Allylation and propargylation of aldehydes mediated byin situgenerated zinc from the redox couple of Al and ZnCl2in 2N HCl

Article abstract of DOI:10.1039/d1nj00978h

A simple one pot allylation and propargylation of aldehydes mediated by zinc(0), which isin situgenerated from the redox couple of Al and ZnCl₂in 2N HCl, is demonstrated to afford the corresponding homoallyl and homopropargyl alcohols with exce

Full text of DOI:10.1039/d1nj00978h

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Allylation and propargylation of aldehydes
mediated by in situ generated zinc from the redox
couple of Al and ZnCl2 in 2N HCl†
Cite this: New J. Chem., 2021,
45, 7163
a
Bibhas Mondal,ab Utpal Adhikari,b Partha Pratim Hajraa and Ujjal Kanti Roy
*
Received 26th February 2021,
Accepted 13th March 2021
A simple one pot allylation and propargylation of aldehydes mediated by zinc(0), which is in situ generated
from the redox couple of Al and ZnCl2 in 2N HCl, is demonstrated to afford the corresponding homoallyl
and homopropargyl alcohols with excellent yields.
DOI: 10.1039/d1nj00978h
rsc.li/njc
efficient and provide convenient procedures with improved yields
remains quite a challenge for synthetic organic chemists.
Introduction
The construction of a C–C bond is the essence of organic
Metal-mediated allylations and propargylations are well-
synthesis in building up the backbone of organic compounds. recognized examples of organic reactions in aqueous media.4
Allylation and Propargylation of carbonyl compounds with allylic However, excessive metal is used in most of the cases and the
and propargylic organometallic reagents are well known processes corresponding metal salt is generated as a waste material. Lots of
with a broad spectrum of synthetic applications.1 For the allyla- research has been done on developing metal mediated allylation
tion and propargylation of aldehydes and ketones, various metals and propargylation reactions in aqueous media. Zinc is very cheap
e.g. Mg, Li, Zn, B, Al, Si, Mn, Ti, Cr, Zr, Cu and Sn are used as and has low toxicity among the successful metals; it is also less
metallic reagents.2 The synthesis of homoallylic and homopro- sensitive to air and water due to its highly negative reduction
pargylic alcohols by allylation and propargylation of carbonyls has potential (ꢀ1.216) receiving special priority both from the eco-
attracted significant interest in synthetic organic chemistry nomic and scientific points of view to mediate the Barbier-type
because of their use as building blocks in the synthesis of natural reactions.5 There are very few reports where zero or low valent
products. The addition of allyl, propargyl or allenyl nucleophiles reactive metals are generated from a redox couple followed by the
to organic electrophiles like carbonyls, imines, and epoxides is a in situ generation of reactive nucleophilic organometallic species.
well-recognized tool for the formation of a carbon–carbon bond in Examples include redox reagent systems such as Indium(III)–
the chemical society.3 Carbonyls are one of the most useful and aluminium, tin(II)–aluminum etc.6 It is hoped that the regenera-
versatile substrates in synthetic organic chemistry due to their tion of Zn(0) from Zn(II) by aluminum or manganese (combined
high reactivity. Several techniques have been reported for the with TMS-Cl) would be a useful method because they have lower
activation of carbonyls with a variety of organometallic nucleo- electronegativity than Zn and can easily reduce in principle a
philes. Moreover, a number of methods have been reported for divalent zinc to a zero valent one. Aqueous reactions have been
the metal mediated allylation and propargylation of carbonyls. extensively considered to avoid flammable, toxic, or carcinogenic
Except a few, all these methods suffer from a lack of efficiency and organic solvents.7 Chemists are trying to synthesize various low
simplicity. As a result, further scope to test the reactivity of novel valent metals preferably having a particular shape and size in nano
reagent systems to bring about efficient and selective allylation scale because of their applications as nano catalysts. On the other
and propargylation of carbonyls, epoxides and imines still exists. hand, organozincs also have a huge number of applications in
The development of simple and novel reagents that are more organometallics. Particularly, allylic and propargylic organozincs
are extremely important in organic synthesis. So far, to the best of
our knowledge, the synthesis of zero valent metallic zinc from a
bivalent one followed by the in situ generation of allylic and
a Department of Chemistry, Kazi Nazrul University, Asansol-713340, India.
E-mail: uroccu@gmail.com; Tel: +91-9732657939
b Department of Chemistry, National Institute of Technology Durgapur,
propargylic organozincs is completely unknown (Charts 1
and 2).8-26 In this work our idea is to generate a zero valent
Durgapur-713209, India
zinc metal from a bivalent, easily available, cheap, moderately
stable (to air and moisture) and commercial zinc source. We tried
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00978h
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Chart 1 Previous works of allylation reactions using zinc metal.
Chart 2 Previous works of propargylation reactions using zinc metal.
a list of reducing agents based on the literature to reduce in situ
the bivalent, easily available, cheap, moderately stable (to air
and moisture) and commercial zinc source in different solvent
systems including aqueous medium (neutral/acidic/alkaline).
Here, we want to explore a new strategic theme for the nucleo-
philic allylation and propargylation reactions via the insertion of
this redox generated zinc(0) in situ with allylic and propargylic
halides. The strategy involves the oxidative addition of allylic or
propargylic halide (RX) across a reactive zero valent metal [M], to
form a nucleophilic reagent R–[M]–X. The in situ generated
nucleophile reacts with suitable electrophile like carbonyls to give
homoallylic and homopropargylic alcohols. This process is highly
efficient, simple, and convenient to provide homoallylic and
homopropargylic alcohols with improved yields. Initial success of
the strategy for carbonyl allylation and propargylation prompted
us to investigate the process and its mechanism in depth.
reduction route i.e. the bottom up approach for the in situ
generation of this highly active metallic zinc. To generate an
active zero valent zinc via chemical redox conditions, the
combination of Zinc(II) chloride with a suitable reducing agent
was examined. For this purpose, we tried with aluminium
powder, manganese, Mn/TMS-Cl, NaBH4, hydrazine hydrate
etc. as the reducing partner. In a one pot reaction, aluminium
powder (1.5 mmol) was added portion wise to a mixture of
ZnCl2 (1.5 mmol) in 2 ml of 2 N HCl and the mixture was stirred
at room temperature till hydrogen gas liberation ceased.
