[Bmim] [Br] as a solvent and activator for the ga-mediated barbier allylation: Direct formation of an N -heterocyclic carbene from Ga metal

Article abstract of DOI:10.1021/jo3020775

The room-temperature ionic liquid (RTIL) [bmim][Br] has been found to be an excellent green and inexpensive medium for the Ga-mediated allylation of aromatic and aliphatic aldehydes and ketones. The RTIL activated the metal via formation of a Ga-N-heteroc

Full text of DOI:10.1021/jo3020775

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Article
[bmim][Br], as a Solvent and Activator for the Ga-mediated Barbier
Allylation: Direct Formation of a N-Heterocyclic Carbene from Ga-metal
Dibakar Goswami, Angshuman Chattopadhyay, Anubha Sharma, and Subrata Chattopadhyay
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo3020775 • Publication Date (Web): 30 Nov 2012
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[bmim][Br], as a Solvent and Activator for the Ga-mediated Barbier Allylation: Direct
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Formation of a N-Heterocyclic Carbene from Ga-metal
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Dibakar Goswami, Angshuman Chattopadhyay, Anubha Sharma and Subrata Chattopadhyay*
BioꢀOrganic Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Fax: +91 22 25505151; eꢀmail: schatt@barc.gov.in
ABSTRACT: The room temperature ionic liquid (RTIL) [bmim][Br] has been found to be an
excellent green and inexpensive medium for the Gaꢀmediated allylation of aromatic and aliphatic
aldehydes and ketones. The RTIL activated the metal via formation of a GaꢀNꢀheterocyclic
carbene complex that assisted to carry out the reaction at an ambient temperature, and with only
0.5 equiv. of Ga and 1.2 equiv. of allyl bromide with respect to the carbonyl substrates. The
present protocol required much shorter time than those reported in literature using other metals
and solvents, and proceeded with good yields and excellent selectivity.
INTRODUCTION
Allylation of carbonyl compounds is a key carbonꢀcarbon bond forming reaction in
organic synthesis, and various protocols have been developed both in organic and aqueous
media.1 The Barbier type protocols involving in situ reaction of allylic halides with different
metals are more convenient for this, since the reactive metalꢀallyl complexes are produced in
organic, aqueous and room temperature ionic liquid (RTIL) media. More recently, indium has
become the metal of choice for this reaction, and is being used extensively.2 In contrast, reports
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on the galliumꢀmediated allylation are limited, despite advantages such as low first ionization
potential (5.99 eV), nonꢀtoxicity, ease of handling (as a liquid at room temperature), and less air
and moistureꢀsensitivity of Ga. The earlier reports with Gaꢀmediated Barbier type reactions are
primarily restricted to reactions in water and/ or organic solvents,3 but not in RTIL. In addition,
use of preformed allylꢀGa dihalides and allyl transfer reaction are also reported.4aꢀc
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The key issue for this type of transformations is acceleration of the electron transfer from
the metals to the allylic halides. This is generally accomplished by Rieke’s activation method,5a
the metalꢀgraphite method5b as well as addition of a catalytic amount of a second metal to the
target metal.5cꢀh The problem is severe with Ga requiring its activation by heating when carried
out in THF,3a,g or sonication3f under solvent free conditions even in the presence of additional
chemical activators. Currently, the RTILs have emerged as good, ecoꢀfriendly alternative
reusable solvents for a wide range of organic transformations. These have excellent stability to
air, moisture and heat, possess very low vapour pressure, and can dissolve various organic and
inorganic materials.6 Also, they can be prepared easily, and their properties can be tailored by
changing the ionic components. In general, only the hydrophobic RTILs are used for the organic
reactions including the Barbierꢀtype protocols.7,8 However, the hydrophilic RTILs such as
[bmim][Br] appear better suited for the reaction. It was envisaged that the stronger oxidizing
power of [bmim][Br] might facilitate a dipolar interaction between Ga metal and the
imidazolium moiety of [bmim][Br], leading to the required metal activation. The standard
reduction potential of [bmim][Br] (0.641 V, measured by cyclic voltammetry using a standard
Calomel electrode as reference) was also suggestive of such an interaction. Hence we
hypothesized that a combination of Ga and [bmim][Br] might trigger the allylation reaction
without the need of any metal activation by chemical additives and/ or energy sources
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(thermal/ultrasonic). In a preliminary communication, we have reported that the Gaꢀmediated
allylation of (R)ꢀcyclohexylideneglyceraldehyde can be achieved in [bmim][Br] within ~4ꢀ5 h
with excellent diastereoselectivity.9 The present study was primarily aimed to assess the potential
of the allylation protocol with a large number of aliphatic/aromatic aldehydes and ketones and
establish the mechanism of the reaction. This has also led to a novel method for activation of
metallic Ga using [bmim][Br] as the solvent as well as a reacting partner, and gave some insights
on the mechanism of the Ga activation.
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RESULTS AND DISCUSSION
Our initial studies (Scheme 1, Table 1) carried out with benzaldehyde (1a) as the
substrate in various media and under different conditions (stoichiometry, presence or absence of
additives etc.) revealed [bmim][Br] as the most efficient medium for the Gaꢀmediated allylation.
