[bmim][Br] as an Inexpensive and Efficient Medium for the Barbier-Type Allylation Reaction Using a Catalytic Amount of Indium: Mechanistic Studies

Article abstract of DOI:10.1002/ejoc.201800043

Barbier-type allylation reactions of aldehydes and ketones have been carried out with both unsubstituted and γ-substituted allyl bromides using only a catalytic amount (0.1 equiv.) of In metal in [bmim][Br], but not in H₂O, organic solvents, or

Full text of DOI:10.1002/ejoc.201800043

A Journal of
Accepted Article
Title: [bmim][Br] as an Inexpensive and Efficient Medium for
the Barbier-type Allylation using a Catalytic Amount of In:
Mechanistic Studies
Authors: Papiya Dey, Mrunesh Koli, Dibakar Goswami, Anubha
Sharma, and Subrata Chattopadhyay
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800043
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10.1002/ejoc.201800043
European Journal of Organic Chemistry
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[bmim][Br] as an Inexpensive and Efficient Medium for the
Barbier-type Allylation using a Catalytic Amount of In:
Mechanistic Studies
Papiya Dey, Mrunesh Koli, Dibakar Goswami*, Anubha Sharma and Subrata Chattopadhyay*
aBio-Organic Division, Bhabha Atomic Research Centre, Mumbai - 400 085, India
Fax: +91 22 25505151; e-mail: dibakarg@barc.gov.in; schatt57@gmail.com
Abstract: The Barbier type allylation of aldehydes/ketones can be
carried out with both unsubstituted and γ-substituted allyl bromides
using only a catalytic amount (0.1 equiv.) of In metal in [bmim][Br],
but not in H2O, organic solvents and other room temperature ionic
liquids. The reactions do not require any metal activator, and
proceed with chemo- and regio-selectivities. Our temporal 1H NMR
studies suggested that besides acting as a solvent, [bmim][Br] also
activates In metal surface by electron polarization to generate both
CH2=CHCH2─In (I) and CH2=CHCH2─InBr2 (II) from allyl bromide
and In. Of the active allylating intermediates, species II is
regenerated by an in situ reduction of InBr3 by an imidazolium-based
N-heterocyclic carbene, both produced during the process,
accounting for the catalytic action of In metal.
has a very low vapor pressure, dissolves most of the organic
substrates and is reusable.7
The inertness and low first ionization potential of In metal
coupled with the extensive tolerance of organoindium species
towards various functional groups make it a popular choice as a
metal for Barbier-type allylation.8 However, most of the previous
efforts employed stoichiometric amounts of In metal for the
reaction.8b,9 The carbonyl allylation protocols using catalytic In
metal relied on using preformed allyl-metal complexes under
strictly anhydrous conditions, and were primarily restricted to
limited substrates.10 Recently, Schneider et al. have reported a
method for allylation of ketones in water with preformed
allylboronates in the presence of In catalyst.11a In another
report,11b Barbier-type allylation of aldehydes and ketones in
DMF was accomplished using a catalytic amount of In metal in
presence of excess Al metal (foil). Similarly, combination of
catalytic amount of InCl3 with stoichiometric amount of Al metal
has also been used for this purpose.11c Catalytic use of
elemental Ga for allylation of acetals, ketals and aminals has
also been reported wherein a Ag (I) co-catalyst was used for
regeneration of Ga metal.11d Thus, these methods11b,c could not
decrease the total amount of metals, and hence did not reduce
the amounts of disposable metallic wastes.
Introduction
The Barbier-type allylation, carried out with allylic halides, metals
(In, Zn, Sn, Ga etc.) and carbonyl substrates in organic
solvents,1a H2O1b and room temperature ionic liquids (RTILs),1c-h
as well as under solvent-free conditions,1i-k is widely investigated
in organic syntheses due to its convenience and applications in
biological and pharmaceutical chemistry.2 Nevertheless, an ideal
version of the reaction involving stoichiometric amounts of the
substrates and catalytic amounts of the metal remains elusive.
In H2O, the greenest and cheapest solvent on earth, a majority
of the allylmetals are hydrolytically unstable,3 warranting use of a
large excess of the reagents for completion of the reactions.
Even the solvent-free synthetic procedures require an excess of
reagents.4 The mechanochemical organic synthesis promises to
offer stoichiometric control, but is operationally difficult and is in
its infancy.5 Another limitation of Barbier-type allylation is the
need of using toxic metal activators such as metal salts and
mineral acids.
In the present investigation, we explored the possibility of
using the In/[bmim][Br] combination for carbonyl allylation using
stoichiometric quantities of the substrates. The study also
provided new mechanistic insights in the allyl-In chemistry in the
chosen RTIL, eventually leading to a protocol with catalytic
quantity of In. To the best of our knowledge, catalytic Barbier-
type allylation of carbonyls using simple allylic bromides, and
without requiring any co-catalyst has never been accomplished.
Results and Discussion
We have previously carried out1f Barbier-type allylation of
carbonyls using sub-stoichiometric (50 mol%) of Ga metal, near
stoichiometric amounts of the reactants and the inexpensive
RTIL, [bmim][Br], both as the metal activator and solvent. As an
imidazolium-based RTIL with small alkyl groups, [bmim][Br] is
expected to be safe.6a More recently, we reported a highly syn-
selective Bi-mediated crotylation of aldehydes in [bmim][Br],
using near stoichiometric amounts of the reactants.1h It is worth
mentioning that, although [bmim][Br] is commercially available
as a solid, stable existence of a supercooled liquid state of
[bmim][Br] could be achieved by cooling down freshly prepared
[bmim][Br] below its melting point.6b,c Moreover, the chosen RTIL
Initially, we carried out the In-mediated reaction between allyl
bromide and benzaldehyde (1a) in H2O, THF and three different
RTILs, including [bmim][Br], and the results (Scheme 1, Table 1)
established the efficacy of [bmim][Br] as the best solvent. All the
reactions were carried out at 2 mmol scale of 1a in H2O/organic
solvent (5 mL) or RTIL (4 mL). The reactions in H2O, THF and
THF-H2O mixture (1:1 v/v) were slow even under ultrasonic
activation, affording the product 2a in 59-69% yields after 18-20
h (Table 1, entries 1-5). Also, a large excess of In (5 equiv.) and
allyl bromide (3 equiv.) were required. Reducing their amounts
led to poorer yields (data not shown). In contrast, the reaction
could be accomplished much faster (4 h) in [bmim][Br] with only
1.5 equiv. of the bromide and 1.2 equiv. of In to obtain 2a in 89%
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10.1002/ejoc.201800043
European Journal of Organic Chemistry
FULL PAPER
yield (Table 1, entry 6). For comparison, we also carried out the
allylation of 1a separately in two other RTILs, [bmim][BF4], and
[bmim][PF6]. As reported earlier, the reactions in these RTILs
took a much longer time.1c,12a,b However, we got 2a in poorer
yields than those reported (Table 1, entries 7, 8). This kind of
yield variability in different batches of RTIL has been observed
previously.12a
Earlier, allylation of 1a in [bmim][BF4] using allyl bromide and
sub-stoichiometric amount of Sn proceeded with very poor yield
(14%).12b This reinforces the importance of our protocol.
The catalytic protocol (0.1 equiv. of In) was equally
effective with both aromatic and aliphatic aldehydes/ketones 1b-
s furnishing the products 2b-s in good yields (Table 2). The
nature of the substituents in the aromatic ring or alkyl chain
length did not have any appreciable effect on the reaction
course. Allylation of ketones proceeded with marginally less, but
appreciable (75-87%) yields. The reaction proceeded with
complete chemoselectivity with the conjugated aldehyde 1i and
ketone 1q to furnish the corresponding 1,2-addition products 2i
OH
Allyl bromide/ In/ solvent
PhCHO
1a
Ph
and
2q
exclusively.