Among the tried methods, this method is most suitable
because the in situ generated AlCl3 is soluble in hydrochloric
acid and reduced metallic zinc is insoluble in the solution.
Thus, the metallic zinc can be easily separated for characteriza-
tion purpose. Metallic lustre of zinc suspension was observed.
Results and discussion
Synthesis and characterization of nano zinc by the redox couple XRD analysis
of Al and ZnCl2
The XRD (Fig. 1) analysis showed that the precipitate from
We were interested in generating highly active metallic zinc for ZnCl2 solutions consisted of pure Zn(0) with a good crystalline
this purpose and using it in situ. We followed the chemical structure. The crystal planes of metallic zinc which were
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active in situ generated metallic zinc for this purpose. We used
commercial ZnCl2 salt as the zinc source. We followed the
chemical reduction process i.e. the bottom up approach for the
in situ generation of highly active metallic zinc. To generate an
active zero-valent zinc via chemical redox conditions, the combi-
nation of Zinc(II) chloride with a suitable reducing agent was
examined (Table 1).
For this purpose, we examined aluminium powder, manga-
nese, Mn/TMSCl, NaBH4, hydrazine hydrate etc. as the reducing
partner. We found that efficient allylation can take place only by
reducing ZnCl2 with aluminium powder, and in the first attempt
we obtained a 10% yield of homoallyl benzyl alcohol when ZnCl2
(1 mmol) was reduced by Al powder (1 mmol) in water (3 ml) at
room temperature (entry 1). The yield of homoallyl alcohol
increases by increasing the solubility of the organic reactants
by adding a miscible organic solvent. A better yield (20%) of
homoallyl benzyl alcohol is obtained when a mixture of H2O–
THF (2: 1) is used as the solvent (entry 2). The effect of THF
addition is more prominent compared to DMF and MeOH (entry
2 vs. entries 3 and 4). Saturated NH4Cl in THF (2: 1) can act as a
better solvent for this type of in situ reduction process (entry 5).
But, in our study, this solvent system could not give any better-
ment with respect to the yield of homoallyl benzyl alcohol. We
then tried simultaneously a basic and an acidic media for this
purpose. HCl medium offers drastic improvement of the yield
(entries 8 and 9) while NaOH medium results in a lower yield
(entry 7). The effect of organic acid additives like HCO2H, HOAc
and benzoic acid is found to be negative (entries 10–12). Thus,
allyl bromide (1 mmol) and benzaldehyde (1 mmol) in the
presence of ZnCl2 in an acidic aqueous medium renders a new
pathway for homoallyl alcohol synthesis. Trial with a manganese
metal powder and Mn/TMS–Cl instead of aluminium decreases
the yield (entries 13–15). Reducing agents like NaBH4 and
hydrazine hydrate instead of aluminium powder yielded trace
amounts of homoallyl alcohol under similar reaction conditions
(entries 16 and 17). In a one pot reaction, aluminium powder
(1.5 mmol) was added portion wise to a mixture of ZnCl2
(1.5 mmol) in 2 ml of 2 N HCl and the mixture was stirred at
room temperature till hydrogen gas liberation ceased. Then THF
(1 ml), allyl bromide (1.5 mmol) and benzaldehyde (1 mmol)
were added and the reaction mixture was stirred at room
temperature to afford the corresponding homoallylic alcohol in
good yield (80%). The yield was found to further improve to 86%
when the temperature was increased to 70 1C (entries 21 vs. 20).
Optimized reaction conditions are those given in entry 21.
Fig. 1 XRD pattern of the material.
identified included (002), (100), (101), (102), (103), (110) and
(004) and perfectly matched with pure Zn metal. Our synthe-
sized Zn nano wire is phase pure. The average crystallite size of
the sample is calculated from the broadening of well defined
peaks of the sample in the XRD pattern by applying the Debye–
Scherrer equation
hDi = 0.9l/(b1/2 cos y)
where hDi is the size of the average crystallite, ‘l’ (1.54 Å) is the
wavelength of the incident X-ray radiation and ‘y’ is the Bragg
angle. Here, ‘b1/2’ is the full-width at half-maximum (FWHM) of
the XRD peak (2y = 36.241, FWHM = 0.30). The average
estimated crystallite size of the sample is B200 nm which
indicates the nanocrystalline nature of the sample.
SEM analysis
The tuned synthesis of highly active metallic zinc for allylation
and propargylation reaction was further analyzed by FESEM
(field emission scanning electron microscopy) (Fig. 2). The
particles show a very uniform spherical morphology. The inset
picture shows a selected area scanning.
In situ generation of zinc(0) and its reactivity
To optimize this facile allylation process, we struggled a lot after
initial success and performed several experiments. A typical
optimization for the allylation of benzaldehyde is described
here. Active metallic zinc is a facile reagent for the Barbier-type
allylation reactions. We were really interested in finding highly
Generality of the reaction: allylation of aldehyde
A variety of aldehydes and allyl bromides can be used for the
allylation reaction using the optimised conditions (Table 1,
entry 21). Table 2 summarizes the details of the carbonyl
allylation results. Both the aromatic and aliphatic aldehydes can
be utilized in this strategy. For the aromatic aldehydes, homoallylic
alcohols can be prepared easily in good to excellent yields, and the
functionalities including chloro and methoxy groups (entries 1, 7, 9
and 11) are tolerated under these mild conditions. Aliphatic
aldehydes are also allylated (entries 3, 5, 8 and 10). Regioselectivity
Fig. 2 FESEM image of the material.
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Table 1 Standardisation of allylation of aldehyde
Sl. no.
Reagents
Solvent
Temp. (1C)
Yield (%)
1
2
3
Al (1 eq.), ZnCl2 (1 eq.)
Al (1 eq.), ZnCl2 (1 eq.)