The reaction carried out in H2O or THF even in the presence of activators proceeded
slowly giving the product 2a in 63% and 49% yields respectively (Table 1, entries 1ꢀ3), while
even under sonication in THF, 2a was obtained in 71ꢀ73% yield (Table 1, entries 4,5) in 14ꢀ16 h.
The reactions in H2O or THF required a large excess of Ga (5 equiv.) and allyl bromide (3
equiv.) as well as extended reaction time, while reduction of these parameters led to significantly
poorer results (data not shown).
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Table 1. Effect of solvents and additives on Gaꢀmediated allylation of 1aa
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4
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7
8
entry
allyl
bromide
(equiv.)
3.0
metal
solvent
Additive
time (h)
yield (%)
(equiv.)
of 2ab
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1
2
5.0
5.0
5.0
5.0
5.0
2.0
2.0
2.0
2.0
1.0
0.5
H2O
H2O
LiCl+KIc
16
24
24
16 d
14 d
20
20
20
4
63
47
49
73
71
64
62
64
82
84
84
3.0
ꢀꢀ
3
3.0
THF
LiCl+KIc
4
3.0
THF
ꢀꢀ
5
3.0
THF
LiCl+KIc
6
1.2
[bmim][BF4]
[bmim][PF6]
ꢀꢀ
ꢀꢀ
7
1.2
8
1.2
[bmim][BF4] [bmim]Br]c
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1.2
[bmim][Br]
[bmim][Br]
ꢀꢀ
ꢀꢀ
ꢀꢀ
10
11
1.2
4
1.2
[bmim][Br]
4
aThe reactions were carried out in 2 mmol scale. Yield of isolated product. 1.0 equiv. of
b
c
LiCl+KI or 20 mol% [bmim][Br]. dunder ultrasonic irradiation.
The commonly used RTILs, [bmim][BF4], and [bmim][PF6], even in the presence of
[bmim][Br] (20 mol%) as the activator furnished 2a in 62ꢀ64% yields after 20 h (Table 1, entries
6ꢀ8). However, allylation of 1a in [bmim][Br] was very fast (4 h) furnishing 2a in 82ꢀ84% yield,
without requiring any metal activator (Table 1, entries 9,10). Further, the reaction in [bmim][Br]
could be accomplished with only 1.2 equiv. of the bromide and 1 equiv. of Ga. Subsequently the
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allylation of 1a was achieved using only 0.5 equiv. of Ga to obtain 2a in a similar yield (Table 1,
entry 11). The results are significant, considering that with subꢀstoichiometric amounts of Sn,
allylation of 1a in [bmim][BF4] proceeded with very poor yield.8 To the best of our knowledge
similar attempt with Ga has not been reported so far in any RTIL.
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The scope of the subꢀstoichiometric protocol (0.5 equiv. Ga) was further explored with
several aromatic and aliphatic aldehydes 1bꢀk as well as ketones 1lꢀq (Scheme 1, Table 2). The
protocol was equally effective with both aromatic and aliphatic aldehydes. With the aromatic
aldehydes, the products 2bꢀh were obtained in good (72ꢀ87%, Table 2, entries 1ꢀ7) yields,
irrespective of the presence of any electron withdrawing or donating group, or a heteroatom in
the aromatic ring. However, no reaction was observed with 2ꢀhydroxyꢀ4ꢀethoxybenzaldehyde,
containing a free phenolic group (data not shown). This is consistent with a previous report
where alkyl gallium compounds were reported to react preferably with the acidic phenolic
moiety.10 Likewise, the aliphatic aldehydes also furnished the corresponding allylated products
2i-2k in high yields (Table 2, entries 8ꢀ10). The lower yield of 2i might be due to its volatility.
The protocol was also useful with aromatic or aliphatic ketones 1l-q furnishing the products 2l-q
in 67ꢀ81% yields (Table 2, entries 11ꢀ16) without any significant role of the steric and/ or
electronic factors. Earlier, the Zn, Bi, and Inꢀmediated allylation of acetophenone (1l) under a
solventꢀfree condition was largely unsuccessful,8a although the Inꢀmediated allylation of 1l and
1q was reported in water.8b Earlier, the Gaꢀmediated allylation of ketones could be achieved only
after metal activation.3a,f Thus, our protocol is more versatile and could be used even for the
bulky and electronically demanding substrates such as 1m-1p.
