Recyclable Polyisobutylene-Bound HMPA as an Organocatalyst in Recyclable Poly(α-olefin) Solvents

Article abstract of DOI:10.1002/cctc.202001207

This work describes the synthesis of a PIB-bound hexamethylphosphoramide (HMPA) analog and its applications as a recyclable catalyst in allylation of aldehydes and reduction of enones in a recyclable poly(α-olefin) (PAO) polymeric solvent. Kinetic studies of the allylation reaction show that this PIB-bound HMPA analog is as active as HMPA in dichloromethane and PAO and that this PIB-bound catalyst is comparably reactive in heptane and in a PAO solvent. The PIB-bound HMPA catalyst has high phase selective solubility in PAO versus a polar solvent. By using this catalyst in a nonvolatile separable PAO solvent, this catalyst recyclability can be coupled to solvent recyclability, something that is less feasible in a conventional heptane solvent. The result is good recycling of catalyst and solvent through at least 5 cycles using simple gravity-based liquid/liquid extractions. This is in contrast to HMPA or conventional solvents which are less recyclable.

Full text of DOI:10.1002/cctc.202001207

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Recyclable Polyisobutylene-Bound HMPA as an
Organocatalyst in Recyclable Poly(α-olefin) Solvents
[a]
[a]
Ying-Hua Fu and David E. Bergbreiter*
This work describes the synthesis of a PIB-bound hexameth-
ylphosphoramide (HMPA) analog and its applications as a
recyclable catalyst in allylation of aldehydes and reduction of
enones in a recyclable poly(α-olefin) (PAO) polymeric solvent.
Kinetic studies of the allylation reaction show that this PIB-
bound HMPA analog is as active as HMPA in dichloromethane
and PAO and that this PIB-bound catalyst is comparably reactive
in heptane and in a PAO solvent. The PIB-bound HMPA catalyst
has high phase selective solubility in PAO versus a polar solvent.
By using this catalyst in a nonvolatile separable PAO solvent,
this catalyst recyclability can be coupled to solvent recyclability,
something that is less feasible in a conventional heptane
solvent. The result is good recycling of catalyst and solvent
through at least 5 cycles using simple gravity-based liquid/
liquid extractions. This is in contrast to HMPA or conventional
solvents which are less recyclable.
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Introduction
bioderived, they are highly recyclable and are safer than
conventional more volatile solvents.
[5–7]
However, they are
Over 90% of all chemical products have at least one catalytic
step in their manufacturing processes. However, while catalysts
simply large alkanes and as alkanes, their applicability in
synthesis and more specifically to organocatalytic processes
involving polar catalysts has to be experimentally established.
Our prior work has shown that vinyl-terminated PIB is
broadly useful as a precursor for formation of recyclable ligands,
catalysts, and reagents that can be easily separated from
[1]
by definition are not consumed in reactions, the sustainability
and recyclability of organocatalysts continue to be problematic.
That in part reflects the fact that organocatalysts often have
[
2,3]
higher mol% loading than transition metal catalysts.
These
[8–14]
issues also are important because more efficient catalyst
recycling strategies are needed to reduce waste production and
because catalyst separation can minimize hazards if a catalyst
has inherent toxicity or contaminates products.
products.
Such separations are effective because terminally
functionalized PIB oligomers have excellent phase selective
solubility in nonpolar alkanes versus polar solvents like
acetonitrile (MeCN), MeOH and water. They are also highly
soluble in poly(α-olefin)s (PAOs) like PAO432, the fully saturated
trimer formed from trimerization of 1-decene. Our prior work
has also shown that PIB-bound species are even more phase
selectively soluble in PAOs than heptane in liquid/liquid
separations involving polar organic phases.
has also shown that terminally functionalized PIB-bound
catalysts or reagents in the same solvents as their low molecular
weight analogs have equivalent reactivity.
An additional issue in homogeneous catalysis is the solvent
used. Solvents are the major component of most chemical
processes and are required in homogeneous catalysis. While
most reported organocatalytic reactions historically used con-
ventional solvents like tetrahydrofuran (THF) and dichloro-
methane (DCM), green solvents are now available as substitutes.
Such green solvents are commonly derived from renewable
[15]
[5,6]
This prior work
[4,5]
[5,9–14,16–20]
resources. An advantage of these green solvents is that they
do not have any net greenhouse gas (GHG) impact because as
However,
when PIB is used in an alkane, reactivities are less predictable
because alkanes have not historically been studied as solvents
for reactions of polar catalysts. Indeed, prior studies have shown
that switching from a solvent like THF or CH Cl to an alkane
bio-derived solvents they are derived from CO in contrast to
2
solvents derived from a non-renewable petrochemical source
However, greenhouse gas generation (GHG) inexorably occurs if
these greener solvents are not recycled. Most organic solvents
including these green solvents are also volatile. Volatile organic
solvents (VOCs) often have health, safety and environmental
risks. Accordingly, a recent thrust of our research has been
exploring the use of highly recyclable nonvolatile
poly(α-olefin)s (PAOs). While such solvents are not at present
2
2
like heptane or its recyclable analog PAO can lead to higher,
different, equivalent or no reactivity for a recyclable PIB-bound
catalyst in a pure alkane because of the minimal ability of
alkanes to solvate reactants, transition states, or
[5,10,13,20]
products.
