Lewis acid-catalyzed oxidative allylation: A new approach for the synthesis of homoallylic alcohols and amines directly from alcohols

Article abstract of DOI:10.1002/adsc.201000713

A new approach for the synthesis of homoallylic alcohols and amines directly from alcohols via one-pot sequential oxidation-Barbier reaction and oxidation-condensation-Barbier reactions, respectively, is reported. The protocol involves the one-pot ferric chloride-catalyzed oxidation of alcohols to the corresponding aldehydes with chloramine-T followed by indium-mediated Barbier allylation with allyl bromide to afford homoallylic alcohols in 70-90% overall yields. The ferric chloride-catalyzed condensation of aldehydes and oxidation by-product p-toluenesulfonamide followed by indium-mediated Barbier-type allylation of the resulting aldimines with allyl bromide affords homoallylic amines in 60-80% overall yields in the same reaction vessel. The present work demonstrates a new one-pot approach toward homoallylic alcohol and amine synthesis directly from alcohols. Copyright

Full text of DOI:10.1002/adsc.201000713

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DOI: 10.1002/adsc.201000713
Lewis Acid-Catalyzed Oxidative Allylation: A New Approach for
the Synthesis of Homoallylic Alcohols and Amines Directly from
Alcohols
a
a
a,
Vishnu P. Srivastava, Rajesh Patel, and Lal Dhar S. Yadav *
a
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
Fax : (+91)-532-246-0533; phone: (+91)-532-250-0652; e-mail: ldsyadav@hotmail.com
Received: September 19, 2010; Revised: December 16, 2010; Published online: March 10, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000713.
Abstract: A new approach for the synthesis of ho-
moallylic alcohols and amines directly from alco-
hols via one-pot sequential oxidation–Barbier reac-
tion and oxidation–condensation–Barbier reactions,
respectively, is reported. The protocol involves the
one-pot ferric chloride-catalyzed oxidation of alco-
hols to the corresponding aldehydes with chlor-
yl/imine compounds to produce homoallylic alcohols/
amines has been extensively studied.
[
5]
Recently, indium has emerged as a metal of high
potential in organic synthesis because it has first ioni-
zation potential enough low to generate organoindi-
um reagents with organic halides relative to the other
metallic elements near it in the periodic table and
enough high to resist to boiling water or alkali.
Indium does not form oxides readily in air and can be
handled safely without apparent toxicity. Indeed, it
appears that indium is the most reactive metal for
such reactions.
ACHTUNGTRENNUNGa mine-T followed by indium-mediated Barbier ally-
lation with allyl bromide to afford homoallylic alco-
hols in 70–90% overall yields. The ferric chloride-
catalyzed condensation of aldehydes and oxidation
by-product p-toluenesulfonamide followed by
indium-mediated Barbier-type allylation of the re-
sulting aldimines with allyl bromide affords homoal-
lylic amines in 60–80% overall yields in the same
reaction vessel. The present work demonstrates a
new one-pot approach toward homoallylic alcohol
and amine synthesis directly from alcohols.
A longstanding endeavour of organic chemists has
been to develop methodology to minimize the use of
hazardous chemicals, exhaustive, and multi-pot pro-
cesses. Over the past few decades a lot of work has
been done to achieve eco-friendly methodology using
[
6]
green catalysts, solvents, and technology. Recently, a
few methods have been developed to replace the
direct use of volatile, toxic, or unstable aldehydes
with relatively more stable, less volatile, and less toxic
Keywords: alcohols; catalysis; chloramine-T; homo-
allylic alcohols; Lewis acids; oxidation
[
7]
alcohols. On the other hand, iron is an abundant,
economical, and environmentally friendly metal,
which has shown increasing promise as a catalyst in
[
8]
many organic transformations such as oxidation, re-
Homoallylic alcohols and amines are versatile precur- duction, condensation, coupling, addition, and substi-
sors for various organic transformations. They are tution reactions.
