Rasta resin-triphenylphosphine oxides and their use as recyclable heterogeneous reagent precursors in halogenation reactions

Article abstract of DOI:10.3762/bjoc.10.143

Heterogeneous polymer-supported triphenylphosphine oxides based on the rasta resin architecture have been synthesized, and applied as reagent precursors in a wide range of halogenation reactions. The rasta resin-triphenylphosphine oxides were reacted with either oxalyl chloride or oxalyl bromide to form the corresponding halophosphonium salts, and these in turn were reacted with alcohols, aldehydes, aziridines and epoxides to form halogenated products in high yields after simple purification. The polymersupported triphenylphosphine oxides formed as a byproduct during these reactions could be recovered and reused numerous times with no appreciable decrease in reactivity.

Full text of DOI:10.3762/bjoc.10.143

Rasta resin–triphenylphosphine oxides and their use
as recyclable heterogeneous reagent precursors
in halogenation reactions
Xuanshu Xia and Patrick H. Toy*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1397–1405.
Department of Chemistry, The University of Hong Kong, Pokfulam
Road, Hong Kong, People’s Republic of China
doi:10.3762/bjoc.10.143
Received: 13 February 2014
Accepted: 18 May 2014
Published: 20 June 2014
Email:
Patrick H. Toy* - phtoy@hku.hk
* Corresponding author
This article is part of the Thematic Series "Organophosphorus chemistry".
Guest Editor: P. R. Hanson
Keywords:
Appel reaction; halogenation; organophosphorus; polymer-supported
reagent; rasta resin; triphenylphosphine oxide
© 2014 Xia and Toy; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Heterogeneous polymer-supported triphenylphosphine oxides based on the rasta resin architecture have been synthesized, and
applied as reagent precursors in a wide range of halogenation reactions. The rasta resin–triphenylphosphine oxides were reacted
with either oxalyl chloride or oxalyl bromide to form the corresponding halophosphonium salts, and these in turn were reacted with
alcohols, aldehydes, aziridines and epoxides to form halogenated products in high yields after simple purification. The polymer-
supported triphenylphosphine oxides formed as a byproduct during these reactions could be recovered and reused numerous times
with no appreciable decrease in reactivity.
Introduction
One of the major drawbacks of the Wittig [1] and Mitsunobu In addition to being relatively easily reduced, halophospho-
[2,3] reactions is that they result in the formation of a stoichio- nium salts 3a,b are also useful reagents in a wide range of reac-
metric quantity of triphenylphosphine oxide (1) as a byproduct. tions, such as those illustrated in Scheme 2: (1) the conversion
From an atom economy perspective this is less than ideal, and
from an environmental point of view it would be good if 1
could be simply reduced to triphenylphosphine (2) for reuse [4].
In this regard Tanaka and co-workers have studied the possi-
bility of applying the reaction first reported by Masaki and
Fukui [5] in which 1 can be treated with oxalyl chloride (or bro-
mide) to form halophosphonium salt 3a (or 3b), which in turn
Scheme 1: The Masaki–Fukui reaction and halophosphonium salt
reduction.
can be reduced to 2 under more mild reaction conditions than
can 1 (Scheme 1) [6,7].
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
Scheme 3: Catalytic Appel reactions reported by Denton and
co-workers.
sized rasta resin–Ph3P (14) in various Wittig reactions that
required only filtration and solvent removal for product purifi-
cation (Figure 1) [27-29]. Additionally, 14 was converted into
phosphonium salt 15, which proved to be an efficient and
highly recyclable catalyst for aldehyde and ketone cyanosilyla-
tion reactions from which the products could also be obtained
Scheme 2: Representative reactions involving halophosphonium salts
3a,b.
of alcohols 4 to alkyl halides 5 (the Appel reaction), (2) the pure after only filtration and solvent removal [30]. It should be
conversion of aldehydes 6 to 1,1-dihaloalkanes 7, (3) halogena- noted that the grafts of the rasta resins reported are random
tion of aziridines 8 to form 2-haloamines 9, (4) halogenation of co-polymers, and the structures drawn for them are not mean to
epoxides 10 to form 1,2-dihaloalkanes 11, (5) and the dehydra- indicate that they are block co-polymers. The format for their
tion of oximes 12 to form nitriles 13.
presentation is used merely to indicate their monomer incorpor-
ation ratios.