Interestingly,
allylation
of
4-
25 oC
2a
methylcyclohexanone (1m) under the above conditions
exclusively furnished 2m with equatorial OH group. This is in
sharp contrast with previous reports where allyl indium reagents
preferably produced the axial-product.12c,d Allylation of the 1,2-
diketone 1p furnished the monoallylated product 2p only. The
ester group was unaffected when ethyl levulinate (1r) was
allylated to give the product 2r, further confirming the
chemoselectivity and enhancing the scope of the protocol. The
Scheme 1. In-mediated allylation of benzaldehyde (1a) under different
conditions.
Table 1. Reaction profile of In-mediated allylation of 1a in various solventsa
Entry
Allyl
bromide
(equiv.)
3
In
Solvent
Time
(h)
Yield (%)b
(equiv.)
reaction
with
the
chiral
substrate,
(R)-
cyclohexylideneglyceraldehyde (1s) proceeded in good yield,
albeit with poor diastereoselectivity. However, reaction with 2-
hydroxybenzaldehyde produced a myriad of products, and the
desired product was obtained in <20% yield (data not shown).
The allylation products could be conveniently isolated by
extracting the reaction mixture three times with Et2O followed by
concentration. The reactions were clean without any side-
products and/ or starting materials. The RTIL mole fraction was
earlier found to govern the reaction yields, which was ascribed
to the change in the ionic character of the medium.13 Hence, we
also carried out the reaction using different amounts of
[bmim][Br] (1-3 mL/mmol). However, the change in the RTIL
mole fractions didn't affect the reaction yields significantly (data
not shown). The RTIL, recovered after Et2O extraction, was
reused at least 5 times without any significant yield loss (the
yield varied from 83-80% in successive cycles). During the
process, although [b2Brmim][Br] is accumulated as a non-
metallic bi-product, its amount [b2Brmim][Br] is very small
compared to the volume of [bmim][Br] employed as a solvent.
More importantly, it does not interfere in the catalytic cycle.
Recently we found that the Bi-mediated crotylation of aldehydes
can be carried out even with a lesser amount (~1.25 mL/mmol)
of [bmim][Br] with marginal yield loss.1f Consistent with this,
allylation of 1a at 10 mmol scale could be carried out in
[bmim][Br] (12 mL) to obtain 2a in 86% yield, and the reaction
was complete in 26 h, establishing practical feasibility of the
protocol (0.1 equiv. of In). Even at 10 mmol scale, the RTIL was
used for three cycles without any loss of yield. To avoid even
any marginal yield loss, the reaction medium was replenished
each time with 2 mL of fresh RTIL, as part of it was consumed
during the process (vide infra).
1
2
5
H2O
H2Oc
18
16
18
18
20
4
59
64
65
69
61
89
64
62
89
86
83
8
3
5
3
3
5
THF
THFc
4
3
5
5
3
5
THF-H2Od
[bmim][Br]
[bmim][BF4]
[bmim][PF6]
[bmim][Br]
[bmim][Br]
[bmim][Br]
H2O
6
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.2
1.2
1.2
0.6
0.5
0.1
0.1
0.1
0.1
7
20
20
12
16
24
72
48
72
8
9
10
11
12
13
14
[bmim][PF6]
[bmmim][Br]
9-10
NRe
[a] The reactions were carried out using 2 mmol of 1a. [b] Isolated yields of
the products. [c] Reaction was carried out under ultrasonication. [d] 1:1 (v/v)
THF-H2O was used. e)NR= No Reaction
Next, we attempted the allylation of 1a in [bmim][Br] under
the above reaction conditions, but using sub-stoichiometric
amounts (0.6-0.1 equiv.) of In, and the results are shown in
Table 1, entries 9-14. Although the reaction became
progressively slower with reduced amounts of In, 2a was
obtained in appreciable yields. Interestingly the reaction could
be accomplished even with 0.1 equiv. of In, when 2a was
obtained in 83% yield in 24 h (Table 1, entry 11). However,
further reduction of the amount of allyl bromide led to incomplete
reactions (data not shown), probably due to the volatility of allyl
bromide. Such an allylation protocol using the combination of
catalytic amount of In(0) with allyl bromide has never been
reported. Use of inexpensive allyl bromide makes the protocol
more economical, attractive and amenable for scale-up. The In
(0.1 equiv.)-mediated reactions of 1a and allyl bromide in water
and [bmim][PF6] were very sluggish and produced 2a in very
poor yields (Table 1, entries 12-13). The above reaction in 1-
butyl-2,3-dimethylimidazolium bromide ([bmmim][Br]), lacking
the imidazole C-2 hydrogen was abortive (Table 1, entry 14).
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10.1002/ejoc.201800043
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Table 2. Reaction profile of allylation of carbonyls in [bmim][Br] using catalytic amount of Ina
R2
O
In (10 mol%)
OH
Br
R1
[bmim][Br]
25 oC / 24 h
R1
R2
1
2
Entry
Substrate
R1
R2
Product
Yieldb
(%)
1
2
1b
1c
1d
1e
1f
2-Cl-C6H4
4-Cl-C6H4
4-F-C6H4
H
H
H
H
H
H
H
H
H
H
2b
2c
2d
2e
2f
92
93
88
91
92
90
82
81
85
85
88
82
80
78
75
75
87
85
3
4
2-Br-C6H4
3-OMe-C6H4
3,5-diOMe-C6H3
C6F5
5
6
1g
1h
1i
2g
2h
2i
7
8
C6H5-CH=CH
2-furfuryl
9
1j
2j
10
11
12
13
14
15
16
17
18
1k
1l
C9H19
2k
2l
cyclohexanone
1m
1n
1o
1p
1q
1r
2mc
2n
2o
2p
2q
2r
4-methylcyclohexanone
C6H5
CH3
C6H5
C6H5
C6H5
C6H5
CH3
C6H5-C=O
C6H5-CH=CH
CH2CH2CO2C2H5
1s
(R)-cyclohexylideneglyceraldehyde
2s
(R,R):(R:S) :: 37:63d
[a] The reactions were carried out using 2 mmol of the carbonyl substrates and 1.5 equiv. of allyl bromide. [b] Isolated yields of the products. [c] The
product with equatorial-OH group was obtained. [d] Based on the isolated yields of the individual diastereomers.
Next, the above protocol was extended for reaction of
crotyl bromide with some of the above mentioned aldehydes
(1a,1b,1f,1k,1s), ketones (1l,1n-p) and some additional
substrates (1t,1u). In all the cases, the products 3a-k were
obtained in good yields (72-92%, Table 3). Most of the
substrates, except benzil (1p) furnished the corresponding γ-
adducts exclusively. With 1p (Table 3, entry 8), an inseparable
mixture of the major γ-adduct 3h along with the α-adduct (E/Z
mixture) was obtained in 67:33 ratio.
further mechanistic studies. Hence, we probed the
1
organometallic species, responsible for the reaction by H NMR
analyses and the results are disclosed below.
Mechanistic studies: For this, the reactions between In (1
mmol) and allyl bromide (1 mmol) in D2O / THF-d8 (3 mL) were
1
followed by recording the H NMR spectra of the aliquots of the
reaction mixture over a time period. In case of [bmim][Br], a
large excess of allyl bromide (12 mmol) was used in 2 mL of
1
RTIL to mimic the catalytic conditions. The H NMR spectra of
Except for 3j (Table 3, entry 10), the syn/anti ratios of the
1
products were determined by comparing their H NMR spectra
the reaction mixture carried out in D2O (vide SI, Fig. 2) showed
appearance of a new doublet (J = 8.4 Hz) at δ 1.64 ppm at the
expense of the –CH2Br signal at δ 3.9 ppm, after 10 min. The
intensity of the peak (δ 1.64 ppm) attained its maximum in 25
min, and thereafter declined gradually to vanish in 2 h. No new
olefinic resonances were noticed. The 1H NMR doublet at δ 1.64
ppm was assigned to CH2=CHCH2─In (I), based on an earlier
report.3a Depletion of the 1H NMR resonances (δ 1.64 ppm) was
associated with an increase in the intensity of the resonances at
~ δ 4.37 ppm, suggesting formation of allyl alcohol, as detected
earlier.3a These results clearly revealed that the active In-
species I is hydrolytically unstable, and may account for the
need of a large excess of In and allyl bromide, when the
allylation is carried out in H2O. Recently, Haddad et al. reported
formation of a mixture of allylindium intermediates in the reaction
of allyl bromide with In metal in D2O.1g They detected species I
with those reported (vide experimental). For 3j, the more
downfield doublet in the aliphatic region (δ 0-1.5 ppm) was
assigned to the syn-isomer by comparing its H NMR spectrum
1
with that of 3a, and the syn/anti ratio was determined. The
exclusive formation of γ-adduct in most of the reactions is in
contrast to a previous report11a where exclusive α-adducts were
obtained using α-methylallyl boronates in presence of catalytic
amount of In metal. Overall, the above results clearly
established [bmim][Br] as the best RTIL among those chosen for
the In-mediated carbonyl allylation. Most importantly, the
reaction requires only 0.1 equiv. of In metal. By contrast, a large
excess of reagents was required for completion of the reaction in
H2O and THF. The special role of [bmim][Br] in activating In
metal, and also ensuring metal recyclability were of interest for
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10.1002/ejoc.201800043
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by recording the 1H NMR spectrum of the reaction mixture in
D2O. Besides I, formation of other allylindium intermediates were
also suggested.1g However, we could not detect any allylindium intermediate other than I in D2O.
