The potential of methylsiloxanes as solvents for synthetic chemistry applications

Article abstract of DOI:10.1039/c3gc42036a

The potential use of volatile methylsiloxanes (VMSs) as solvents for chemicals synthesis has been explored. Assessment of the environmental impact of these VMS solvents is made and found to be significantly lower than those of the non-polar organic solvents that they have the potential to replace. The polarities of the VMSs, as expressed by empirical polarity measurements, and miscibilities with other liquids are found to be similar to those of alkane solvents. Finally, some uses of VMSs as solvents for both organic and inorganic transformations are described. The VMSs provide environmentally more sustainable (greener) alternatives to the nonpolar solvents that they have the potential to replace.

Full text of DOI:10.1039/c3gc42036a

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The potential of methylsiloxanes as solvents for
synthetic chemistry applications†
Cite this: DOI: 10.1039/c3gc42036a
Mohd Azri Ab Rani, Nadine Borduas, Victoria Colquhoun, Robert Hanley,
Henry Johnson, Solène Larger, Paul D. Lickiss,* Veronica Llopis-Mestre, Selina Luu,
Martin Mogstad, Philipp Oczipka, James R. Sherwood, Tom Welton* and Jun-Yi Xing
The potential use of volatile methylsiloxanes (VMSs) as solvents for chemicals synthesis has been explored.
Assessment of the environmental impact of these VMS solvents is made and found to be significantly
lower than those of the non-polar organic solvents that they have the potential to replace. The polarities
of the VMSs, as expressed by empirical polarity measurements, and miscibilities with other liquids are
found to be similar to those of alkane solvents. Finally, some uses of VMSs as solvents for both organic
and inorganic transformations are described. The VMSs provide environmentally more sustainable
(greener) alternatives to the nonpolar solvents that they have the potential to replace.
Received 30th September 2013,
Accepted 20th November 2013
DOI: 10.1039/c3gc42036a
www.rsc.org/greenchem
group substituents. They have been given specific exemption
from regulation as Volatile Organic Compounds (VOCs) in the
The sustainability of solvent use is being scrutinised by acade- USA and were approved to replace ozone-depleting sub-
1
Introduction
1
3
1
–3
mia and industry alike with an ever increasing intensity.
stances such as methylchloroform under the Significant New
1
4
Such a high profile within green chemistry research can be jus- Alternatives Policy, SNAP. In the corresponding European
tified by the volume of waste arising from industrial solvent directive on VOCs no such exemptions have been granted for
4
use. It has been estimated that solvents accounted for 35% of organic solvents with vapour pressures equal to or greater than
1
5
all the volatile organic compounds (VOCs) that were released 0.01 kPa at 293.15 K. Poly(dimethylsiloxanes), PDMS, have
5
into the atmosphere from the UK in 2002 and it is becoming negligible volatilities placing these safely outside of the
increasingly apparent that minimising the impact of solvent requirements for VOC status in Europe. Siloxanes are ubiqui-
1
6–22
use should be regarded amongst the highest of priorities tous, finding widespread use in a variety of applications,
within green chemistry. including antifoams, lubricants, adhesives, hydraulic fluids,
2
3
The development of new classes of solvent, and the develop- and for degreasing clothes and circuit boards, yet their
ment of synthetic chemistry in these has been a significant potential as solvents for use in synthetic chemistry has
6
area of research since green chemistry was first defined.
remained largely unexplored. There are, however, some scat-
Research utilising non-polar neoteric solvents is currently tered reports in the literature of siloxanes being used as reac-
7
–9
dominated by the use of supercritical carbon dioxide,
to a lesser extent, fluorous solvents.
and, tion solvents, for example, the use of PDMS in the asymmetric
1
0–12
Although these hydrogenation of tiglic acid and poly(methylphenylsiloxane) as
solvent systems have become popular the specialist equipment a solvent for the Ru-catalysed synthesis of DMF, the use of
required to achieve supercritical conditions for CO2 and the (Me Si) O as both reagent and solvent in glycosylation reac-
modification of reaction components often required for com- tions, the use of (Me Si) O in the synthesis of an oxygen-
3 2
patibility with the fluorous phase are significant barriers to centred hexatantalum cluster complex, together with several
their wider application. These problems prompted us to other examples as described below. There are also several
2
4
3
2
5
2
26
explore the potential applications of other non-polar liquids as examples of the use of (Me
alternative solvents. as a solvent for recrystallization. For example, the crystallization
Volatile methylsiloxanes (VMSs) are well known compounds of [(η -Me C ) Co] has been carried out in (Me Si) O in prefer-
3 2
Si) O, not as a reaction solvent, but
5
5
5 2
3
2
with a repeating silicon–oxygen backbone unit, with methyl ence to hexane or toluene to give crystals suitable for X-ray crys-
2
7
tallography (see also below for further examples). Here we
demonstrate that Volatile Methyl Siloxanes (VMSs) can be used
as replacements for conventional non-polar organic solvents in
sustainable chemical processes with neither the need for
specialist equipment nor modification of reagents.
Department of Chemistry, Imperial College, London SW7 2AZ, UK.
E-mail: p.lickiss@imperial.ac.uk, t.welton@imperial.ac.uk
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c3gc42036a
1
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1
.1 Properties of volatile methylsiloxanes
In siloxane nomenclature the terminal Me
nated with the abbreviation M and the –(SiMe
addition to a selection of the solvents scrutinised in the orig-
inal publication, this work presents an equivalent analysis of
seven VMSs. The assessment utilises three categories to define
each of the three EHS indicators of health, safety, and environ-
ment to create a numerical score for each solvent. Low scores
indicate that the solvent is preferred from an EHS perspective.
In a deviation from the original method, the parameter pre-
viously used to quantify the air hazard category within the
environment indicator has been replaced by the photochemi-
cal ozone creation potential (POCP) of each solvent.
This allows for a fair comparison between all the entries.
Therefore the full list of categories that have been used to
assess the greenness of VMSs as solvents is as follows. In the
safety indicator; release potential, fire or explosion risk,
and reaction or decomposition potential. In the health
indicator; acute toxicity, chronic toxicity, and irritation. In the
environment indicator; persistency, air hazard (POCP), and
water hazard.
3
SiO– group is desig-
O)– repeating
2
1
6
unit as D. Hence linear and cyclic species can be referred to
by using a representative combination of letters (Table 1). It
should be noted that the cyclic (Me SiO) is a solid at room
2 3
temperature and is relatively susceptible to ring-opening reac-
tions, it has not, therefore, been included in this study. Silox-
anes bearing alkyl groups other than methyl, and aryl groups
1
6,20
are also well known
but they are generally more expensive,
and more viscous and high boiling than the methyl analogues
and so they have also been excluded from this study. (Low
molecular weight siloxanes containing Si–H or Si–halogen
bonds are reactive and so are unsuitable as solvents for many
reactions.) The physical properties of siloxanes do bear some
resemblance to traditional organic solvents (Table 2), but their
dissimilarities were recognised as being significant enough to
prompt this investigation. It should be noted that the potential
2
3
By this assessment, it is clear that VMSs may be used to
reduce the EHS impact of a chemical process and so to make
it greener, if all other aspects of the process, such as energy
use, remain unchanged. What is also evident is that there is
significant variation within the VMSs regarding their EHS pro-
of VMSs as solvents was described in 2000, but little activity
on the topic seems to have occurred since then.
1
.2 Health and safety considerations
The environmental health and safety (EHS) profiles of VMSs
can be graphically represented using the concept previously
defined by Capello et al. to rank organic solvents (Fig. 1). In
3 4 5
files. Three of the analysed VMSs (MD M, D and D ) present
some health risks of comparable magnitude to some of the tra-
ditional organic solvents included in the assessment, although
these are less than those of the non-polar solvents for which
3
4
3
the VMSs are likely replacements (see below). Whereas MD M
Table 1 The structures of six VMSs
35–37
and D are irritants, D is believed to impair fertility
and
5
4
to exhibit estrogenic behaviour in rats and mice (see, for
example, ref. 38 and 39). The remaining VMSs have no associ-
ated health risks according to this particular methodology.