Capitalizing on the fact that 1 is formed as a byproduct from
3a,b in each of these reactions, Denton and co-workers have
recently combined the Masaki–Fukui reaction with many of the
functional group transformation outlined in Scheme 2, in one-
pot processes in which the role of 1 is referred to as that of a
catalyst [8-12]. For example, catalytic Appel reactions were
achieved by slowly adding separate solutions of oxalyl chloride
and alcohols 4 to a solution of 1 (Scheme 3) [8,9]. In these reac-
tions, the simultaneous slow addition of oxalyl chloride and
alcohol substrate 4 to a sub-stoichiometric quantity of 3a was
necessary in order to minimize formation of undesired ester
side-products formed by the reaction of 4 with the acid chloride.
Furthermore, chromatographic purification of the alkyl halide
product 5 was required. Thus, while the procedures reported by
Denton et al. might be conceptually interesting, they may not be
particularly convenient to perform, especially on larger scales
than what was originally reported.
Figure 1: Rasta resins 14 and 15.
Thus, considering our prior success in using 14 and 15, we
We have had a long-term interest in the use of organic poly- wanted to oxidize 14 to 16, and in turn use this as a heteroge-
mers as supports for reagents and catalysts [13], and have neous precursor to reagents 17a,b for use in the halogenation
reported the use of various polymer-supported phosphines as reactions described in Scheme 2. We anticipated that using a
reagents, organocatalysts, and ligands in order to simplify pro- full equivalent 17a,b generated in situ would eliminate the need
duct isolation [14-18]. Most recently we have studied the use of for slow addition of the oxalyl halide to form the halophospho-
the rasta resin polystyrene architecture [19-26] as a platform for nium salt, and thus allow for the reactions to be performed more
reagents and catalysts [27-33], and have used easily synthe- conveniently than in the catalytic procedures of Denton and
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
co-workers. Furthermore, since 16 would be the byproduct of TLC analysis, and they were all finished in 4–7 hours. Upon
the reactions, it could be recovered by filtration at the end of the completion, the reaction mixtures were cooled to room tempera-
reactions and reused directly. Herein we report the realization of ture and then filtered. Finally, the filtrates were concentrated at
this strategy and describe simple procedures for alcohol, aziri- reduced pressure to afford the desired products that were essen-
dine, aldehyde and epoxide halogenation reactions from which tially pure based on 1H and 13C NMR spectroscopic analyses.
the desired products are easily isolated and the phosphine oxide Chromatographic purification of the resultant alkyl halide prod-
byproduct is readily recycled.
ucts was not required. As can be seen in Table 1, both primary
(entries 1–8) and secondary (entries 9 and 10) benzylic alco-
hols with various substituents all afforded excellent yield of the
Results and Discussion
Rasta resin 16 was prepared by oxidation of 14, which was corresponding chloride and bromide using this procedure.
prepared as previously reported [28], using H2O2 (Scheme 4). Similar high yields were also obtained from reactions using pri-
The loading level of 16 was determined by elemental analysis to mary (Table 1, entries 11–14) and secondary aliphatic alcohols
be 0.97 mmol/g, and gel-phase 31P NMR spectroscopic analysis (Table 1, entries 15 and 16). A reaction performed on a scale
of 16 showed only a single peak at 29.4 ppm, indicating that the ten times larger afforded excellent yield as well (Table 1, entry
phosphine groups were completely oxidized.
17). The recovered polymer was washed sequentially with
water, MeOH, THF, diethyl ether and hexane. After drying, 31P
NMR spectroscopy confirmed its identity as 16.