Hexamethylphosphoramide (HMPA) is both an organocata-
lyst and a useful polar aprotic solvent. Its applications as a
catalyst and solvent have been described in reactions like
[a] Y.-H. Fu, Prof. D. E. Bergbreiter
Department of Chemistry
Texas A&M University
[
21–25]
[24,26]
[27–32]
allylations,
aldol reactions,
aldol additions,
reductions,
reductive
[29,30]
[33]
[34]
reductive alkylations, alkyne additions,
[35]
[36]
P.O. Box 30012, College Station
77842-3012 Texas, (USA)
E-mail: bergbreiter@tamu.edu
ring opening reactions and radical polymerizations. How-
ever, whether HMPA is used as a catalyst or as a solvent, its use
like that of other dipolar aprotic solvents has been limited
because of its toxicity and the difficulty of separating it from
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202001207
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products. The carcinogenic and mutagenic toxicity of HMPA is
of special concern since separating HMPA either from an
organic phase, from a product, or from an aqueous waste phase
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[37–39]
after a reaction is problematic.
Specifically, HMPA has a
high boiling point and is miscible both with many organic
solvents and with water, making it difficult to separate after a
reaction.
There is a long history wherein polymer supports have been
used to design safer versions of dipolar aprotic solvents
[40,41]
including HMPA.
The earliest reports of a polymer-bound
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phosphoramide described synthesis of a DVB-crosslinked poly-
styrene (PS) supported phosphoric triamide as an organo-
catalyst for biphasic reactions like S 2 reactions and
N
[42]
reductions.
Similar PS-supported HMPA analogs were also
Figure 1. Scheme for use of a recyclable alkane soluble PIB-bound
hexaalkylphosphoramide analog of HMPA (PIB-HMPA) in catalysis using
PAO432 as a recyclable alkane solvent.
shown to be useful organocatalysts in catalytic aldol
[43–45]
reactions.
However, while these polystyrene supported
HMPA resins could be recycled by a simple filtration, polystyr-
ene supported HMPA resins’ activity sometimes decreased on
[43]
recycling.
These PS-bound HMPA analogs also required a
conventional volatile organic solvent. Other examples of Results and Discussion
polymers containing HMPA analogs included a polymeric
phosphoramide (poly(HMPA)) that was formed by post-mod-
The synthesis of a PIB-bound hexaalkylphosphoramide (PIB-
HMPA, 5) is shown in Scheme 1. In this synthesis, a commer-
[46]
ification of linear poly(ethylenimine) or by a ring opening
polymerization
phosphorylpropylenimine.
of
N-bis(dimethylamino)
While this polymer analog of
cially available alkene terminated PIB oligomer 1 (M =1000 Da)
n
[
47]
was first converted to a PIB-bound alcohol 2 which was then
allowed to react with iodine in presence of triphenylphosphine
and imidazole to generate the PIB-bound iodide 3. The iodide 3
was used to form a secondary amine 4 by allowing it to react
with excess propylamine in a theomorphic solvent mixture
(heptane/DMF). A liquid/liquid separation isolated the PIB-
bound secondary amine 4 as a heptane solution with any
excess amine and any polar byproducts phase separating into
the DMF-rich phase. Lithiation of a THF solution of the amine 4
with n-butyllithium formed a lithium amide in situ that was
HMPA reportedly had good solubility in water, methanol,
acetonitrile, N,N-dimethylformamide, chloroform and dichloro-
methane, its applicability as a cosolvent or catalyst in synthesis
was not described. Later work by the Haag group showed that
a dendritic HMPA analog that had a phosphoramide covalently
attached onto a hyperbranched polyglycerol (hPG) as a support
was useful as a soluble polymeric catalyst for allylation and
[25]
Mukaiyama aldol reactions. This polymer-bound HMPA ana-
log could be separated by membrane filtration. However, those
reactions and the membrane filtration required relatively large
amounts of conventional volatile organic solvents.
HMPA-catalyzed reactions whether with HMPA itself or with
polymer-bound analogs all use conventional solvents like
toluene, tetrahydrofuran (THF) and dichloromethane (DCM).
Moreover, the recyclability of HMPA as an organocatalyst has
not always been well described or is not simple and efficient.
Accordingly, to provide a more sustainable way to use HMPA as
an organocatalyst and to further explore the potential of
alternative nonvolatile PAOs as fully recyclable solvents with
this sort of highly polar organocatalyst, we synthesized a PAO-
anchored polyisobutylene (PIB)-bound HMPA analog as an
organocatalyst and explored its reactivity and use for aldehyde
allylation and enone reduction in PAO432. The results below
show that this PIB-bound HMPA analog has comparable
reactivity to HMPA in both a conventional solvent and in this
recyclable PAO432 solvent. We show that both this PIB-bound
HMPA-like organocatalyst and the PAO solvent phase can be
readily separated and recycled through at least five cycles
separating products and the PAO phase containing the PIB-
bound catalyst by simple gravity separations (Figure 1).
Scheme 1. Synthesis of hexaalkylphosphoramide-terminated PIB oligomer 5
(
PIB-HMPA).
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allowed to react with excess N,N,N’,N’-tetramethyl-phosphoro-
diamidic chloride to form 5. After solvent removal, the crude
product 5 was isolated as a viscous oil that was further purified
by trituration with MeOH to remove any non-PIB bound
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phosphoramide species. The product was characterized by H,
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31
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C and P NMR spectroscopy. P NMR spectroscopy in
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particular was important as it allowed us to compare the
NMR spectra of HMPA with that of 5 (See Figure S1). In this
P
P
3
1
3
1
spectrum, 5 had essentially the same P NMR chemical shift as
HMPA consistent with formation of a hexaalkylphosphoramide.