Therefore it is recommended that solvent choice between
VMSs should be scrutinised with an emphasis on both practi-
cal and EHS considerations, just as it should within more tra-
ditional solvent classes. Generally the VMSs with greater
molecular weights are safer to handle due to their lower
vapour pressures and higher flash points. These results can be
compared to the conclusions presented in the Pfizer medicinal
Hexamethyldisiloxane (M
2
)
Octamethyltrisiloxane (MDM)
Decamethyltetrasiloxane (MD
2
M) Poly(dimethylsiloxane) (PDMS)
2
chemistry solvent ‘traffic light’ selection guide, in which sol-
vents are also organised according to their respective EHS pro-
files. The EHS profiles of VMSs most closely resemble those of
solvents deemed to be green in this system.
4 5
Octamethylcyclotetrasiloxane (D ) Decamethylcyclopentasiloxane (D )
Table 2 The physical properties of VMSs and some common solvents (measured at 25 °C if temperature dependent unless otherwise stated)
Physical properties
M
2
MDM
D
4
D
5
Hexane
Diethyl ether
Toluene
Ref.
−
1
Molecular weight/g mol
Density/g cm
Melting point/°C
Boiling point/°C
Vapour pressure/kPa
F.P./°C
162.38
0.762
−67
236.53
0.820
−80
296.62
0.950
18
176
0.13
69
2.19
0.056
17.93
370.77
0.953
−44
86.18
0.661
−95
69
20.2
−22
0.30
9.8
74.12
0.714
−116
35
71.7
−45
0.22
92.14
0.867
−95
111
3.8
4
0.56
519
28.52
−3
28, 29
20, 29
20, 29
21, 29
20, 29, 30
22, 29
22, 29
100
153
210
5.61
−9
0.50
0.93
15.7
0.52
37
0.85
0.034
16.95
0.023
77
3.70
0.017
18.04
Dynamic viscosity/mPa s
−1
Aq. solubility/mg L
64200
17.10
−
1
Surface tension/mN m (20 °C)
18.40
31, 32, 33
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Fig. 1 The EHS profiles for a variety of traditional solvents and VMSs.
5
3
1
.3 Environmental properties
lives are up to 6 days and 2 days respectively, which com-
pares very favourably with commonly used organic solvents.
Volatile methylsiloxanes already enter the environment in The hazard posed by VMSs to aquatic environments has been
many ways due to their significant uses in household and per- judged to be minimal, validated by German Wassergefähr-
2
1
sonal care products, cleaning agents and industrial products. dungsklassen (WGK) water hazard classifications. The high
The distribution and fate of VMSs in the air and in aqueous molecular weight PDMS oligomers lack sufficient volatility to
environments has, therefore, been extensively studied, [see, for be directly transported into the atmosphere. These compounds
example, ref. 40–47] and as a result their environmental tend to associate themselves with the sludge deposits of water
3
7
impacts are widely recognised. A Canadian study, has for treatment plants or in soil. Abiotic hydrolysis is the fastest
example, concluded that D5 meets some toxicity and persist- known decomposition pathway for PDMS, from which volatile
4
8
54
ence criteria but not its bioaccumulation criterion while a silanols are formed. Dimethylsilanediol, Me
recent update of the Equilibrium Criterion Model provides a methylsilanol, Me SiOH, are the eventual products upon de-
wide range of physical and environmental parameters for polymerisation of the siloxane backbone.
2 2
Si(OH) , and tri-
3
4
9
D5. In the UK the cyclic siloxanes have been assessed for per-
Once in the atmosphere the low molecular weight VMSs
5
0–52
sistence, bioaccumulation and toxicity.
However, further photodegrade under the action of hydroxyl radicals to form
studies are clearly needed on the bioaccumulative effects of silanols. Therefore, the environmental consequences of VMS
VMSs as not all studies are in agreement (see for example, ref. contamination are dictated by the properties of their silanol
4
0). This current study does not attempt to review in detail the degradation products. The rate at which Me
3
SiOH photolyti-
environmental aspects of siloxanes, however, they do need to cally decomposes has been experimentally determined to be
5
8
be borne in mind. Being both hydrophobic and less dense greater than that observed for a selection of siloxanes. Exclu-
than water, if low molecular weight VMSs enter into an sion of VMSs from VOC regulations in the United States of
aqueous environment they preferentially form a thin film on America is justified by the benign nature of their ultimate
the water’s surface and rapidly evaporate (Table 3). Experi- photodegradation products, namely carbon dioxide, silica, and
5
3,59
mental data for D
4
and D
5
demonstrate that their aquatic half- water (Scheme 1).
Siloxanes have a tendency to terminate
Table 3 The environmental characteristics of VMSs and some traditional solvents. Theoretical values are presented in parentheses
Environmental properties
M
2
MDM
D
4
D
5
Hexane
Diethyl ether
Toluene
Ref.
Aquatic t1/2/days
Log KOW
WGK
(0.28)
4.2
1
(0.34)
4.8
1
1–6
6.98 ± 0.13
1
0
1–2
8.07 ± 0.22
1
0
7
4.0
3
30
0.89
1
6
2.7
2
21, 53, 55
20, 29, 56
21, 34
POCP
0
0
48
47
64
21, 57
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Table 4 Kamlet–Taft polarity parameters for various organic solvents
and VMSs
Class
Compound
α
β
π*
Ref.
a
Linear siloxane
Linear siloxane
Linear siloxane
M
2
0
0
0
0.06
0.11
0.19
0.01
0.02
0.11
This work
This work
This work
a
a
MDM
PDMS
2000 g mol−
)
1
(
a
a
Cyclic siloxane
Cyclic siloxane
Alcohol
Alkane
Ester
Ether
Ketone
D
D
4
0
0
0.12
0.14
0.63
0.04
0.50
0.52
0.54
0.04
0.07
0.72
This work
This work
62, 72
62, 72
62, 72
62, 72
72, 73
5
Methanol
n-Hexane
Ethyl acetate
Diethyl ether
Acetone
0.56
a
a
0
0
a
0
0.54
0.28
0.70
a
0
a
0
a
Negative values set to zero.
dependent process. To this end, M2 has previously been
applied in the purification of metal phosphide complexes,
either as a recrystallisation solvent or to otherwise wash out
5
3
Scheme 1 The degradation of a generic VMS fragment.
6
8–71
impurities from crude products.
free radical reactions, rather than acting as a radical source
5
3
and do not facilitate low level ozone formation. Hence VMSs 2.2 Solubilities of volatile methylsiloxanes
2
1
have a POCP rating of zero.
The solubility of numerous organic solvents in VMSs was
tested to complement the polarity measurements. In cases
Silanol species may alternatively be removed from the
atmosphere by wet deposition and returned to an aqueous
environment. Although Me SiOH is volatile, Me Si(OH) may
1
where 1 : 1 (v : v) mixtures were not miscible, H NMR spec-
3
2
2
troscopy was used to derive the solubility of both the organic
solvent in the VMS and vice-versa. Many common laboratory
solvents are soluble with equal volumes of the VMSs examined
only very slowly return to the atmosphere, such is its affinity
towards water. The decomposition of aqueous Me Si(OH) has
been shown to occur via an oxidative demethylation mechan-
ism. It should also be acknowledged that incineration is
likely to be an option for VMS disposal after both industrial
use and the retrieval of contaminated sludge from waste water
treatments. The combustion products of VMSs, CO
SiO , are the same as those observed for their photodegrada-
tion, and are significantly less toxic than those observed for
2
2
(
(
Tables 5 and 6). Exceptions include dimethylformamide
DMF) and pyridine in all cases, with acetic acid, dimethyl car-
6
0
bonate, ethanol and methanol preferentially forming liquid–
liquid biphasic systems with the higher molecular weight
VMSs only. The solubility of gases in VMSs is comparable to
that in organic solvents, but a greater uptake rate has been
2 2
, H O and
2
74
reported, suggesting that VMSs may be good alternative sol-
2
1
many other organic compounds.
vents for reactions involving gases (Table 7).
2.3 Applications in synthesis
2
Results and discussion
A number of organic and inorganic transformations were
attempted in various VMSs to assess their potential as reaction
2
.1 Polarity measurements
Polarity has traditionally been one of the primary methods of
6
1
describing the physical properties of solvents. It includes all
Table 5 The room temperature solubility (v/v) of organic solvents in
possible solvent-solute interactions except those that give rise VMSs from 1 : 1 binary mixtures
to a chemical transformation. We have chosen the Kamlet–Taft
PDMS
2000 g
system for describing solvent polarity of hydrogen bond acidity
(
6
2–65
(
α) and basicity (β) and dipolarity/polarisability (π*).
These
−1
Solute
M
2
3
MD M
mol
)
D
4
D
5
were derived from the UV-vis spectra of three dyes, namely Nile
red, p-nitroaniline, and N,N-diethyl-p-nitroaniline (Table 4).