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Table 2. Ga mediated allylation of aldehydes and ketones in [bmim][Br]a
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8
Entry Substrate
R1
3ꢀOMeꢀC6H4
4ꢀ(CH3)2CHꢀC6H4
4ꢀC6H5ꢀ C6H4
4ꢀBrꢀC6H4
C6F5
R2
H
Product
2b
2c
Time (h) Yield (%)b
1
2
6
5
84
87
83
77
83
72
79
67
84
95
71
67
77
81
78
77c
1b
1c
1d
1e
1f
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H
3
H
5
2d
2e
4
H
6
5
H
5
2f
6
3ꢀIndolyl
H
12
8
1g
1h
1i
2g
7
C6H5CH=CH
(CH3) 2CH
CH3(CH2)5
CH3(CH2)8
C6H5
H
2h
2i
8
H
6
9
H
5
1j
2j
10
11
12
13
14
15
16
17
H
5
1k
1l
2k
2l
CH3
C6H5
C6H5
Ph
10
16
8
C6H5
1m
1n
1o
1p
1q
1a
2m
2n
2o
C6H5CO
C6H5CH=CH
Cyclohexanone
8
7
2p
2q
2r
2ꢀMethylcyclohexanone
C6H5
7
H
7
65d
aThe reactions were carried out in 2 mmol scale. Yields of the isolated products. Exclusively
b
c
d
transꢀproduct (H and OH trans) was obtained. The reaction was carried out with Eꢀ
PhCH=CHCH2Br to get antiꢀ2r as the sole product.
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Chemoꢀ and stereoselective allylation of multifunctional compounds is one of the most
fundamental goals in constructing complex molecules. Our new protocol of allylation proceeded
with complete chemoselectivity with the conjugated carbonyls, 1h and 1o, furnishing the
respective 1,2ꢀaddition products 2h and 2o only, while only the mono allylated product 2n was
obtained with the diketone 1n. Interestingly, allylation of 2ꢀmethylcyclohexanone (1q) under the
above conditions furnished transꢀ2q exclusively in excellent yield (Table 2, entry 16). Likewise,
reaction of 1a with cinnamyl bromide afforded the γꢀaddition product, antiꢀ2r only (Table 2,
entry 17). The products obtained from the allylation in [bmim][Br] could be conveniently
isolated by extraction with Et2O followed by concentration. The reactions proceeded smoothly
without any side reactions such as reduction and coupling, and furnished the products devoid of
side products and/ or starting materials. Only with the ketone 1m, the reaction was incomplete
allowing the recovery of 18% of the substrate. In general, the reaction yields were more than
those reported in other solvents.
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The above results clearly established [bmim][Br] as the best RTIL among those chosen
for the Gaꢀmediated allylation. The reactions were much faster in [bmim][Br] even without any
metal activator, and could be executed with only a subꢀstoicheometric quantity of Ga, providing
economic and environmental advantages. In contrast, excess reagents and toxic metal activators
such as acid or fluoride are required in H2O. Finally, unlike in H2O or THF, the reaction in
[bmim][Br] takes place at an ambient temperature, which is conducive while using more volatile
reagents such as allyl bromide.
Mechanistic studies: For the mechanistic insight of the activating role of [bmim][Br], we probed
the nature of the organometallic species responsible for the reaction. For this, the course of the
reaction between Ga (1 mmol) and allyl bromide (1 mmol) in [bmim][Br] (2 mL) was
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periodically followed up to 8 h, by H NMR spectra. The intensity of the NMR peak due to the
CH2ꢀGa protons in the allylꢀgallium species was quantified by comparing with that of the CH2Br
signal of allyl bromide.
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The H NMR spectra of the reaction mixtures obtained in [bmim][Br] showed a new
doublet at δ 1.63 (J = 6.4 Hz) with simultaneous reduction of the signal at δ 3.9. In addition, new
olefinic multiplets at δ 4.59 at the expense of that at δ 4.78 (of allyl bromide) also emerged, and
1
these resonances together accounted for two protons over the entire period of studies. The H
NMR doublets for the CH2ꢀGa protons suggested5h (CH2CH=CH2)2GaBr (I) as the active allylꢀ
Ga species, which was confirmed by its isolation, followed by spectral (1H and C NMR), and
13
chemical analyses. An analogous compound, (CH2CH=CH2)2GaCl, synthesized following a
reported procedure,11 also provided a similar 1H NMR spectrum. The sesquibromide,
(CH2CH=CH2)3GaBr3, produced5h in THF, was not formed12 in [bmim][Br]. The integration of
the NMR signals for I reached a maximum (~48%) in ~ 3.5 h, and remained steady even up to 8
h. Addition of 1a to the reaction mixture led to complete depletion of the signals, confirming I as
the active organometallic species. The composition of I as (CH2CH=CH2)2GaBr also explains
the need of only 0.5 equiv. of Ga for the reaction.
With regard to the activation of Ga metal, we envisaged a dipole induced dipole
1
interaction with [bmim][Br]. After incubating for 10 min only, the H NMR spectrum of the
reaction mixture of [bmim][Br] (2 mL) and Ga (1.0 mmol) showed upfield shifts of the
imidazole Hꢀ2 (3.8 Hz), and Hꢀ4 and Hꢀ5 (each 3.4 Hz) protons. Similar trends were also seen
for the imidazole carbons, where an upfield shift of 10 Hz for Cꢀ2 and 6 Hz for both Cꢀ4 and Cꢀ5
were noticed. The shifts were more prominent at 30 min. These results suggested partial
polarization of the Ga metal electrons towards the imidazolium core of [bmim][Br] which would
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activate the Gaꢀmetal. The shift followed the expected trend, considering maximum positive
charge density at Cꢀ2 of the imidazole ring. The NMR shifts of similar magnitudes are generally
attributed to the charge transfer phenomenon.13 Possibly the combination of Ga and [bmim][Br]
initially forms ion pairs or more complex ionꢀaggregates with an activated Ga. The results are
consistent with the respective redox potentials of [bmim][Br] and Gaꢀmetal. The metalꢀcatalyzed
Barbier reaction is proposed to be mediated through radicals on metal surface as well as metal
surface activation especially in aqueous acidic media.14 Earlier surface reaction with In metal in
H2O has been reported.15 The occurrence of a charge transfer in RTILs is not unprecedented.16a,b
Especially the imidazoliumꢀbased ILs have remarkable ability to promote electron transfer
reactions.16c However, the direct involvement of a metal in this process is not reported so far.