[
1]
used in numerous reactions such as Prins cyclization,
Meanwhile, the oxidation of alcohols is an impor-
[2]
[3]
b-fragmentation, natural product synthesis, and tant method for forming aldehydes. The oxidation of
also represent useful building blocks for the synthesis primary alcohols using different oxidants has been
[
9]
of biologically and synthetically important com- carried out by many workers. However, some of the
[4]
pounds such as b-amino acids, 1,3-amino alcohols, 1- more historically prominent methods have significant
[
10]
amino-3,4-epoxides, pyrrolidines, and piperidines. The drawbacks. They suffer from difficult work-up, re-
coupling of an allyl halide and a carbonyl/imine com- quire low temperature, use of solvents that can be dif-
pound in the presence of a metal is a fundamental ficult to remove, are toxic in nature, and produce haz-
procedure for a new CÀC bond formation to produce ardous waste. o-Iodoxybenzoic acid (IBX) is a mild
homoallylic alcohols/amines. During the past two de- and selective reagent for this transformation, but its
cades low-valent metal-mediated allylation of carbon- solubility in common solvents remains a problem for
[
11,12]
which new solutions are still to be sought.
Advan-
Adv. Synth. Catal. 2011, 353, 695 – 700
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
695

Vishnu P. Srivastava et al.
COMMUNICATIONS
Scheme 1. Direct synthesis of homoallylic alcohols and amines from alcohols.
tageously, chloramine-T acts as an oxidizing agent in Table 1. Optimization of catalyst for the synthesis of homo-
[a]
both acidic and alkaline solutions, and leaves TsNH2 allylic alcohols.
and NaCl as by-products, whose presence would not
adversely affect the ensuing Barbier reaction, hence it
is a well suited oxidant for the present study. Further-
more, the oxidation by-product TsNH could be uti-
lized in the subsequent step to afford N-tosylimines,
which would be reacted with an allyl halide to obtain
homoallylic amines in a one-pot operation.
2
[b]
[c]
Entry
Metal
Lewis acid
Time [h]
Yield [%]
1
2
3
4
5
6
7
8
9
In
In
In
Zn
Zn
Zn
–
–
–
In
Zn
–
FeCl3
9
90
45
57
65
ZnCl2
CoCl2
15
15
20
24
20
5
15
12
24
30
5
Several modifications including the use of different
metals, catalysts, and solvents have been made to im-
FeCl3
[
13]
prove the Barbier reaction. Recently, Zhang et al.
reported an electrochemical synthesis of homoallylic
ZnCl2
trace
52
CoCl2
FeCl3
ZnCl2
CoCl2
–
[
13i,j]
[
d]
alcohols from alcohols in a one-pot operation.
Consideration of the above valid points and continua-
– (95)[
d]
– (50)
[
14]
[d]
tion of our quest for developing one-pot protocols,
– (60)
[
e]
intrigued us to develop a one-pot protocol for the 10
trace[
e]
1
1
1
2
–
trace
synthesis of homoallylic alcohols/amines starting di-
rectly from alcohols employing chloramine-T as the
oxidant (Scheme 1).
Our initial experiment was carried out for the reac-
tion of benzyl alcohol (1a) with allyl bromide (3)
using chloramine-T (2) as an oxidant at room temper-
ature. A systematic study was first undertaken to
define the best reaction conditions and to examine
the role of the Lewis acid in the oxidation of alcohol
[
f]
FeCl3
N.R.
[a]
Reaction conditions: benzyl alcohol (1 mmol), chlor-
AHCTUNGTREGUNaNN mine-T (1 mmol), DCM (3 mL), In (1 mmol) and allyl
bromide (1.5 mmol).
10 mol% of catalyst.
[
[
[
[
b]
c]
d]
e]
Yields of the isolated pure compounds.
Isolated yield of benzaldehyde.
Benzaldehyde was formed in 8–13% yield but 4a was
only detected in traces.