We next explored the phase separability of 5. Our prior
work has shown that PIB-bound dyes can be separated in a
gravity-based liquid/liquid separation using heptane and a
polar solvent like methanol with >99.9% of the PIB-bound
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[6]
species partitioning into the alkane phase. We anticipated
similar phase selective solubility would accrue to 5. That proved
to be the case. In the experiment that probed the phase
separability of 5, we vigorously mixed a 0.05 M solution of 5
with a methanol solution that contained 0.03 M phosphoric
Figure 2. Kinetic studies of allylation of benzaldehyde with 10 mol% of
HMPA or 5 in various solvents at 25°C: no catalyst in dichloromethane (red
circle), HMPA in dichloromethane (orange circle); 5 in dichloromethane (light
green triangle), 5 in heptane (green triangle); 5 in PAO432 (malachite green
square).
3
1
acid. P NMR spectroscopy analysis of the methanol phase after
8 h showed that there was no detectable 5 present indicating
that the concentration of 5 in MeOH was <0.001 M (Figure S2,
99.8% phase selective solubility for 5 in PAO432 vs. MeOH). A
4
that was indistinguishable from that for 5 in PAO432. The overall
conclusion is that 5 as a catalyst for this allylation chemistry is
equivalent to HMPA in CH Cl , in heptane or PAO432.
>
similar experiment was also carried out with heptane as the
alkane phase. In this case, we did detect a small concentration
of 5 in the MeOH phase (>95% phase selective solubility for 5
in heptane vs. MeOH, see Figure S3).
2
2
After confirming the comparative reactivity of 5 versus
HMPA and showing that 5 is reactive in PAO432, we explored the
recyclability of 5 in benzaldehyde allylation in PAO432. This
included developing procedures for separation of 5 in PAO432
from the products using MeCN as an extraction solvent. To this
end, we examined the phase selective solubility of 5 and the
product (1-phenylbut-3-en-1-ol) in a biphasic PAO432/MeCN
mixture. In this experiment, 0.25 g of 5 and 0.1 mL of 1-
phenylbut-3-en-1-ol were dissolved in 2 mL of PAO432 and then
mixed vigorously with 2 mL of MeCN. After a gravity separation,
an aliquot (100 μL) from each phase was dissolved in CDCl3
with 0.2 M of dichloroethane as an internal standard and further
Next, we examined the use of 5 as a catalyst. The initial
reaction we studied was the allylation of benzaldehyde in
CH Cl using allyltrichlorosilane. This reaction has been reported
2
2
using HMPA as a catalyst and has been studied by others using
polymer-supported formamides as catalysts so it provides a
good way to evaluate the utility of this PIB-bound HMPA analog
[44,48–50]
as an organocatalyst.
We first compared the reactivity of
5
with HMPA in CH Cl in order to confirm that 5 acts like HMPA
2
2
in this allylation chemistry (Scheme 2). We then examined the
reactivity of 5 as an allylation catalyst in heptane and PAO432
Overall, we examined five systems; no catalyst in CH Cl , HMPA
1
1
.
analyzed by H NMR spectroscopy. Based on these H NMR
spectra (see Figure S4), >99% of 5 stayed the in PAO432 phase
while more than 98% of 1-phenylbut-3-en-1-ol was present in
the MeCN phase. This result is in accord with our prior
observations that PIB derivatives have little or no solubility in
2
2
in CH Cl , 5 in CH Cl , 5 in heptane, and 5 in PAO432.
2
2
2
2
These studies of the conversion versus reaction time profile
for the allylation of benzaldehyde are shown in Figure 2. This
work like others’ prior work with HMPA as a catalyst included
[6,10]
acetonitrile phases.
This phase selective solubility wherein 5
[51]
an equivalent of N,N-diisopropylethylamine (DIPEA). As ex-
pected, no reaction occurred without catalyst. Allylations with 5
is in the PAO432 phase and the small organic compound is in the
MeCN phase assured an excellent separation of catalyst from
product via a liquid/liquid (PAO432/MeCN) separation in recy-
cling studies in the following reactions. However, a complica-
tion was observed in an examination of the PAO phase in a
in CH Cl2 were comparable but marginally faster than with
2
HMPA in CH Cl . Allylations with 5 in PAO converted
432
2
2
benzaldehyde into 1-phenylbut-3-en-1-ol quantitatively essen-
tially as fast as was the case for HMPA in CH Cl . An experiment
4
32
3
1
similar separation by P NMR spectroscopy of an experiment
where allyl trichlorosilane was present. Under those conditions,
we observed that the original signal for 5 at 25.3 ppm in PAO432
disappeared and that a new signal at 32.3 ppm appeared (see
Figure S5). Since allyl trichlorosilane can readily hydrolyze to
form HCl with adventitious water, we considered that this
downfield shift might be the result of protonation of the
phosphoramide catalyst by adventitious HCl formed from the
chlorosilane. Since a protonated catalyst 5 is incapable of
activating allyltrichlrosilane to form six-member ring transition
2
2
using 5 in heptane showed that allylation occurred at a rate
Scheme 2. Allylation of benzaldehyde catalyzed by the PIB-bound organo-
catalyst 5.
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[
51]
state, we added an additional step and treated the recovered
PAO432 led to a consistently high conversion for five cycles.