On continued stirring for 1ꢀ1.5 h, the mixture of [bmim][Br] and Ga eventually produced
a gray solid that can be purified by vacuum sublimation (bath temperature 40ꢀ45 oC/0.1 torr). Its
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1
IR spectrum (thin film) displayed a strong –OꢀH stretching band (3545 cmꢀ1), while a H NMR
13
singlet at δ 7.21 (2H) and the C NMR resonances at δ 123.3 and δ 121.7 (olefin) along with δ
175.9 (carbene) revealed the presence of a Nꢀheterocyclic carbene (NHC) moiety. The Raman
spectrum confirmed the GaꢀC (697 cmꢀ1) and GaꢀO (330 and 662 cmꢀ1) bonds, while excluding
any GaꢀGa and GaꢀOꢀGa bonds.17aꢀc The electron impact mass spectrum showed a [M+ꢀ57] ion
peak at m/z 262 (17%), a [M+ꢀBr] ion peak at m/z 240 (11%), and a major fragmentation peak at
m/z 184 (18%) (GaBr(OH)2) with appropriate isotopic patterns. In addition, the fragmentation
peak at m/z 138 (19%) accounted for the NHC moiety. The studies on the thermal stability of the
o
solid by differential scanning calorimetry (DSC) showed a gradual mass loss starting at ~120 C
o
accompanied by a large endothermic thermal transition at ~162 C producing Ga metal. The
broad DSC profile indicated the powder as a Nꢀheterocyclic carbene (NHC)ꢀGa complex. Based
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on these, the intermediate was assigned the structure [NHCꢀGaBr(OH)2] (II) (Scheme 2). The
NMR data was also clean without showing presence of any isomers, consistent with the proposed
monocarbene structure of the complex. The XRD analysis of II was inconclusive except for
showing its amorphous nature. Despite our intense efforts, we failed to isolate it in crystalline
form. However, the spectral, elemental and thermal data of the solid established its proposed
structure unambiguously.
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The probable reaction sequence involved in the process of Gaꢀactivation and the
allylation reaction is shown in Scheme 2. Possibly, an electron transfer from Ga to O2, present in
the RTIL18 occurs and results in the formation of Ga+ and O2 , which is stabilized by the C2ꢀ
ꢀ.
hydrogen of [bmim].19 The electron transfer is “facilitated” by an interaction between Ga and
ꢀ.
[bmim]. Subsequent transfer of the C2ꢀproton from [bmim.] to [O2 ] would then give the
.
.
[bmim]ꢀNHC and HO2 . Another such cycle with [HO2 ] can also be envisaged to produce more
[bmim]ꢀNHC and H2O. In view of its good σꢀdonating ability, the generated NHC would
stabilize the Ga (I) species. Further, the poor stability of the sterically less hindered cyclic
carbene and Ga(I) would eventually furnish the species II by oxidation. This type of reaction is
well established in MꢀNHC chemistry, but the mechanism remains unclear.20 To support the
hypothesis, we incubated Ga in deaerated [bmim][Br], where the formation of II was not
observed. As expected, purging the same reaction mixture with O2 produced the same Gaꢀ
intermediate II, which is consistent with the proposed mechanism.
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H
H
H
H
H
Br-
-
+δ
(i)
4
Ga
+
+
4
4
N
N
Br
N
N
N
N
C4H9
+δ
C4H9
H
H
Ga
Transient
H
H
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H
H
H
-
.
2
- 2 GaBr
-
(ii)
+δ
[HO
+ 2
]
2
2 O
2
Br +
2
2
N
N
+
N
N
C4H9
+δ
C4H9
C4H9
..
H
Ga
H
. -
O2
H
H
H
H
H
H
-
.
2
-
- 2 GaBr
+δ
[HO
]
2
N
N
2
Br +
2 H2O + O2
+
2
2
N
N
+
C4H9
+δ
N
N
C4H9
C4H9
..
H
(iii)
Ga
H
-
HO2
H
H
H
H
Br-
[OVERALL REACTION]
+ 2 H2O
4 GaBr
+
4
Ga +
4
+
O2
+
4
N
N
N
N
C4H9
C4H9
..