Al (1.5 eq.), ZnCl2 (1.5 eq.),
Al (1.5 eq.), ZnCl2 (1.5 eq.),
Al(1.5 eq.), ZnCl2 (1.5 eq.), NH4Cl
Al (1.5 eq.), ZnCl2 (1.5 eq.), 1N HCl
Al (1.5 eq.), ZnCl2 (1.5 eq), 1N NaOH
Al (1 eq.), ZnCl2 (1 eq.), 2N HCl
Al (1.5 eq.), ZnCl2 (1.5 eq.), 3N HCl
Al(1.5 eq.), ZnCl2 (1.5 eq.), HCO2H (solid, 4 eq.)
Al (1.5 eq.), ZnCl2 (1.5 eq.), 2N HOAc
Al (1.5 eq.), ZnCl2 (1.5 eq.), PhCO2H (solid, 5 eq.)
Mn (1.5 eq.), ZnCl2 (1.5 eq.), 2N HCl
Mn (5 eq.), ZnCl2 (1.5 eq.), 2N HCl
Mn and TMSCl (5 eq.), ZnCl2 (1.5 eq.), 2N HCl
NaBH4 (1.5 eq.), ZnCl2 (1.5 eq.), 2N HCl
Hydrazine hydrate (1.5 eq.), ZnCl2 (1.5 eq.), 2N HCl
Al (2 eq.), ZnCl2 (1.5 eq.), 2N HCl,
Al (1.5 eq.), ZnCl2 (2 eq.), 2N HCl,
Al (1.5 eq.), ZnCl2 (1.5 eq.), 2N HCl,
Al (1.5 eq.), ZnCl2 (1.5 eq.), 2N HCl
H2O
H2O–THF (2 : 1)
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
70
10
20
15
12
20
45
10
75
65
12
10
Trace
10
10
15
Trace
Trace
80
H2O–DMF (2 : 1)
H2O–MeOH (4 : 1)
H2O–THF (2 : 1)
1N HCl–THF (2 : 1)
1N NaOH–THF (2 : 1)
2N HCl–THF (2 : 1)
1N HCl–THF (2 : 1)
H2O–THF (2 : 1)
4
5a
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
2N HOAc–THF (2 : 1)
H2O–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
2N HCl–THF (2 : 1)
80
80
86
Condition: aSat. NH4Cl in THF
study using 3-phenyl allyl bromide, crotyl bromide and 3,3- and negligible conversion of ketone was observed even after
dimethylallyl bromide showed that a g-substitution product 24 h.
was obtained exclusively (entries 1–4, and 10) with very poor
diastereoselectivity.
However, ketones did not undergo allylation under this
reaction condition, and negligible conversion of ketone was
Comparative reactivity study of different types of zinc
depending on surface morphology
The activity of this in situ generated Zinc towards the allylation
and propargylation of simple aldehydes might be attributed to
its special type of metallic Zn(0). A preliminary stoichiometric
comparative reactivity study of different types of zinc based on
observed even after 24 h.
Generality of the method: propargylation of aldehydes
The scope of the reaction was successfully extended to the surface morphology was performed using Rieke zinc,28 com-
propargylation of aldehydes. In the following condition model mercial zinc powders, granules, foils etc. under aqueous acidic
the reaction of 3-bromo-prop-1-yne 4E and benzyl aldehyde 2F conditions (Table 4). To 3 ml of 2 : 1 HCl (2N) : THF, metallic
yielded 72% of 1-phenyl-pent-4-yn-2-ol 5G (Table 3, entry 7). zinc (1.5 mmol, 98 mg), allyl/propargyl bromide (1.5 mmol,
Complete absence of isomeric allenyl alcohol suggests that 0.15 ml) and benzaldehyde (1 mmol, 106 mg) were added and
under the reaction conditions, metallotropic rearrangement stirred at 80 1C for 17 hours. The aqueous reaction mixture was
between propargylzinc and allenylzinc is completely arrested.27 extracted twice with diethyl ether (2 ꢁ 10 ml). The combined
The generality of the method was further tested for the reaction organic layer was washed with brine (10 ml), dried over Na2SO4,
of 2F, 2A, and 2G–2L varying with the corresponding propargyl filtered, and concentrated under vacuum. The crude product
bromide instead of 4E, 4A–4D affording moderate to good yields was purified by column chromatography on silica gel (petro-
of the corresponding homopropargyl alcohols 5A–5I (Table 3).
leum ether–ethyl acetate 9 : 1). Our synthesized metallic Zn(0)
Methoxy (OMe-) and halide (Br & Cl) groups are tolerated in showed the highest reactivity towards the allylation and pro-
the reaction conditions. Both the aromatic and aliphatic alde- pargylation of benzaldehyde in water. Rieke zinc was prepared
hydes can be utilized in this methodology. For the aromatic according to the procedure mentioned in the literature in dry
aldehydes, homopropargylic alcohols can be prepared easily in THF using 2 mmol ZnCl2 (supporting information). THF was
good to excellent yields. The functionalities including bromo, removed by evaporation from the activated Rieke zinc solution.
chloro, and methoxy groups etc. (entries 1, 8 and 9) on alde- Then 2N HCl–THF (2 : 1, 3 ml), allyl/propargyl bromide (2 mmol,
hydes are tolerated under these mild conditions. Aliphatic 0.2 ml) and benzaldehyde (1 mmol, 106 mg) were subsequently
aldehydes are also propargylated for the synthesis of homo- added to the residual activated zinc (1.5 mmol) and stirred at
propargylic alcohols (entries 3, 5 and 6). However, ketones did 80 1C for 17 h to give the corresponding homoallyl alcohol
not undergo propargylation under these reaction conditions, (yield = 68%). Our synthesized special zero valent metallic zinc
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Table 2 Allylation of aldehydesa
Table 3 Propargylation of aldehydesa
#
1
Bromide Aldehyde Product
Time (h) Yield%
#
1
Bromide Aldehyde Product
Time (h) Yield% Syn/antib
4A
4B
2A
2I
15
16
81
63
1A
2A
16
72
00 : 100
2
2
3
4
1B
1A
1B
2B
2C
2D
16
16
16
77
71
89
52 : 48
00 : 100
46 : 54
3
4
4C
4A
2J
16
16
55
48
2K
5
6
7
8
4D
4E
4E
4E
2H
2H
2F
15
18
16
14
78
68
72
72
5
6
1C
1C
2E
2F
14
14
83
86
—
—
2L
9
4E
2G
17
80
7
8
9
1C
1C
1D
2A
2G
2A
16
13
14
91
65
75
—
—
—
Condition: Al 1.5 mmol, ZnCl2 1.5 mmol, Aldehyde 1 mmol, propargyl
bromide 1.5 mmol, 2 N HCl 2 ml, 1 ml THF, and 70 1C.
allylation and propargylation of benzaldehyde in acidic water–
THF (2 : 1).
