Gallium metal mediated allylation of carbonyl compounds and imines under solvent-free conditions

Article abstract of DOI:10.1016/j.tetlet.2003.10.188

Gallium metal is effective in mediating the allylation of various carbonyl compounds and imines under solvent-free conditions, with the application of sonic energy, affording the corresponding homoallylic alcohols and amines. The imines themselves were al

Full text of DOI:10.1016/j.tetlet.2003.10.188

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 243–248
Gallium metal mediated allylation of carbonyl compounds
and imines under solvent-free conditions
a,
Philip C. Andrews, Anna C. Peatt and Colin L. Raston
a
b
*
a
Centre for Green Chemistry/School of Chemistry, Monash University, PO Box 23, Victoria 3800, Australia
School of Biomedical and Chemical Sciences, University of Western Australia, Crawley, Perth, WA 6009, Australia
b
Received 25 September 2003; revised 22 October 2003; accepted 30 October 2003
Abstract—Gallium metal is effective in mediating the allylation of various carbonyl compounds and imines under solvent-free
conditions, with the application of sonic energy, affording the corresponding homoallylic alcohols and amines. The imines them-
selves were also prepared under solventless conditions in high yield, thereby establishing a two-step solvent-free synthesis of
homoallylic amines. In comparison, indium metal produced a mixture of the desired homoallylic secondary amine and the bis-
allylated species via an iminium ion intermediate.
ꢀ
2003 Elsevier Ltd. All rights reserved.
1. Introduction
has been given to the production of benign synthetic
6
protocols for the allylation of carbonyl compounds, the
We have recently established that homoallylic alcohols
can be readily prepared from carbonyl compounds and
allylic halides under metal mediated solvent-free condi-
allylation of imines has been somewhat neglected. The
contributing factors to this include: the low reactivity of
unactivated imines towards nucleophilic addition, their
tendency to deprotonate when they are derived from
enolisable carbonyl compounds and their sensitivity to
1
2
tions using In, Bi, Zn and Sn in Barbier–Grignard type
reactions. This is an important addition to the variety of
procedures which utilise benign reaction media, such as
water and ionic liquids. From a green perspective, the
solvent-free alternative alleviates entirely the need to
dispose of, or recycle, the reaction media. Furthermore,
the use of water as a reaction media for such 1,2-addi-
tions inhibits the scope of the reaction to only hydro-
lytically robust substrates such as carbonyl compounds,
4
water. Given these properties (particularly their
hydrolytic instability), solvent-free approaches to the
synthesis of allylated amines are attractive in providing
such compounds, and the drive towards the develop-
ment of more benign synthetic protocols.
In extending our previous investigations, we now focus
on the potential use of gallium metal in solvent-free
allylation reactions. Gallium is of particular interest for
several reasons over and above the fact that in compar-
ison with the other group 13 elements its role as a
potential mediator in organometallic reactions has been
3
thus excluding compounds such as most imines.
The 1,2-addition of organometallic reagents to imines is
an established route to homoallylic amines, which are
important fundamental building blocks in many bio-
4
7
logically active compounds. The traditional synthetic
largely ignored. Its low first ionisation potential of
procedures for their synthesis involve the exclusion of
air and moisture, the use of dry solvents and highly
reactive organometallic reagents, mostly allylic com-
5.99 eV (cf. Li 5.39, Mg 7.65 eV) makes it useful for single
electron transfer reactions, which combined with its low
vapour pressure and the fact it is liquid at low temper-
atures (mp 29.8 ꢁC), makes it an attractive candidate for
5
pounds of Li, Mg, Cu and Zn. Though much attention
8
metal mediated reactions. Furthermore, commercially
available metallic Ga 99.99% is a relatively inexpensive
9
metal. However, as with the imines themselves, the
organometallic complexes of Ga can be unstable to water
leading to a significant reduction in reactivity through
Keywords: Gallium; Indium; Imines; Allylation; Solvent free; Alde-
hydes.
*
Corresponding author. Tel.: +61-3-9905-5509; fax: +61-3-9905-4597;
e-mail: phil.andrews@sci.monsh.edu.au
10
hydroxide and galloxane formation, although the
0
040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.188