Nile red was used as an alternative to the more commonly
used Reichardt’s dye because poor solubility of the latter in Dimethyl carbonate
the VMSs prevented its use. With near-zero values for all three
Acetic acid
Dichloromethane
Diethyl ether
≥1.00
≥1.00
≥1.00
≥1.00
0.008
≥1.00
≥1.00
≥1.00
≥1.00
≥1.00
0.132
≥1.00
0.146
≥1.00
≥1.00
≥1.00
0.015
≥1.00
≥1.00
≥1.00
0.078
≥1.00
0.209
≥1.00
0.096
≥1.00
≥1.00
0.292
0.023
0.175
≥1.00
≥1.00
0.026
≥1.00
0.169
≥1.00
≥1.00
≥1.00
≥1.00
≥1.00
0.032
≥1.00
≥1.00
≥1.00
≥1.00
≥1.00
0.393
≥1.00
0.182
≥1.00
≥1.00
≥1.00
0.026
≥1.00
≥1.00
≥1.00
0.173
≥1.00
0.294
≥1.00
Dimethylformamide
Ethanol
Ethyl acetate
of these parameters, the VMSs can be said to be similar to
alkanes, not ethers as might be expected from their chemical n-Hexane
Methanol
Methyl ethyl ketone
Pyridine
structures. Hence VMSs are typified by their low polarity,
6
6,67
hydrophobic nature and surprisingly low viscosity,
and are
likely to be good replacements for alkanes in any polarity Toluene
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Table 6 The room temperature solubility (v/v) of VMSs in organic sol-
vents from 1 : 1 binary mixtures
PDMS
(
2000 g
−1
Solvent
M
2
3
MD M
mol
)
D
4
D
5
Acetic acid
≥1.00
≥1.00
≥1.00
≥1.00
0.775
≥1.00
≥1.00
≥1.00
≥1.00
≥1.00
0.987
≥1.00
0.195
≥1.00
≥1.00
≥1.00
0.079
≥1.00
≥1.00
≥1.00
0.329
≥1.00
0.092
≥1.00
0.000
≥1.00
≥1.00
0.052
0.000
0.249
≥1.00
≥1.00
0.017
≥1.00
0.011
≥1.00
≥1.00
≥1.00
≥1.00
≥1.00
0.155
≥1.00
≥1.00
≥1.00
≥1.00
≥1.00
0.282
≥1.00
0.253
≥1.00
≥1.00
≥1.00
0.076
≥1.00
≥1.00
≥1.00
0.631
≥1.00
0.176
≥1.00
Dichloromethane
Diethyl ether
Dimethyl carbonate
DMF
Ethanol
Ethyl acetate
n-Hexane
Methanol
Methyl ethyl ketone
Pyridine
Toluene
Table 7 Gas solubilities in D4 and various organic solvents
3
Gas solubility ×10 (w/w) in the given solvents at
atmospheric pressure
Diethyl
ether
Gas
D
4
n-Hexane
Toluene
Ref.
H
2
0.07 at
0 °C
0.46 at
5 °C
0.308 at
5 °C
3.98 at
0 °C
0.016 at
21 °C
0.750 at
20 °C
0.566 at
20 °C
6.22 at
21 °C
0.016 at
25 °C
0.843 at
25 °C
0.640 at
25 °C
16.3 at
25 °C
0.007 at
25 °C
0.321 at
25 °C
0.244 at
25 °C
5.07 at
25 °C
74–77
75–77
75–77
Scheme 2 Five examples of organic transformations conducted in a
VMS solvent. Reaction 1; exhaustive amine alkylation. Reaction 2; Diels–
Alder cycloaddition where the dienophile is either methyl acrylate (R =
OMe) or acrolein (R = H). Reaction 3; enolate alkylation where PTC =
phase transfer catalyst. Reaction 4; alcohol oxidation where PTC =
3
O
2
2
CO
CO
2
75–77 phase transfer catalyst. Reaction 5; Grignard reaction and subsequent
2
3
carboxylation.
solvents (Scheme 2). We are not here attempting to provide a solvents upon the rates of amine alkylations are well understood,
comprehensive list of all reactions that could be attempted in the reaction being favoured by polar, non-hydrogen bonding sol-
61,79
VMSs nor to complete systematic investigations of all potential vents.
2 4
The rate of alkylation in both M and D , although
solvent effects on these reactions, but rather to give an indi- faster than in n-hexane, is slow compared to that observed in
cation of the range of possibilities available. There are several tetrahydrofuran, again indicating their low polarities.
VMSs that could be used for these reactions. We have selected
2.3.2 Diels–Alder cycloaddition. The effects of solvents
M
2
as our prototypical VMS and used a variety of different upon the rates and selectivities of Diels–Alder cycloadditions
6
1
VMSs in each of the reactions, again to exemplify the range of are also well understood. Hence, the reactions between cyclo-
possibilities available. We did investigate reactions that have pentadiene and either methyl acrylate or acrolein were studied
previously been used to characterize solvent behaviours,
6
1
in a variety of VMSs and organic solvents (Scheme 2, Reaction
before going on to study other widely used synthetic pro- 2). The rates of the reactions leading to the endo-adducts are
cedures. No siloxane specific effects were observed within this accelerated by greater polarity of the solvent and by hydrogen
set of reactions; while some reactions gave isolated yields bonding to the carbonyl group of the dienophile, whereas reac-
exceeding those obtained using non-polar organic solvents, tions to the exo-adduct are less affected by the solvent. This, in
many gave comparable yields. For completeness, reactions that turn, leads to a greater selectivity to the endo-adducts in polar,
were attempted, but gave significantly poorer results than in hydrogen bond donor solvents. The VMSs gave selectivities
non-polar organic solvents are recorded in the ESI.†
that were remarkably similar to each other, and to that
2
.3.1 Amine alkylation. In parallel with a rare literature observed for the hydrocarbon solvents but lower than those
7
8
example of VMSs being used to facilitate synthetic chemistry,
obtained in ethers and ketones (Table 9). This again confirms
the alkylation of triethylamine with iodomethane was conducted that the VMSs should be considered to be non-polar, non-
8
0
in M and D (Scheme 2, Reaction 1) and compared with a tra- hydrogen bond donor solvents.
2
4
ditional non-polar solvent, hexane, and a more polar one, THF.
The ammonium salt product precipitates cleanly from the VMS Phase-transfer catalysis (PTC) is a well established technique
2.3.3 Phase transfer catalysis and emulsion formation.
8
2
solution and is isolated by filtration, without need for further that has many commercial applications. It has recently been
purification beyond removal of the volatiles. The effects of recognised to have the potential to give rise to green synthetic
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Table 8 Isolated yields for the reaction between triethylamine and significantly to ca. 60 h and 16 h respectively. Thus, while this
methyl iodide at room temperature
is a crude and qualitative study, it is clear that the rate of sep-
2
aration with M is greater than that of toluene. While this type
Experiment
Solvent
Duration
Yield/%
of behaviour is difficult to quantify, the clear difference indi-
cates that VMSs are less likely to form and maintain emulsions
in general. This may be a useful feature to consider for large
scale extractions and biphasic reactions where emulsion for-
mation can be problematic.
1
2
3
4
5
1
2
3
1
2
3
4
1
2
3
10 min
30 min
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
65 h
10 min
10 min
10 min
16
32
100
100
100
97
92
95
76
77
73
97
88
92
M
2
Tetraalkylammonium salts are the most commonly used
D
4
8
2
phase transfer catalysts. Hence, the oxidation of ethyl man-
delate to ethyl-oxo-phenylacetate by calcium hypochlorite in
the presence of a tetraalkylammonium halide phase transfer
catalyst was attempted (Scheme 2, Reaction 4). Separation of
the product in the VMS phase was easily achieved and the
product was then isolated by removal of the solvent. The oxi-
dising agent and by-products remained in the aqueous phase.
n-Hexane
THF
92
The reaction can be performed in M to give an improved yield
2
8
3
(
(
53 and 60% for two repeats) over the reaction in cyclohexane
40 and 44% for two reactions) although in both cases the
routes. Its advantages for commercial application (increased
rates of reaction and product specificity, low energy require-
ments, use of inexpensive catalysts and bases) are also advan-
tages for sustainable chemistry. The immiscibility of VMSs
with water leads to these being candidate solvents for the non-
product was heavily contaminated with starting material,
about 53 and 42% respectively for cyclohexane and M solvents
2
respectively. Hence, for this reaction the VMS can be used as a
replacement solvent for with a slightly improved yield.