Table 3. Reaction profile of allylation of carbonyls in [bmim][Br] using catalytic amount of Ina
R2
O
In (10 mol%)
OH
Br
R1
[bmim][Br]
25 oC / 24 h
R1
R2
3
1
Entry
Substrate
R1
C6H5
R2
Product
Yieldb
(%)
92
γ (syn:anti):α (E:Z)c
1
2
3
4
5
6
7
8
1a
1b
1f
H
H
H
H
3a
3b
3c
3d
3e
3f
100 (40:60) : 0 (-)
100 (48:52) : 0 (-)
100 (46:54) : 0 (-)
100 (49:51) : 0 (-)
----
2-Cl-C6H4
3-OMe-C6H4
C9H19
91
87
1k
1l
87
Cyclohexanone
C6H5
82
1n
1o
1p
CH3
C6H5
78
100 (65:35) : 0 (-)
----
C6H5
3g
3h
75
C6H5
C6H5-C=O
72
67 (44:56) : 33 (73:27)
9
1s
(R)-cyclohexylideneglyceraldehyde
3i
85
γ [(2R,3R,4R):(2R,3S,4R):(2R,3S,4S)]:α(E:Z) :: 100 [9:13:78] : 0(-)d
10
11
1t
4-C2H5-C6H4
C6H13
H
H
3j
90
100 (50:50) : 0 (-)
1u
3k
85
100 (40:60) : 0 (-)
[a] The reactions were carried out using 2 mmol of the carbonyl substrtates and 1.5 equiv. of crotyl bromide. [b] Isolated yields of the products. [c]
The regioisomeric and diastereomeric ratios were obtained from the 1H NMR spectra. [d] Based on the isolated yields of the individual
diastereomers.
Next, we followed the reaction in THF as above. Due to the
overlapping resonances of the solvent in the region of interest (δ
1.5-1.9 ppm), formation of species I could not be inferred
reached a maximum in ~ 3 h, and remained steady up to 8 h.
The intensities of the NMR peak due to the CH2 In protons in
the species I and II were quantified by comparing with that of the
N-CH3 singlet signal originating from the RTIL. Earlier, a rapid
conversion of the initially formed species I to II was reported in
[bpy][BF4],12a especially in the presence of [bpy][Br]. However,
no such transformation was noticed in [bmim][Br], as their
relative ratio remained 1:1 over the entire study period. These
results are consistent with the fact that the combination of
propargyl bromide and In generated allenylindium and
allenylindium dibromide in both aqueous media and THF.16
Addition of 1a (10 fold excess) to the reaction mixture in
[bmim][Br] led to the complete disappearance of the 1H NMR
signal at δ 1.67 ppm (recorded in D2O), but no NMR signal of the
product 2a was observed due to its insolubility in D2O. But when
the 1H NMR spectrum of the same aliquot was recorded in
CDCl3, the signals due to 2a along with those of II and unreacted
1a were evident. The temporal profiles (16 h and 24 h) of the 1H
NMR spectra of the reaction aliquots recorded in D2O showed
absence of I in the reaction mixture. However, the 1H NMR
spectrum of aliquot (16 h), recorded in CDCl3 showed the 1H
NMR signals of II and 2a (~70% conversion, cf. 1H NMR
spectrum). Even after 24 h, when the reaction was complete, a
significantly intense signal of II was observed. Although we are
unsure of the relative reactivities of the species I and II with the
carbonyls, the above results suggested that between them, only
II was regenerated and helped completion of the reaction.
1
conclusive (vide SI, Fig. 3). But the H NMR spectrum recorded
after 4 h, showed appearance of the new doublets (J = 8.8 Hz)
at δ 1.98 ppm and olefinic multiplets at δ 4.57-4.85 ppm. This
suggested formation of some allyl-In species. Although the allyl-
In species remained stable up to 6.5 h (cf. time-dependent 1H
NMR spectra), its formation required a long induction time.
Earlier, the reaction of In and allyl bromide in organic media and
RTIL produced I along with CH2=CHCH2─InBr2 (II), the latter
showing a 1H NMR signal at ~ δ 2.01.12a,14 Keeping these in view,
and appearance of the new olefinic resonances at ~ δ 4.5 ppm,
formation of the species II in THF was inferred.
Previously, Chan et al. reported simultaneous formation of
I and II in [emim][BF4] and [bpy][BF4].12a,15 The species I is
insoluble in CDCl3.1g Hence, to detect all the possible allylindium
intermediates in [bmim][Br] in the present studies, the reaction
mixture aliquots were concentrated in vacuo (~50 Torr),
dissolved separately in CDCl3 and D2O, and their 1H NMR
spectra recorded every hour over a period of 8 h. The spectra
recorded in CDCl3 (vide SI, Fig. 4) showed a new doublet (J =
8.5 Hz) at δ 2.15 ppm along with new olefinic multiplets at δ 6.19
ppm and 4.72 ppm, while that recorded in D2O (vide SI, Fig. 5)
showed a doublet (J = 8.5 Hz) at δ 1.67 ppm. As above, the
allyl-In species detected in CDCl3 was assigned structure II,
whereas the species detected in D2O was assigned structure I.
Thus, analogous to the observations in other RTILs,1g we found
that both I and II are simultaneously generated in [bmim][Br].
The total integration of the signals at δ 2.15 ppm and 1.67 ppm
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10.1002/ejoc.201800043
European Journal of Organic Chemistry
FULL PAPER
in the formation of the carbene. Similar carbene formation was
also reported even at pH 8.3 during deuterium exchange of the
C-2 proton in D2O.20a-c Such type of unsaturated carbenes are
stable20d in their monomeric forms. Earlier dimesitylcarbene was
used as a reducing agent to produce Au nanoparticles from Au+3
solution.20e Under the present conditions, the NHC reduces InBr3
to produce InBr along with its oxidized product, 2-bromo-1-butyl-
3-methylimidazolium bromide, [b2Brmim][Br] (eqs. 6-7). To
confirm this, the crude reaction mixture was successively
extracted with Et2O (to isolate the product alcohol) followed by
ice-cold CHCl3, and the residue dried under vacuum to obtain a
~1:1 mixture of [bmim][Br] and[b2Brmim][Br], which was
characterized by 1H and 13C NMR spectroscopy. Besides the
H
H
H
H
Br-
+δ
+
In
N
N
Br
+
N
N
C4H9
+δ
C4H9
H
In
H
[bmim][Br]
Transient
Scheme 2. Activation of In-metal by [bmim][Br].