244
P. C. Andrews et al. / Tetrahedron Letters 45 (2004) 243–248
possible catalytic role of these compounds should not be
ignored. Furthermore, we have investigated the use of
both indium and gallium metal in the solvent-free allyl-
ation of imines to assess whether protocols developed
for the allylation of carbonyl compounds could be
extended to imines. The imines we have investigated were
also prepared under solvent-free conditions via conden-
sation reactions of aldehydes with aniline, thereby
establishing a completely solvent-free synthetic protocol
for the synthesis of homoallylic amines.
Table 1. Solvent-free allylation of various carbonyl compounds med-
iated by gallium metal
a
Carbonyl compounds
Isolated yield (%)
PhCH@O97
2
-(MeO)–PhC(H)@O86
PhC(H)@C(H)C(H)@O74
Ph(Me)C@O34
b
(Ph) C@O<10
2
b
2
-(OH)PhC(H)@O
-(OH)PhC(H)@O
0
b
4
0
a
1
All products gave satisfactory IR, H NMR and EIMS as compared
to previously reported spectra.
b
1
Estimated by H NMR and GC–MS.
2
. Synthesis of homoallylic alcohols
We first investigated the allylation of carbonyl com-
pounds using gallium metal with the aim of establishing
its suitability for the allylation of imines and to compare
it with indium as a mediator in such reactions. A typical
experiment involved the addition of allyl bromide
yield of homoallylic alcohol on work-up, though a
complication may be loss of the allyl bromide from the
reaction mixture at elevated temperatures. In turning to
a less volatile substrate, the reaction of crotyl bromide
was investigated, resulting in a high yield of the homo-
allylic alcohol (c-allylation product), ca. 94%, which is
comparable to that obtained on sonication of the reac-
tion mixture, ca. 91%. Therefore, it can be inferred that
while the role of sonication is important when using
highly volatile allyl bromide it actually does not impact
significantly on the overall yield of the reaction. The
continual production of a clean gallium metal surface
via slight heating is sufficient to ensure an efficient
conversion of the aldehyde to the homoallylic alcohol.
The low melting point of gallium therefore negates the
need for sonication when using substituted allyl bro-
mides, an option which is not practical for other metals
(
7.25 mmol) to a mixture of commercially available
gallium metal (5 mmol) and a carbonyl compound
5 mmol) under an inert atmosphere of nitrogen. The
(
mixture was sonicated for 12 h, before quenching with
water (ca. 0.5 mL) and extraction of the alcohol into
diethyl ether, Scheme 1. The resulting alcohol was then
purified by flash chromatography (10:1 hexane–ethyl
acetate) mixture.
Under sonication the reactions proceeded smoothly and
efficiently with yields ranging from 74% to 97%, Table 1,
for aromatic, –OMe substituted aromatic and aliphatic
aldehydes. However, the system was not effective with
ketones or hydroxybenzaldehydes. With the exception
of the hydroxybenzaldehydes the addition reactions
gave comparable yields to those obtained when using
2
we have thus far investigated, such as Sn.
Slightly lower yields of ca. 74% were observed for the
a,b-unsaturated aldehyde, cinnamaldehyde. However, in
terms of selectivity only the 1,2-addition product was
observed. Conversely, aldehydes containing –OH sub-
stitution were inert to this system, with both 2- and
4-hydroxy substituted benzaldehydes producing none of
the desired homoallylic alcohol. This is consistent with
the acidity of such substituted phenolic groups and
studies showing that alkyl gallium compounds react
selectively at the phenolic OH rather than adding to the
1
indium metal in previous solvent-free reactions. In
contrast, in the absence of sonication and with the
reaction mixtures simply agitated by stirring, only rela-
tively low yields were obtained. Given the similar first
ionisation potentials (IP) of indium, 5.70 eV, and gal-
lium, 5.99 eV, and the liquid state of gallium just above
room temperature, it was surprising that gallium re-
quired sonication whilst for indium simply stirring was
effective. This raises the issue of whether sonication for
gallium is required to clean the metal surface, or whether
sonication influences the chemical reactivity of the gal-
lium metal. Accordingly, we carried out the reaction of
benzaldehyde in the absence of sonication at 35 ꢁC
11
carbonyl moiety. Also, consistent with our previous
analogous studies for Bi, Zn, In and Sn, reactions
involving ketones undergo allylation to a much lesser
extent (e.g., acetophenone ca. 30%). Ketones, such as
benzophenone, which exhibit reduced electrophilicity,
result in low yields, <10%, reflecting a lower reactivity of
the carbonyl group. In comparison, similar reactions
(
above the melting point of the metal), whereby clean
metal surface should constantly be available for the
reactive substrate. However, this resulted in a very low
12
using an allylic gallium reagent in THF–hexane and
OH
O
i) Ga/
ii) H O
Br
+
R
2
R
H
R'
R, R' = Alkyl or Aryl groups
Scheme 1. Solvent-free allylation of various carbonyl compounds mediated by gallium metal.

