatic substituents, however a much lower
selectivity was achieved. These results suggest the importance of the
aromatic substituent on the allene.13 The absolute stereochemistry
toluene, 0 oC
CH3CN, 0 oC
CHCl3, ꢀ40 oC
CHCl3, rt
NIS
NIS
15
24
1.3
23
67
26
NIS
I(collidine)2PF6
CHCl3, ꢀ40 oC
ICl
I2
CHCl3, rt
toluene, rt
I2
a
DBDMH: 1,3ꢀdibromoꢀ5,5ꢀdimethyl hydantoin, NBS: Nꢀbromo
succineimide, NIS: Nꢀiodo succineimide, b 0.05ꢀ0.1 mmol of 2a was used.
Er was determined by HPLC.
c
Further optimization studies initially focused on the use of
additives to increase the product yield (Table 2). We believed that
the moderate yield of the reaction under the above conditions could
be caused by deactivation of the catalyst by hydrogen iodide
generated from I2 during the lactonization reaction. We therefore
examined the effect of several basic additives for trapping hydrogen
iodide. With inorganic bases, such as K2CO3 and Li2CO3, the yield
of 3a increased but enantioselectivity decreased (entries 1 and 2).
Conversely, the use of bulky organic bases, such as 2,6ꢀdiꢀtertꢀ
butylpyridine (DTBP) increased the yield while maintaining
enantioselectivity (entry 3). The reaction with 2,4,6ꢀcollidine, gave
lower selectivity and yields (entry 4), illustrating the importance of
bulkiness of pyridine base. Because further screening of additives
and molecular ratios of I2 and DTBP gave no further improvements
in enantioselectivity, we investigated the effect of the phenyl group
substituents of the trisimidazoline catalyst. After screening (see
Electronic Supplementary Information), we found that introduction
of substituents on the 2ꢀposition of the phenyl group of Phꢀtris (1a)
improved the enantioselectivity and a trisimidazoline 1b (DMPꢀtris,
Figure 1) gave better results (entry 5, 81:19 er). Further
improvement in enantioselectivity was achieved by conducting the
reaction at 0 °C, although this improvement was small (entry 6,
83:17 er), likely because of the low solubility of 2a under these
conditions. We then tested the iodolactonization reaction of a more
lipophilic substrate 2b (R = 4ꢀtBuC6H4) under the same conditions.
Table 3 Generality of iodolactonization of allenoic acida
Entry
2
R
Time (h)
48
3
Yield (%)
er b
C6H5
2a
2b
2c
2d
2e
2f
1
2
3a
3b
3c
3d
3e
3f
83
89
88
78
85
78
53
85
82
35
74
83:17
91:9
4ꢀtBuꢀC6H4
4ꢀTBSOCH2ꢀC6H4
4ꢀTMSꢀC6H4
4ꢀtBuOꢀC6H4
4ꢀFꢀC6H4
18
3
24
90:10
90:10
51:49
84:16
81:19
82:18
83:17
87:13
67:33
4
24
5
24
6
42
4ꢀCF3ꢀC6H4
7
2g
2h
2i
48
3g
3h
3i
3,5ꢀdiMeꢀC6H3
3ꢀTBSOCH2ꢀC6H4
2ꢀMeꢀC6H4
8
24
9
48
We found that 2b dissolved at
0 °C and provided good
enantioselectivity as expected (entry 7, 91:9 er). The selectivity was
decreased by conducting the reaction at lower temperatures (−20 °C,
entry 8), and so the reaction conditions described in entry 7 were
taken as the optimal conditions.
10
11
2j
72
3j
PhCH2CH2
2k
24
3k
a
The reaction was carried out with 2 (0.1ꢀ0.04 mmol), I2 (2.5 equiv), 1b (10
b
mol %) and DTBP (2.5 equiv) in toluene (0.05 M) at 0 oC. Er was
determined by HPLC.
2 | J. Name., 2012, 00, 1-3
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of major enantiomer of 3a was determined to be (S), which is the system proceeds mainly via a πꢀallyl cation intermediate and that the
same configuration with previously reported bromolactone produced trisimidazoline may facilitate enantioselective trapping of the
by the bromolactonization reaction using trisimidazoline catalyst.2i,4
intermediates (Scheme 3). The selectivity trend that moderately
It is interesting how the enantioselectivity was induced with electron rich aromatic rings gave better results (Table 3), though
allenoic acid substrates in this reaction. We think the intermediates strongly electron donating 2e gave poor results, could be due to this
generated from the allenes with the iodine source should be proposed reaction pathway.
particularly important. As described above, one possibile
intermediate is a 3ꢀmembered ring iodonium ion (Scheme 1).