These synthetic studies used a 6.5 mol% loading of 5 versus the
10 mol% loading of 5 which was used in the kinetic study
(Figure 2). This lower catalyst loading leads to only ca. 80%
conversion with 5 in PAO432 over 24 h. However, this level of
conversion was constant through 5 cycles, showing the
recyclability of 5 is very high and that the activity of 5 in cycles
one and five. If a significant amount of 5 had leached from PAO
into the acetonitrile phase or if 5 were not recyclable, this
conversion would have dropped. We also noted that there was
no visible change in volume of the PAO432 phase suggesting
complete recycling of the solvent, something we noted in other
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PAO phase with 4 N NaOH. After this base treatment, the
P
NMR signal in the PAO phase changed back to 25.3 ppm
suggesting that experiments to recycle catalyst 5 should
include a second NaOH treatment to insure that the PAO phase
is recycling the active catalyst 5. While we had expected that
the additive DIPEA would have been a strong enough base to
deprotonate any protonated phosphoramide that formed, that
appeared to not be what happened so all of the experiments in
Table 1 used this NaOH washing step. We speculated that this
requirement may reflect the fact that the differences in the
pKa’s of the conjugate acids for the phosphoramide and DIPEA
in this PAO medium might not be as large as the differences in
pKa of these two conjugate acids in water.
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[5]
work.
To verify the synthetic utility of this chemistry, we carried
out allylations of several other aromatic aldehydes in addition
to benzaldehyde. Since Figure 2 had shown that 10 mol%
catalyst afforded higher conversions, all of these synthetic
reactions were carried out with this catalyst loading. In each
case, the acetonitrile phases from five reaction cycles using 5 in
PAO432 cycles was isolated and combined and the allylation
The performance and recyclability of the allylation of
benzaldehyde catalyzed by HMPA or 5 in alkane solvents
heptane or PAO432) is shown in Table 1. In an initial experiment,
(
we first examined HMPA-saturated PAO432 as a reaction
medium. Separate experiments showed that PAO432 saturated
with HMPA had ca. 0.02 M HMPA. Thus, HMPA could in principle
itself function as a catalyst in PAO432. In the event, allylation of
benzaldehyde by allyl trichlorosilane did occur with a 51%
conversion of benzaldehyde over 24 h at room temperature.
However, on recycling, the conversion dropped to <0.1%. In
contrast, 5 either in heptane or PAO432 was a recyclable catalyst
for the allylation chemistry. The difference between using 5 in
heptane versus in PAO432 was that recycling in heptane was
simply less successful. While 5 in heptane afforded a higher
conversion (96%) of benzaldehyde in heptane in the first cycle,
the conversion decreased in cycles 2–5. This presumably reflects
the ca. 5% catalyst loss of 5 due to leaching of 5 into the
heptane-saturated acetonitrile phase. In contrast, using 5 in
1
13
products were isolated and characterized by IR, H and C NMR
spectroscopy. As shown in Table 2, PIB-HMPA 5 is an efficient
catalyst for the allylation of all of these aromatic aldehydes. The
allylic alcohol products were obtained in a steady and
moderately high conversion. In the product layers from each
3
1
cycle, there is no observable signal for 5 in a P NMR spectrum,
suggesting no catalyst leaching to the product layer occurred
1
with these different aromatic aldehydes. Additionally, H NMR
spectroscopy showed negligible PAO432 was present in the
[
6]
product layer (MeCN phase) in agreement with our prior work.
Average isolated yields of products for each of these reactions
1
were consistent with the observed conversions measured by H
NMR spectroscopy.
As noted in the introduction, HMPA and its polymer bound
analogs catalyze a variety of reactions. To confirm that the
catalyst 5 has similar versatility, we also studied using 5 as a
Table 1. Recyclability of 5 in the allylation of benzaldehyde by allyl
trichlorosilane in heptane or PAO432. The conversions were calculated by
taking and analyzing the MeCN layer from a biphasic separation (MeCN/
PAO432 or MeCN/heptane).
recyclable catalyst in
a
conjugate reduction in PAO432
(Scheme 3) since a similar reduction catalyzed by HMPA in
[29]
Entry Catalyst
Cycle Solvent Catalyst Loading
mol%]
Conversion
[%]
CH Cl has been reported.
2
2
[
α,β-Unsaturated carbonyl compounds including benzylide-
1
2
3
HMPA
HMPA
PIB-HMPA
5
1
2
1
PAO432
PAO432
heptane 6.5
4
–
51
<0.1
96
neacetone, (E)-1,3-diphenyl-2-propen-1-one (chalcone), diben-
zylideneacetone, β-ionone, (E)-2-benzylidenecyclopentan-1-one
and methyl cinnamate where examined as substrates. Amongst
these substrates, β-ionone and methyl cinnamate were soluble
4
5
6
7
8
9
1
PIB-HMPA
5
PIB-HMPA
5
PIB-HMPA
5
PIB-HMPA
5
PIB-HMPA
5
PIB-HMPA
5
PIB-HMPA
5
PIB-HMPA
5
2
3
4
5
1
2
3
4
5
heptane
heptane
heptane
heptane
PAO432
–
95
87
81
74
83
77
74
80
84
in PAO432. The other substrates had poor solubility in PAO432
.