10 1E
11 1C
2G
2H
16
16
75
90
50 : 50
—
Plausible reaction pathway
The plausible reaction mechanism is demonstrated schemati-
cally in Scheme 1. Allyl bromide oxidatively adds to zero valent
zinc which is in situ generated by the redox reaction of Al and ZnCl2
in 2N HCl to form reactive allyl zinc(II)bromide. Allyl zinc(II)bromide
in situ reacts with aldehyde to form homoallyloxyzinc(II) bromide
which again hydrolyses under the reaction conditions to form
the desired homoallyl alcohol. Metallic zero valent zinc thus
Conditions: Al 1.5 mmol, ZnCl2 1.5 mmol, Aldehyde 1 mmol, allyl
bromide 1.5 mmol, 2 N HCl 2 ml, 1 ml THF, and 70 1C.
using a cheap zinc salt in combination with a suitable reducing generated is more reactive compared to commercial zinc. Plau-
redox partner (Al) showed the highest reactivity towards the sible mechanistic outcome demands a catalytic cycle for zinc.
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Table 4 Comparative reactivity study of different types of zinc depending on surface morphology
Sl. no.
Zinc reagents
Time (h)
Allylation yield (%)
Propargylation yield (%)
1
2
3
4
5
6
Rieke zinc synthesized
Zinc powder 325 mesh (SRL)
Zinc powder 200 mesh (Merck)
Zinc granules 30 mesh (Sigma Aldrich)
Zinc foil 0.38 mm (commercial)
17
17
17
17
17
17
68
53
48
27
18
86
60
43
37
15
trace
72
Zinc(0) by ZnCl2 and Al (this work, condition of Table 1, entry 21)
Condition: aldehyde 1 mmol, allyl/propargyl bromide 1.5 mmol, 2N HCl–THF (2 : 1) 3 ml
But in our study zinc is always needed in stoichiometric amount.
Significant differences in yield between catalyzed and stoichiometric
reaction conditions were observed. In the reaction conditions, the
final product is not alcohol but zinc alkoxide, and thus more than
stoichiometric amounts of zinc are necessary to mediate the reac-
tion. If the generated zinc(II) alkoxide species i.e. homoallyloxyzinc(II)
bromide can be reduced to zinc(0) with the formation of alcohol
under these conditions by aluminium powder then only catalytic
amounts of zinc chloride could do the work. But, this is not
happening under our reaction conditions. Aluminium powder
cannot reduce zinc(II) alkoxide species to metallic zinc under our
reaction conditions.
Similarly propargyl bromide oxidatively adds to zerovalent
zinc, in situ formed by the redox reaction of Al and ZnCl2 in 2N
HCl, to form reactive propargyl zinc(II) bromide which in situ
reacts with aldehyde to form homopropargyloxyzinc(II) bromide
which again hydrolyses under the reaction conditions to form
the desired homoproparglyl alcohol.
Scheme 1
organometallic substrate, solvation and nature of the metal.
Deployment of this strategy for the generation of a propargyl or
allenyl-metal (Scheme 2) is expected to be complicated by
metallotropic rearrangement (B to B0) and selectivity during
the reaction with an electrophile (B to C; B0 to C). For the
carbonyl compounds as the electrophile, similar to the pathway
demonstrated by Tamaru29 and Marshall30 A0 - B - C addition
of propargylic zinc has been depicted in the scheme for the
synthesis of homopropargylic alcohols. Tuning the regioselec-
tivity to homopropargylic alcohol (C) instead of allenic alcohol by
Zinc chloride is reduced to generate metallic zinc. Metallic
zero valent zinc thus generated is more reactive compared to
commercial zinc. The formation of homopropargylic alcohols
as end-organic product suggests that the reaction proceeds via
the in situ generation of propargylic zinc reagent followed by
carbonyl addition reaction.
The next issue relates to the metallotropic rearrangement of
an organometallic reagent and its nucleophilic reactivity
towards an electrophile. In this scenario, the selectivity of the
end-organic product is determined by the degree of control in
all the reactions occurring in tandem. The selectivity of the
addition is determined by the position of equilibrium between
the two organometallic intermediates i.e. allenylzinc and pro-
pargylzinc species. The relative rate of addition to carbonyl
compounds by an organometallic species is thus dependent on
the steric bulk of the electrophile, substitution pattern on the
Scheme 2
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the same set of metal and other reaction conditions remains a
13C NMR (CDCl3): d 59.25, 76.48, 118.68, 126.72, 127.97,
pertinent challenge. Formation of the homopropargylic alcohol 128.21, 128.42, 132.96, 137.49, 140.14, 140.26.
as the only product suggests the possible arresting of allenylic-
zinc intermediate B as major one (Scheme 2).
ESI-MS: for C16H15ClO [M], [M ꢀ OH]+ = 241.09(35Cl) &
243.09(37Cl).
HRMS calculated for the fragment ion C16H14Cl [M ꢀ OH]+ =
241.0784 found 241.0790(35Cl) & 243.0754 found 243.0769(37Cl).
Anal (C16H15ClO) calcd, C: 74.27, H: 5.84; found, C: 74.59,
H: 5.62.
Experimental
General procedure
4-Methyl-1-phenyl-hexa-1,5-dien-3-ol (3B):32 (syn:anti 52 : 48).
Using the general procedure but with 3-bromo-2-methyl-propene
1B (203 mg, 1.5 mmol), and 3-phenyl-propenal 2B (132 mg,
1.0 mmol), the title compound 3B (145 mg, 77%) was obtained
as a light yellow oily liquid.