Alternatively, πꢀallyl cation can also be formed. For asymmetric
induction to occur, a key step should be different by the respective
intermediates: In the case of a mechanism via an iodonium ion, the
stereochemistry should be determined when the iodine adds to C–C Scheme 3 Proposed reaction pathway
double bond of the allene because the following nucleophilic attack
may be expected to occur in an antiꢀfashion. Alternatively, for a
mechanism proceeding via a πꢀallyl cation, the lactone forming step
would be the crucial for determining the stereochemistry because of
the planarity of the πꢀallyl cation. Ma’s group has reported that the
formation of either of these intermediates depends on the iodine
source.7 In their study on diastereoselective iodolactonization of
allenoic acids, reactions using NIS were reported to proceed via
iodonium ion, while reactions with I2 proceed via πꢀallyl cation.
Therefore, we infer that our reaction system using I2 may also
proceed via a πꢀallyl cation intermediate.
To obtain further insight into the iodolactonization of allenoic
acids, the reaction with (DHQD)2PHAL, which is a widely used
organocatalyst for asymmetric halocyclizations,1j was also
investigated (Scheme 4). Two conditions using NIS and I2, which
respectively correspond to the conditions of entry 2 in Table 1 and
entry 4 in Table 2, were applied to allenoic acid 2a. Interestingly, the
contrasting selectivity trend to the reaction with trisimidazoline was
observed: NIS gave low but significant enantioselectivity (59:41 er),
while I2 gave a racemic product. It was then implied that πꢀallyl
cation intermediate could not appropriate to induce the
enantioselectivity with (DHQD)2PHAL.14 These observations
suggest the importance of appropriate combination of catalysts and
halogen sources for enantioselective halocyclization with allenes,
which afford 2 possible haloꢀintermediates.
Scheme 1 Possible intermediates generated from allene
To obtain further insight into the mechanism, the reaction was
carried out using the trisubstituted allenoic acid 2l, which contained
a 1:1 mixture of allene stereoisomers. Under the enantioselective
iodolactonization conditions given in Table 3 the racemic allenoic
acid 2l gave the lactone 3l in 76% yield with 79:21 er (Scheme 2)
even though the conditions were optimized for trisubstituted allenoic
acids. It was noted that the product was obtained as a Zꢀisomer in a
highly selective manner (>20:1) determined by NOSEY.
Scheme 4 Reaction with (DHQD)2PHAL
In conclusion, we report the first example of an
enantioselective iodolactonization reaction of allenoic acids.
The combination of a trisimidazoline catalyst and I2 is found to
be effective. The involvement of a πꢀallyl cation intermediate in
this system is proposed, based on mechanistic studies using a
trisubstituted allenoic acid. This work illustrates the importance
of controlling the haloꢀ intermediate for enantioselective
halocyclization reaction with allenes. Further investigations
toward understanding the details of this reaction mechanism are
now underway.
This work was financially supported by a GrantꢀinꢀAid for
Scientific Research on Innovative Areas “Advanced Molecular
Transformations by Organocatalysts” and Platform for Drug
Discovery, Informatics, and Structural Life Science from
MEXT.
Scheme 2 Reaction with trisustituted allenoic acid 2l
These observations suggest that a πꢀallyl cation intermediate is
involved in this reaction system for the following reasons. If the
reaction underwent
a mechanism involved an iodonium ion
Notes and references
Graduate School of Pharmaceutical Sciences, Osaka University, 1ꢀ6
intermediate predominantly, the yield of one enantiomer should be
up to 50% given the observed selective formation of a Zꢀisomer and
the expected antiꢀaddition of carboxylic acid to the iodonium
intermediate. However, based on the results (76% yield and 79:21
er), a major enantiomer was produced in 60% yield. In our previous
report on the enantioselective bromolactonization reactions of the
alkenoic acids with trisimidazoline, we proposed that a chiral
carboxylate formed with trisimidazoline could introduce asymmetry
into the racemic bromonium ion intermediates and allow
discrimination between the enantiomers.2i,4 Because the observations
in this study agree with our previous reports, we consider that the
enantioselective iodolactonization of allenoic acids in this reaction
a
Yamadaꢀoka, Suita, Osaka, 565ꢀ0871, Japan. Fax: +81 6 6879 8229; Tel:
+81 6 6879 8226; Eꢀmail:murai@phs.osakaꢀu.ac.jp, fujioka@phs.osakaꢀ
u.ac.jp.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/c000000x/
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8
(a) X. Zhang, C. Fu, Y. Yu, S. Ma, Chem. Eur. J. 2012, 18, 13501.
(b) B. Lü, X. Jiang, C. Fu, and S. Ma, J. Org. Chem. 2009, 74, 438.
(c) X. Jiang, C. Fu, and S. Ma, Chem. Eur. J. 2008, 14, 9656.
Stereoselective halolactonization reactions of axially chiral allenoic
acids by chirality transfer approach have been reported. See, ref 6a
and 7, and references cited therein.
4 | J. Name., 2012, 00, 1-3
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