–
We addressed this by adding 1.8–3.4 M ethyl acetate (EtOAc) as
a cosolvent. EtOAc was chosen for three reasons. First, EtOAc
has good miscibility with PAO432, it is considered a green
–
–
6.5
–
PAO432
0
PAO432
–
11
PAO432
–
1
2
PIB-HMPA
PAO432
–
Scheme 3. Reduction of an α,β-unsaturated carbonyl compound (1 equiv.)
with trichlorosilane (2 equiv.) in PAO432/EtOAc using 18 mol% 5 as a catalyst.
5
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Table 2. Scope of allylation reactions of aromatic aldehydes with different substituents catalyzed by PIB-supported catalyst 5 (10 mol%) in PAO432. The
conversions were calculated by taking and analyzing the MeCN layer from a biphasic separation (MeCN/PAO432). The average product yields were calculated
after combining the MeCN layers from cycle 1 to 5 and purifying via a column chromatography.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Substrate
Product
Conversion [%]
Cycle 1
Average
yield
[%]
Cycle 2
87
Cycle 3
84
Cycle 4
93
Cycle 5
89
9
8
8
2
1
2
83
75
70
67
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
78
73
72
74
73
87
90
86
71
85
78
78
74
solvent, and its volatility means it can be removed from the
went 1,4-reduction to form a saturated ketone in high
conversion (Table 3). In each case, the polar MeCN product-
containing phase was analyzed by H and P NMR spectro-
scopy. Similar to the results of the allylation, there was no
[52]
product solution and potentially recycled. In the presence of
catalyst 5 and trichlorosilane as a reducing agent, benzylidenea-
cetone, chalcone, dibenzylideneacetone and β-ionone all under-
1
31
Table 3. Scope of Lewis base-catalyzed conjugate reduction of various α,β-unsaturated carbonyl compounds.
Substrate
Condition
Product
Conversion [%]
Yield
[a]
[%]
PAO432
EtOAc (1.8 M),
,
[b]
94
83
2
h
PAO432
EtOAc (1.8 M),
,
>99
84
>99
2
h
PAO432
EtOAc (3.4 M),
,
80
4
h
PAO432
4 h
,
,
>
99
98
–
PAO432
EtOAc (1.8 M),
2 h
16
1
PAO432,
2 h
<0.1
–
1
[
a] Isolated yield. In cases of (E)-2-benzylidenecyclopentan-1-one and methyl cinnamate, the products were not further isolated after reactions due to the
poor conversions. [b] Averaged yield after 6 recycles of the reduction.
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significant leaching of catalyst or the PAO solvent leaching to Conclusion
the polar product phase. However, in the cases using (E)-2-
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
benzylidenecyclopentan-1-one and methyl cinnamate as sub-
strates, the reduction reactions proceed in low conversion. The
poor conversion of (E)-2-benzylidenecyclopentan-1-one could
be attributed to the unfavored six-membered cyclic transition
state between trichlorosilane and the s-cis conformation of (E)-
This work describes a PIB supported HMPA analog as a safer
and recoverable version of a highly toxic HMPA low molecular
weight catalyst. The phosphoramide terminated PIB oligomer 5
served as a catalyst in allylation reactions using a variety of
aromatic aldehyde substrates. Kinetic studies showed that
catalyst 5 was comparable in reactivity to HMPA in CH Cl and
[29]
2
-benzylidenecyclopentan-1-one. In any case, a similar low
2
2
conversion (<1%) of (E)-2-benzylidenecyclopentan-1-one in an
that its reactivity in heptane or a recyclable PAO solvent was
HMPA-catalyzed reduction in CH Cl was also seen. As for the
comparable to the reactivity of HMPA in CH Cl . Catalyst 5 was
2
2
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
reduction of methyl cinnamate, the reduction using HMPA as
the catalyst to reduce the unsaturated ester in CH Cl in
also a competent alternative to HMPA as a recyclable catalyst
for the conjugate reduction of various enones by HSiCl . In all of
2
2
3
presence of HSiCl also showed a negligible conversion (<1%)
these reactions, catalyst 5 along with the PAO reaction solvent
3
to the desired reduced product. These failures suggested that
both failures were due to the inapplicability of these α,β-
unsaturated carbonyl compounds in this type of reduction
was successfully recovered by a simple liquid/liquid (PAO432/
MeCN) extraction for at least 5 cycles with no significant loss in
activity.
rather than a limitation of 5 or a nonpolar milieu like PAO432
.
To further establish the synthetic utility of these trans-
formations and to examine recyclability of 5 and the PAO Experimental Section
solvent in these reductions, we examined reduction of
Chemicals and Instruments. Reagents were all purchased from
benzylideneacetone in more detail. In these reactions we first
removed the EtOAc at reduced pressure using a rotary
evaporator. The product was then separated from the PAO432
phase by adding MeCN using the same work-up procedure
used above in the allylation chemistry. Then fresh substrates
and EtOAc were added to the PAO432 phase with 5 that was
separated from the acetonitrile product phase. As shown in
Figure 3, 5 could be recycled with consistently high conversions
for 6 cycles. As was true for the allylation chemistry, the
recyclability of catalyst 5 and the solvent system makes this a
greener process than the prior work that used HMPA and
dichloromethane. We also showed that this product afforded
good isolated yields by combining the product phases from all
six cycles and isolating the product. The isolated yield was
averaged 81%/cycle.
commercial sources and used without further purification. Tetrahy-
drofuran (THF) was distilled from sodium benzophenone ketyl.