1
a followed by allylation of the resulting aldehyde in
[f]
No reaction when FeCl (1 mmol) was used in the ab-
3
dichloromethane (DCM) as solvent (Table 1). Among
sence of chloramine-T.
the different metals/Lewis acids tested, In/FeCl gave
3
the best result (Table 1, entry 1).
The presence of the Lewis acid is extremely impor- alcohols 4a (Table 1, entry 5). When CoCl is used as
2
tant for the oxidation of alcohol 1a to the correspond- a catalyst, the oxidation of benzyl alcohol (1a) pro-
ing aldehyde (Table 1, entries 7–9). In the absence of ceeded slowly and a low yield of the aldehyde was ob-
a Lewis acid no significant oxidation of alcohol to al- tained but the allylation reaction proceeds smoothly,
dehyde was observed (Table 1, entries 10 and 11, yield however, the overall yield is low (Table 1, entry 6).
of benzaldehyde 8–13%). It was noted that 1a could
not be oxidized to benzaldehyde with a Lewis acid investigated in different solvents. Among the solvents
for example, FeCl ) in the absence of chloramine-T. investigated, DCM gave the best result (Table 2,
The catalytic activity of FeCl was also intensively
3
(
3
In addition, the presence of a suitable Lewis acid also entry 4). In aqueous solvent In/FeCl afforded the pi-
3
accelerates the allylation of the oxidation product sig- nacol coupling product 6a as the side reaction product
nificantly (Table 1, entries 4–6 and 10, 11). On the along with the Barbier product (Table 2, entry 2). On
other hand, a combination of Zn/FeCl gives a moder- the other hand, when In/FeCl was used in an aprotic
3
3
ate yield of homoallylic alcohol 4a (Table 1, entry 4) solvent, only the Barbier product was obtained
whereas Zn/ZnCl gives only a trace of homoallylic (Table 2, entries 1 and 3–7).
2
696
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 695 – 700

Lewis Acid-Catalyzed Oxidative Allylation: A New Approach
Table 2. Optimization of solvent.
When the reaction mixture was stirred overnight, the
reduction product 7 was obtained as the major prod-
uct (Table 3, entry 9). The best result was obtained
with 1:1.5 ratio of indium and allyl bromide in DCM
using 10 mol% of FeCl (Table 3, entry 2).
3
The mechanism for the formation of reduction
product 7 presumably involves two steps. In the first
step iron ACHUNTGRNENUG( III) chloride is reduced by indium metal to
form a low-valent iron species, which in the subse-
quent step would reduce the allylic alcohol 4a to the
corresponding saturated alcohol 7. This presumption
is in conformity with the earlier reports on the mecha-
[a]
Entry
Solvent
THF
Time [h]
Yield [%]
58
40/(30)
30
90
30
nism of reduction using ironACHTGNUTRENNUNG
1
2
3
4
5
6
7
20
17
25
9
15
20
15
[
15]
[b]
metal.
THF:H O
2
toluene
DCM
DMF
iron(II) in the reaction mixture.
The present optimized synthesis of homoallylic al-
cohol 4a involves the stirring of an equimolar mixture
of benzyl alcohol and chloramine-T in DCM using
CH CN
62
38
3
1,4-dioxane
FeCl as catalyst (10 mol%) at room temperature for
3
[
[
a]
b]
4 h followed by addition of indium powder (1 equiv.)
and allyl bromide (1.5 equiv.) and stirring the reaction
mixture for 5 h.
Yield of isolated pure compound 4a.
Isolated yield of pinacol 6a.
The substrate scope was then investigated under
Variation of the ratio of metal, Lewis acid, and allyl the optimized conditions. A variety of alcohols such
bromide had a significant impact not only on the con- as substituted benzyl alcohols, 2-furylmethanol, cyclo-
version but also on the reaction outcome. With 2 hexylmethanol, and n-octanol were tested. For benzyl
equivalents of indium and 3 equivalents of allyl bro- alcohol and substituted benzyl alcohols containing
mide (3) in the presence of 10 mol% of FeCl the ho- either electron-withdrawing or electron-donating sub-
3
moallylic alcohol reaction product 4a was formed stituents (Table 4, entries 1–6), the reaction proceeded
along with the reduction product 7 (Table 3, entry 8). smoothly and afforded homoallylic alcohols in good
to excellent yields. 2-Furylmethanol was transformed
into 4g in 78% yield (Table 4, entry 7) whereas a rela-
Table 3. Optimization of molar ratio of indium:FeCl :allyl
3
bromide.