The procedures given below for carbonyl propargylation were
followed in similar cases. All products showed satisfactory
spectral and analytical data.
Allylation of aldehyde
1H NMR (CDCl3): d 1.06 (d, 3H, J = 5.13 Hz), 1.10 (d, 3H, J =
5.19 Hz), 1.87 (brs, 1H), 2.36–2.51 (m, 1H), 4.06 (t, 1H, J =
6.84 Hz), 4.22 (t, 1H, J = 5.31 Hz), 5.11–5.23 (m, 2H), 5.74–5.95
(m, 1H), 6.16–6.29 (m, 1H), 6.61 (d, 1H, J = 15.97 Hz), 7.21–7.42
(m, 5H).
To an opened round bottom flask, aluminium powder (1.5 mmol)
was added portion wise to a mixture of ZnCl2 (1.5 mmol) in 2 ml
of 2 N HCl. The mixture was stirred at room temperature till
hydrogen gas liberation ceased. Then THF (1 ml), allyl bromide
(1.5 mmol) and benzaldehyde (1 mmol) were added sequentially
and the reaction mixture was stirred at 70 1C to afford the
corresponding homoallyl alcohol. The reaction was stopped when
the aldehyde had been completely consumed (TLC monitoring,
16 h) and THF was removed by evaporation. The aqueous reaction
mixture was extracted twice with diethyl ether (2 ꢁ 10 ml). The
combined organic layer was washed with brine (10 ml), dried over
Na2SO4, filtered, and concentrated under vacuum. The crude
product was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate 9 : 1) to afford 65–91% of the
corresponding homoallyl alcohol.
13C NMR (CDCl3): d 14.85, 15.97, 43.88, 44.60, 75.79, 76.14,
115.93, 116.54, 126.48, 127.63, 128.55, 130.00, 130.22, 131.17,
131.65, 136.78, 139.94, 140.20.
ESI-MS: for C13H16O [M], [M ꢀ OH]+ = 171.09.
HRMS calculated for the fragment ion C13H15 [M ꢀ OH]+ =
171.1174 found 171.1176.
Anal. (C13H16O) calcd, C: 82.94, H: 8.57; found, C: 82.80,
H: 8.83.
3-Phenyl-dodec-1-en-4-ol (3C):33 Using the general proce-
dure but with (1-bromomethyl-vinyl)-benzene 1A (296 mg,
1.5 mmol), and nonanal 2C (142 mg, 1.0 mmol), the title
compound 3C (185 mg, 71%) was obtained as a light yellow oily
liquid.
Propargylation of aldehydes
1H NMR (CDCl3): d 0.85 (t, 3H, J = 6.40 Hz), 1.10–1.45
(m, 10H), 1.73 (brs, 1H), 3.26 (t, 1H, J = 8.16 Hz), 3.72–3.80
(m, 1H), 5.15–5.28 (d, 2H), 6.01–6.15 (m, 1H), 7.16–7.35 (m, 5H).
13C NMR (CDCl3): d 14.03, 22.59, 29.17, 29.47, 31.78, 34.36,
57.30, 73.91, 117.68, 126.53, 127.94, 128.59, 138.31, 141.69.
ESI-MS: for C18H28O [M], [M ꢀ OH]+ = 243.22.
HRMS calculated for the fragment ion C18H17 [M ꢀ OH]+ =
243.2113 found 243.2116.
To an opened round bottom flask aluminium powder
(1.5 mmol) was added portion wise to a mixture of ZnCl2
(1.5 mmol) in 2 ml of 2 N HCl. The mixture was stirred at room
temperature till hydrogen gas liberation ceased. Then THF
(1 ml), proparglyl bromide (1.5 mmol) and benzaldehyde (1 mmol)
were added sequentially and the reaction mixture was stirred at
70 1C to afford the corresponding homoallyl alcohol. The reaction
was stopped when the aldehyde had been completely consumed
(TLC monitoring) and THF was removed by evaporation. The
aqueous reaction mixture was extracted twice with diethyl ether
(2 ꢁ 10 ml). The combined organic layer was washed with brine
(10 ml), dried over Na2SO4, filtered, and concentrated under
vacuum. The crude product was purified by column chromato-
graphy on silica gel (petroleum ether/ethyl acetate 9 : 1) to afford
the corresponding homopropargyl alcohol.
Anal (C18H28O) calcd, C: 83.02, H: 10.84; found, C: 82.95,
H: 10.64.
2-Methyl-1-p-tolyl-but-3-en-1-ol (3D):34 (syn : anti = 46 : 54).
Using the general procedure but with 3-bromo-2-methyl-
propene 1B (203 mg, 1.5 mmol), and 4-methyl-benzaldehyde
2D (120 mg, 1.0 mmol), the title compound 3D (157 mg, 89%)
was obtained as a light yellow oily liquid.
1H NMR (CDCl3): d 0.87 (d, 3H, J = 6.81 Hz), 1.02 (d, 3H, J =
6.82 Hz), 2.07 (brs, 1H), 2.35 (s, 3H), 2.46–2.50 (m, 1H), 4.32 (d,
Analytical data
1-(4-Chloro-phenyl)-2-phenyl-but-3-en-1-ol (3A):31 Using the 1H, J = 7.99 Hz), 4.56 (d, 1H, J = 5.64 Hz), 5.01–5.10 (m, 2H),
general procedure but with (1-bromomethyl-vinyl)-benzene 1A 5.16–5.25 (m, 2H), 5.74–5.78 (m, 1H), 7.12–7.26 (m, 4H).
(296 mg, 1.5 mmol), and 4-chloro-benzaldehyde 2A (141 mg,
1.0 mmol), the title compound 3A (186 mg, 72%) was obtained 115.26, 116.48, 126.47, 126.74, 128.71, 128.88, 136.85, 137.17,
13C NMR (CDCl3): d 14.28, 16.49, 21.09, 44.57, 46.10, 77.70,
as a light yellow oily liquid.
139.50, 139.66, 140.43, 140.82.