Dichloromethane (DCM) was purified using a SPBT-1 Bench Top
Solvent Purification System before use. (E)-1,3-Diphenyl-2-propen-
1
-one (chalcone), dibenzylideneacetone and (E)-2-benzylidenecyclo-
pentan-1-one were prepared by an aldol condensation. Alkene-
terminated PIB with a molecular weight (M ) of 1000 Da is
n
commercially available and was obtained from Texas Polymer
[
53]
Corp.
molecular weight (M
31
The poly(α-olefin) (PAO) solvent used had a reported
) of 432 Da and was obtained from Exxon
n
[54]
1
13
Mobil.
H, C, and P NMR spectra were recorded on a Varian
500 MHz spectrometer interfaced to a Linux computer and were
worked up using VNMR-J software and on Bruker Avance Neo 400
instruments. Chemical shifts are reported in ppm and referenced to
the residual proton in CDCl , and spin multiplicities are indicated by
the following symbols: s (singlet), d (doublet), t (triplet), dd (doublet
of doublet), and
phosphoric acid (85 wt% in H O) at 0 ppm was used as an external
3
31
m
(multiplet). For
P NMR spectroscopy,
2
standard. Infrared spectra (IR) were recorded on a Shimadzu
IRAffinity-1S IR spectrophotometer. A multi-detector Viscotek gel
permeation chromatograph (GPC) and a Viscotek LT400 L column
was used to analyze the molecular weight of the polymer products
at a flow rate of 1 mL/m using THF as the eluent. A value for dn/
dc=0.093 was utilized to calculate the PIB molecular weight and
[
55]
molecular weight distribution. The GPC data analysis was carried
out using Omnisec software version 4.7.
Hydroxy-Terminated PIB Oligomer 2. Compound 2 was prepared
[8]
using a reported procedure which yielded 9.98 g of 2 (98% yield).
1
H NMR (CDCl ) δ: 0.75–1.46 (m, 208H), 3.33 (dd, 1H), 3.50 (dd, 1H).
3
1
3
C NMR (CDCl
) δ: 19.7, multiple peaks between 29.1 and 38.3,
3
49.5, multiple peaks between 56.8 and 59.6, 69.7.
Iodide-Terminated PIB Oligomer 3. Compound 3 was prepared
[8]
using a reported procedure on a 26 g scale to afford 3 as a viscous
1
oil (89% yield). H NMR (CDCl ) δ: 1.00–1.43 (m, 220H), 3.13 (dd, 1H),
3
1
3
3
2
.27 (dd, 1H). C NMR (CDCl ) δ: 20.8, 23.9, multiple peaks between
3
9.1 and 38.3, 52.7, multiple peaks between 56.9 and 59.4.
Propylamine-Terminated PIB Oligomer 4. To a 50 mL heptane
solution of iodide-terminated PIB oligomer (3) (20 g, 20 mmol) was
added a 50 mL DMF solution of propylamine (18 mL, 200 mmol) to
form a biphasic reaction mixture. Upon heating to 90°C, the
biphasic mixture became single phase. After 24 h, the reaction
Figure 3. Recyclability data of 5 in catalytic reduction of benzylideneacetone
with HSiCl in PAO432 containing 1.8 M of EtOAc as a cosolvent.
3
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mixture was cooled to room temperature to reform a biphasic
mixture. The two layers were separated, and the heptane phase
was washed with DMF (2×25 mL), water (2×25 mL), and brine (1×
2 mmol) was then added into the reaction mixture. The reaction
was stirred at room temperature and aliquots (50 μL) of the
reaction mixture were removed at different time intervals. Each
aliquot was diluted with 1 mL of CH Cl and washed with 1 mL of
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
25 mL). The heptane layer was then dried with MgSO , and the
4
2
2
solvent was removed under reduced pressure using a rotary
evaporator. The product viscous oil was purified by column
aqueous ammonium chloride (NH Cl). The mixture was separated
4
by centrifugation, the two phases were separated, and the CH Cl2
2
chromatography (Brockmann aluminum oxide) to afford 19.2 g of 4
phase was concentrated at reduced pressure using a rotary
1
1
(
(
96% yield). H NMR (CDCl ) δ: 1.00–1.41 (m, 308 H), 2.18 (1H), 2.31
evaporator. The residue dissolved in CDCl and analyzed by H NMR
3
3
13
dd, 1 H), 2.48 (dd, 1 H), 2.53 (m, 2 H); C NMR (CDCl ) δ: 11.8, 14.0,
spectroscopy.
3
21.3, 22.7, 23.2, multiple peaks between 29.0 and 38.1, 47.3, 51.7,
51.9, multiple peaks between 58.4 and 59.5.
Benzaldehyde Allylation and Recycling of Catalyst 5. A scintilla-
tion vial containing a solution of benzaldehyde (0.1 mL, 1 mmol),
DIPEA (0.5 mL), catalyst 5 (0.25 g, 0.0625 mmol) and PAO432 (2 mL)
was prepared. Allyltrichlorosilane (0.29 mL, 2 mmol) was added into
the reaction mixture via a syringe. After 24 h stirring, the reaction
was quenched by adding 5 mL of MeCN. The reaction mixture was
then transferred into a centrifuge tube the phases separated with a
15 min centrifugation. The bottom layer (MeCN phase) separated
and the conversion was analyzed as described above. The PAO
phase was washed with 5 mL of NaOH (4N) and returned to a
scintillation vial for the next cycle. To isolate the product, the MeCN
phase from the five cycles was combined, the solvent was removed
at reduced pressure and the crude 1-phenylbut-3-en-1-ol was
purified by a column chromatography. The product was all
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
PIB-HMPA 5. Propylamine-terminated PIB oligomer 4 (9.93 g,
9
.9 mmol) was dissolved in 50 mL of anhydrous THF and allowed to
n
react with. 5 mL of BuLi (2.5 M) which was added dropwise. The
reaction solution was allowed to stir for 2 h at room temperature.