H
" In-situ formed NHC"
[bmim][Br]
C4H9
H
OH
Ga Br
OH
N
N
H
II
O
Ga
OH
R1
R2
R1
R2
GaBr
Br
2
II
H2O
I
Scheme 2
Several authors have suggested a link for the fast chemical conversion of imidazolium
ion to the NHC on the surface of a nanoparticle.21 Our hypothesis of direct adduct formation of a
NHC with Ga metal is consistent with this. This also corroborates with the results of theoretical
DFT calculation wherein a similar oxidative addition of the imidazolium salts to Pt(0) has been
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shown to be exothermic.22 In this study, we have also carried out the experiments with
[bmim][Br] (2 mL) and incremental amounts of Ga (up to 5.0 mmol). However, the gray solid II
was obtained in 3ꢀ4% yields only. This suggested that compound II makes a coating on the Gaꢀ
metal, preventing its further reaction with [bmim][Br]. Addition of allyl bromide removes the
coating of II making the activated Gaꢀmetal available to continue the reaction.
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Since their description by Wanzlick23 and Arduengo,24 stable NHCs have been a
significant area of study and several crystalline and wellꢀcharacterized NHC complexes with
main group and especially transition metals have been synthesized.25 The NHCs can stabilize
thermally labile or low oxidation state metal fragments, and the complexes can be used for
various organic transformations. The use of NHCs as good σꢀdonor molecules to stabilize
trivalent group 13 compounds is well established.25b, 26 Over the past few years, the chemistry of
metastable gallium complexes has been an active research area in organometallic chemistry. A
large number of NHCꢀcomplexes with metal (Al, In, and Ga) hydrides and halides have since
been synthesized.27 However, formation of GaꢀNHC directly from the metal is novel and opens
various new possibilities in organic synthesis as well as materials development. Sterically nonꢀ
hindered NHCs are often sensitive to air and moisture, making their isolation and use
difficult.24a,28 However, in our case its formation was achieved in a hygroscopic RTIL, and also
required O2.
Earlier, the Pdꢀcarbene complexes have been shown to catalyze the Heck and related
CꢁC bond forming reactions.29 We have also isolated the Gaꢀcarbene complex II and reacted
with allyl bromide to synthesize the active allylating Ga species I. The exact mechanism of the
catalysis is at present, far from clear. It may in fact turn out to follow a much more complex
pathway. The GaꢀNHC complex might be the precursor, which would generate a zeroꢀvalent Gaꢀ
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NHC species as the likely active catalyst, as suggested for the Pdꢀcatalyzed Heck reaction in
[bmim][Br].
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CONCLUSION
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Overall, a novel method for Ga metal mediated allylation of carbonyls in [bmim][Br] has
been accomplished. To the best of our knowledge, this is the first example of a Gaꢀmediated
allylation of carbonyls in any RTIL. The imidazolium cation is usually considered as a simple
inert component of a solvent system, and its possible involvement in a catalytic cycle, especially
with a free a metal is rarely an issue. We have found that [bmim][Br] reacts with Ga to give the
GaꢀNHC complex (II). This also provides a method of baseꢀfree generation of NHCs through
aerobic oxidation of a free metal, although a similar protocol has been reported with transition
metal complexes.15c This is very interesting since strong bases can cause various side reactions.
We expect the present results to open up many applications towards synthesizing stable NHCs in
RTILs and applying those as versatile heterogeneous catalysts in many organic reactions.
EXPERIMENTAL SECTION
Typical procedure for the Ga-mediated allylation reaction in [bmim][Br]. A mixture of Ga
and allyl bromide (quantities specified in Tables) in [bmim][Br] (2 mL/mmol) was stirred at
room temperature for 1 h, followed by addition of the aldehyde. The reaction mixture was stirred
at room temperature for the time specified in the Tables 1 and 2. After completion of the reaction
(cf. TLC), the mixture was extracted with Et2O (3 × 10 mL), the ether extract evaporated in
vacuo and the residue purified by column chromatography (silica gel, hexane/EtOAc) to give the
respective products. All the products were fully characterized by IR, 1H and 13C NMR spectra as
well as elemental analysis. The Gaꢀmediated allylation was also carried out in H2O and THF
under the conditions specified in Table 1.
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NMR experiments. A mixture of the Ga (1.0 mmol) and allyl bromide (1.0 mmol) in
[bmim][Br] (2 mL) was magnetically stirred at room temperature. Aliquots (35 ꢀL) of reaction
mixture were taken at different time intervals, and the 1H NMR spectra were recorded in CDCl3
or D2O.
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1-Phenyl-but-3-en-1-ol 2a.8 yield: 0.249 g (84%); colourless liquid; IR (flim): 3468, 922 cmꢀ1;
1H NMR (200 MHz, CDCl3): δ 1.92 (broad s, 1H), 2.45ꢀ2.56 (m, 2H), 4.74 (t, J = 6.8 Hz, 1H),
13
5.10ꢀ5.25 (m, 2H), 5.65ꢀ5.93 (m, 1H), 7.26ꢀ7.40 (m, 5H); C NMR (50 MHz, CDCl3): δ 43.8,
73.2, 118.4, 125.8, 127.5, 128.4, 134.4, 143.8.