The above results also suggested activation of In metal
specifically by [bmim][Br], but not by other RTILs, chosen for the
study. For investigating this, we recorded the 1H NMR spectrum
of [bmim][Br] (2 mL) after incubating with In (1.0 mmol) for 0.5 h
(vide SI, Fig. 6), which showed upfield shifts of its imidazole
protons (~ 39 Hz for H-2, H-4 and H-5). The magnitudes of the
NMR shifts relates to the charge transfer (CT) process,17a-c and
the imidazolium-based ILs are known to promote electron-
transfer reactions.17d Thus, the upfield shift of the 1H NMR
resonances suggested electron polarization from In metal to
[bmim][Br] that would activate the metal surface via formation of
ion-pairs or more complex ion aggregates with a partially
charged activated In (Scheme 2). Similar activation of Ga and Bi
metals in [bmim][Br] were observed earlier by us,1f,h and is
consistent with higher standard reduction potential of [bmim][Br]
(0.641 V, measured against standard calomel electrode as
reference)1f compared to that of In metal, supporting such a
possibility of electron polarization between In and the RTIL. The
Barbier type of reactions are known to proceed via formation of
radicals on the activated metal surfaces.18a Addition of allyl
bromide to the above mixture regenerated the RTIL, because
1
resonances for [bmim][Br], the H NMR spectrum of the mixture
showed two doublets (each J = 2.0 Hz) at δ 7.99 ppm and 8.16
ppm, while its 13C NMR spectrum showed resonances at δ
123.3, 124.9 and 130.9 ppm. These suggested formation of
[b2Brmim][Br] in the reaction mixture. This was further confirmed
by preparing its chloro-analog, [b2Clmim][Cl], according to a
reported procedure,20f and comparing its 1H and 13C NMR
spectra with those for the above mixture. This unambiguously
proved the formation of [b2Brmim][Br], and, thus, of the
reduction of In(III) to In(I) in situ. The InBr produced in eqs. 5-7,
subsequently reacts with allyl bromide to furnish II (eq. 8),
maintaining the catalytic cycle without further addition of In metal.
For additional confirmation, commercially available InBr was
1
reacted with allyl bromide in [bmim][Br]. A new H NMR doublet
at δ 2.12 confirmed formation of the species II, as was reported
earlier in the reaction of InBr with allyl bromide in
[bpy][BF4]/[bpy][Br].12a
One limitation of the study is that we could not identify the
NHC intermediate, because of its low concentration and also its
fast reaction with InBr3. It was also impossible to identify
formation of its precursor, [bmim][OH] from the complex reaction
mixture. Nevertheless, in an indirect attempt to support our
hypothesis, we stirred a mixture of InBr3 (10 mol%), allyl bromide
(1 mmol) and [bmim][OH]21 (10 mol%) in [bmim][Br] (2 mL) for
30 min. The 1H NMR spectrum (CDCl3) of an aliquot of the
reaction mixture showed the peaks due to the species II. This
can only happen by the [bmim][OH]-mediated reduction of InBr3
to InBr, followed by its reaction with allyl bromide. Earlier, similar
reduction of the group 13-metal halides by an NHC-complex has
been reported.22
1
the H NMR resonances of its imidazole protons were restored.
This also confirmed that the proposed electron polarization
generated
a transient activated In species, facilitating its
subsequent reaction with allyl bromide to furnish the active
allylating species I and II. However, similar electron polarization
was not evident either in [bmim][BF4] or in [bmim][PF6], which
unequivocally proved that this kind of metal activation is possible
only in [bmim][Br], and not in other similar RTILs used in this
study. Possibly, as stated earlier, the reduction potential of the
RTILs plays a role in the electron polarization. The reduction
potentials of RTILs are reported to be highly dependent on the
nature of the anions,18b,c particularly the fluorine containing
anions are known to enhance their electrochemical stability
against reduction.18c
The formation of the species I and II and the catalytic role
of In can be rationalized considering the reactions shown in
Scheme 3. Initially, the activated In metal reacts with allyl
bromide to furnish both I and II (eq. 1). Addition of species I to
the carbonyl substrate, followed by hydrolysis19 furnishes the
product homoallylic alcohol, along with [bmim][OH] and InBr
(eqs. 2-3). On the other hand, a similar reaction course of II
yields the product along with [bmim][OH] and InBr3 (eqs. 4-5).
The [bmim][OH], generated in situ, has been reported to be a
1:1 mixture of N-heterocyclic carbene (NHC) and water.20a In situ
dissociation of the C-2 hydrogen of the imidazolium ring results
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10.1002/ejoc.201800043
European Journal of Organic Chemistry
FULL PAPER
In
InBr2
Br
2 In
+
+
2
(eq. 1)
(eq. 2)
II
I
(activated)
OIn
R2
In
R1COR2
+
R1
I
OH
OIn
(eq. 3)
H O
2
[bmim][Br]
+
[bmim][OH]
InBr
+
+
+
R1
R1
R2
R2
OInBr2
R2
(eq. 4)
(eq. 5)
InBr2
R1COR2
+
R1
II
OH
OInBr2
[bmim][OH]
InBr3
+
+
+ H2O
[bmim][Br]
+
R1
R1
R2
R2
(eq. 6)
(eq. 7)
(eq. 8)
+
H2O
[bmim][OH]
N
N
C4H9
[b2Brmim][Br] + InBr
InBr2
InBr3
+
N
N
C4H9
Br
2
2
+
2 InBr
II
Overall reaction:
OH
Br
2 In
4
2R1COR2
[bmim][OH]
+ +
+
+
H O
2
[b2Brmim][Br]
+
2
[bmim][Br]
+
2
R1
InBr2
(activated)
R2
+
2
II
+
Br
N
N
[b2Brmim][Br]
=
C4H9
Br
Scheme 3. Plausible mechanism for carbonyl allylation with catalytic In.
wastes. Although
a very small amount (1 mmol/mmol of
Conclusions
substrate, calculated to be only 1.7% of the RTIL recovered after
the reaction) of the non-metallic bi-product, [b2Brmim][Br], is
accumulated in the reaction mixture, it does not interfere in the
catalytic cycle as evident from the recyclability of the RTIL. The
reaction mechanism for the catalytic cycle of In metal and its
activation by [bmim][Br] has also been established. Eventual
formation of a monomeric NHC, which ultimately reduces In(III)
to In(I), has been found to play a pivotal role in maintaining the
Overall, we have developed a catalytic Barbier-type protocol for
the indium-mediated allylation of carbonyls achieving excellent
yield. The reaction could be accomplished even with 10 mol% of
In-metal, thereby reducing the total metallic wastes unlike most
of the previous methods of Barbier-type carbonyl allylation.
Additionally we have carried out the reactions with only 1.5 equiv.
of the allylic halides that would minimize generation of organic
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10.1002/ejoc.201800043
European Journal of Organic Chemistry
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1-(4-Chlorophenyl)-but-3-en-1-ol (2c)23a: Yellow liquid; IR: 3416, 914 cm-
catalytic cycle. Overall, the present protocol outscores myriads
of previously reported methods of Barbier-type carbonyl
allylation, both in terms of procedural advantages and
conceptual novelty.
1
;
1H NMR (500 MHz, CDCl3): δ = 2.06 (broad s, 1H), 2.36-2.42 (m, 1H),
2.62-2.66 (m, 1H), 5.16-5.22 (m, 3H), 5.83-5.92 (m, 1H), 7.19-7.23 (m,
1H), 7.27-7.34 (m, 2H), 7.56-7.58 (m, 1H); 13C NMR (125 MHz, CDCl3): δ
= 42.0, 69.6, 118.6, 127.0, 127.1, 128.4, 129.4, 131.7, 134.2, 141.2.
1-(4-Fluorophenyl)-but-3-en-1-ol (2d)23a: Yellow liquid; IR: 3385, 919 cm-
1
;
1H NMR (500 MHz, CDCl3): δ = 2.12 (broad s, 1H), 2.45-2.53 (m, 2H),
Experimental Section
4.73 (t, J = 6.0 Hz, 1H), 5.14-5.18 (m, 2H), 5.75-5.83 (m, 1H), 7.02-7.07
(m, 2H), 7.27-7.34 (m, 2H); 13C NMR (125 MHz, CDCl3) δ = 43.9, 72.6,
115.1, 115.3, 118.7, 127.4, 127.5, 134.1, 139.5, 161.2, 163.1.
All chemicals were procured from Sigma Aldrich and used as received.