Crown ethers are also widely used to facilitate phase trans-
8
4
polar phase in PTC. Their anti-foaming properties are also
likely to be an advantage in this application.
8
2
fer in aqueous-organic biphasic systems. Hence, the reaction
It was noticed that when carrying out various reactions
using the VMS solvents, with the exception of PDMS, separ-
ation from water usually occurred faster than with other
organic solvents. It is unclear why this should be the case but
it may be due to the VMSs’ low surface tension, lower polarity
or larger molecular size and it is consistent with the well
between diethylmalonate and n-butyl bromide was performed
in an aqueous-M biphasic system in the presence of potassium
2
carbonate and 18-crown-6 (Scheme 2, Reaction 3). The yields
were similar to those for the same reaction in cyclohexane
(
Table 10). Elevated temperatures were necessary to achieve the
8
4
required phase distribution of the catalyst during the reaction.
Upon cooling the low concentration of the phase transfer cata-
known use of siloxanes as industrial defoaming agents.
brief qualitative study comparing solvents’ emulsion behaviour
A
2 2
lyst in the M layer, together with the hydrophobicity of M , led
was performed using M and toluene (as this was used some
2
to minimal washing being required for the removal of the
phase transfer catalyst from the non-polar VMS phase.
of the synthesis reactions) to ascertain whether there are any
significant differences between siloxanes and hydrocarbons.
An ultrasonic bath was used to emulsify biphasic mixtures of
water and solvent and the rate at which the emulsion cleared
into two separate solvent layers was observed. For mixtures
As they contain no double bonds and are generally chemi-
cally inert they should not undergo reduction themselves.
Hence, two aqueous biphasic sodium borohydride reductions,
of (1) hexanal and (2) cyclohexanone were also performed with
both traditional organic solvents and siloxanes, using tetra-
butylammonium bromide as the phase transfer catalyst. The
yields using each solvent are presented in Table 11.
containing 5 ml each of toluene–water and M –water the time
2
taken for the emulsions to clear was ca. 24 h and 2 h respect-
n
ively. On adding 25 mg Bu
4
NBr as a phase transfer catalyst to
each solution the emulsion clearance times were increased
The biphasic reduction of 1-hexanal to 1-hexanol was
carried out in M and PDMS for comparison with hexane, and
2
1
CH
2
Cl
2
. Monitoring by H-NMR spectroscopy showed the loss
Table 9 Endo/exo selectivity ratios observed for the cycloaddition
reaction between cyclopentadiene and two dienophiles at 25 °C
Endo/
Endo/
Table 10 Isolated yields for the reaction between n-butyl bromide and
diethyl malonate after 4 h
Solvent
exoacrolein exomethyl acrylate Ref.
Acetone
3.6
n/a
3.0
3.0
2.9
2.9
2.8
2.8
2.4
3.4
2.9
2.6
2.6
2.6
2.6
2.6
2.5
2.7
81
81
Diethyl ether
Experiment
Solvent
Temperature/°C
Yield/%
PDMS (RMM = 2000 g mol−
1
)
This work
This work
This work
This work
This work
81
M
2
1
2
1
2
1
2
M
2
90
90
90
86
88
80
79
83
82
MDM
D
D
4
D
4
5
n-Hexane
Toluene
Cyclohexane
81
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Table 11 Biphasic sodium borohydride reduction reactions performed
Table 12 N-functionalisation of phthalimide
in VMSs and organic solvents
Alkylating agent
Benzyl chloride
Solvent
Yield/%
Reaction
-Hexanal–1-hexanol
Solvent
Yield/%
Conversion/%
D
4
89
22
14
51
17
1
M
2
78.8
76.8
a68.5
>99
>99
>99
>99
>99
>99
>99
Hexane
CH Cl
MDM
Toluene
Hexane
CH Cl
PDMS
2
2
Isobutyl bromide
2
2
Cyclohexanone–cyclohexanol
Toluene
MDM
Pentane
43.4
a46.8
base, PTC and any remaining phthalimide. The reaction
mixture was allowed to cool and in the cases of the VMSs and
the hexane as solvents the product crystallised from solution
yielding a high purity product. The product remained in the
a
Isolated yield unavailable.
2 2
of the aldehyde proton in the 9.5–10.0 ppm region and that organic phase of reaction carried out in CH Cl , thus requiring
reaction in all the solvents was complete in 20 min. It is poss- separation of the organics and rotary evaporation to remove
ible that some product was being lost during the solvent the solvent.
removal step and, as PDMS is unsuitable for removal by evap-
The N-alkylation of phthalimide with isobutyl bromide was
oration, in this case the product was removed by distillation. performed in MDM and in toluene (Table 12). The reaction in
Even after distillation, some PDMS persisted in the product MDM gave the better yield of the two and it is again likely that
and an accurate yield was difficult to determine, although it the higher boiling point assisted the reaction progress.
1
was clear from the H NMR spectrum that the reaction had However, although monitoring of the reaction by tlc showed
gone to completion. It was also noticed that removal of the the complete consumption of isobutylbromide the yields are
PTC seemed to be easier from the siloxanes than from the low. In this case the relatively low boiling point of the bromide
more polar CH Cl
2 2
, possibly due to the more polar solvent sol- may have lead to losses by evaporation. Despite this the reac-
vating the PTC better. The reduction of cyclohexanone to cyclo- tions in the VMS biphasic systems outperformed their organic
1
hexanol was carried out in a similar way with in situ H-NMR solvent equivalents, and demonstrate the suitability of VMSs
spectroscopy indicating complete conversion to cyclohexanol for this type of reaction. An advantage here is that solubility in
in all solvents. Again all of the reactions were completed in a VMSs is not key to the success of the reaction, with neither
short period of time at room temperature. The moderate iso- phthalimide nor the reaction products showing significant
lated yields of product from this reaction may be due to the solubility in VMSs. The ease of product separation in the case
slight solubility of the product in water leading to losses of VMSs is an advantage over some other solvents: polar sol-
during the work-up procedure. From these results it can be vents such as DMF and alcoholic solvents may require further
seen that the reduction of carbonyl functional groups can be work-up steps as product and any remaining starting material
achieved in both traditional solvents and in siloxanes in this remain in the same phase.
biphasic system and that the PTC is active in all cases. Despite
2.3.5 Grignard reaction. A Grignard reaction was
the monitoring of the reactions showing that they had gone to attempted to assess the ability of VMSs to facilitate organo-
completion, difficulties in separating the solvents from the metallic chemistry (Scheme 2, Reaction 5). This was selected
products led to a reduction in isolated yields.
in preference to an alkyllithium reagent such as BuLi as this is
8
7
2
.3.4 N-functionalisation of phthalimide. Phthalimide well known to lead to ring-opening reactions of siloxanes.
8
5
derivatives have many applications in chemistry and so their For this reason the reaction was carried out only in M . Yields
2
syntheses in both organic solvents and siloxanes were for the various reactions are shown in Table 13. Reactions in
attempted. The N-functionalisation of imides is generally per- neat M2 yielded no product indicating that, in spite of the
formed by the reaction of the imide with an alkyl halide in the presence of lone pair-bearing oxygen atoms, the siloxanes do
presence of a base such as NaH, BuLi or KOH, using solvents not act as even moderate bases. However, using a siloxane–
such as DMSO, DMF, THF, acetone and xylene and the use of a THF mix of solvents allows a yield as good as, if not better
PTC has also successfully been used to perform a number of than the conventional ether solvents. Thus, again a siloxane
8
6
N-alkylations of imides. The N-benzylation of phthalimide may be used at least as a partial substitute for a traditional
was attempted in D , hexane and CH Cl using TBAB as the solvent.
4
2
2
catalyst and at the reflux temperature of each solvent for 3 h.