1-(3-Methoxyphenyl)-but-3-en-1-ol 2b.30a yield: 0.296 g (84%); colourless liquid; IR (flim):
1
3418, 916 cmꢀ1; H NMR (200 MHz, CDCl3): δ 2.18 (broad s, 1H), 2.49ꢀ2.53 (m, 2H), 3.80 (s,
3H), 4.69 (t, J = 6.2 Hz, 1H), 5.10ꢀ5.19 (m, 2H), 5.70ꢀ5.91 (m, 1H), 6.79ꢀ6.91 (m, 3H), 7.21ꢀ7.29
(m, 1H); 13C NMR (50 MHz, CDCl3): δ 43.7, 55.2, 73.2, 111.3, 112.9, 118.1, 118.3, 129.4,
134.4, 145.6, 159.7.
1-(4-Isopropylphenyl)-but-3-en-1-ol 2c.30b yield: 0.331 g (87%); colourless liquid; IR (flim):
3388, 915 cmꢀ1; 1H NMR (200 MHz, CDCl3): δ 1.26 (d, J = 6.8 Hz, 6H), 2.08 (broad s, 1H), 2.51
(t, J = 6.6 Hz, 2H), 2.88ꢀ2.94 (m, 1H), 4.70 (t, J = 6.6 Hz, 1H), 5.12ꢀ5.21 (m, 2H), 5.76ꢀ5.84 (m,
1H), 7.26 (q, J = 8.0 Hz, 4H); 13C NMR (50 MHz, CDCl3): δ 24.0, 33.8, 43.7, 73.2, 112.0, 118.1,
125.8, 126.4, 134.7, 141.3, 148.2.
1-(4-Phenylphenyl)-but-3-en-1-ol 2d.30c yield: 0.372 g (83%); colourless liquid; IR (flim):
3583, 911 cmꢀ1; 1H NMR (200 MHz, CDCl3): δ 2.18 (broad s, 1H), 2.53ꢀ2.59 (m, 2H), 4.78 (t, J
13
= 6.6 Hz, 1H), 5.15ꢀ5.24 (m, 2H), 5.75ꢀ5.95 (m, 1H), 7.24ꢀ7.41 (m, 5H), 7.45ꢀ7.61 (m, 4H); C
NMR (50 MHz, CDCl3): δ 43.8, 73.0, 118.5, 126.3, 127.0, 127.1, 127.3, 128.8, 134.4, 140.4,
140.8, 142.9.
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1-(4-Bromophenyl)-but-3-en-1-ol 2e.30d yield: 0.350 g (77%); colourless liquid; IR (flim):
3389, 910 cmꢀ1; 1H NMR (200 MHz, CDCl3): δ 2.14 (broad s, 1H), 2.34ꢀ2.53 (m, 2H), 4.67 (t, J
= 6.6 Hz, 1H), 5.10ꢀ5.17 (m, 2H), 5.65ꢀ5.86 (m, 1H), 7.20 (d, J = 8.0 Hz, 2H), 7.45 (d, J = 8.2
Hz, 2H); 13C NMR (50 MHz, CDCl3): δ 43.8, 72.5, 118.8, 121.2, 127.5, 131.4, 133.9, 142.8.
1-Pentafluorophenyl-but-3-en-1-ol 2f.30e yield: 0.395 g (83%); colourless liquid; IR (flim):
3408, 926 cmꢀ1; 1H NMR (200 MHz, CDCl3): δ 2.50ꢀ2.82 (m, 3H), 5.05ꢀ5.17 (m, 3H), 5.62ꢀ5.79
(m, 1H); 13C NMR (50 MHz, CDCl3): δ 41.1, 65.6, 116.3, 119.4, 132.4, 137.5, 140.5, 144.7.
1-(3-Indolyl)-but-3-en-1-ol 2g. yield: 0.269 g (72%); colourless liquid, IR (flim): 3458, 910 cmꢀ
1; 1H NMR (200 MHz, CDCl3): δ 2.08 (broad s, 1H), 2.36ꢀ2.49 (m, 1H), 2.81ꢀ2.97 (m, 1H), 4.05ꢀ
4.15 (m, 1H), 5.04ꢀ5.15 (m, 2H), 5.82ꢀ5.96 (m, 1H), 6.95ꢀ7.06 (m, 1H), 7.08ꢀ7.16 (m, 2H), 7.36ꢀ
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7.40 (m, 1H), 7.57ꢀ7.63 (m, 1H), 7.89ꢀ7.94 (m, 1H); C NMR (50 MHz, CDCl3): δ 41.3, 60.3,
110.9, 114.5, 116.2, 118.8, 119.0, 121.0, 121.8, 138.7. Anal. Calcd. for C12H13NO: C, 76.98; H,
7.00; N 7.48%. Found: C, 76.81; H, 6.92; N, 7.34%.
