Bismuth mediated barbier synthesis of α-homoallylic alcohols via a sigmatropic rearrangement in [bmim][HSO4]

Article abstract of DOI:10.1080/00397911.2020.1854785

A novel protocol for the Bismuth metal mediated regioselctive crotylation of aldehydes yielding α-homoallylic alcohols has been developed using 20mol% [bmim][HSO₄] as a metal activator, as well as a mediator for regioconversion from the initially formed γ-regiomer. The present protocol minimizes the use of conventional solvents and proceeds with good yields and excellent regioselectivities.

Full text of DOI:10.1080/00397911.2020.1854785

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Bismuth mediated barbier synthesis of
α-homoallylic alcohols via a sigmatropic
rearrangement in [bmim][HSO4]
Sucheta Chatterjee , Papiya Dey , Seema V. Kanojia , Subrata Chattopadhyay
& Dibakar Goswami
To cite this article: Sucheta Chatterjee , Papiya Dey , Seema V. Kanojia , Subrata Chattopadhyay
& Dibakar Goswami (2020): Bismuth mediated barbier synthesis of α-homoallylic alcohols
via a sigmatropic rearrangement in [bmim][HSO4], Synthetic Communications, DOI:
10.1080/00397911.2020.1854785
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1854785
Bismuth mediated barbier synthesis of a-homoallylic
alcohols via a sigmatropic rearrangement in [bmim][HSO4]
a
a
a
a
ꢀ
ꢀ
Sucheta Chatterjee , Papiya Dey , Seema V. Kanojia , Subrata Chattopadhyay ,
and Dibakar Goswamia,b
b
aBio-Organic Division, Bhabha Atomic Research Centre, Mumbai, India; Homi Bhabha National Institute,
Mumbai, India
ABSTRACT
ARTICLE HISTORY
Received 3 July 2020
A novel protocol for the Bismuth metal mediated regioselctive croty-
lation of aldehydes yielding a-homoallylic alcohols has been devel-
oped using 20 mol% [bmim][HSO4] as a metal activator, as well as a
mediator for regioconversion from the initially formed c-regiomer.
The present protocol minimizes the use of conventional solvents and
proceeds with good yields and excellent regioselectivities.
KEYWORDS
Bismuth; [bmim][HSO4];
crotylation; ionic liquid;
regioselectivity
GRAPHICAL ABSTRACT
Introduction
Crotylation of carbonyls is one of the most fundamental C–C bond formation reactions, to
furnish homoallylic alcohols with two stereogenic centers in a single step (Scheme 1). Since
its inception, a plethora of synthetic methodologies for crotylation of carbonyls have
emerged to address issues such as regio-, enantio- and diastereoselectivities.[1] These meth-
odologies include either addition of preformed metal-crotyl complexes to carbonyls, or the
operationally simple in-situ Barbier type reactions.[1] Amongst these, Barbier type crotylation
Scheme 1. Possible products from crotylation of an aldehyde.
CONTACT Dibakar Goswami
dibakarg@barc.gov.in
Bio-Organic Division, Bhabha Atomic Research Centre,
Mumbai 400085, India.
Supplemental data for this article can be accessed on the publisher’s website.
Equal contributors.
ß 2020 Taylor & Francis Group, LLC
ꢀ