Other reagents were of AR grade. [bmim][Br] and [b2Clmim][Cl] were
synthesized following known procedures.1h,20f The organic extracts were
dried over anhydrous Na2SO4. The IR spectra were recorded as films
with a BRUKER Tensor II spectrophotometer. The 1H and 13C NMR
spectra were recorded with a Bruker AC-200 (200 MHz) or a Varian 500
MHz NMR spectrometer. For characterization, the NMR spectra were
processed using either MestReNova Lite-11.0.4, ACD/1D NMR
Processor or Bruker TOPSPIN software. The optical rotations were
recorded with a JASCO DIP-360 digital polarimeter. The elemental
analyses were carried with an Elementar Vario micro cube. Melting
points (mp) were measured in a Büchi B-540 apparatus.
1-(4-Bromophenyl)-but-3-en-1-ol (2e)1f: Yellow liquid; IR: 3384, 919 cm-1;
1H NMR (500 MHz, CDCl3): δ = 2.25 (broad s, 1H), 2.41-2.50 (m, 2H),
4.67-4.69 (m, 1H), 5.12-5.15 (m, 2H), 5.72-5.80 (m, 1H), 7.20-7.22 (m,
2H), 7.45-7.47 (m, 2H); 13C NMR (125 MHz, CDCl3): δ = 43.7, 72.6,
118.8, 121.2, 127.6, 131.4, 133.9, 142.8.
1-(3-Methoxyphenyl)-but-3-en-1-ol (2f)1f: Colorless liquid; IR: 3417, 917
cm-1; 1H NMR (500 MHz, CDCl3): δ = 2.06 (broad s, 1H), 2.46-2.56 (m,
2H), 3.82 (s, 3H), 4.72 (dd, J = 7.5 and 5.0 Hz, 1H), 5.14-5.20 (m, 2H),
5.78-5.86 (m, 1H), 6.81-6.84 (m, 1H), 6.93-6.95 (m, 2H), 7.25-7.28 (m,
1H); 13C NMR (125 MHz, CDCl3): δ = 43.8, 55.2, 73.2, 111.3, 113.0,
118.1, 118.4, 129.4, 134.4, 145.6, 159.7.
Reaction monitoring by NMR. A mixture of In (1 mmol) and allyl
bromide (1 mmol) in D2O / THF-d8 (3 mL) was magnetically stirred at
room temperature. Aliquots (35 µL) of reaction mixture were taken at
different time intervals, and the 1H NMR spectra recorded. In case of
[bmim][Br], a mixture of In (1 mmol) and allyl bromide (12 mmol) were
stirred in [bmim][Br] (2 mL), and the 1H NMR spectra of the aliquots (35
µL, dried at ~50 Torr) of the reaction mixture recorded separately in
CDCl3 and D2O over a time period.
1-(3,5-Dimethoxyphenyl)-but-3-en-1-ol (2g)23b: Yellow liquid; IR: 3421,
920 cm-1; 1H NMR (500 MHz, CDCl3): δ = 2.05 (broad s, 1H), 2.45-2.53
(m, 2H), 3.79 (s, 6H), 4.66 (dd, J = 7.5 and 5.0 Hz, 1H), 5.13-5.18 (m,
2H), 5.77-5.85 (m, 1H), 6.36-6.38 (m, 1H), 6.51-6.52 (m, 2H); 13C NMR
(125 MHz, CDCl3): δ = 43.7, 55.3, 73.3, 99.4, 103.7, 118.3, 134.4, 146.5,
160.8.
General procedure for In-mediated allylation of 1a in aqueous or
organic media. In metal (1.15 g, 10.0 mmol) was added to a mixture of
1a (212 mg, 2.0 mmol) and allyl bromide (726 mg, 6.0 mmol) in H2O or
THF or H2O-THF (1:1 v/v) (5 mL), and stirred magnetically for the time
specified in Table 1. Similar reactions were also carried out in H2O and
THF under ultrasonication. The mixture was extracted with Et2O (2 × 10
mL), dried, concentrated in vacuo, and subjected to column
chromatography (silica gel, 0-15% EtOAc/hexane) to obtain pure 2a
(yields in Table 1).
1-Pentafluorophenylbut-3-en-1-ol (2h)1f: Yellow thick liquid; IR: 3384, 968
cm-1; 1H NMR (500 MHz, CDCl3): δ = 2.34-2.35 (m, 1H), 2.58-2.64 (m,
1H), 2.75-2.80 (m, 1H), 5.09-5.19 (m, 3H), 5.71-5.79 (m, 1H); 13C NMR
(125 MHz, CDCl3) δ = 41.3, 65.7, 116.4, 119.5, 132.5, 136.5, 138.5,
139.5, 141.6, 143.7, 145.7, 145.8.
(E)-1-Phenylhexa-1,5-dien-3-ol (2i)1f: Colorless liquid; IR: 3417, 919 cm-1;
1H NMR (500 MHz, CDCl3): δ = 1.86 (broad s, 1H), 2.37-2.47 (m, 2H),
4.35-4.39 (m, 1H), 5.16-5.22 (m, 2H), 5.83-5.91 (m, 1H), 6.25 (dd, J =
15.5 and 6.5 Hz, 1H), 6.62 (d, J = 15.5 Hz, 1H), 7.23-7.26 (m, 1H), 7.31-
7.34 (m, 2H), 7.38-7.40 (m, 2H); 13C NMR (125 MHz, CDCl3): δ = 42.0,
71.7, 118.5, 126.5, 127.6, 128.6, 130.4, 131.6, 134.0, 136.7.
General procedure for In-mediated allylation reactions in RTILs. A
mixture of In metal and allyl bromide (quantities specified in Tables 1-3)
in the RTIL (2 mL/mmol) was stirred at room temperature for 0.5 h,
followed by addition of the aldehyde (2 mmol). After stirring the mixture at
room temperature for completion of each reaction (cf. TLC), the mixture
was extracted with Et2O (3 × 10 mL), dried, concentrated in vacuo and
the residue purified by column chromatography (silica gel,
EtOAc/hexane) to give the respective products.
1-(Furan-2-yl)-but-3-en-1-ol (2j)23c: Brown liquid; IR: 3385, 918 cm-1; 1H
NMR (500 MHz, CDCl3): δ = 2.14 (broad s, 1H), 2.61-2.64 (m, 2H), 4.74-
4.76 (m, 1H), 5.14-5.21 (m, 2H), 5.77-5.85 (m, 1H), 6.26 (t, J = 2.5 Hz,
1H), 6.33-6.34 (m, 1H), 7.38-7.39 (m, 1H); 13C NMR (125 MHz, CDCl3): δ
= 40.1, 66.9, 106.1, 110.1, 118.5, 133.7, 142.0, 156.0.
1-Phenylbut-3-en-1-ol (2a)1f: Colorless liquid; IR: 3419, 918 cm-1; 1H
NMR (500 MHz, CDCl3): δ = 2.10 (broad s, 1H), 2.50-2.54 (m, 2H), 4.72-
4.75 (m, 1H), 5.14-5.19 (m, 2H), 5.78-5.86 (m, 1H), 7.26-7.31 (m, 1H),
7.34-7.37 (m, 4H); 13C NMR (125 MHz, CDCl3): δ = 43.8, 73.3, 118.3,
125.8, 127.5, 128.4, 134.5, 143.9.
1-Tridecen-4-ol (2k)1f: Colorless liquid; IR: 3554, 919 cm-1; 1H NMR (500
MHz, CDCl3): δ = 0.87 (t, J = 7.0 Hz, 3H), 1.26-1.34 (m, 13H), 1.42-1.48
(m, 3H), 1.61 (broad s, 1H), 2.10-2.16 (m, 1H), 2.28-2.32 (m, 1H), 3.61-
3.66 (m, 1H), 5.12-5.15 (m, 2H), 5.79-5.87 (m, 1H); 13C NMR (125 MHz,
CDCl3): δ = 14.1, 22.7, 25.7, 29.3, 29.6, 29.7, 31.9, 36.9, 42.0, 70.8,
117.9, 134.9.