Appel reactions using 16
With 16 in hand, we initially used it to perform Appel reactions
by first converting it into either 17a or 17b in situ (Scheme 4). Aldehyde halogenation reactions using 16
To do this, 16 was suspended/swollen in dichloromethane, and With the success of the Appel reactions, we further examined
then the appropriate oxalyl halide was added. Once gas evolu- the utility of 16 by studying its use in aldehyde halogenation
tion ceased, alcohol 4 was added, and the reaction mixture was reactions. As before, 16 was converted into 17a or 17b in situ,
heated to reflux. Progress of the reactions was monitored by and aldehyde 6 was added upon cessation of gas evolution.
Scheme 4: Synthesis and applications of rasta resins 16 and 17a,b.
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
Table 1: Appel reactions using 16.a
Entry
Starting material
Product
Yield (%)
1
2
3
5Aa, X = Cl
5Ab, X = Br
5Ba, X = Cl
98
92
93
4A
4B
4
5
6
5Bb, X = Br
5Ca, X = Cl
5Cb, X = Br
91
93
90
4C
7
8
5Da, X = Cl
5Db, X = Br
5Ea, X = Cl
5Eb, X = Br
98
93
85
89
4D
4E
9
10
11
12
13
14
5Fa, X = Cl
5Fb, X = Br
5Ga, X = Cl
5Gb, X = Br
95
90
98
92
4F
4G
15
16
5Ha, X = Cl
5Hb, X = Br
95
91
4H
17b
5Ha, X = Cl
92
aReaction conditions: 0.6 mmol 16, 0.6 mmol oxalyl halide, 0.5 mmol 4 in 5 mL CH2Cl2, reflux. bReaction conditions: 6 mmol 16, 6 mmol oxalyl halide,
5 mmol 4 in 50 mL CH2Cl2, reflux.
After 72 hours at reflux, the reactions were stopped, filtered and Aziridine halogenation reactions using 16
the products were purified by column chromatography We next examined the use of 16 as a precursor to 17a and 17b
(Table 2). Unlike the Appel reactions discussed above, some of in aziridine halogenation reactions [34]. Using reaction condi-
these reactions did not proceed to completion, even when the tions similar to those used for the Appel and aldehyde halogena-
reaction time was lengthened. Generally, it was observed that tion reactions, a variety of N-tosyl aziridines 8 were success-
the bromination reactions afforded higher product yields than fully converted into the corresponding chloro- or bromotosy-
did the corresponding chlorination reactions (Table 2, entries lamides 9 in excellent yields (Table 3). The trans configura-
1–8), except when electron-rich aldehyde starting materials tions of the 9Ba and 9Bb were confirmed by X-ray diffraction
were used (Table 2, entries 9–12). These results are generally analysis of the isolated products, and as was true for the Appel
similar to what was previously reported by Denton and reactions described above, all the products were obtained in
co-workers when similar substrates were used [12].
high purity simply by filtration to remove the polymer, and
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
Table 2: Halogenation reactions of aldehydes using 16.a
Entry
Starting material
Product
Yield (%)
1
2
3
7Aa, X = Cl
7Ab, X = Br
7Ba, X = Cl
54b
75b
65b
6A
6B
4
5
7Bb, X = Br
7Ca, X = Cl
7Cb, X = Br
7Da, X = Cl
7Db, X = Br
7Ea, X = Cl
7Eb, X = Br
7Fa, X = Cl
7Fb. X = Br
83b
64b
89b
61b
93b
94c
20d
85b
77b
6C
6D
6E
6F
6
7
8
9
10
11
12
aReaction conditions: 0.3 mmol 16, 0.3 mmol oxalyl halide, 0.1 mmol 6 in 3 mL CHCl3, reflux. bIsolated yield after flash silica gel column chromatog-
raphy. cIsolated yield after filtration and concentration under reduced pressure. dDetermined by 1H NMR spectroscopy.