2
S. CHATTERJEE ET AL.
procedures are operationally simple, and the most convenient way to prepare homoallylic
alcohols. However, most of the reported Barbier protocols produce only c-homoallylic alco-
hols,[1,2] and the reports describing regioselective formation of a-homoallylic alcohols are
limited,[3,4] which restricts their potential use in natural product synthesis.
The a-regioselective Barbier-type crotylation of aldehydes was first achieved using
SnBr2 in a mixture of dicholoromethane (DCM)-H2O solvents.[3a] Loh et al. extensively
studied the regioselectivity of the reaction using several metals in different organic/aque-
ous media and found the reactions to be sluggish.[3b,c] The reactions, carried out in
water at higher temperatures proceeded with low yields.[3b] The a-regioselective crotyla-
tion of benzaldehyde was also achieved in water, using Sn/NaBF4, but the generality of
the protocol was unexplored.[3d] In addition, DMPU has been shown to promote the
formation of a-homoallyl alcohols at high temperatures in Zn-mediated crotylation.[3e]
More recently, exclusive a-adducts were obtained using costly praseodymium metal in
THF.[3f] Generally, it is understood that the formation of a-crotylated products using
Barbier protocol needed either higher temperature or presence of a protic solvent.
In addition to the above direct protocols, there are reports where both carboxylic
acids and Lewis acids have been used as a promoter for allyl transfer reaction of
c-homoallyl alcohol to the a-regiomer through a 2-oxonia [3,3]-sigmatropic rearrange-
ment.[4] However, except for a few, most of these methods use preformed allylmetal
complexes, or employ a two-step process. It is generally accepted that allylation of alde-
hydes yielding exclusive a-adducts are rather difficult to perform, especially adopting a
Barbier-type one-step protocol. Also, in many instances, the products were found to be
contaminated with organometallic residues arising from the preformed allylmetal com-
plex and/or Lewis acids, which were difficult to separate.[4b] Therefore, a general and
versatile protocol using a catalytic amount of nonmetallic promoter for one-step
Barbier-type a-regioselective allylation is highly solicited.
The room temperature ionic liquids (RTILs) with Lewis or Bronsted acidic properties
are easily available, and find applications in organic reactions, as well as in CO2
fixation, as ionogels, in metal processing, etc.[5,6] We hypothesized that the acidic RTILs
can be effectively used for developing a protocol for the targeted a-regioselective
Barbier-type carbonyl crotylation. To prove our hypothesis, we carried out the Barbier-
type crotylation of carbonyls in a Lewis acidic IL ([bmim][AlCl4]), a Bronsted acidic IL
([bmim][HSO4]), and a hydrophilic acidic IL ([bmim][Br]). We chose [bmim][Br] as a
reaction medium since it can activate several metals, enabling the Barbier-type carbonyl
allylation to be carried out at room temperature with stoichiometric or even
sub-stoichiometric amounts of metals/reagents.[7a–d] Moreover, it is acidic because of its
imidazolium C-2 hydrogen associated with bromide as the counter anion. Herein, we
report the results of our studies that led to an unprecedented, efficient protocol for
a-regioselective Bismuth mediated allylation of aldehydes at room temperature, using
near stoichiometric amounts of the reactants and only 20 mol% [bmim][HSO4].
Results and discussion
Initially, we carried out crotylation of benzaldehyde (1a) as a model substrate in differ-
ent solvents including H2O, 10% aqueous HCl, DCM-H2O, and the above RTILs using