Then N,N,N’,N’-tetramethylphosphorodiamidic chloride (2 mL,
14 mmol) was added and the reaction mixture was stirred for 6 h at
room temperature. The solvent was then removed under reduced
pressure using a rotatory evaporator until ca. 10 mL of reaction
residue remained. This viscous residue was then added to a 40-fold
excess volume of cold methanol. The supernatant MeOH was
removed by decantation. The insoluble oil that remained was re-
dissolved in 10 mL of THF. A second solvent trituration using a 40-
fold excess volume of cold methanol to afford the desired product.
The MeOH was again removed and the viscous residue was
1
13
characterized by H, C NMR and IR spectroscopy.
1-Phenylbut-3-en-1-ol: The product was isolated by flash chroma-
collected and dried under vacuum overnight to afford 7.89 g of 5
1
tography (CH
of product/cycle (81%)). H NMR (500 MHz, CDCl
m, 2H), 4.74 (m, 1H), 5.17 (m, 2H), 5.82 (m, 1H), 7.27–7.37 (m 5H).
Cl /hexanes=8/1, v/v) as colorless liquid (0.6 g, 0.12 g
2 2
1
as a dark orange oil. H NMR (CDCl , δ): 2.62–2.87 (m, 16H), 1.0–1.41
3
13
, δ): 2.19 (1H), 2.53
3
(m, 272H). C NMR (CDCl
, δ): 11.6, 14.1, multiple peaks between
3
(
2
4
2
1
0.6 and 32.5, 34.6, 35.9, multiple peaks between 36.0 and 38.2,
13
31
C NMR (125 MHz, CDCl
3
, δ): 43.7, 73.3, 118.3, 125.7, 127.5, 128.3,
5.8, multiple peaks between 51.1 and 59.5. P NMR (CDCl , δ):
3
À 1
À 1
À 1
134.4, 143.8. IR (neat) data: 3386, 3066, 3031, 2909, 1641 cm .
5.3 ppm. GPC data: M =1,700 gmol , M =2,800 gmol , Đ=
n
w
.65. The exact amount of the phosphoramide per gram was
Lewis Base-Catalyzed Allylation of Other Aromatic aldehydes. These
reactions were all carried out on a 0.1 mmol scale following the
procedure used for benzaldehyde. 5 used here was the other batch
from the experiment of benzaldehyde allylation and recycling of
catalyst 5. The HMPA amount was calculated accordingly (0.25 g,
1
determined by H NMR spectroscopy using dichloroethane as an
internal standard and varied from 0.4 to 0.25 mmol/g.
Phase Selectivity Solubility of Catalyst 5 in PAO432 or heptane
versus MeOH. 0.25 g of 5 was dissolved in 2 mL of PAO432. Then the
PAO432 phase was vigorously mixed with 2 mL of 0.03 M of H PO in
1
mmol). Products from the combined MeCN phases from 5 cycles
3
4
were further purified by a column chromatography and character-
ized by H, C NMR and IR spectroscopy.
MeOH for 48 h. The biphasic mixture was transferred into a
centrifuge tube and underwent a 15-min centrifugation. 0.4 mL of
1
13
1
the MeOH and PAO phase was taken individually and directly
432
1-(p-Tolyl)but-3-en-1-ol: H NMR (500 MHz, CDCl , δ): The product
3
31
analyzed by P NMR spectroscopy. The phase selectivity of 5 in
heptane with MeOH was also studied and analyzed via the same
procedure. The catalyst 5 showed high phase selectivity solubility
in PAO432 (>99.9%) and the MeOH phase showed no detectable
was isolated by a flash chromatography (EA/hexanes=1/4, v/v) as
colorless liquid (0.57 g, 0.11 g of product/cycle (70%)). 1.97 (d, J=
3
Hz, 1H), 2.35, (br, s, 3H), 2.51 (m, 2H), 4.71 (m, 1H), 5.14 (m, 2H),
13
5.81 (m, 1H), 7.16 (d, J=8 Hz, 2H), 7.25 (d, J=8 Hz, 2H).
C NMR
signal from 5. The internal standard H PO also had no leaching to
3 4
(125 MHz, CDCl , δ): 21.1, 43.7, 73.2, 118.2, 125.7, 129.1, 134.6,
3
the PAO432. However, a phase selectivity solubility of 5 in heptane
versus MeOH showed ca. 4.8% 5 leached from heptane to the
MeOH phase.
137.2, 140.9. IR (neat) data: 3391, 3076, 2935, 2929, 2924, 1640,
À 1
1
514 cm .