1-(2-Chlorophenyl)-but-3-en-1-ol (2b)23a: Colorless liquid; IR: 3385, 918
cm-1; 1H NMR (500 MHz, CDCl3): δ = 2.15 (broad s, 1H), 2.36-2.42 (m,
1H), 2.62-2.66 (m, 1H), 5.16-5.21 (m, 3H), 5.83-5.91 (m, 1H), 7.19-7.22
(m, 1H), 7.28-7.34 (m, 2H), 7.56-7.58 (m, 1H); 13C NMR (125 MHz,
CDCl3): δ = 42.0, 69.7, 118.6, 127.0, 127.1, 128.4, 129.4, 131.8, 134.2,
141.2.
1-Allylcyclohexanol (2l)1f: Colorless liquid; IR: 3405, 912 cm-1; 1H NMR
(500 MHz, CDCl3): δ = 1.25-1.28 (m, 1H), 1.40-1.61 (m, 10H), 2.20-2.22
(m, 2H), 5.09-5.15 (m, 2H), 5.84-5.93 (m, 1H); 13C NMR (125 MHz,
CDCl3): δ = 22.2, 25.8, 37.4, 46.7, 70.9, 118.6, 133.7.
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10.1002/ejoc.201800043
European Journal of Organic Chemistry
FULL PAPER
1-Allyl-4-methylcyclohexanol (2m):23d Colorless liquid; IR: 3385, 919 cm-
2-Methyl-1-(2-chlorophenyl)-but-3-en-1-ol (3b)23h
: Colorless liquid; IR:
1
;
1H NMR (500 MHz, CDCl3):δ = 0.92 (d, J = 6.5 Hz, 3H), 1.02-1.10 (m,
3416, 914 cm-1; 1H NMR (500 MHz, CDCl3): δ = 0.98 (syn) and 1.02 (anti)
(two d, J = 6.5 and 7.0 Hz, 3H), 2.01 (syn) and 2.18 (anti) (two broad s,
1H), 2.57-2.61 (anti) and 2.71-2.75 (syn) (two m, 1H), 4.96 (syn) (d, J =
6.5 Hz, 0.5H), 5.09-5.19 (m, 2.5H), 5.79-5.86 and 5.89-5.96 (two m, 1H),
7.18-7.22 (m, 1H), 7.26-7.35 (m, 2H), 7.47-7.54 (m, 1H); 13C NMR (125
MHz,CDCl3): δ = 12.7, 16.4, 42.5, 45.0, 73.2, 73.5, 115.4, 116.8, 126.6,
128.1, 128.2, 128.4, 129.3, 132.0, 132.6, 139.6, 140.2, 140.4, 140.6.
2H), 1.39-1.48 (m, 3H), 1.60-1.72 (m merged with broad s, 5H), 2.30 (d, J
= 7.5 Hz, 2H), 5.12-5.18 (m, 2H), 5.84-5.93 (m, 1H); 13C NMR (125 MHz,
CDCl3): δ = 21.2, 31.3, 31.5, 37.2, 42.1, 71.5, 118.6, 133.7.
2-Phenylpent-4-en-2-ol (2n)1f: Colorless liquid; IR: 3424, 914 cm-1; 1H
NMR (500 MHz, CDCl3): δ = 1.56 (s, 3H), 2.09 (broad s, 1H), 2.49-2.54
(m, 1H), 2.68-2.72 (m, 1H), 5.12-5.16 (m, 2H), 5.59-5.68 (m, 1H), 7.24-
7.27 (m, 1H), 7.34-7.37 (m, 2H), 7.45-7.46 (m, 2H); 13C NMR (125 MHz,
CDCl3): δ = 29.9, 48.5, 73.6, 119.5, 124.7, 126.6, 128.1, 133.7, 147.6.
2-Methyl-1-(3-methoxyphenyl)-but-3-en-1-ol (3c)23c: Colorless liquid; IR:
3416, 914 cm-1; 1H NMR (500 MHz, CDCl3): δ = 0.89 (anti) and 1.02 (syn)
(two d, J = 7.0 Hz, 3H), 2.04 (broad s, 1H), 2.46-2.51 (anti) and 2.56-2.59
(syn) (two m, 1H), 3.82 (s, 3H), 4.34 (anti) and 4.59 (syn) (two d, J = 7.5
and 5.5 Hz, 1H), 5.05-5.09 and 5.17-5.22 (two m, 2H), 5.74-5.85 (m, 1H),
6.80-6.84 (m, 1H), 6.88-6.92 (m, 2H), 7.23-7.27 (m, 1H); 13C NMR (125
MHz, CDCl3) δ = 13.9, 16.5, 44.5, 46.0, 55.2, 77.2, 77.8, 112.2, 112.3,
112.7, 113.1, 115.3, 116.6, 118.9, 119.2, 129.0, 129.1, 140.4, 140.6,
144.2, 144.4, 159.6.
1,1-Diphenylbut-3-en-1-ol (2o)1f: Colorless liquid; IR: 3416, 914 cm-1; 1H
NMR (500 MHz, CDCl3): δ = 2.57 (broad s, 1H), 3.09-3.11 (m, 2H), 5.19-
5.28 (m, 2H), 5.64-5.73 (m, 1H), 7.22-7.27 (m, 2H), 7.32-7.35 (m, 4H),
7.46-7.48 (m, 4H); 13C NMR (125 MHz, CDCl3) δ = 46.7, 76.9, 120.5,
126.0, 126.8, 128.2, 133.4, 146.5.
2-Hydroxy-1,2-diphenylpent-4-en-1-one (2p)23e: White solid (mp: 90-91
oC); IR: 3453, 1677, 924 cm-1; 1H NMR (500 MHz, CDCl3): δ = 2.96-3.00
(m, 1H), 3.12-3.16 (m, 1H), 4.19 (s, 1H), 5.00-5.14 (m, 2H), 5.69-5.78 (m,
1H), 7.27-7.34 (m, 3H), 7.38-7.46 (m, 3H), 7.50-7.51 (m, 2H), 7.72-7.74
(m, 2H); 13C NMR (125 MHz, CDCl3): δ = 44.0, 81.4, 120.2, 125.6, 128.0,
128.8, 130.1, 132.3, 132.6, 134.7, 141.8, 200.8.
3-Methyltridec-1-en-4-ol (3d) 23i: Colorless liquid; IR: 3384, 912 cm-1; 1H
NMR (500 MHz, CDCl3):δ = 0.88 (t, J = 7.0 Hz, 3H), 1.03 and 1.04 (two d,
J = 5.5 and 5.0 Hz, 3H), 1.27-1.39 (m, 14H), 1.48-1.56 (m merged with
broad s, 3H), 2.18-2.24 (anti) and 2.26-2.30 (syn) (two m, 1H), 3.37-3.41
(anti) and 3.48-3.51 (syn) (two m, 1H), 5.07-5.13 (m, 2H), 5.73-5.84 (m,
1H); 13C NMR (125 MHz, CDCl3): δ = 14.0, 16.3, 22.6, 25.7, 26.1, 29.3,
29.6, 31.9, 34.0, 34.3, 43.4, 44.1, 74.7, 115.1, 116.1, 140.4, 141.1.
(E)-1,3-Diphenylhexa-1,5-dien-3-ol (2q)1f: Colorless thick liquid; IR: 3456,
1640, 969, 918 cm-1; 1H NMR (500 MHz, CDCl3):δ = 2.33 (broad s, 1H),
2.77-2.87 (m, 2H), 5.19-5.24 (m, 2H), 5.69-5.77 (m, 1H), 6.54 (d, J = 16.0
Hz, 1H), 6.66 (d, J = 16.0 Hz, 1H), 7.22-7.33 (m, 4H), 7.36-7.40 (m, 4H),
7.52-7.54 (m, 2H); 13C NMR (125 MHz, CDCl3): δ = 47.1, 75.7, 120.0,
125.4, 126.5, 126.9, 127.5, 128.3, 128.4, 128.5, 133.1, 135.3, 136.8,
145.3.