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3
Table 1. Reaction course of the metal mediated crotylation of 1a.a
Metal
(mmol)
Crotyl bromide
(mmol)
Regioselectivityb Yieldc
Entry
1d
2
Metal
In
In
Solvent
Activator Time (h)
(a:c)
(%)
H2O
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
6.0
6.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
36
18
48
48
48
48
48
48
72
24
24
24
24
48
48
48
48
48
48
48
48
48
32
48
24
24
24
85:15
82:18
58:42
65:35
72:28
62:38
55:45
82:18
91:9
4:96
2:98
2:98
1:99
10:90
5:95
3:97
2:98
30:70
34:66
NR
62:38
75:25
89:11
NR
52
48
57
62
68
41
49
25
30
86
85
32
89
85
84
36
85
64
52
–
Aq. HCl (10%)
3
4
5
6
7
8
9
In
DCM-H2O
Aq. HCl (10%)
DCM-H2O
Aq. HCl (10%)
DCM-H2O
Aq. HCl (10%)
DCM-H2O
[bmim][Br]
[bmim][Br]
[bmim][Br]
Sn
Sn
Bi
Bi
Zn
Zn
In
Sn
Zn
Bi
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26e
27f
[bmim][Br]
In
[bmim][Br]-10% HCl
[bmim][Br]-10% HCl
[bmim][Br]-10% HCl
[bmim][Br]-10% HCl
[bmim][AlCl4]
[bmim][AlCl4]
[bmim][AlCl4]
[bmim][AlCl4]
[bmim][HSO4]
[bmim][HSO4]
[bmim][HSO4]
[bmim][HSO4]
[bmim][HSO4]
[bmim][HSO4]
Sn
Zn
Bi
In
Sn
Zn
Bi
69
78
74
–
87
86
87
In
Sn
Zn
Bi
Bi
Bi
99:1
99:1
99:1
KF
NH4HF2
aAll reactions were carried out at 2 mmol scale in H2O/organic/mixed solvents (5 mL) or RTIL (4 mL) at 25 ꢁC, unless
c
otherwise mentioned; bBased on the isolated yields of the individual regioisomers; Combined yields of all the regiom-
d
ers; The reactions were carried out at 40 ꢁC for 12 h using 0.2 mL water;[22] e,fThe reaction was carried out in the
presence of aqueous 10% KF or 10% NH4HF2 as the additives respectively; NR: no reaction.
a host of metals (In, Zn, Sn and Bi) and commercially available crotyl bromide. The
choices of solvents other than the RTILs and the metals were based on previous reports
on a-selective Barbier-type carbonyl allylation.[3,8] A mixture of the a:c adducts (2a and
3a) in different ratios was obtained in all the cases (Table 1). Consistent with the previ-
ous results,[3c] the In-mediated reaction in H2O gave 2a in a poor yield, even at a
higher temperature (entry 1). The reaction yield did not improve appreciably in aqueous
10% HCl or DCM-H2O mixture (entries 2–3), and the regioselectivity in DCM-H2O
was poor. With the other metals (Zn, Sn and Bi) also, the reactions in these media were
sluggish and gave 2a in poor to modest (25–68%) yields (entries 4–9). Even in presence
of metal activators viz. KF and NH4HF2, no significant improvement in Bismuth metal
mediated crotylation reaction was observed (data not shown). The regioselectivity was
good only with Zn in DCM-H2O (entry 9), but with poor yield. Notably, all these reac-
tions required large excesses of the metal and crotyl bromide for completion, which
otherwise remained incomplete with lesser amounts of the reagents (data not shown).

4
S. CHATTERJEE ET AL.
Table 2. Effect of [bmim][HSO4] amounts on the Bismuth mediated crotylation of 1a.a
Entry
[bmim][HSO4] amount
Time (h)
Regioselectivity (a:c)b
Yield (%)c
1
2
3
4
5
6
4 mL
2 mL
1 mL
50 mol%
20 mol%
10 mol%
24
28
28
36
48
120
99:1
98:2
98:2
99:1
99:1
94:6
87
85
85
87
84
71
aAll reactions were performed with 1a (2 mmol), crotyl bromide (3.0 mmol), and Bi (2.0 mmol) at 25 ꢁC; bBased on the
1H NMR analyses of the crude reaction mixtures after the stipulated time period. cCombined yields of all the
regioisomers.
By contrast, the reaction was complete in the chosen RTILs with only 1.5 equiv. of the
bromide and near-stoichiometric amounts of the metals. The reactions in [bmim][Br]
were faster (24 h), but preferentially produced the undesired c-regioisomer, even in the
presence of aqueous HCl (entries 10–17). The In/Bi/Sn-catalyzed crotylation proceeded
with better a-regioselectivity in the Lewis acidic RTIL than in [bmim][Br] but gave the
product in only 52–69% yields (entries 18–21). Instead, the Bronsted acidic RTIL,
[bmim][HSO4], was found to be an excellent medium for a-regioselective crotylation of
1a (entries 22–27), particularly with Bismuth metal (entries 25–27). Presence of an add-
itional Bismuth metal activator (KF or NH4HF2) had no effect on the reaction yield and
time (entries 26 and 27), while Zn metal was ineffective in both the acidic RTILs
(entries 20 and 24). The observed metal-dependent regioselectivity alteration in the car-
bonyl crotylation is synthetically interesting, but not unprecedented.
Next, the above reaction was carried out at 2 mmol scale using different amounts of
[bmim][HSO4] (Table 2, entries 1–6). Progressive reduction of the solvent volume did
not have any appreciable effect on the reaction yield and regioselectivity, except for
slowing down the reaction. The reaction was complete even with 10 mol% of
[bmim][HSO4] but only at 120 h. The reaction occurs with unaffected yield and selectiv-
ity even with drastic reduction in the amount of RTIL making the protocol practically
solvent-free, which is remarkable. Considering both yield and reaction time, a combin-
ation of Bi (1.0 equiv.), crotyl bromide (1.5 equiv.), and 20 mol% [bmim][HSO4] was
found ideal to carry out crotylation of 1a. Hence, this combination was used for all the
subsequent studies carried out using the same conditions. It is worth mentioning that
purity of [bmim][HSO4] is very crucial in achieving the high a-selectivity in the crotyla-
tion, as reported earlier.[9] The impurities present in the ILs and/or their precursors
may be responsible for the altered regioselectivity. After encountering the problem, we
always used the ILs, prepared according to literature procedures (vide SI).[7b,10]
The catalytic protocol of regioselective crotylation was subsequently extended to other
aldehydes 1b–m (Table 3, entries 2–13). The protocol was equally effective with both
aromatic and aliphatic aldehydes furnishing the products 2b–m in good yields
(72–86%) and excellent a-regioselectivity. The substituents in the aromatic ring did not
have any appreciable bearing on the reaction course. However, reaction with 2-hydroxy-
benzaldehyde was unsuccessful (data not shown). The reaction of cinnamyl bromide
with 1a in [bmim][HSO4] yielded the a-regioisomer 2n almost exclusively (entry 14,
Table 3). The reaction mixture was quenched with water-EtOAc, extracted with EtOAc,
concentrated in vacuum and subjected to column chromatography (vide Experimental
section) to obtain the respective products in 72–86% yields. The reactions were clean