1H NMR (CDCl3): d 2.32 (brs, 1H), 3.47 (t, 1H, J = 8.46 Hz),
ESI-MS: for C12H16O [M], [M ꢀ OH]+ = 159.12. HRMS
4.80 (d, 1H, J = 7.96 Hz), 5.21–5.31 (m, 2H), 6.14–6.32 (m, 1H), calculated for the fragment ion C12H15 [M ꢀ OH]+ = 159.1174
7.01–7.22 (m, 9H).
found 159.1166.
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Anal. (C12H16O) calcd, C: 81.77, H: 9.15; found, C: 81.53,
H: 8.89.
1H NMR (200 MHz, CDCl3): d 0.93 (s, 3H), 0.98 (s, 3H), 4.39
(s, 1H), 5.01–5.17 (m, 2H), 5.8–5.94 (m, 1H), 7.19–7.3 (m, 4H);
1-Anthrylpent-4-en-2-ol (3E): Using the general procedure 13C NMR (54.6 MHz, CDCl3): 20.95, 24.35, 42.27, 79.98, 114.24,
but with 3-bromo-2-methyl-propene 1C (181 mg, 1.5 mmol), 127.66, 129.13, 133.15, 139.25, 144.73; anal. (C12H15ClO) calcd,
and anthracen-9-yl-acetaldehyde 2E (220 mg, 1.0 mmol), the C: 68.40, H: 7.18; found, C: 68.43, H: 7.19.
title compound 3E (218 mg, 83%) was obtained as a light yellow
oily liquid.
3-Methyl-1-phenyl-pent-4-en-2-ol (3J):37,38(syn : anti 42 : 58).
Using the general procedure but with 1-bromo-but-2-ene 1D
1H NMR (200 MHz, CDCl3): d 1.72 (br s, 1H), 2.42–2.48 (m, (203 mg, 1.5 mmol), and phenyl-acetaldehyde 2G (120 mg,
2H), 3.79–3.83 (m, 2H), 4.12–4.15 (m, 1H), 5.16–5.26 (m, 2H), 1.0 mmol), the title compound 3J (132 mg, 75%) was obtained
5.84–5.93 (m, 1H), 7.42–7.56 (m, 4H), 7.97–8.02 (m, 2H), 8.28– as a light yellow oily liquid.
8.36 (m, 3H).
1H NMR (200 MHz, CDCl3): d 1.01–1.04 (d, 3H, J = 6.8 Hz),
13C NMR (54.6 MHz, CDCl3): d 34.74, 41.93, 72.18, 118.40, 1.59 (br s, 1H), 2.17–2.29 (m, 1H), 2.45–2.59 (m, 1H), 2.71–2.83
124.64, 124.89, 125.71, 126.57, 129.17, 130.53, 130.62, 131.52, (m, 1H), 3.54-3.65 (m, 1H), 4.99–5.08 (m, 2H), 5.69–5.86 (m, 1H),
134.74.
7.08–7.26 (m, 5H); 13C NMR (CDCl3): d [14.45, 16.20] (anti + syn),
ESI-MS: for C19H18O [M], [M + H]+ = 263.14, [M ꢀ OH]+= 40.73, [42.95, 43.10] (anti + syn), [75.52, 75.63] (anti + syn),
245.13.
[115.03, 115.86] (anti + syn), 126.57, 128.28, 129.18, 138.87,
1-Phenyl-but-3-en-1-ol (3F):35 Using the general procedure [139.79, 140.84] (anti + syn); EIMS m/z (rel abundance): 176
but with 3-bromo-propene 1C (182 mg, 1.5 mmol), and benzal- (M.+, o1), 159 [(M ꢀ OH)+, 100], 131 (64), 121 (8) 117 (50), 103
dehyde 2F (106 mg, 1.0 mmol), the title compound 3F (127 mg, (76), 91 (92), 77 (72), 55 (30), 65 (12).
86%) was obtained as a light yellow oily liquid.
1-Phenyl-but-3-en-1-ol (3K): Using the general procedure but
1H NMR (200 MHz, CDCl3): d 2.12 (s br, 1H), 2.48–2.55 with 3-bromo-propene 1C (182 mg, 1.5 mmol), and 4-methoxy
(m, 2H), 4.73 (t, 1H, J = 7.0 Hz), 5.12–5.21 (m, 2H), 5.71–5.92 benzaldehyde 2G (136 mg, 1.0 mmol), the title compound 3K
(m, 1H), 7.23–7.37 (m, 5H); 13C NMR (54.6 MHz, CDCl3): 43.85, (164 mg, 86%) was obtained as a light yellow oily liquid.
73.34, 118.41, 125.84, 127.57, 128.44, 134.48, 143.9; anal.
(C10H12O) calcd, C: 81.04, H: 8.16; found, C: 81.09, H: 8.20.
1H NMR (400 MHz, DMSO-d6): d 2.27–2.41 (m, 2H), 3.72
(s, 3H), 4.5–4.54 (m, 1H), 4.94–4.96 (m, 2H), 5.68–5.78 (m, 1H);
1-(4-Chloro-phenyl)-but-3-en-1-ol (3G):36 Using the general 6.85–6.87 (d, 2H, J = 8.56); 7.21–7.23 (m, 1H);13C NMR (100
procedure but with 3-bromo-propene 1C (182 mg, 1.5 mmol), MHz, DMSO-d6): 43.44, 54.45, 71.60, 113.10, 116.35, 136.91,
and 4-chloro-benzaldehyde 2A (141 mg, 1.0 mmol), the title 135.57, 137.50, 157.98; anal. (C11H14O2) calcd, C: 74.13, H: 7.92;
compound 3G (166 mg, 91%) was obtained as a light yellow oily found, C: 74.14, H: 7.93.
liquid.
1-(4-Chloro-phenyl)-2,2-dimethyl-3-butyn-1-ol (5A):31 Using
1H NMR (CDCl3): d 2.32 (brs, 1H), 3.47 (t, 1H, J = 8.46 Hz), the general procedure but with 3-bromo-3-methyl-but-1-yne
4.80 (d, 1H, J = 7.96 Hz), 5.21–5.31 (m, 2H), 6.14–6.32 (m, 1H), 4A (221 mg, 1.5 mmol), and 4-chloro-benzaldehyde 2A
7.01–7.22 (m, 9H).