1-(1-Methyl-2-propenyl)-cyclohexanol (3e)23j: Colorless liquid; IR: 3384,
912 cm-1; 1H NMR (500 MHz, CDCl3): δ = 1.02 (d, J = 7.0 Hz, 3H), 1.16-
1.19 (m, 1H), 1.40-1.45 (m, 2H), 1.48-1.56 (m, 5H), 1.57-1.63 (m, 3H),
2.16-2.19 (m, 1H), 5.04-5.08 (m, 2H), 5.79-5.87 (m, 1H); 13C NMR (125
MHz, CDCl3): δ = 14.1, 21.8, 25.8, 34.4, 35.0, 48.3, 72.4, 115.8, 140.4.
3-Methyl-2-phenylpent-4-en-2-ol (3f)23k: Colorless liquid; IR: 3473, 914
cm-1; 1H NMR (500 MHz, CDCl3):δ = 0.89 (syn) and 0.99 (anti) (two d, J
= 7.0 and 6.5 Hz, 3H), 1.55 (s, 3H), 2.01 (syn) and 2.09 (anti) (two broad
s, 1H), 2.54-2.58 (syn) and 2.59-2.64 (anti) (two m, 1H), 5.11-5.15 (m,
2H), 5.70-5.77 (anti) and 5.81-5.88 (syn) (two m, 1H), 7.24-7.28 (m, 1H),
7.34-7.37 (m, 2H), 7.42-7.47 (m, 2H); 13C NMR (125 MHz, CDCl3): δ =
14.1, 14.8, 25.9, 28.5, 48.8, 49.0, 75.7, 75.8, 116.3, 116.6, 125.2, 125.5,
126.4, 126.6, 127.9, 139.9, 140.0, 147.0.
Ethyl 4-Hydroxy-4-methylhept-6-enoate (2r):23f Colorless liquid; IR: 3443,
1730, 948 cm-1; 1H NMR (500 MHz, CDCl3): δ = 1.25 (t, J = 7.0 Hz, 3H),
1.40 (s, 3H), 1.66 (broad s, 1H), 1.94-1.98 (m, 1H), 2.12-2.17 (m, 1H),
2.42 (d, J = 7.5 Hz, 2H), 2.75 (t, J = 6.5 Hz, 2H), 4.13 (q, J = 7.0 Hz, 2H),
5.15-5.19 (m, 2H), 5.74-5.83 (m, 1H); 13C NMR (125 MHz, CDCl3): δ =
14.1, 26.1, 28.0, 37.9, 45.2, 60.5, 85.8, 119.6, 132.0, 172.6.
1,2-Cyclohexylidenedioxy-5-hexen-3-ol (2s): (2R,3R)-isomer: Colorless
20
liquid; [α]D24 +3.6 (c 1.10, CHCl3) (lit.23g [α]D +3.5 (c 1.06, CHCl3)); IR:
2-Methyl-1,1-diphenylbut-3-en-1-ol (3g)23l:White solid (mp: 52-53 oC); IR:
3553, 915 cm-1; 1H NMR (500 MHz, CDCl3): δ = 1.16 (d, J = 7.0 Hz, 3H),
2.51 (broad s, 1H), 3.66-3.71 (m, 1H), 5.25-5.33 (m, 2H), 6.00-6.07 (m,
1H), 7.28-7.31 (m, 2H), 7.40-7.45 (m, 4H), 7.61-7.72 (m, 4H); 13C NMR
(125 MHz, CDCl3): δ = 13.6, 44.4, 79.4, 117.4, 125.8, 126.0, 126.6, 126.7,
128.2, 128.3, 128.4, 130.2, 132.6, 139.4, 145.9, 146.9.
3454, 1641, 927 cm-1; 1H NMR (500 MHz, CDCl3): δ = 1.39-1.40 (m, 2H),
1.57-1.62 (m, 8H), 2.22-2.32 (m merged with broad s, 3H), 3.55-3.59 (m,
1H), 3.71-3.74 (m, 1H), 3.97-4.04 (m, 2H), 5.09-5.14 (m, 2H), 5.81-5.90
(m, 1H); 13C NMR (125 MHz, CDCl3): δ = 23.8, 24.0, 25.1, 34.8, 36.2,
38.3, 65.7, 71.6, 78.0, 109.9, 117.7, 134.0. (2R,3S)-isomer: Colorless
20
liquid; ([α]D24 +7.6 (c 1.02, CHCl3) (lit.23g [α]D +7.4 (c 1.15, CHCl3)); IR:
3455, 1642, 926 cm-1; 1H NMR (500 MHz, CDCl3): δ = 1.37-1.39 (m, 2H),
1.57-1.61 (m, 8H), 2.13-2.22 (m merged with broad s, 2H), 2.29-2.34 (m,
1H), 3.75-3.77 (m, 1H), 3.88-3.92 (m, 1H), 3.97-4.02 (m, 2H), 5.11-5.16
(m, 2H), 5.79-5.87 (m, 1H); 13C NMR (125 MHz, CDCl3): δ = 23.8, 24.0,
25.1, 34.8, 36.2, 37.6, 64.8, 70.5, 77.7, 109.6, 118.2, 134.0.
23m
2-Hydroxy-3-methyl-1,2-diphenylpent-4-en-1-one (3h)
: White solid
(mp: 61-62 oC); IR: 3453, 1719, 919 cm-1; 1H NMR (500 MHz, CDCl3): δ
= 1.09 (d, J = 7.0 Hz, 3H), 2.81-2.85 and 3.09-3.13 (two m, 1H), 3.83
(broad s, 1H), 5.11-5.17 (m, 2H), 5.77-5.84 (m, 1H), 7.26-7.40 (m, 6H),
7.58-7.60 (m, 2H), 7.74-7.77 (m, 2H); 2-Hydroxy-1,2-diphenylhex-4-en-1-
one (Inseparable α-isomer 3h')23n 1H NMR (500 MHz, CDCl3): 1.37 (Z)
:
2-Methyl-1-phenylbut-3-en-1-ol (3a)1h: Colorless liquid; IR: 3415, 914 cm-
1
;
1H NMR (500 MHz, CDCl3): δ = 0.89 (anti) and 1.03 (syn) (two d, J =
and 1.62 (E) (two d, J = 7.0 and 6.0 Hz, 3H), 3.51-3.54 (m, 2H), 4.09
(broad s, 1H), 5.32-5.37 and 5.42-5.48 (two m, 2H), 7.26-7.40 (m, 2H),
7.44-7.54 (m, 6H), 7.97-7.99 (m, 2H); 13C NMR (125 MHz, CDCl3): δ =
14.7, 17.9, 36.6, 43.1, 44.6, 81.6, 84.6, 116.9, 123.4, 124.5, 125.5, 125.7,
126.5, 127.6, 127.8, 128.0, 128.4, 128.7, 128.8, 128.9, 129.1, 129.6,
129.8, 130.0, 130.1, 131.4, 132.3, 132.5, 132.6, 133.1, 134.7, 134.9,
135.9, 139.0, 140.9, 142.0, 201.0, 201.8.
7.0 Hz, 3H), 2.05 (broad s, 1H), 2.48-2.52 (anti) and 2.58-2.61 (syn) (two
m, 1H), 4.37 (anti) and 4.62 (syn) (two d, J = 7.5 and 5.5 Hz, 1H), 5.05-
5.08 and 5.18-5.23 (two m, 2H), 5.74-5.86 (m, 1H), 7.27-7.37 (m, 5H);
13C NMR (125 MHz, CDCl3):δ = 14.0, 16.5, 44.6, 46.2, 77.3, 77.9, 115.4,
116.6, 126.5, 126.8, 127.3, 127.6, 128.0, 128.2, 140.3, 140.6, 142.5.