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5
Table 3. Bismuth mediated crotylation/cinnamylation of the aldehydes 1a–j in [bmim][HSO4].a
Entry
Substrate
R1
R2
Product
Yield (%)b
Regioselectivity [a-(E:Z):c]c
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1a
C6H5
2-Cl-C6H4
2-F-C6H4
3-F-C6H4
3-OMe-C6H4
4-Et-C6H4
4-Br-C6H4
PhCH2
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
84
75
82
81
78
86
72
80
85
84
82
78
73
81
99(57:43):1
98(47:53):2
96(61:39):4
99(63:37):1
94(31:69):6
99(22:78):1
95(31:69):5
90(70:30):10
99(32:68):1
99(33:67):1
99(43:57):1
97(81:19):3
99:1
9
n-C5H11
n-C9H19
n-C8H17
10
11
12
13
14
2-Furfural
Crotonaldehyde
C6H5
91(89:11):9
aAll reactions were performed with aldehyde (2 mmol), crotyl bromide (3.0 mmol), Bi (2.0 mmol) and [bmim][HSO4]
(20 mol%) at 25 ꢁC; bCombined yields of all the regioisomers; cBased on the isolated yields of the individual
regioisomers.[11]
Table 4. Bismuth mediated reaction of aldehydes with trans-2-bromo-1-octene in [bmim][HSO4].a
Entry
Substrate
R1
R2
Product
Yield (%)b
Regioselectivity [a:c]c
1
2
1o
1a
n-C6H13
C-6H5
C5H11
C5H11
2o
2p
81
82
98:2
99:1
aAll reactions were performed with aldehyde (2 mmol), bromide (3.0 mmol), Bi (2.0 mmol) and [bmim][HSO4] (20 mol%)
at 25 ꢁC; bCombined yields of all the regioisomers; Based on the isolated yields of the individual regioisomers.
c
without any side-products, but ꢂ2–4% of the starting aldehydes was left unreacted in
all the cases. The a/c-ratios of the products were estimated after isolation by column
chromatography.[11] However, because of their negligible yields, we did not determine
the syn:anti ratio of the c-adducts, 3a–j. We also scaled up the optimized protocol by
carrying out crotylation of 1a at 10 mmol scale and obtained 2a (84% yield) in 48 h.
To further extend the applicability of our protocol, we reacted both aliphatic and aro-
matic aldehydes 1o and 1a with trans-1-bromo-2-octene, an allylic halide with an alkyl
chain substituted in the c-position (Table 4). In both cases, the a-homoallylic alcohols
2o and 2p were obtained as the major products, indicating that the substitution in the
starting bromide may not have any significant effect on the outcome of the reaction.