2.3.6 Hydrogenation reactions. Several catalytic hydrogen-
In this case the high yield obtained from the reaction per- ation reactions were also performed in VMSs to examine their
formed in D (Table 12) is likely to be due to the relatively high suitability for this class of reaction (Table 14). Two solvents,
4
boiling point of D4 (176 °C compared to 69 and 40 °C for M and methanol were chosen, M being the only VMS that
2
2
hexane and CH
2
Cl
2
respectively).
allows easy removal by rotary evaporation. Details of the com-
Performing this reaction in a VMS may also lead to improve- pounds used and reaction conditions are detailed in Table 13.
ments from a work-up point of view: on completion of the For the nitrobenzene and cycloheptene 5% Pd on carbon was
reactions, water was added to the reaction flask to dissolve the the catalyst. Nitrobenzene is highly polar and exhibited only
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Table 13 Isolated yields for the carboxylation of bromocyclohexane via
a Grignard reaction
2.3.7 Tin mediated allylation. Metal mediated allylation
reactions can be carried out using a range of metals and in sol-
8
9
vents as varied as water and ionic liquids, see for example. It
was thus thought of interest to attempt this type of reaction in
siloxanes. As this type of reaction has been proven to occur
well in water as the sole solvent it was decided not to attempt
it in a VOC solvent but to use three reactions: one in a VMS,
one biphasic, and one in water. The reason for involving a
VMS at all is that, even though the reaction performs well in
water alone, the isolation of the product from the reaction
mixture still requires the use of a solvent and it was thought
that a biphasic system would facilitate the workup.
Solvent
Repeats
Yield/%
M
2
M
2
M
2
1
1
1
2
1
2
1
2
0
0
–THF (4.5 : 0.5)
–THF (4 : 1)
46
49
28
30
41
43
THF
Diethyl ether
moderate solubility in M2 and a second phase containing
The tin mediated allylation of benzaldehyde was performed
nitrobenzene was visible at the start of the reaction. However, at room temperature over 18 h in both water and water–M2
when complete the reaction mixture was homogenous and the (1 : 1 v/v) and gave moderate yields in both cases (Table 15). On
reaction had occurred as well in M2 as it had in methanol. workup, the product from the reaction performed in M was
2
1
Analysis of the product from the reaction in MeOH by H-NMR isolated by an extraction and separation technique but the
spectroscopy indicated low levels (0.6%) of N-methylaniline, reaction in water, when extracted with ethyl acetate, formed a
indicated by the N–CH proton peak at 2.87 ppm but this was viscous emulsion with the remaining tin which required fil-
3
1
not seen in the product obtained when using M
2
as solvent.
tration prior to separation. H-NMR spectroscopy of products
Cycloheptene was subjected to hydrogenation using a from both reactions indicated the presence of benzoic acid. In
similar technique and again the rate of reaction in both M2 the biphasic reaction ca. 10% of the acid was present while
and methanol was similar. Complete conversion to cyclo- from the water alone up to 50% acid was present, presumably
heptane was achieved by both solvents after 90 minutes with the extended reaction time in the presence of water had led to
no observable byproducts. To examine effectiveness of M as a hydrolysis of the initial aldehyde.
2
solvent for use with other catalysts the benzaldehyde was
Allylation of 1-hexanal was carried out in a similar manner
hydrogenated using platinum dioxide (Adam’s catalyst). A pro- to benzaldehyde and after 3 h the reactions were found to be
cedure from the literature was followed where, along with complete by TLC analysis. In this case no unwanted acid for-
Adam’s catalyst, triethylamine was used to promote the hydro- mation occurred indicating that the prolonged reaction time
8
8
genation. Carbonyl groups are often more difficult to reduce of the benzaldehyde allylation may have resulted in the side
than an olefins and a longer reaction time was required for product formation. Again, the reaction performed in water
1
this reaction. Analysis after 1 hour indicated little conversion. required filtration prior to extractive workup. H-NMR spectro-
Reactions were allowed to continue overnight and after 19 h scopic analysis of the products revealed the desired product
both reactions were complete. A filtration through a silica plug had been formed. In the case of the biphasic reaction some
was performed to remove both catalyst and promoter and the PTC remained and the isolated yield has been corrected for
solvent was then removed to give the product.
From the isolated yields obtained for each reaction it is for the reaction performed in water and the H-NMR spectrum
clear the VMS solvent M is a potentially useful solvent in indicated a relatively clean product. The allylation reaction
this in Table 15. Diethyl ether was used to isolate the product
1
2
which to perform catalytic hydrogenations on compounds of yields were thus marginally improved when using the siloxane
different polarity and containing a range of functional groups. solvent for the reaction and the workup procedure was also
7
4
The high rate of gas uptake by VMSs, mentioned above may simplified.
well play an important role in these reactions.
2.3.8 Carbonyl ligand substitution reactions. The displace-
ment of a carbonyl ligand(s) from a carbonyl complex is a
common step in the synthesis of transition metal compounds.
Syntheses of a number of inorganic compounds, 6, 7, 8, by the
displacement of carbonyl ligands was therefore attempted
Table 14 Pd-catalysed hydrogenation reactions
b
Reaction
Solvent
Yield/%
Conversion /%
Time
a
a
Cycloheptene to
cycloheptane
Nitrobenzene to
aniline
Benzaldehyde to
benzyl alcohol
M
2
>99
>99
>99
>99
>99
>99
90 min
90 min
140 min
140 min
Table 15 Allylation reactions in water in M2 and in water–M2
Methanol
M
2
79
71
74
64
Substrate
Solvent
Yield/%
Time
Methanol
c
M
2
19 hc
Methanol
19 h
Benzaldehyde
Water
Water–M
M
2
69
79
0
18 h
18 h
18 h
3 h
3 h
3 h
2
a
Yields not measured due to problems in separating the product from
the solvent. Conversion determined by H-NMR spectroscopy. 1-Hexanal
Hydrogen balloons deflated over night and were refilled the following
morning and the reaction continued.
b
1
Water
Water–M
72
91
0
c
2
M
2
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Table 16 Examples of carbonyl ligand substitution reactions using various solvents
Reaction scheme
Solvent
Time (h)
Temp. (°C)
Yield/%
M
Toluene
2
2
2
120
120
81, 84
81, 86
M
2
18
18
120
120
38, 40
36, 42
Petroleum ether
D
4
24
24
150
150
90, 92
81, 88
Dibutyl ether
using VMSs as solvents (for the formation of the chromium tri- chronic toxicity (unavailable for VMSs), was replaced by POCP.
carbonyl complex a small amount of THF was also added to The metrics are not completely dissimilar, but crucially POCP
both solvents). In each case, VMSs gave similar results to the is known for a variety of volatile compounds. Data were
traditional organic solvents in facilitating the synthesis of the obtained from the material safety datasheet (MSDS) of each
complexes described below (Table 16). For these reactions VMS with the exception those parameters described in Tables
replacement of an aromatic solvent, an alkane and an ether 2 and 3. Worst case scenarios for water persistency have been
has been achieved showing that the siloxanes may have wide used for the VMSs, extrapolated from theoretical half-lives in
applicability.
cases where experimentally determined data were not
available.
2
1
In order to fit the required parameters to the EHS ranking
methodology the physical properties and/or legislative status
of the solvents are converted, linearly or logarithmically, into
3
Conclusions
VMSs are solvents of similar polarities and process perform- an index value for each category (i.e. a zero to one scale). The
ances to alkane solvents. Further to this, their health and respective equations to do so are listed in Table 17. If the
environmental impacts are lower than for many solvents. resulting value exceeds one, or is less than zero, the limits of
Additionally, VMSs can be substituted for a variety of conven- the index scale were taken instead in accordance with the orig-
tional solvents without the need for specialist equipment, nor inal method. In instances where the parameter being assessed
adaptation of reagents and procedures. VMSs are currently is not expressed numerically (e.g. solvent risk phrases) the con-
5
5
commercially available on a large scale. Hence, VMSs have the version was weighted as previously. The highest priority para-
potential to be useful replacements for alkanes, and other sol- meter available was used to define each category. In cases
vents, in synthetic chemistry applications, both for organic in- where equal priority parameters are available, the highest
organic transformations.