P. C. Andrews et al. / Tetrahedron Letters 45 (2004) 243–248
245
the gallium mediated allylation of carbonyl compounds
restricted to only hydrolytically robust C@N containing
compounds such as the sulfonyl imines. Moreover, the
13
3
in water give similar or slightly improved yields.
Another noteworthy observation is that no side prod-
ucts resulting from Wurtz or pinacol coupling are ob-
served, only unreacted starting material being detected.
use of water as bulk solvent generates an aqueous waste
stream, which is an important consideration when
18
developing green chemistry reactions.
3
. Solvent-free synthesis of imines
4. Synthesis of homoallylic secondary amines
Traditionally imines have been synthesised via the
addition of amines and carbonyl compounds under
The reaction methodology, Scheme 2, which proved
successful with various aldehydes, was repeated with the
imines. The corresponding homoallylic amines were
obtained in yields ranging from 32% to 94%, Table 3, for
imines derived from aromatic, –OMe substituted aromatic
and aliphatic aldehydes. These yields are comparable
14
azeotropic conditions.
Recently, however, several
benign synthetic protocols have been developed for the
synthesis of imines including the use of a water sus-
15
pension, several solid state procedures including clay
catalysed synthesis using microwave irradiation and
solid state synthesis involving ultra-high intensity
16
17
grinding. The imines in this study, listed in Table 2,
were produced by grinding/mixing together various
aldehydes with aniline in a mortar and pestle in the
presence of a catalytic amount of toluene-4-sulfonic
acid, Scheme 2.
Table 3. Synthesis of homoallylic secondary amines using gallium
metal
Imine
Yield of homoallylic
secondary amine (%)
a
PhC(H)@NPh
94
87
72
4-(MeO)–PhC(H)@NPh
2-(MeO)–PhC(H)@NPh
2-(OH)–PhC(H)@NPh
Residual acid was removed by washing quickly with a
small amount of water and the remaining solid product
recrystallised from MeOH affording the desired imines
in high yields, ca. 90–100%, Table 2. The reaction times
of 10–30 min are on par with those previously reported,
with only a slight reduction in yields observed for those
b
<5
PhC(H)@N–2,6-(Me)–Ph
PhCH@NCH CH Ph
(Ph) C@NH
PhC(H)@C(H)C(H)@NPh
44
32
2
2
b
2
<10
0
15–17
a
1
imines derived from cinnamylaldehyde.
Although
All products gave satisfactory IR, H NMR and EIMS as compared
to previously reported data.
the water suspension method required no acidic catalyst
to produce high yields, the procedure is obviously
b
1
Estimated by H NMR and GC–MS.
Table 2. Synthesis of imines from amines using solventless protocols
a
Aniline
Aldehyde
Imine
Yield (%)
PhNH
PhNH
PhNH
PhNH
PhNH
2
2
2
2
2
PhCH@OPhC(H)
@NPh
99
97
94
96
89
54
46
2 -H( O) –HP )h– CP (h HC )H @O2-(O
2-(MeO )) –– PP hh CC (HH @) O2-(MeO
4-(MeO )) –– PP hh CC (HH @) O4-(MeO
PhCH@CHCH@OPhC(H)
PhCH@OPhCH
@NPh
@NPh
@NPh
@C(H)C(H)@NPh
@NCH CH Ph
@N–2,6-(Me)–Ph
PhCH
2
CH
2
NH
2
2
2
2,6-(Me)–PhNH
2
PhCH@OPhCH
a
1
All products gave satisfactory IR, H NMR and EIMS as compared to previously reported data.
R
O
HO3S
Me (ca. 5%)
Ph
N
C
+
PhNH2
R'
R
R'
R
i) Ga/
ii) H2O
12 h
Br
Ph
N
C
+
HN
C
R
Ph
R'
R'
R, R', = alkyl, aryl or H
Scheme 2. A two-step solvent-free route to homoallylic amines.