(E)-1-Phenylhexa-1,5-dien-3-ol 2h.3g yield: 0.275 g (79%); colourless liquid; IR (flim): 3425,
1
916 cmꢀ1; H NMR (200 MHz, CDCl3): δ 1.84 (broad s, 1H), 2.33ꢀ2.42 (m, 2H), 4.31ꢀ4.41 (m,
1H), 5.13ꢀ5.22 (m, 2H), 5.75ꢀ5.93 (m, 1H), 6.18ꢀ6.29 (dd, J = 6.2 Hz and 16.0 Hz, 1H), 6.61 (d,
J = 16.0 Hz, 1H), 7.23ꢀ7.40 (m, 5H); 13C NMR (50 MHz, CDCl3): δ 41.8, 71.6, 118.3, 126.3,
127.5, 128.4, 130.2, 131.4, 133.9, 136.5.
2-Methylhex-5-en-3-ol 2i.30f yield: 0.153 g (67%); colourless liquid; IR (flim): 3419, 908 cmꢀ1;
1H NMR (200 MHz, CDCl3): δ 0.90 (d, J = 5.4 Hz, 6H), 1.61ꢀ1.70 (m, 1H), 2.01ꢀ2.16 (m, 2H),
2.22 (broad s, 1H), 3.31ꢀ3.38 (m, 1H), 5.06ꢀ5.14 (m, 2H), 5.71ꢀ5.92 (m, 1H); 13C NMR (50
MHz, CDCl3): δ 17.5, 18.7, 33.0, 38.8, 75.3, 117.7, 135.4.
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1-Decen-4-ol 2j.30g yield: 0.262 g (84%); colourless liquid; IR (flim): 3408, 912 cmꢀ1; 1H NMR
(200 MHz, CDCl3): δ 0.85 (t, J = 6.4 Hz, 3H), 1.22ꢀ1.60 (m, 10H), 1.92 (broad s, 1H), 2.02ꢀ2.29
(m, 2H), 3.52ꢀ3.65 (m, 1H), 4.98ꢀ5.17 (m, 2H), 5.65ꢀ5.93 (m, 1H); 13C NMR (50 MHz, CDCl3):
δ 14.0, 22.6, 25.6, 29.3, 31.8, 36.8, 41.9, 70.6, 117.8, 134.9.
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1-Tridecen-4-ol 2k.30h yield: 0.376 g (95%); colourless liquid; IR (flim): 3462, 908 cmꢀ1; H
NMR (200 MHz, CDCl3): δ 0.91 (t, J = 6.4 Hz, 3H), 1.29ꢀ1.47 (m, 16H), 1.75 (broad s, 1H),
2.08ꢀ2.37 (m, 2H), 3.59ꢀ3.68 (m, 1H), 5.12ꢀ5.18 (m, 2H), 5.76ꢀ5.93 (m, 1H); 13C NMR (50
MHz, CDCl3): δ 14.0, 22.6, 25.6, 29.3, 29.6, 31.8, 36.7, 41.8, 70.6, 117.7, 134.9.
2-Phenylpent-4-en-2-ol 2l.3a yield: 0.230 g (71%); colourless liquid, IR (flim): 3499, 925 cmꢀ1;
1H NMR (200 MHz, CDCl3): δ 1.54 (s, 3H), 1.98 (broad s, 1H), 2.44ꢀ2.74 (m, 2H), 5.08ꢀ5.18 (m,
2H), 5.56ꢀ5.64 (m, 1H), 7.23ꢀ7.46 (m, 5H); 13C NMR (50 MHz, CDCl3): δ 29.8, 48.4, 73.6,
119.4, 124.7, 126.5, 128.1, 128.5, 133.6, 147.6.
1,1-Diphenylbut-3-en-1-ol 2m.30i yield: 0.300 g (67%); colourless liquid, IR (flim): 3434, 929
cmꢀ1; H NMR (200 MHz, CDCl3): δ 2.04 (broad s, 1H), 3.08 (d, J = 8 Hz, 2H), 5.15ꢀ5.28 (m,
1
2H), 5.56ꢀ5.73 (m, 1H), 7.21ꢀ7.35 (m, 7H), 7.43ꢀ7.47 (m, 3H); 13C NMR (50 MHz, CDCl3):
δ 46.5, 77.0, 120.3, 125.8, 126.7, 128.0, 133.2, 146.3.
2-Hydroxy-1,2-diphenylpent-4-en-1-one 2n.30j yield: 0.388 g (77%); colourless liquid, IR
(flim): 3478, 917 cmꢀ1; H NMR (200 MHz, CDCl3): δ 2.16 (broad s, 1H), 2.91ꢀ3.19 (m, 2H),
1
4.96ꢀ5.13 (m, 2H), 5.63ꢀ5.80 (m, 1H), 7.24ꢀ7.52 (m, 8H), 7.71ꢀ7.75 (m, 2H); 13C NMR (50
MHz, CDCl3): δ 43.7, 81.2, 111.9, 120.2, 125.4, 127.9, 128.7, 129.9, 132.1, 132.6, 134.3, 141.5,
200.6.