scoring (least green) parameter was applied to give the category
index value. Finally a summation of each of the nine effect cat-
egory index values from the three indicators was performed to
give the EHS profile score, as represented in Fig. 1. The entry
for PDMS was based on data for the oligomer with a molecular
4
Experimental
4
.1 Calculation of environmental health and safety profiles
−1
weight of approximately 2000 g mol , although all the PDMSs
The methodology applied by which solvents may be ranked have comparably low EHS profiles. Also with regard to PDMS
according to their EHS profiles has been documented in the (2000 g mol− ), the experimental determination of the flash
1
3
4
literature, with a dissemination of the procedure also avail- point was only conducted to a temperature of 110 °C, at which
5
5
able online. To allow a comparison between traditional sol- point the test was abandoned (as reported in the MSDS issued
vents and VMSs, the parameter previously measuring the by the supplier Alfa-Aesar). Hence, although it may be con-
−1
severity of any air hazard posed by a solvent, the index value of siderably higher, the flash point of PDMS (2000 g mol ) was
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Table 17 Conversion equations to map VMS parameters (x) onto the component categories (y) of the EHS scale
Indicator
Safety
Category (y)
Priority Parameter (x)
Conversion (x to y)
log 10ðxÞ ꢀ log 10ð0:0001Þ
y ¼
Release potential
1
2
Partial pressure/bar
log 10ð10Þ ꢀ log 10ð0:0001Þ
2
25 ꢀ x
Boiling point/°C
y ¼
y ¼
2
50
25 ꢀ x
00
2
Safety
Safety
Health
Fire/explosion risk
1
2
Flash point/°C
R-Phrases
2
R10 corresponds to y = 0.750
R11 or R30 corresponds to y = 0.875
R12, R15, R17 or R18 corresponds to y = 1
Reaction or decomposition potential
1
2
NFPA Reactivity
R-Phrases
NFPA-1 corresponds to y = 0.600
NFPA-2 corresponds to y = 0.800
NFPA-3 or NFPA-4 corresponds to y = 1
R5, R19, R44 corresponds to y = 0.800
R1, R2, R3, R6 or R17 corresponds to y = 1
log 10x ꢀ log 10ð10Þ
y ¼ 1 ꢀ
Acute toxicity
1
2
IDLH/mg m−
3
log 10ð100000Þ ꢀ log 10ð10Þ
EU Classes
Xn corresponds to y = 0.375
T corresponds to y = 0.625
T+ corresponds to y = 0.875
GK-5 corresponds to y = 0.125
2
GK
GK-4 corresponds to y = 0.375
GK-3 corresponds to y = 0.625
GK-2 corresponds to y = 0.875
GK-1 corresponds to y = 1
2
R-Phrases
R20, R21 or R22 corresponds to y = 0.375
R23, R24, R25 or R31 corresponds to y = 0.625
R26, R27, R28, R29 or R32 corresponds to y = 0.875
log 10x ꢀ log 10ð0:1Þ
y ¼ 1 ꢀ
3
Health
Chronic toxicity
1
2
MAK-CH/mg m−
log 10ð10000Þ ꢀ log 10ð0:1Þ
EU Classes
Xn corresponds to y = 0.300
T corresponds to y = 0.500
T+ corresponds to y = 0.700
GK-5 corresponds to y = 0.100
2
GK
GK-4 corresponds to y = 0.300
GK-3 corresponds to y = 0.500
GK-2 corresponds to y = 0.700
GK-1 corresponds to y = 0.800
R31 or R33 corresponds to y = 0.500
R42 or R43, y = 0.600
2
R-Phrases
R29, R32 or R48 corresponds to y = 0.700
R40, R62, R63 or R64 corresponds to y = 0.800
R45, R46, R47, R49, R60 or R61 corresponds to y = 1
Health
Irritation
1
1
EU Class
T corresponds to y = 0.625
T+ corresponds to y = 0.875
R36, R37 or R38 corresponds to y = 0.625
R34 corresponds to y = 0.750
R35 corresponds to y = 0.875
R-Phrases
log 10x ꢀ log 10ð5Þ
y ¼ 1 ꢀ
1
2
1
LD50 (dermal)/mg kg−
log 10ð50000Þ ꢀ log 10ð5Þ
log 10ðxÞ ꢀ log 10ð1Þ
Environment Persistency
Aquatic half-life/days
y ¼
log 10ð100Þ ꢀ log 10ð1Þ
x
00
Environment Air hazard
1
1
2
POCP
y ¼ 1
log 10x ꢀ log 10ð0:1Þ
y ¼ 1 ꢀ
1
Environment Water hazard
L(E)C50 acute/mg L−
log 10ð1000Þ ꢀ log 10ð0:1Þ
R-Phrases
R52 corresponds to y = 0.375
R51 corresponds to y = 0.625
R50 corresponds to y = 0.875
WGK-1 corresponds to y = 0.125
WGK-2 corresponds to y = 0.500
WGK-3 corresponds to y = 0.875
2
WGK
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set to 110 °C, accounting for the majority of its EHS profile stirred at 25 °C in an atmosphere of nitrogen for 24 h. An
3
score.
aliquot (0.05 cm ) was taken by which the stereoisomeric ratio
of the product, bicyclo[2.2.1]hept-5-ene-2-carboxylic acid
methyl ester (2a) or bicyclo[2.2.1]hept-5-ene-2-carbaldehyde
4
.2 Determination of Kamlet–Taft parameters
(
2b), could be established by GC analysis (30 m RH-WAX GC
The determination of the Kamlet–Taft parameters α, β, and π*
was performed using the following dyes; Nile red (7-diethyl-
amino-3,4-benzophenoxazine-2-one), p-nitroaniline, and N,N-
diethyl-p-nitroaniline. The UV-visible spectra of the individual
dyes in each VMS were analysed in the following fashion.
Solvent polarisability, as described by π*, was determined
column, i.d. 50 μm, injector and detector temperatures 250 °C,
oven temperature 120 °C, total run-time 15 min) see Table 9.
3
Diethyl-2-butylmalonate (3). n-Butyl bromide (1.1 cm ,
0
1
.01 mol) was added to a solution of diethylmalonate (1.5 mL,
0 mmol), 18-crown-6 (0.08 g, 0.3 mmol), potassium carbonate
(
3.32 g, 24 mmol), and Adogen-464 (0.1 mL, 0.2 mmol) in a
solvent (M , D , cyclohexane, 25 mL). The mixture was heated
to 90 °C for 4 h under N atmosphere, then by quenching with
3
−1
using eqn (1) where ν/10 cm is the wavenumber of the
6
4
2
4
single light absorbance of the dye N,N-diethyl-p-nitroaniline.
Absorbance wavenumbers of dyes in non-VMS solvents were
2
water (15 mL). The two layers were separated and the organic
layer was washed with saturated aqueous potassium dihydro-
gen phosphate (2 × 15 mL), and the volatiles were removed
under reduced pressure. A pale yellow oily residue was
obtained in the yields given in Table 10. NMR: δH (400 MHz in
6
4,73
obtained from the literature.
νVMS ꢀ νCyclohexane
π* ¼
ð1Þ
νDimethylsulphoxide ꢀ νCyclohexane
The hydrogen bond basicity (β) of the VMSs was determined
3
−1
CDCl
1H, t, CHC(O)), 1.90 (2H, td, CHCH2), 1.33 (2H, tt,
CHCH CH ), 1.34 (2H, m, CH CH CH ), 0.90 (3H, t,
CH CH CH ) ppm. MS: m/z = 216 ([EtO CC(C
5%), 171 ([EtO )HCO] , 40%), 160 ([EtO
00%).
4.3.3 Preparation of ethyl-oxo-phenylacetate. Calcium
3 2 3 2 3
) 1.28 (6H, t, OCH CH ), 4.21 (4H, q, OCH CH ), 4.22
using eqn (2) where Δν/10 cm is the wavenumber of the
single light absorbance of the dye p-nitroaniline subtracted
from that of N,N-diethyl-p-nitroaniline. As previously, results
derived from the absorbance wavenumbers in non-VMS sol-
(
2
2
2
2
3
+
2
2
3
2
4
H
9
)HCO
2
2
Et] ,
+
+
6
2,73
2
CC(C
4
H
9
2
CCHCO
Et] ,
vents were obtained from the literature.
1
ꢀ
ꢁ
ΔνVMS ꢀ ΔνCyclohexane
β ¼ 0:76
ð2Þ
ΔνDimethylsulphoxide ꢀ ΔνCyclohexane
hypochlorite 0.715 g, 5 mmol) was added to a stirred solution
of ethylmandelate (1.07 mL, 6.62 mmol), water (25 mL),
Adogen-464 (0.07 mL, 0.13 mmol) in a solvent (M , cyclo-
2
hexane 25 mL) over a period 15 min while the reaction temp-
erature was maintained below 30 °C. The mixture was stirred
2
at room temperature under N atmosphere for 1 h, followed by
further 2 h at 60 °C. The mixture was quenched by saturated
sodium chloride solution (50 mL). The aqueous phase was
extracted and washed with additional amount of reaction
solvent (3 × 15 mL). The combined organic phase was washed
with water (10 mL), dried over magnesium sulphate, and evap-
The hydrogen bond acidity (α) of the VMSs was determined
3
−1
using eqn (3) where ν/10 cm is the light absorbance wave-
7
2
number of the dye Nile red. Reichardt’s betaine dye was
unsuitable due to its low solubility in the VMSs, hence Nile
red was used instead. The absorbance wavelengths of Nile red
in traditional organic solvents were also obtained from the
7
2
literature.