246
P. C. Andrews et al. / Tetrahedron Letters 45 (2004) 243–248
Table 4. Estimated yield of formation of bis-allylated (2) and mono-allylated (1) species under various conditions using indium metal as the mediator
for the allylation of the imine N-benzylidene aniline
a
a
Entry
Method
Homoallylic alcohol
Homoallylic secondary
a
amine (1)
Bis-allylated amine (2)
1
2
3
4
1 equiv of allyl bromide
1.5 equiv of allyl bromide
No reaction, recovery of starting material
31%
15%
10%
36%
22%
89%
32%
63%
0%
3 equiv of allyl bromide
Generation of allyl indium species before quench
a
1
Estimated by H NMR and GC–MS.
with the results of similar reactions with indium metal
19–21
carried out in THF, DMF and alcoholic solvents.
R
Br
C
N
Ph
+
Furthermore, in comparison to other organometallic
allylation reactions there is no requirement for pre-
synthesis of the allyl metallic species. The imines pre-
pared from aromatic carbonyl compounds formed in the
highest yields, while the a,b-unsaturated imine, unfor-
tunately, produced only the homoallylic alcohol, pre-
sumably from allylation of the aldehyde derived from
the gallium salt catalysed hydrolysis of the imine. For
comparison, yields were substantially reduced for ben-
zophenone imine (ca. <10%) again reflecting deactiva-
tion of the C@N bond. Similar to the –OH substituted
aldehydes, imines derived from hydroxy substituted
aldehydes did not undergo any 1,2-addition reactions.
R'
M/
R
MBr
R
C
N
Ph
R'
Br-
C
N Ph
R'
MBr
MBr
N
C
5
. Gallium versus indium
R
Ph
R'
In an attempt to expand previous solvent-free reaction
protocols for the allylation of carbonyl compounds
mediated by indium metal, we investigated the solvent-
free approach to the synthesis of allylamines. In these
reactions, a mixture of the imine and indium powder,
and 1.5 equiv of allyl bromide was sonicated for 12 h
before quenching with water (ca. 0.5 mL). In compari-
son to the equivalent reactions mediated by gallium
metal, indium metal produced surprising results. A
mixture of products resulted, with the mono-allylated
species, 1, and the bis-allylated (allylphenyl-(1-phenyl-
but-3-enyl)amine), species, 2, being detected and iso-
lated using flash chromatography. Furthermore,
according to GC–MS data the two products were each
produced in approximately 40% yield, Scheme 3 and
Table 4. The homoallylic alcohol is derived from the
hydrolysis of any remaining imine and the subsequent
reaction of the reformed aldehyde with the residual allyl
indium halide species, which itself is relatively stable in
an aqueous environment.
H+
C
N
R
Ph
R'
H
N
C
R
Ph
R'
2
1
22
Scheme 3. Formation of bis-allylated amine in a solventless protocol
with indium as the mediator.
allowing the allylic indium halide species to form before
the addition of the imine (Table 4, entry 4).
The bis-allylated product, 2, presumably arises from the
formation of an iminium salt, which subsequently
undergoes competitive nucleophilic attack from the allyl
indium halide species, Scheme 3. This has been previ-
ously postulated as a possible route in similar allylation
reactions, and is further supported by the increase in
yield, ca. 60%, of 2 when a large excess of allyl bromide is
present (ca. 3 equiv), which encourages formation of the
iminium salt. Given that iminium salts are observed
when reacting alkyl halides and enamines under refluxing
In an attempt to prevent the bis-allylated species, 2,
being formed, and to elucidate the mechanism of its
formation, the procedure was modified whereby only
2
3
1
equiv of the bromide was added initially (Table 4,
entry 1). However, this resulted only in recovery of
starting material, presumably due to the lack of allyl
indium halide formation. Formation of bis-allylated 2
was entirely suppressed by addition of allyl bromide to
indium metal prior to any addition of amine, thus
2
4
conditions, we attempted to form deliberately the