Table 4. Synthesis of homoallylic alcohols 4 and amines 5 ac-
[a]
cording to Scheme 1.
[b]
[c,d]
Entry
R
Product Time [h]
Yield [%]
1
2
3
Ph
p-ClC H
p-MeOC H
m-ClC H
p-MeC H
o-ClC H
2-furyl
cyclohexyl
n-C H
Ph
p-ClC H
p-MeOC H
p-MeC H
4a
4b
4c
4d
4e
4f
4g
4h
4i
5a
5b
5c
5d
5e
9
90
85
82
76
88
73
78
70
72
78
68
80
79
60
10
11
10
9
14
12
17
16
12
15
14
13
19
6
4
6
4
6
4
5
6
7
8
6
4
6
4
Entry Indium
equiv.)
FeCl3
(mol%)
Allyl bromide
(equiv.)
Yield
[a]
(
[%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1.5
2
2
10
10
10
10
15
20
10
10
10
1.2
1.5
2
83
90
90
91
88
85
9
7
15
10
11
12
13
14
6
4
3
6
4
1.5
1.5
3
3
3
6
4
o-ClC H
6
4
[
b]
78 (10)[
[
[
[
a]
b]
c]
b]
See Experimental Section for general procedure.
40 (35)
[
b,c]
Total stirring time.
– (89)
[13]
All the products are known compounds and were char-
acterized by comparison of their mp, TLC, IR and
[
[
[
a]
Yields of the isolated pure compound 4a.
Isolated yield of reduction product 7.
Reaction time 16 h, yield of 7.
b]
c]
1
HNMR data with those of authentic samples.
[d]
Yields of the isolated pure compounds.
Adv. Synth. Catal. 2011, 353, 695 – 700
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
697

Vishnu P. Srivastava et al.
COMMUNICATIONS
Scheme 2. Proposed pathway for the formation of homoallylic alcohols and amines.
tively low conversion of aliphatic alcohols was ob-
served under the present reaction conditions (Table 4, for the synthesis of homoallylic alcohols and amines
entries 8 and 9). directly from alcohols. The protocol involves the one-
In summary, we have developed a new approach
We also investigated the possibility of substitution pot FeCl -catalyzed oxidation of alcohols to aldehydes
3
of the hydroxy group of homoallylic alcohol 4a with with chloramine-T followed by indium-mediated
oxidation by-product p-toluenesulfonamide to afford Barbier allylation to afford homoallylic alcohols. Sim-
homoallylic amine 5a under the present reaction con- ilarly, the condensation of oxidation product alde-
ditions by stirring the reaction mixture for 40 h at hydes with oxidation by-product p-toluenesulfon-
room temperature and also at reflux in the presence
ACHTUNGTRENNGNUa mide affords N-tosylimines, which undergo an
of 4 ꢁ molecular sives, but no appreciable formation indium-mediated Barbier-type allylation with allyl
of 5a was observed (Scheme 2, Path a). Then, we bromide to afford homoallylic amines in the same re-
turned our attention toward imine formation via con- action vessel. Thus, the present work opens up a new
densation of the oxidation product aldehyde and the and efficient one-pot synthetic route to homoallylic
oxidation by-product p-toluenesulfonamide followed alcohols and amines.
by the allylation of the resulting imine in the same re-
action vessel (Scheme 2, Path b). Thus, stirring an
equimolar mixture of benzyl alcohol and anhydrous Experimental Section
chloramine-T in the presence of 10 mol% FeCl using
3
dry DCM as solvent at room temperature for 4 h fol- General Procedure for One-Pot Synthesis of Homo-
lowed by addition of 4 ꢁ molecular sieves and reflux allylic Alcohols 4 Directly from Alcohols
for 1.5 h afforded the corresponding imines. Then, the
reaction mixture was cooled to room temperature,
indium (1 equiv.) and allyl bromide (1.5 equiv.) were
added and the mixture was further stirred for 6.5 h to
afford the corresponding homoallylic amine in good action mixture was stirred for 5–12 h at room temperature.