6
S. CHATTERJEE ET AL.
Scheme 2. Possible pathway for the regioconversion of c-adduct to a-adduct.
Mechanistic considerations
Based on our previous studies on Bismuth-mediated carbonyl allylation in
[bmim][Br],[7b] we presumed that a dipole-induced dipole interaction between the imi-
dazolium core of [bmim][HSO4] and Bi may be responsible for the metal activation
1
(vide Scheme 2). To get an insight into such an interaction, H NMR spectrum of a
stirred (10 min) mixture of Bi (1.0 mmol) and [bmim][HSO4] (20 mol%) was recorded
1
and compared with that of pure [bmim][HSO4]. As reported earlier,[7b] the H NMR
spectrum, recorded in D2O-DMSOd6, showed upfield shifts of the [bmim][HSO4] pro-
tons. As expected, the protons attached to the imidazolium ring showed maximum
shifts [H-2 (20 Hz), H-4 and H-5 (18 Hz each)]. As expected in case of a charge transfer
phenomena between imidazolium core and Bismuth metal, the shifts gradually
decreased with increasing distance from the imidazolium core. Earlier also, we reported
that the imidazolium-based ionic liquids promoted electron transfer reactions
remarkably.[7b]
For identification of the active crotylbismuth intermediate, a mixture of Bismuth metal
(1 mmol), crotyl bromide (1.5 mmol) and [bmim][HSO4] (20mol%) was stirred for 1.5 h at
1
25ꢁC, and the H NMR spectra of the reaction mixture aliquots were recorded in CD2Cl2
(vide SI) both at room temperature and after cooling the sample to ꢃ70ꢁC. Due to the flux-
1
ional behavior of the allylbismuth species,[7b,12] the H NMR spectrum, recorded at room
temperature did not providꢁe any information regarding the crotyl-Bi species. However, the
ꢀ
spectrum recorded at ꢃ70 C showed a doublet at d 2.66ppm (vis-a-vis at d 2.87ppm in
[bmim][Br]),[7b] indicating the formation of an g1-Bismuth-crotyl coordination complex.
Hence, as in the case of [bmim][Br],[7b] we presumed crotylbismuth dibromide (vide
Scheme 2) as the active species in the present case also.
Further, to understand the origin of the a-adduct, we monitored the progress of the
reaction between 1f (2 mmol) and crotyl bromide (3.0 mmol) in [bmim][HSO4]
1
(20 mol%) by H NMR spectroscopy. Aliquots from the reaction mixture was taken at
different time intervals (as denoted in Figure 1) and was extracted with EtOAc as
1
described before. The organic extract was concentrated under vacuum and H NMR
was recorded in CDCl3. Initially, formation of the c-homoallylic alcohol (terminal olefin

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7
1
Figure 1. Partial H NMR spectra of the aliquots (taken at different time intervals) from the reaction
performed with 1f (2 mmol), crotyl bromide (3.0 mmol), bismuth (2.0 mmol) and [bmim][HSO4]
(20 mol%) at 25 ꢁC. The peaks have been assigned to the corresponding product.
peaks at d 5.8 and d 5.1 ppm) was noticed, which eventually got converted to the
desired a-product, as indicated by the appearance of the internal olefin peaks at d 5.4
and d 5.6 ppm in place of the terminal olefin peaks (Figure 1). To further explore the
regioinversion, a mixture of the known[7c] c-crotylated adduct of 1f was stirred with
20 mol% [bmim][HSO4] for 48 h also gave the a-adduct 2f (yield 92%), but only in the
presence of 1f. This also explains the recovery of some amount of unreacted aldehydes
in all our studies. To investigate whether the regioconversion is an intra- or intermo-
lecular process, a crossover experiment was carried out by stirring a mixture of the
c-adduct of 1f and 1b in [bmim][HSO4] (Scheme 3) for 24 h. After 24 h, the starting
aldehydes 1f/1b, along with the a-adducts, 2f and 2b were isolated from the reaction
mixture. This crossover experiment clearly emphasized the obligatory presence of an
aldehyde in the transformation and revealed that the rearrangement of the c-adduct to
the a-adduct is an intermolecular process.
Based on these observations, we propose a mechanism (Scheme 2) for the intermolecular
regio-conversion of the initially formed c-adduct to its regioisomer in presence of the
Bronsted acid. This proposed mechanism is similar to what was proposed by Loh et.al.[3c]
The initially activated Bismuth metal (via a dipole-induced-dipole interaction with the imi-
dazolium core of the RTIL) produces the active organometallic species I in-situ. This, in
turn, initially reacts with the aldehyde to form the c-adduct, which reacted with the
unreacted and/or in situ regenerated aldehyde under the acidic conditions to form the hemi-
acetal III and further loses a water molecule to form the oxonium intermediate IV
(Scheme 2). This undergoes successive retro-ene and sigmatropic rearrangements to furnish
2f (Scheme 1). Acidity of the medium as well as the aldehyde plays an important role in
such transformation. The aldehyde in Scheme 2 may be the residual aldehyde during the