(141 mg, 1.0 mmol), the title compound 5A (169 mg, 81%)
13C NMR (CDCl3): d 59.25, 76.48, 118.68, 126.72, 127.97, was obtained as a light yellow oily liquid.
128.21, 128.42, 132.96, 137.49, 140.14, 140.26.
1H NMR: d 1.09 (s, 3H), 1.24 (s, 3H), 2.21 (s, 1H), 2.37 (brs,
ESI-MS: for C16H15ClO [M], [M ꢀ OH]+ = 241.09(35Cl) & 1H), 4.45 (s, 1H), 7.31 (s, 4H); 13C NMR: 143.23, 133.68, 128.96,
243.09(37Cl).
127.73, 88.89, 79.36, 71.04, 37.61, 25.84, 24.32; EIMS m/z (rel
HRMS calculated for the fragment ion C16H14Cl [M ꢀ OH]+ = abundance): 208 (1), 166 (2), 141 (100), 113 (21), 77 (75), 68 (75),
241.0784 found 241.0790(35Cl) & 243.0754 found 243.0769(37Cl). 51 (15), 39 (9); HRMS calculated for C12H13OCl (m/z) 208.0655
Anal (C16H15ClO) calcd, C: 74.27, H: 5.84; found, C: 74.59, found (m/z) 208.0648; anal (C12H13OCl) calcd, C: 69.07, H: 6.28;
H: 5.62.
1-Phenyl-pent-4-en-2-ol (3H):37,38 Using the general proce-
found, C: 69.81, H: 6.13.
1-(4-Formyl-phenyl)-2-ethyl-2-methyl-3-butyn-1-ol (5B): (mix-
dure but with 3-bromo-propene 1C (182 mg, 1.5 mmol), and ture of 2 diastereomers). Using the general procedure but with
phenyl-acetaldehyde 2G (120 mg, 1.0 mmol), the title com- 4-bromo-4-methyl-hex-2-yne 4B (263 mg, 1.5 mmol), and benzene-
pound 3H (107 mg, 65%) was obtained as a light yellow oily 1,4-dicarbaldehyde 2I (134 mg, 1.0 mmol), the title compound 5B
liquid.
1H NMR (200 MHz, CDCl3): d 1.81 (s br, 1H), 2.22–2.35 (m, 2H),
(145 mg, 63%) was obtained as a light yellow oily liquid.
1H NMR: d 0.95–1.73 (m, 8H), 2.26–2.27 (s, 1H), 2.70 (brs,
2.67–2.86 (m, 2H), 3.85–3.93 (m, 1H), 5.13–5.21 (m, 2H), 5.78–5.9 1H), 4.58, 4.61(s, 1H), 7.51–7.80 (m, 4H), 9.94 (s, 1H); 13C NMR:
(m, 1H), 7.21–7.36 (m, 5H); 13C NMR (54.6 MHz, CDCl3): 41.22, 191.60, 147.28, 146.80, 135.90, 128.84, 128.49, 127.09, 87.89,
43.34, 71.75, 118.13, 126.51, 128.57, 129.48, 134.75, 138.46; anal. 87.33, 78.89, 78.69, 72.69, 42.48, 42.08, 30.58, 29.66, 21.80,
(C11H14O) calcd, C: 81.44, H: 8.70; found, C: 81.38, H: 8.67.
21.00, 9.03; HRMS calculated for C14H16O2 (m/z) 216.2783
1-(4-Chloro-phenyl)-2,2-dimethyl-but-3-en-1-ol (3I):39 Using found (m/z) 216.2775; anal (C14H16O2) calcd, C: 77.75, H: 7.46;
the general procedure but with 1-bromo-3-methyl-but-2-ene found, C: 77.61, H: 7.57.
1B (224 mg, 1.5 mmol), and 4-chloro-benzaldehyde 2A
1-Furan-2-yl-4-trimethylsilanyl-but-3-yn-1-ol (5C): Using the
(141 mg, 1.0 mmol), the title compound 3I (158 mg, 75%) general procedure but with (3-bromo-prop-1-ynyl)-trimethyl-silane 4C
was obtained as a light yellow oily liquid.
(287 mg, 1.5 mmol), and tetrahydro-furan-2-carbaldehyde 2J (100 mg,
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1.0 mmol), the title compound 5C (117 mg, 55%) was obtained as a
light yellow oily liquid.
1-(4-Bromo-phenyl)-but-3-yn-1-ol (5H):43 Using the general
procedure but with 3-bromo-propyne 4E (179 mg, 1.5 mmol)
1H NMR: d 0.14 (s, 9H), 2.30 (brs, 1H), 2.78 (d, 2H, J = and 4-bromo benzaldehyde 2L (185 mg, 1.0 mmol), the title
6.24 Hz), 4.84 (t, 1H, J = 6.26 Hz), 6.31–6.34 (m, 2H), 7.36–7.38 compound 5H (162 mg, 72%) was obtained as a light yellow oily
(m, 1H);
liquid.
1H NMR (400 MHz, CDCl3) d 2.08–1.98 (m, 1H), 2.38 (br s,
1H), 2.65–2.49 (m, 2H), 4.84–4.71 (m, 1H), 7.28–7.14 (m, 2H),
7.50–7.35 (m, 2H); 13C NMR (100.6 MHz, CDCl3) d 29.64, 71.58,
71.87, 80.34, 122.03, 127.71, 131.78, 141.59; anal. C10H9BrO
13C NMR: 154.80, 142.09, 110.14, 106.46, 102.48, 88.10,
66.13, 27.69, -0.05; ESI-MS: for C11H16O2Si [M], [M + H]+
209.10, [M ꢀ OH]+ = 191.09.
=
HRMS calculated for the fragment C11H15OSi [M-OH]+
=
191.0892 found 191.0883; anal (C11H16O2Si) calcd, C: 63.42, calcd, C: 53.36, H: 4.03; found, C: 53.37, H: 4.04.
H: 7.74; found, C: 63.33, H: 7.57.