(E)-1,3-Diphenylhexa-1,5-dien-3-ol 2o. yield: 0.405 g (81%); colourless liquid; IR (flim):
3431, 911 cmꢀ1; H NMR (200 MHz, CDCl3): δ 2.17 (broad s, 1H), 2.82 (d, J = 6.8 Hz, 2H),
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5.18ꢀ5.27 (m, 2H), 5.64ꢀ5.85 (m, 1H), 6.50ꢀ6.72 (m, 2H),7.23ꢀ7.42 (m, 8H),7.52ꢀ7.56 (m, 2H);
13C NMR (50 MHz, CDCl3): δ 47.1, 75.7, 120.1, 125.0, 125.5, 126.4, 127.0, 127.3, 127.4,
127.8, 128.2, 128.3, 132.5, 135.2, 137.4, 145.3. Anal. Calcd. for C18H18O: C, 86.36; H, 7.25%.
Found: C, 86.60; H, 7.01%.
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1-Allylcyclohexanol 2p.30k yield: 0.218 g (78%); colourless liquid; IR (flim): 3451, 913 cmꢀ1;
1H NMR (200 MHz, CDCl3): δ 1.23 (broad s, 1H), 1.45ꢀ1.61 (m, 10H), 2.20 (d, J = 7.4 Hz, 2H),
5.04ꢀ5.14 (m, 2H), 5.77ꢀ5.98 (m, 1H); 13C NMR (50 MHz, CDCl3): δ 21.5, 25.5, 30.3, 36.3,
37.9, 45.2, 72.5, 117.6, 134.0.
1-Allyl-2-methylcyclohexanol 2q.30l yield: 0.237 g (77%); colourless liquid; IR (flim): 3484,
1
908 cmꢀ1; H NMR (200 MHz, CDCl3): δ 0.79 (d, J = 2.4 Hz, 3H), 1.16ꢀ1.50 (m, 9H), 1.70
(broad s, 1H), 2.14 (d, J = 7.4 Hz, 2H), 4.93ꢀ4.99 (m, 2H), 5.65ꢀ5.82 (m, 1H); 13C NMR (50
MHz, CDCl3): δ 14.6, 21.4, 25.4, 30.2, 35.8, 37.8, 45.1, 72.3, 117.5, 133.9.
1,2-Diphenylbut-3-en-1-ol 2r.30m yield: 0.291 g (65%); colourless liquid; IR (flim): 3429, 910
cmꢀ1; 1H NMR (200 MHz, CDCl3): δ 1.95 (broad s, 1H), 3.55 (t, J = 8.0 Hz, 1H), 4.84 (d, J = 7.8
Hz, 1H), 5.18ꢀ5.39 (m, 2H), 6.01ꢀ6.35 (m, 1H), 7.17ꢀ7.45 (m, 10H); 13C NMR (50 MHz,
CDCl3): δ 58.9, 77.0, 118.2, 126.1, 126.4, 126.5, 127.2, 127.5, 127.7, 128.2, 128.4, 137.7, 140.4,
141.7.
1
Diallylgallium bromide I. yield: 0.093 g (40%); colourless liquid; H NMR (200 MHz,
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CDCl3): δ 1.69 (d, J = 6.4 Hz, 4H), 4.91ꢀ5.06 (m, 4H), 5.71ꢀ5.91 (m, 2H); C NMR (50 MHz,
CDCl3): δ 33.1, 114.6, 138.1. EIMS: m/z (%) 152 [{M − Br}+, 11].
N-Butyl-N-methyl-imidazolinylidinegalliumdihydroxybromide II. yield: 0.013 g (4%); grey
o
amorphous solid; mp: >250 C; IR (KBr): 3545, 3466, 3229, 2853, 1616, 1187, 924, 637, 538,
479 cmꢀ1; 1H NMR (200 MHz, CDCl3): δ 0.99 (t, J = 7.4 Hz, 3H), 1.21ꢀ1.28 (m, 2H), 1.74 (broad
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s, 2H), 1.87ꢀ1.92 (m, 2H), 4.03 (s, 3H), 4.24 (t, J = 7.4 Hz, 2H), 7.21 (s, 2H); 13C NMR (50
MHz, CDCl3): δ 12.9, 18.9, 31.6, 36.2, 49.3, 121.7, 123.3, 175.9. EIMS: m/z (%) 264 [{M −
C4H9}+, 17], 241 [{M − Br}+, 11], 184 [{Ga(OH)2Br}+, 18], 138 [{C8H14N2}+, 19]; Raman: 697,
330, 662, 980, 1079, 1332, 1374 cmꢀ1. Anal. Calcd. for C8H16BrGaN2O2: C, 29.85; H, 5.01; N,
8.70%. Found: C, 29.64; H, 5.18; N, 8.61%.
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Supporting Information: 1H and 13C NMR spectra for all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
REFERENCES
1.
(a) Panek, J. S. In Comprehensive Organic Synthesis. Schreiber, S. L. Ed.; Pergamon:
Oxford, 1991, vol. 1, 595; (b) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207–2293;
(c) Marshall, J. A. Chem. Rev. 1996, 96, 31–48; (d) Organic Synthesis in Water. (Ed.
Grieco, P. A.), Blackie, London, 1998, and references therein; (e) Yamaguchi, M. In
Main Group Metals in Organic Synthesis. Yamamoto, H.; Oshima, K., Eds., WileꢀVCH:
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