2
0:47 ꢀ ð1:95π*Þ ꢀ νVMS
α ¼
ð3Þ
1:01
1
orated using a rotary evaporator to give the product. H NMR
spectroscopy and mass spectrometry showed the presence of
both the desired product and the starting material in the
4
.3 Preparation of organic compounds
4
.3.1 Triethylmethylammonium iodide (1). To a stirred material isolated, no attempt to separate them was made. Data
3
solution of triethylamine (2.0 cm , 0.014 mol) in the chosen for the product: δH (400 MHz in CDCl ) 1.45 (3H, t, Ha), 4.48
solvent (M
added methyl iodide (1.5 cm , 0.024 mol). The mixture was (1H, tt, He)/ppm. MS: m/z = 178 ([PhC(O)CO
stirred at room temperature until ≥80% conversion of triethyl- ([PhCOH] , 100%), 77 ([C H ] , 70%).
3
3
2 4
, D , hexane, or tetrahydrofuran, 5.0 cm ) was (2H, q, Hb), 8.04 (2H, dddd, Hc), 7.54 (2H, dddd, Hd), 7.69
3
+
2
Et] , 7%), 105
+
+
6
5
amine had occurred. The resultant white precipitate was iso-
4.3.4 Cyclohexylcarboxylic acid (5). Thoroughly dried
lated by suction filtration, and dried in a vacuum oven to give glassware was used together with freshly dried solvents. Mag-
triethylmethylammonium iodide (1), see Table 8. NMR: δH nesium turnings (0.44 g, 18 mmol) were added to the chosen
(
(
400 MHz in D
2
O) 3.23 (6H, q, NCH
2
), 2.85 (3H, s, NCH
3
), 1.21 anhydrous solvent (M
2
, diethyl ether, tetrahydrofuran, or mix-
and THF, 3.5 mL) and stirred vigorously at room
temperature. A solution of cyclohexyl bromide (2.05 mL,
+
2
9H, t, NCH CH )/ppm. MS: m/z (ESI) 116, 117 [NEt Me] ; 359 tures of M
+
3
3
2
[(NEt Me) I] .
3
2
4
.3.2 Bicyclo[2.2.1]hept-5-ene-2-carboxylic acid methyl 16.6 mmol) was dissolved in the same solvent or solvent
ester (2a) and bicyclo[2.2.1]hept-5-ene-2-carbaldehyde (2b). A mixture and added dropwise to the magnesium under an N
2
3
solution of freshly cracked cyclopentadiene (0.150 cm , atmosphere at such a rate that a gentle reflux of the solvent
3
0
.0019 mol) and methyl acrylate (0.175 cm , 0.0019 mol) or was maintained. After the addition was complete, the reaction
3
acrolein (0.127 cm , 0.0019 mol) in the chosen solvent mixture was heated at 70 °C for 30 min. The mixture was
2.1 cm ) was introduced into a sealed vial. The mixture was allowed to cool to room temperature, and an excess of carbon
3
(
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dioxide was bubbled through the reaction vessel with rapid aqueous phase with until neutral pH was determined by
stirring. To the resultant wax was added diethyl ether (5 mL) litmus paper. The aqueous phase was extracted with solvent
3
and aqueous hydrochloric acid (2 N, 17 mL). The aqueous (10 cm ) the non-aqueous fractions combined, washed with
3
phase was extracted, washed with diethyl ether (3 × 15 mL), water (2 × 10 cm ), dried over sodium sulphate (0.8 g) and
1
and the combined organic phase was washed with aqueous evaporated under reduced pressure to give 1-hexanal. H-NMR
3
sodium hydroxide (2 N, 5 × 5 mL). The washings of the organic (400 MHz, CDCl ) δ 0.92 (3H), 1.30–1.42(6H), 1.55–1.63 (2H),
phase was cooled to 0 °C and acidified with concentrated 3.67 (2H).
aqueous HCl until precipitation formed. The white precipitate
4.3.9 Reduction of cyclohexanone using sodium boro-
was isolated by suction filtration to give cyclohexyl carboxylic hydride. To a stirred biphasic solution of the appropriate
9
0
3
3
acid in the yields given in Table 11. Mpt. 29 °C (lit. 30 °C ) solvent (toluene, pentane or MDM, 10 cm ), water (10 cm ),
NMR: δH (400 MHz in CDCl ) 2.36 (1H, tt, CHCO ), 1.71–2.45 tetrabutylammonium bromide (0.1610 g, 0.5 mmol), sodium
3
2
(
4H, m, CH
2
CH), 1.40–1.71 (4H, m, CH
2
CH
2
CH), 1.30 (2H, m, hydroxide (0.1200 g, 3 mmol), cyclohexanone (0.4907 g,
+
CH CH CH
2
2
2
CH)/ppm. MS: m/z = 128 ([C
6
H
11CO
2
H] , 63%), 83 5 mmol) and sodium borohydride (0.1890 g, 5 mmol) was
+
+
+
(
(
(
[C H ] , 70%), 73 ([C H CO H] , 80%), 55 ([C H ] , 100%), 41 added. The mixture was stirred at room temperature for
6
11
2
4
2
4
7
+
−1
[C
C–H), 1705 (CvO), 1451 (CH
.3.5 Hydrogenation of cycloheptene. Palladium (5%) on mined by litmus paper. The aqueous phase was extracted with
3
H
5
] , 61%). IR: in nujol νmax/cm 2500–3094 (O–H), 2927 15 min. The non-aqueous phase was removed and 2 M HCl(aq)
2
), 1261 (C–O). added to the aqueous phase with until neutral pH was deter-
4
3
carbon (0.03 g, 0.15 mmol) was placed in to a two neck flask solvent (2 × 10 cm ), non-aqueous fractions combined, washed
which was then sealed with septa. Nitrogen gas was flushed with water (2 × 10 cm ), dried over sodium sulphate (0.8 g) and
3
through the vessel for ten minutes to remove oxygen. Cyclo- evaporated under reduced pressure to give cyclohexanol.
1
heptene (0.6731 g, 7 mmol) in the appropriate solvent (metha-
3
H-NMR (400 MHz, CDCl ) δ1.18–1.36 (6H), 1.75(2H), 2.39
3
nol or M
hydrogen atmosphere (balloon) at room temperature for
0 minutes. The reaction mixture was filtered and solvent 12.5 mmol), benzyl chloride (2.000 g, 15.8 mmol), potassium
removed under reduced pressure to give cycloheptane. carbonate (1.92 g, 13.9 mmol), tetrabutylammonium bromide
2
, 10 cm ) was added via syringe and stirred under a (2H), 3.64 (1H).
4.3.10 Benzylation of phthalimide. Phthalimide (1.839 g,
9
1
3
H-NMR (400 MHz, CDCl ) δ 1.57 (14H).
(0.805 g, 2.5 mmol) in the appropriate solvent (10 cm ) were
3
4
.3.6 Hydrogenation of nitrobenzene. Palladium (5%) on placed in a flask. The mixture was heated to reflux for 3 h. The
3
carbon (0.03 g, 0.15 mmol) was placed in to a two neck flask reaction was quenched with water (50 cm ) and cooled in ice,
which was then sealed with septa. Nitrogen gas was flushed filtered, washed with cold water (2 × 20 cm ) and dried in an
3
through the vessel for ten minutes to remove oxygen. Nitro- oven at 100 °C for 12 h to give N-benzyl phthalimide. For the
benzene (0.6153 g, 5 mmol) in the appropriate solvent (metha- reaction in CH
2
Cl
nol or M , 10 cm ) was added via syringe and stirred CH Cl removed, the aqueous portion extracted with CH Cl
2
2
, the reaction mixture was filtered, the
3
2
2
2
2
3
3
2 2
magnetically under a hydrogen atmosphere (balloon) at room (2 × 10 cm ), the CH Cl washed with water (10 cm ), and the
temperature for 140 minutes. The reaction mixture was filtered combined organic extracts dried over sodium sulphate (0.8 g).
and solvent removed under reduced pressure to give phenyl- The solvent was removed under reduced pressure to give
1
1
amine. H-NMR (400 MHz, CDCl
(
4
3
) δ 3.30–3.90 (2H), 6.68–6.74 N-benzyl phthalimide; H-NMR (400 MHz, CDCl
2H), 6.77–6.82 (1H), 7.10–7.22 (2H). 7.26–7.26 (3H), 7.45–7.46 (3H), 7.71–7.74 (3H), 7.86–7.88 (2H);
.3.7 Hydrogenation of benzaldehyde. Platinum dioxide Mpt. 115–119 °C (114–116 °C, ).