1-(4-Methoxy-phenyl)-but-3-yn-1-ol (5I):44,45 Using the gen-
3-Methyl-1-phenyl-pent-4-yn-2-ol (5D):40 Using the general eral procedure but with 3-bromo-propyne 4E (179 mg,
procedure but with 3-bromo-3-methyl-but-1-yne 4A (221 mg, 1.5 mmol) and 4-methoxy benzaldehyde 2G (136 mg, 1.0 mmol),
1.5 mmol), and 2-phenyl-propionaldehyde 2K (134 mg, 1.0 mmol), the title compound 5I (141 mg, 80%) was obtained as a light
the title compound 5B (97 mg, 48%) was obtained as a light yellow yellow oily liquid.
oily liquid.
1H NMR (400 MHz, CDCl3) d 2.08 (t, 1H, J = 2.6 Hz), 2.31 (br
1H NMR: d 1.27 & 1.28 (d, 3H, J = 7.20 Hz and J = 6.94 Hz), s, 1H), 2.61–2.67 (m, 2H), 3.80 (s, 3H), 4.83 (t, 1H, J = 6.4 Hz),
1.84 (brs, 1H), 2.21 (d, 1H, J = 2.43 Hz), 2.55–2.62 (m, 1H), 2.87– 6.85–6.91 (m, 2H), 7.28–7.34 (m, 2H); 13C NMR (100.6 MHz,
2.93 (m, 2H), 3.66–3.74 (m, 1H), 7.19–7.38 (m, 5H);
CDCl3) d 29.61, 55.51, 71.08, 72.21, 81.03, 114.08, 127.23,
134.87, 159.56; anal. C11H12O2 calcd, C: 74.98, H: 6.86; found,
ESI-MS: for C12H14O [M], [M + H]+ = 175.11, [M ꢀ OH]+
=
157.10, HRMS calculated for the quasimolecular ion peak of C: 74.99, H: 6.87.
C
12H14O [M + H]+ = 175.1123 found 175.1145 and that of the
fragment ion C12H13 [M ꢀ OH]+ = 157.1017 found 157.1026;
anal (C12H14O) calcd, C: 82.72, H: 8.10; found, C: 82.95, H: 7.91.
4,4-Dimethyl-2-phenyl-hex-5-yn-3-ol (5E): Using the general
procedure but with 3-bromo-but-1-yne 4D (200 mg, 1.5 mmol),
and phenyl-acetaldehyde 2H (120 mg, 1.0 mmol), the title
compound 5E (136 mg, 78%) was obtained as a light yellow
oily liquid.
Conclusion
In summary, we have demonstrated a facile strategy for the
allylation and propargylation of aldehydes to afford the corres-
ponding homoallyl and homopropargyl alcohols with a two
carbon extension. The reaction involves the addition of highly
reactive nano Zn(0) species coming from the redox couple of Al
& ZnCl2 in THF–H2O solvent. The process is simple and
efficient compared to other processs, and the reagents are easy
to handle as they are not sensitive to air and moisture. The
reagents are cheap and easily available in an undergraduate
laboratory. The yield of the process is good to excellent. The use
of aqueous solvent and nontoxic zinc adds to the environmen-
tally friendly nature of the reaction. All of the above features are
expected to add to the synthetic utility of the present reaction.
Further work is warranted to understand the mechanistic
details of the reaction.
1H NMR: d 1.21 (s, 3H), 1.26 (s, 3H), 1.39 (d, 3H, J = 3.53 Hz),
1.86 (brs, 1H), 2.24 (s, 1H), 3.12–3.16 (m, 1H), 3.51 (d, 1H, J =
1.97 Hz), 7.20–7.30 (m, 5H).
ESI-MS: for C14H18O [M], [M + H]+ =203.14, [M ꢀ OH]+
=
185.13.
HRMS calculated for the fragment C14H17 [M ꢀ OH]+
=
185.1330 found 185.1330.
Anal. (C14H18O) calcd. C: 83.12, H: 8.97; found, C: 82.91,
H: 8.69.
1-Phenyl-pent-4-yn-2-ol (5F):40,41 Using the general procedure
but with 3-bromo-propyne 4E (179 mg, 1.5 mmol), and Phenyl-
acetaldehyde 2H (120 mg, 1.0 mmol), the title compound 5F
(109 mg, 68%) was obtained as a light yellow oily liquid.
1H NMR (400 MHz, CDCl3): d 1.96 (d, 1H, J = 4.4 Hz), 2.12
(t, 1H, J = 2.7 Hz), 2.32–2.52 (m, 2H), 2.79–2.96 (m, 2H), 3.87–
4.05 (m, 1H), 7.12–7.37 (m, 5H); 13C NMR (100.6 MHz, CDCl3) d
26.64, 42.70, 71.03, 71.33, 80.82,126.90, 128.83, 129.61, 137.87;
Anal (C11H12O) calcd, C: 82.46, H: 7.55; found, C: 82.66, H: 7.47.
1-Phenyl-but-3-yn-1-ol (5G):42 Using the general procedure
but with 3-bromo-propyne 4E (179 mg, 1.5 mmol) and benzal-
dehyde 2F (106 mg, 1.0 mmol), the title compound 5G (105 mg,
72%) was obtained as a light yellow oily liquid.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors acknowledge the financial support for this work by
SERB-New Delhi, SR/FT/CS-137/2011 dated 12.07.2012, and
Science & Technology and Biotechnology-WB, 50(Sanc.)/ST/P/
S&T/15G-10/2018 dated 30.01.2019 to UKR.
1H NMR (400 MHz, CDCl3) d 2.05 (t, 1H, J = 2.8 Hz), 2.37
(s, 1H), 2.68–2.54 (m, 2H), 4.85 (td, 1H, J = 6.5, 2.6 Hz), 7.43–
7.20 (m, 5H), 13C NMR (100.6 MHz, CDCl3) d 29.42, 70.94, 72.30,
80.62, 125.70, 127.96, 128.45, 142.39, anal. (C10H10O) calcd, C:
82.16, H: 8.20, O: 10.94; found, C: 82.17, H: 8.21.
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