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10.1002/ejoc.201800043
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1,2-Cyclohexylidenedioxy-4-methyl-5-hexen-3-ol
(3i):
(2R,3R,4R)
Keywords: Allylation, catalytic indium, ionic liquid, [bmim][Br]
isomer: Colorless oil; [α]D25 +46.2 (c 0.96, CHCl3) (lit.23o [α]D +45.9 (c
1.81, CHCl3)); IR: 3486, 927 cm-1; 1H NMR (500 MHz, CDCl3): δ = 1.09
(d, J = 7.0 Hz, 3H), 1.38-1.40 (m, 2H), 1.55-1.61 (m, 8H), 2.26-2.31 (m,
2H), 3.27-3.30 (m, 1H), 3.70-3.74 (m, 1H), 3.95-4.00 (m, 1H), 4.06-4.15
(m, 1H), 5.02-5.09 (m, 2H), 5.69-5.76 (m, 1H); 13C NMR (125 MHz,
CDCl3): δ = 15.7, 23.8, 24.0, 25.1, 34.9, 36.2, 42.3, 66.3, 74.6, 76.7,
24
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2003, 44, 8765–8768.
25
109.6, 115.3, 140.5. (2R,3S,4R) isomer: Colorless oil; [α]D +29.0 (c
24
1.04, CHCl3) (lit.23o [α]D +29.8 (c 1.40, CHCl3)); IR: 3475, 926 cm-1; 1H
NMR (500 MHz, CDCl3): δ = 1.10 (d, J = 6.5 Hz, 3H), 1.38-1.40 (m, 2H),
1.53-1.61 (m, 8H), 2.14 (broad s, 1H), 2.23-2.27 (m, 1H), 3.64-3.66 (m,
1H), 3.86-3.89 (m, 1H), 3.94-3.97 (m, 1H), 4.08-4.11 (m, 1H), 5.03-5.07
(m, 2H), 5.70-5.77 (m, 1H); 13C NMR (125 MHz, CDCl3): δ = 15.5, 23.8,
23.9, 25.1, 34.9, 36.2, 40.7, 64.1, 73.6, 76.3, 109.2, 115.4, 140.1.
(2R,3S,4S) isomer: Colorless oil; [α]D25 +2.2 (c 1.12, CHCl3) (lit.23o [α]D
24
+2.4 (c 1.17, CHCl3)); IR: 3475, 927 cm-1; 1H NMR (500 MHz, CDCl3): δ =
1.06 (d, J = 7.0 Hz, 3H), 1.36-1.38 (m, 2H), 1.54-1.58 (m, 8H), 2.08
(broad s, 1H), 2.37-2.41 (m, 1H), 3.55-3.57 (m, 1H), 3.85-3.88 (m, 1H),
3.94-3.98 (m, 1H), 4.00-4.04 (m, 1H), 5.07-5.11 (m, 2H), 5.79-5.87 (m,
1H); 13C NMR (125 MHz, CDCl3): δ = 16.5, 23.8, 23.9, 25.1, 34.9, 36.3,
40.3, 65.3, 74.8, 76.8, 109.2, 116.0, 139.3.
2-Methyl-1-(4-ethylphenyl)-but-3-en-1-ol (3j): Colorless liquid; IR: 3417,
914 cm-1; 1H NMR (500 MHz, CDCl3): δ = 0.88 (anti) and 1.04 (syn) (two
d, J = 6.5 Hz each, 3H), 1.25 (t, J = 7.5 Hz, 3H), 2.47-2.52 and 2.57-2.61
(m, 1H), 2.64-2.68 (m, 2H), 4.34 (anti) and 4.58 (syn) (two d, J = 8.0 and
6.0 Hz, 1H), 5.05-5.08 and 5.17-5.23 (two m, 2H), 5.73-5.87 (m, 1H),
7.17-7.27 (m, 4H); 13C NMR (125 MHz, CDCl3) δ = 14.2, 15.5, 16.5, 28.5,
44.6, 46.1, 77.8, 115.2, 116.4, 125.8, 126.5, 126.8, 127.5, 127.7, 130.3,
139.7,139.9, 140.5, 140.8, 143.3, 143.6. Anal. Calcd. for C13H18O: C,
82.06; H, 9.53. Found: C, 81.78; H, 9.64.
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3-Methyldec-1-en-4-ol (3k):23h Colorless liquid; IR: 3383, 912 cm-1; 1H
NMR (500 MHz, CDCl3):δ = 0.89 (t, J = 7.0 Hz, 3H), 1.03 and 1.04 (two d
merged together, J = 5.0 Hz each, 3H), 1.25-1.35 (m, 8H), 1.48-1.57 (m
merged with a broad s, 3H), 2.18-2.23 (anti) and 2.26-2.30 (syn) (two m,
1H), 3.37-3.41 (anti) and 3.48-3.50 (syn) (two m, 1H), 5.07-5.13 (m, 2H),
5.72-5.84 (m, 1H); 13C NMR (125 MHz, CDCl3) δ = 14.0, 16.3, 22.6, 25.7,
26.0, 29.4, 31.8, 34.0, 34.3, 43.4, 44.1, 74.7, 115.1, 116.1, 140.4, 141.1.
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Isolation of [b2Brmim][Br]. In a separate experiment of allylation of 1a,
the reaction mixture was extracted with Et2O to isolate 2a, thoroughly
washed with ice-cold CHCl3 (3 × 5 mL), and the residue dried under
vacuum to obtain a product, containing a ~1:1 mixture of both [bmim][Br]
and [b2Brmim][Br]. Light red viscous oil; 1H NMR (500 MHz, CDCl3): δ =
0.91-0.96 (m, 6H), 1.33-1.41 (m, 4H), 1.83-1.90 (m, 4H), 4.06 (s, 3H),
4.09 (s, 3H), 4.28-4.32 (m, 4H), 7.46 (t, J = 1.5 Hz, 1H), 7.57 (t, J = 1.5
Hz, 1H), 7.99 (d, J = 2 Hz, 1H), 8.16 (d, J = 2 Hz, 1H); 13C NMR (125
MHz, CDCl3): δ = 13.3, 13.4, 19.4, 19.5, 31.2, 32.1, 36.9, 37.0, 49.9,
122.1, 123.3, 123.7, 124.9, 128.1, 128.9, 130.9, 137.2.
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Synthesis
of
1-butyl-2-chloro-3-methylimidazolium
chloride
[b2Clmim][Cl]20f. To a cooled (0 °C) suspension of [bmim][Cl] (873 mg, 5
mmol) in THF (10 mL), dropwise addition of MeMgBr (3 M in Et2O, 2 mL,
6 mmol), followed by stirring for another 30 min gave a clear solution. It
was cooled to -78 °C, and a solution of hexachloroethane (1.66 g, 7
mmol) in THF (10 mL) was dropwise added to it. The reaction mixture
was brought to room temperature and stirred further for 18 h. The upper
layer was discarded, the bottom phase washed with dry THF (3 × 5 mL),
and the residue dried under vacuum to yield pure [b2Clmim][Cl] as a light
brown viscous liquid. 1H NMR (500 MHz, CDCl3): δ = 0.74 (t, J = 7.5 Hz,
3H), 1.16-1.23 (m, 2H), 1.64-1.70 (m, 2H), 3.86 (s, 3H), 4.12 (t, J = 7.5
Hz, 2H), 7.83 (d, J = 1.5 Hz, 1H), 7.90 (d, J = 1.5 Hz, 1H); 13C NMR (125
MHz, CDCl3): δ = 13.3, 19.3, 31.0, 37.2, 49.8, 123.1, 124.5, 131.0.
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Table of Contents
FULL PAPER
Barbier
type
allylation
of
Catalytic In (10 mol%) , High yield,
In(0) [catalytic]
aldehydes/ketones can be carried
out with both unsubstituted and γ-
substituted allyl bromides using
Reusable
[bmim][Br]
Papiya Dey, Mrunesh Koli, Dibakar
Goswami*, Anubha Sharma and Subrata
Chattopadhyay*
In
(activated)
only
a
catalytic amount (0.1
Br
Br
equiv.) of In-metal in [bmim][Br].
Product
RCHO
Page No. – Page No.
Br
In
[bmim][Br] as an Inexpensive and
Efficient Medium for the Barbier-type
Allylations using a Catalytic Amount
of In: Mechanistic Studies
InBr
I
InBr2
II
[b2Brmim][Br]
Reduction
C4H9
RCHO
N
InBr3
:
N
Product
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