A mixture of alcohol 1 (2 mmol), chloramine-T (2 mmol)
and FeCl3 (10 mol%) in 3–5 mL of DCM was stirred at
room temperarue for 3–5 h, then indium powder (2 mmol)
and allyl bromide 3 (3 mmol) were added. The resultant re-
yield along with a minor amount of the homoallylic The reaction progress was monitored by TLC. Upon com-
alcohol.
The generality of the method was also investigated
with a variety of alcohols. In all cases the reaction
preceded smoothly and good yields of the correspond-
ing homoallylic amines 5 were obtained (Table 4).
The present protocol therefore appears to be quite
general, atom and pot economical.
pletion (Table 4), the solvent was evaporated under reduced
pressure, the residue dissolved in 10 mL THF, the reaction
mixture was quenched with saturated ammonium chloride
and extracted with diethyl ether (2ꢂ10), dried over anhy-
drous Na SO , filtered, and evaporated to dryness. The resi-
due thus obtained was purified by silica gel column chroma-
tography using ethyl acetate-n-hexane as eluent to afford an
analytically pure sample of 4.
2
4
The isolation of RCHO, TsNH , and N-tosylald-
2
ACHTUNGTRENNUNGi mine during the reaction provides strong evidence
for the reaction pathway proposed in Scheme 2. De- General Procedure for One-Pot Synthesis of Homo-
tails of the mechanism of oxidation of alcohols to al- allylic Amines 5 Directly from Alcohols
[
16]
dehydes with chloramine-T and that of indium-pro-
moted allylation of aldehydes and aldimines are avail-
A mixture of alcohol 1 (2 mmol), chloramine-T (2 mmol)
and FeCl (10 mol%) in 3–5 mL of dry DCM was stirred at
[
13,17]
3
able in the literature.
Probably, the catalyst FeCl3
room temperature for 3–5 h followed by addition of 4 ꢁ mo-
lecular sieve (200 mg) and stirring for 1–2.5 h at the reflux
temperature. Then, the reaction mixture was cooled to room
activates chloramine-T and alcohols through coordi-
nation to accelerate their oxidation with chloramine-
T. Moreover, indium metal, aldehydes, and aldimines temperature and indium powder (2 mmol) and allyl bromide
would also be activated by FeCl for allylation. 3 (3 mmol) were added. The resultant reaction mixture was
3
698
asc.wiley-vch.de
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 695 – 700

Lewis Acid-Catalyzed Oxidative Allylation: A New Approach
stirred for 6–14 h at room temperature. The reaction prog-
ress was monitored by TLC. The isolation, purification, and
characterization of product 5 were done by following the
same procedure as described above for the synthesis of ho-
moallylic alcohols 4.
[6] a) J. H. Clark, D. J. Macquarrie, Handbook of green
chemistry and technology, Wiley-Blackwell, 2008; b) A.
Lapain, D. Constable, Green Chemistry Metrics, Wiley-
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chini, Methods and Reagents for Green Chemistry,
Hoboken NJ, John Wiley & Sons, 2007.
The structures of the products were confirmed by compar-
1
ison of their mp, TLC, IR and H NMR data with those of
authentic samples obtained commercially or prepared by
[13]
the literature methods.
[
7] a) L. Blackburn, R. J. K. Taylor, Org. Lett. 2001, 3,
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Acknowledgements
[
2
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. One of us (R.P.) is
grateful to the CSIR, New Delhi, for the award of a Senior
Research Fellowship.
Synth. Catal. 2005, 347, 703; c) M. Nakanishi, C. Bolm,
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