1-(4-Methoxyphenyl)but-3-en-1-ol: The product was isolated by a
Phase Selectivity Solubility of Catalyst 5 and 1-phenylbut-3-en-1-
ol in PAO432 versus MeCN. 0.25 g of 5 was dissolved in in 2 mL of
PAO432 and vigorously mixed with 2 mL of 0.5 M of 1-phenylbut-3-
en-1-ol in MeCN for 1 min. The biphasic mixture was transferred
into a centrifuge tube and underwent a 15-min centrifugation. An
aliquot (100 μL) was taken from both the MeCN and PAO phase
flash chromatography (EA/hexanes=1/4, v/v) as yellow liquid
1
(0.6 g, 0.12 g of product/cycle (67%)). H NMR (500 MHz, CDCl , δ):
3
1.96 (s, 1H), 2.51 (m, 2H), 3.81 (s, 3H), 4.69 (m, 1H), 5.14 (m, 2H), 5.79
1
3
(
m, 1H), 6.88 (d, J=8.5 Hz), 7.28 (d, J=8.5 Hz). C NMR (125 MHz,
CDCl , δ): 43.7, 55.2, 72.9, 113.7, 118.1, 127.0, 134.6, 136.0, 158.9.IR
3
À 1
4
32
(neat) data: 3428, 3076, 2972, 2932, 1611, 1512 cm .
1
and analyzed by H NMR spectroscopy in CDCl
using 0.2 M of
3
1
-(4-fluorophenyl)but-3-en-1-ol: The product was isolated by a flash
dichloroethane as an internal standard. The amount of 5 that
leached into the MeCN phase was calculated to be <0.1% and the
amount of leaching of 1-phenylbut-3-en-1-ol into the PAO phase
chromatography (EA/hexanes=1/5, v/v) as yellow liquid (0.63 g,
1
0.13 g of product/cycle (75%)). H NMR (500 MHz, CDCl , δ): 2.04 (s,
3
4
32
1
1H), 2.48 (m, 2H), 4.72 (m, 1H), 5.16 (m, 1H), 5.79 (m, 1H), 7.03 (m,
was calculated to be 1.6% based on H NMR spectroscopy.
13
2H), 7.32 (m, 2H). C NMR (125 MHz, CDCl , δ): 43.9, 72.6, 115.1,
3
Typical Procedure for Kinetic Studies of Allylation of Benzalde-
hyde. For the kinetic studies, benzaldehyde (0.1 mL, 1 mmol), DIPEA
1
3
15.2, 118.6, 127.4, 127.5, 134.1, 139.5, 161.1, 163.1. IR (neat) data:
393, 3077, 2976, 2934, 1641, 1603, 1509 cm .
À 1
(
0.5 mL), and catalyst 5 (0.25 g, 0.1 mmol) were dissolved in 2 mL of
Reduction of Various α,β-Unsaturated Carbonyl Compounds by 5
in PAO432 and Catalyst Recycling. A 10-mL round-bottomed flask
CH Cl . HMPA or 5 was then added at a concentration of 10 mol%
2
2
(0.1 mmol of an HMPA equivalent). Allyltrichlorosilane (0.29 mL,
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7
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containing α,β-unsaturated carbonyl compounds (0.5 mmol), ethyl
acetate (0–3.4 M), 5 (0.25 g, 0.1 mmol) and PAO432 (2 mL) was
prepared. Trichlorosilane (0.1 mL, 1 mmol) was then added into the
Keywords: recyclable organocatalyst
polyisobutylene supported catalysts · allylation · reduction
·
poly(α-olefin)s
·
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
reaction mixture via a syringe under N . The reaction was
2
monitored by TLC. After the starting material was consumed or no
significant change was observed, the cosolvent EtOAc was removed
at reduced pressure using a rotary evaporator. Then 5 mL of MeCN
was introduced to form a biphasic mixture of 5 in PAO432 and the
product in MeCN. A 15-min centrifugation separated the phases.
The PAO phase was washed with 5 mL of NaOH and reused as
described above. The bottom MeCN phase was analyzed as
described above to determine conversions and the products were
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
either directly analyzed or purified by
a
silica gel column
[6] M. L. Harrell, T. Malinski, C. Torres-López, K. Gonzalez, J. Suriboot, D. E.
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chromatography and analyzed by H, C NMR and IR spectroscopy.
[
9] D. E. Bergbreiter, Y.-C. Yang, C. E. Hobbs, J. Org. Chem. 2011, 76, 6912–
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4
-Phenyl-2-butanone: The product was isolated by a flash chroma-
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tography (EA/hexanes=1/4, v/v) as colorless liquid (0.37 g, 0.061 g
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1
8084–8087; Angew. Chem. 2014, 126, 8222–8225.
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3
[
[
2.74 (t, J=7.5 Hz, 2H), 2.88 (t, J=7.5 Hz, 2H), 7.16 (m, 3H), 7.26 (m,
1
3
2
1
H). C NMR (125 MHz, CDCl , δ): 29.7, 30.0, 45.1, 126.1, 128.2,
3
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À 1
28.4, 140.9, 207.8. IR (neat) data: 3027, 2959, 1714, 1602 cm .
[
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1
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7.56 (t, J=10 Hz, 1.25 Hz, 1H), 7.96 (d, J=10 Hz, 2H). C NMR
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FULL PAPERS
Y.-H. Fu, Prof. D. E. Bergbreiter*
1 – 10
Recyclable Polyisobutylene-Bound
HMPA as an Organocatalyst in Re-
cyclable Poly(α-olefin) Solvents
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Organocatalysis: A recyclable (PIB)
supported organocatalyst HMPA (PIB-
HMPA) is described and comparable
to HMPA. PIB-HMPA successfully
hyde and the reduction of enones in
poly(α-olefin)s (PAOs). PIB-HMPA/
PAOs can be recycled by a biphasic
extraction and retained the excellent
activities for at least five cycles.
catalyzed the allylation of a benzalde-