P. C. Andrews et al. / Tetrahedron Letters 45 (2004) 243–248
247
iminium salt via the addition of 1.5 equiv of MeI to
N-benzylidene aniline at 50 ꢁC in toluene for 24 h under
an inert atmosphere. This resulted in the formation of the
corresponding iminium salt in a 67% yield. The possi-
bility of the bis-allylated species arising from the reaction
of unreacted allyl bromide and 1 after hydrolysis was
discounted by the in vacuo removal of any excess allyl
bromide prior to the aqueous quench, which still resulted
in significant yields of the bis-allylated product.
under an atmosphere of argon. This was allowed to
sonicate for 12 h, which resulted in the complete con-
sumption of gallium metal and the formation of a dark
2
yellow oil. The reaction was then quenched with H O
(ca. 0.5 mL) and allowed to sonicate for a further
15 min. The reaction mixture was extracted with diethyl
ether (3 · 15 mL), with the combined organic layers
4
being dried over MgSO . This was then purified by
Kugelrohr distillation under high vacuum to afford the
desired homoallylic secondary amine as a clear yellow
oil. The spectral data of this compound was consistent
Why is no bis-allylated product formed when gallium
metal is used? This is still an intriguing question and one
we are still attempting to understand. One possibility is
that the allyl gallium halide species forms at a much
faster rate than the iminium salt thus producing the
homoallylic secondary amine. This would imply that
although the two metals have similar first ionisation
potentials, insertion of gallium into a carbon halogen
bond in forming the reactive allyl metal halide species,
and its subsequent reaction with the unsaturated bond,
is significantly more facile than the corresponding pro-
cess for indium. Another possible process could be that
gallium in some way deactivates the N towards allyla-
tion through complexation or that the allyl bromide is
bound up by the organogallium species in a way that
does not occur with indium. The similarity in the
chemical behaviours of the two metals makes these
explanations, at first sight, counter-intuitive and so is
the target of further investigation.
26
with data that had been previously reported.
6.1.3. Synthesis of allylphenyl-(1-phenyl-but-3-enyl)-
amine. To a mixture of 0.32 g of indium metal
(2.7 mmol) and 0.50 g of benzylidene aniline (2.7 mmol),
1.5 equiv of allyl bromide (4.5 mmol) was added drop-
wise under an atmosphere of argon. This was allowed to
sonicate for 12 h, which resulted in the complete con-
sumption of indium metal and the formation of a dark
yellow oil. The reaction was then quenched with H O
2
(ca. 0.5 mL) and allowed to sonicate for a further
15 min. The reaction mixture was extracted with diethyl
ether (3 · 15 mL), with the combined organic layers
being dried over MgSO . This was then purified by flash
4
chromatography on silica, using 10:1 hexane–ethyl ace-
tate on silica gel, to give the bis-allylated product as a
clear yellow oil in 28% yield. Spectral data of this
compound was in agreement with data that had been
27
previously reported for this compound.
6. Experimental
Reactions employing sonication were performed in the
Transtek Systems, Soniclean 80T model. This sonicator
runs at 240 V with and operating frequency of 50–60 Hz.
GC–MS data were obtained on the Aligent 6890 Series
GC Systems and the Aligent 5973 Network Mass
Selective Detector. Aliquots of the samples (1 lL) were
injected, with inlets having a split ratio of 25:1. Helium
gas was employed at a pressure set at 7.16 psi and flow
rate 26.6 mL/min. The installed column was the HP-
5MS 5% phenyl methyl siloxane with a capillary size of
30.0 m · 250 lm · 0.25 lm. The oven setpoint was 60 ꢁC
(held for 3 min) increasing at a rate of 10 ꢁC/min to the
6
6
.1. Typical procedures
.1.1. Synthesis of 1-phenyl-3-buten-1-ol. In a Schlenk
flask under a nitrogen atmosphere, a suspension of
mmol (0.35 g) of gallium metal and 5 mmol (0.53 g) of
5
benzaldehyde, was allowed to stir rapidly. To this sus-
pension 7.5 mmol (0.91 g) of allyl bromide was added
and allowed to react via sonication for 12 h, resulting in
the production of a brown oil and the complete con-
sumption of the gallium metal. This was then quenched
with 0.5 mL of H O. The reaction mixture was then
2
extracted with 3 · 10 mL of diethyl ether, with the
ethereal layers being combined and dried over MgSO .
1
endpoint of 280 ꢁC. H NMR spectra of compounds
3
were recorded in CDCl on the Varian Mercury 300 at
4
The solution was then filtered and reduced under
reduced pressure, to give a colourless oil. The homoal-
lylic alcohol was isolated by flash chromatography on
silica gel using a 10:1 ratio of hexane to ethyl acetate in
400 MHz. Infra-red spectra were obtained on a Perkin–
Elmer 1600 FTIR.
Acknowledgements
97% yield. Spectral data of this compound was in
accordance with that previously reported.
25
We thank the Centre for Green Chemistry at Monash
University and the Australian Research Council for
their financial support.
6
.1.2. Synthesis of phenyl-(1-phenyl-but-3-enyl)-amine.
To a mixture of 0.11 g of gallium metal (1.6 mmol) and
.28 g of benzylidene aniline (1.6 mmol), 1.5 equiv of
0
allyl bromide (0.29 g/2.4 mmol) was added dropwise
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3
0 ꢁC, 300 MHz) d 8.38 (1H, s, CH), 7.18 (10H, m, aromatic H), 0.45
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(3H, s, –CH ).
3

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