8
S. CHATTERJEE ET AL.
Scheme 3. Crossover experiment.
Scheme 4. Crotylation of 1k using 20 mol% [bmim][HSO4].
course of the reaction, or the aldehyde regenerated in the reaction mixture. The requirement
of slight excess crotyl bromide in the reaction may be attributed to the volatility of the same.
It has been reported[4c] that, in an allyl transfer reaction, E/Z stereoselectivity of the
a-adduct depends on the syn/anti stereochemistry of the precursor c-adduct. However,
in our case, the E/Z selectivity of the products could not be explained solely on the
basis of the stereochemistry of the c-adducts. The possible effects of the metal and solv-
ent on the E/Z-selectivity are unclear. Similar variations in the E/Z-selectivity with vari-
ation in the metal were also observed by Loh et al.[3c] Hence, any detailed
rationalization at this stage, will be speculative.
To further prove the proposed reaction mechanism, and to confirm the role of
secondary –OH group in the mechanism, the crotylation of R-(2,3)-
Cyclohexylideneglyceraldehyde 1q was attempted (Scheme 4). The c-adduct of R-(2,3)-
Isopropylidenglyceraldehyde is known to undergo trans-acetalization in an acidic
medium to form 2,3-disubstituted acetonide.[13] We envisaged that crotylation of R-
(2,3)-Cyclohexylideneglyceraldehyde 1q in [bmim][HSO4] would follow a similar trend.
True to our expectation, crotylation of 1q yielded only the 2,3-disubstituted product 2q.
Further isomerization to the a-adduct required its hemiacetal to undergo the sigma-
tropic rearrangement via a preferable 6-membered cyclic transition state, which was
stereochemically impossible, thereby blocking further isomerization to the a-adduct.
This also supported our proposed mechanism of conversion of c-adduct to a-adduct
through the intermediacy of species II–V (Scheme 2).