0.0794 g, 0.35 mmol) was placed in to a two neck flask which 4.3.11 Alkylation of phthalimide. Phthalimide (0.8827 g,
3
) δ 4.87 (2H),
9
1
(
was then sealed with septa. Nitrogen gas was flushed through 6 mmol), isobutyl bromide (1.039 g, 7.6 mmol), potassium car-
the vessel for ten minutes to remove oxygen. Benzaldehyde bonate (0.922 g, 6.7 mmol), tetrabutylammonium bromide
3
(
0.7494 g, 7 mmol) and triethylamine (0.7083 g, 7 mmol) in (0.3864 g, 1.2 mmol) in the appropriate solvent (10 cm ) were
3
2
the appropriate solvent (methanol or M , 10 cm ) was added placed in a flask. The mixture was heated to reflux for 2 h. The
via syringe and stirred magnetically under a hydrogen atmos- reaction was quenched with water (30 cm ) and cooled in ice,
phere (balloon) at room temperature for 19 h. The reaction filtered, washed with cold water (2 × 10 cm ) and dried in an
3
3
mixture was filtered through a silica plug and solvent removed oven at 100 °C for 12 h to give N-isobutyl phthalimide. For the
1
under reduced pressure to give benzyl alcohol. H-NMR reaction in toluene, the reaction mixture was filtered, the
(
400 MHz, CDCl
3
) δ 4.70 (2H), 7.30–7.40 (5H).
toluene portion separated, the aqueous extracted with toluene
3
4
.3.8 Reduction of 1-hexanal using sodium borohydride. (10 cm ), and the combined organic extracts washed with
3
To a stirred biphasic solution of the appropriate solvent (M , water (10 cm ). These were dried over sodium sulphate (0.8 g)
pdms, hexane or CH
ammonium bromide (0.2576 g, 0.8 mmol), sodium hydroxide phthalimide. H-NMR (400 MHz, CDCl
(
2
3
3
2 2
Cl 10 cm ) water (10 cm ), tetrabutyl- and solvent removed under reduced pressure to give N-isobutyl
1
3
) δ 0.98 (6H), 2.15 (1H),
0.1920 g, 4.6 mmol) 1-hexanal (0.8012 g, 8 mmol), and 3.54 (2H), 7.75 (2H), 7.86 (2H); Mpt: 94–96 °C.
sodium borohydride (0.3024 g, 8 mmol) was added. The
4.3.12 Allylation of benzaldehyde. To a mixture of benz-
mixture was stirred at room temperature for 20–25 min. The aldehyde (0.2162 g, 2.0 mmol), Adogen 464 (0.05 g, 0.2 mmol),
3
non-aqueous phase was removed and 2 M HCl(aq) added to the in the appropriate solvent system (10 cm ) was added tin
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powder (0.2374 g, 3 mmol). The mixture was stirred at room cold finger. After removal of the Mo(CO)
6
from the sublimation
temperature for 3 h. The reaction was quenched with the finger, the temperature of the oil bath was adjusted to 100 °C,
3
appropriate solvent (10 cm ). For the reaction in the M
2
–water and the sublimed purified product was collected in the yields
) 3.06, 2.49 (2H,
binary mixture, the non-aqueous phase was washed twice with given in Table 12. NMR: δH (400 MHz in CDCl
water (2 × 10 cm ), once with saturated sodium chloride solu- dt, CH ), 3.64 (2H, m, CHCH ), 4.96 (2H, m, CHCHCH ), 6.10,
3
3
2
2
2
9
8
6
tion, dried over sodium sulphate (0.8 g) and evaporated under (2H, m, CHCHCHCH
2
92
)/ppm. MS: m/z = 218 ([ Mo(η -C
7
H
8
)-
)(CO)] , 100%). IR: in nujol
2922 & 2852 (C–H), 1903, 1928, 1993 (CuO),
0 cm ), once with saturated sodium chloride solution, dried 1376–146 (CH ).
+
6
+
7 8
reduced pressure to give 1-phenylbut-3-en-1-ol. For the reaction (CO)] , 39%), 212 ([ Mo(η -C H
−
1
in water the product was filtered, washed twice with water (2 × νmax/cm
3
1
2
6
over sodium sulphate (0.8 g) and evaporated under reduced
4.4.3 η -Anisole chromium tricarbonyl (8). Chromium hexa-
pressure to give 1-phenylbut-3-en-1-ol.
carbonyl (1 g, 4.5 mmol), anisole (10 mL, 92 mmol), and THF
4
, dibutyl ether, 60 mL) in
a flask wrapped by aluminium foil. The flask was evacuated
1
H-NMR (400 MHz, CDCl
.84 (1H), 7.33 (m, 5H).
3
) δ 2.55 (3H), 4.77 (1H), 5.18 (2H), (6 mL) were added to the solvent (D
5
4
.3.13 Allylation of 1-hexanal. To a mixture of 1-hexanal and back-filled with nitrogen nine times. The mixture was
0.3004 g, 3.0 mmol), Adogen 464 (0.0.1392 g, 0.3 mmol), in heated at 150 °C for 24 h then cooled to room temperature,
the appropriate solvent system [water or water–M binary and the solvent was removed carefully under reduced pressure.
mixture (1 : 1 v/v), 10 cm ] was added tin powder (0.5341 g, The crude product was redissolved in diethyl ether and the
(
2
3
4
1
.5 mmol). The mixture was stirred at room temperature for solution filtered through a short pad of silica gel. The solvent
6
8 h. The reaction was quenched with the appropriate solvent was removed via rotary evaporation, to give the purified η -
3
(10 cm ). For the reaction in the M –water binary mixture, the anisole chromium tricarbonyl in the yields given in Table 12.
2
3
non-aqueous phase was washed twice with water (2 × 10 cm ), NMR: δH (400 MHz in CDCl
3 3
) 3.75 (3H, s, CH ), 5.58 (2H, d,
once with saturated sodium chloride solution, dried over o-CH), 5.51 (2H, dd, m-CH), 4.91 (1H, m, p-CH)/ppm. MS:
5
3
6
+
53
6
sodium sulphate (0.8 g) and evaporated under reduced m/z = 245 ([ Cr(η -C H OCH )(CO) ] , 67%), 212 ([ Cr(η -
6
5
3
3
+
−1
pressure to give 1-nonen-4-ol. For the reaction in water the
6 5 3
C H )-(CO) ] , 71%). IR: in nujol νmax/cm 2923 & 2852 (C–H),
product was filtered, washed twice with water (2 × 10 cm ), 1975, 1945, 1903, 1857 (CuO), 1528, (aryl C–C),
once with saturated sodium chloride solution, dried over 1376–1465 (CH3).
3
sodium sulphate (0.8 g) and evaporated under reduced
pressure to give 1-nonen-4-ol.
1
H-NMR (400 MHz, CDCl ) δ 0.92 (3H), 1.26–1.41 (6H), 1.48
3
(2H), 2.31 (2H), 3.67 (1H), 5.15 (1H), 5.87 (1H).
References
4
.4 Preparation of inorganic compounds
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denum hexacarbonyl (0.5 g, 1.9 mmol), and piperidine
2.5 mL, 25 mmol) were added to the solvent (M or toluene,
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(
4
2
2
0 mL) and stirred at 120 °C under N atmosphere for 2 h. The
mixture was cooled to room temperature, and the resulting
precipitate was isolated via suction filtration, washed with
light petroleum ether (5 mL) to give the product in the yields
given in Table 12. NMR: δH (400 MHz in DMSO-d ) 2.69 (4H,
6
m, CH N), 1.35–1.69 (8H, m, CH CH )/ppm. MS: m/z = 350
2
2
2
9
8
+
95
+
([(Pip)
2
Mo(CO)
3
] , 51%), 319 ([(Pip)
Mo(CO)(C)] , 62%), 282 ([(Py)
2
Mo(CO)
2
] , 33%), 306
9
8
+
95
+
([(Pip)
2
2
Mo(CO)] , 100%), 261
9
8
+
98
+
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3
5
6
−
1
nujol νmax/cm
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2
3
4
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