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SYNTHETIC COMMUNICATIONSV
9
Experimental section
Typical procedure for solvent-free bismuth mediated crotylation reaction
in [bmim][HSO4]
To a mixture of the aldehyde (2.00 mmol), crotyl bromide (425 mg, 3.0 mmol) and
Bismuth metal powder (418 mg, 2.00 mmol), 20 mol% [bmim][HSO4] was added and
stirred magnetically for 48 h. After that, the reaction mixture was quenched with water
(5 mL) and EtOAc (5 mL), filtered through a silica pad, filtrate extracted with EtOAc
(3 ꢄ 10 mL), the organic layer concentrated in vacuum and subjected to column chro-
matography to obtain the respective pure products (yields and regioisomeric ratio are
mentioned in Table 3).
Analytical data for selected compounds
1-Phenylpent-3-en-1-ol
(2a)[3f]
(yield
83%,
E:Z ¼ 57:43):
FTIR
(film)
3,37,52,91,91,49,21,030 cmꢃ1; H NMR (500 MHz, CDCl3): d 1.61 (Z-) and 1.70 (E-)
(two d, J ¼ 6.5 Hz, 3H), 2.10 (broad s, 1H), 2.37–2.50 and 2.56–2.62 (two m, 2H), 4.68
(Z-) and 4.72 (E-) (two dd, J ¼ 5.0 & 8.0 Hz and 5.5 & 8.0 Hz, 1H), 5.41–5.47 (m, 1H),
5.57–5.67 (m, 1H), 7.27–7.30 (m, 1H), 7.35–7.39 (m, 4H); 13C NMR (125 MHz, CDCl3):
d 12.9, 17.9, 36.9, 42.7, 73.5, 73.9, 125.7, 125.8, 126.8, 127.4, 127.5, 128.3, 129.3,
144.1, 144.2.
1
1-(2-Chlorophenyl)pent-3-en-1-ol (2b)[3g] (yield 74%, E:Z ¼ 47:53): FTIR (film)
3,37,82,91,81,47,01,040 cmꢃ1; H NMR (500 MHz, CDCl3) d 1.62 (E-) and 1.71 (Z-)
1
(two d, J ¼ 7.0 and 6.0 Hz, 3H), 2.24–2.31 (m, 1H), 2.42–2.48 and 2.54–2.57 (two m,
2H), 5.09 (E-) and 5.15 (Z-) (two dd, J ¼ 3.5 & 8.5 Hz and 4.0 & 8.2 Hz, 1H), 5.45–5.53
(m, 1H), 5.59–5.72 (m, 1H), 7.18–7.22 (m, 1H), 7.28–7.34 (m, 2H), 7.54–7.59 (m, 1H);
13C NMR (125 MHz, CDCl3) d 12.9. 17.9, 35.1, 40.9, 69.9, 70.3, 125.4, 126.6, 126.9,
127.1, 127.9, 128.2, 128.3, 129.3, 129.5, 131.7, 131.8, 141.4, 141.5.
1-(4-Ethylphenyl)pent-3-en-1-ol (2f) (yield 85%, E:Z ¼ 22:78): FTIR (film)
3,36,52,96,31,44,61,040 cmꢃ1
;
1H NMR (500 MHz, CDCl3) d 1.25 (t, J ¼ 7.5 Hz, 3H),
1.62 (E-) and 1.70 (Z-) (two d, J ¼ 7.0 and 6.0 Hz, 3H), 2.09 (broad s, 1H), 2.37–2.49
(m, 2H), 2.57–2.59 and 2.64–2.68 (two m, 2H), 4.65 (Z-) and 4.69 (E-) (two dd, J ¼ 5.0
& 8.0 Hz and 5.0 & 7.5 Hz, 1H), 5.41–5.48 (m, 1H), 5.58–5.65 (m, 1H), 7.18–7.21 (m,
2H), 7.27–7.31 (m, 2H); 13C NMR (125 MHz, CDCl3) d 12.9, 15.5, 17.9, 28.5, 36.8, 42.6,
73.4, 73.8, 125.8, 125.9, 127.0, 127.3, 127.8, 129.0, 141.4, 143.4. Anal. Calcd. for
C13H18O: C, 82.06; H, 9.53%. Found: C, 81.74; H, 9.78%.
Conclusion
Overall, a novel, Barbier-type a-regioselective crotylation procedure was established
using a Bronsted acidic RTIL (bmim)[HSO4] in combination with Bismuth metal. The
reaction is highly regioselective and furnishes the products in high yields with both ali-
phatic and aromatic aldehydes. In general, the formation of a-crotylated products by
the Barbier route needed higher temperatures and/or presence of protic solvents. The
present protocol substantially deviates from them and offers distinct procedural

10
S. CHATTERJEE ET AL.
advantages. We expect the present results to open up many applications toward synthe-
sizing a-homoallylic alcohols that are versatile building-blocks of many natural prod-
ucts. The mechanism of the proposed reaction has been well supported by several
1
chemical reactions as well as H NMR spectroscopy.
Complete experimental section including materials, apparatus, details of the synthesis
1
procedures and H and 13C spectral data are described in the Supporting Information.
This material can be found via the “Supplementary Content” section of this
article’s webpage.
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