concentration of the filtrate. Our procedure was also success- halogenation reactions shown in Scheme 2. Since these reac-
fully performed on a gram-scale (Table 3, entry 5), and tions require the use of a base, we designed a bifunctional rasta
substrates 8 possessing a single aryl substituent were halo- resin, RR-NBniPr2-PPh3=O 18 (Scheme 5), which bears both
genated as the less hindered position (Table 3, entries 10–13). triphenylphosphine oxide and tertiary amine moieties, in order
to increase the efficiency and appeal of our method. We have
Recovery and reuse of 16
extensive experience in preparing functionalized resins with
After demonstrating that 16 could effectively serve as a two different catalytic groups [35-38], and prepared 18 by oxi-
precursor to 17a,b, and realizing that it was returned as the dation of 19, which we previously used as a bifunctional
byproduct of the above reactions, we next examined its re- reagent in one-pot Wittig reactions [29]. Gel-phase 31P NMR
cyclability in the chlorination of alcohol 4H and aziridine 8B. spectroscopic analysis of 18 indicated that oxidation of the
The results of these studies are shown in Table 4 and Table 5, phosphine groups was complete, and elemental analysis was
respectively. In these experiments, the polymer recovered by used to determine the loading level of phosphine oxide and
filtration of the reaction mixture was washed and dried, and amine groups to be 1.07 mmol/g and 1.06 mmol/g, respectively.
then used directly for the next reaction cycle. Excellent yields It should be noted that a test reaction between N,N-diisopropyl-
were successfully obtained for 8 runs with both 4H and 8B. benzylamine and H2O2 under similar reaction conditions does
Gel-phase 31P NMR analysis of 16 recovered at the end of these not result in amine oxidation, and this seems to indicate that
experiments indicated no change in its oxidation state.
only the phosphine groups of 19 are oxidized during its conver-
sion to 18.
Epoxide halogenation reactions
With the versatility and excellent reactivity of 16 established, Having successfully synthesized polymer 18, we examined its
we were encouraged to examine our method in the epoxide reactivity in epoxide halogenation reactions (Table 6). As
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
Table 3: Halogenation reactions of aziridines using 16.a
Entry
1
Starting material
Product
Yield (%)
9Aa, X = Cl
9Ab, X = Br
95
93
8A
2
3
4
9Ba, X = Cl
9Bb, X = Br
89
92
8B
8C
8D
5b
6
9Ba, X = Cl
9Ca, X = Cl
95
96
7
8
9
9Cb, X = Br
9Da, X = Cl
9Db, X = Br
98
93
92
10
11
9Ea, X = Cl
9Eb, X = Br
91
93
8E
8F
12
13
9Fa, X = Cl
9Fb, X = Br
93
95
aReaction conditions: 0.6 mmol 16, 0.6 mmol oxalyl halide, 0.5 mmol 8 in 5 mL CH2Cl2, reflux. bReaction conditions: 6 mmol 16, 6 mmol oxalyl halide,
5 mmol 8 in 50 mL CH2Cl2, reflux.
Table 4: Recycling of 16 in the chlorination of 4H.a
Table 5: Recycling of 16 in the chlorination of 8B.a
Run
1
2
3
4
5
6
7
8
Run
1
2
3
4
5
6
7
8
Yield of 9Ba (%)
93 92 95 93 87 91 91 90
96 97 95 95 96 95 90 92
Yield of 5Ha (%)
94 95 93 98 94 95 95 93
99 99 99 99 98 95 93 91
Recovery of 16 (%)
Recovery of 16 (%)
aReaction conditions: 0.6 mmol 16, 0.6 mmol oxalyl chloride, 0.5 mmol
8B in 5 mL CH2Cl2, reflux.
aReaction conditions: 0.6 mmol 16, 0.6 mmol oxalyl chloride, 0.5 mmol
4H in 5 mL CH2Cl2, reflux.
before, 18 was suspended/swollen in solvent prior to addition of epoxides bearing phenyl (Table 6, entries 1 and 2), benzyl
the oxalyl halide. Once gas evolution ceased, epoxide 10 was (Table 6, entries 3 and 4), and alkyl substutients (Table 6,
added, and the reaction mixture was heated to reflux. When entries 5–8) all proceeded to completion, and afforded the
TLC analysis indicated that the reactions were complete, 3–4 corresponding 1,2-dihalides in excellent yields.
hours, the reaction mixtures were filtered and the filtrates were
concentrated to afford products that were essentially pure We also examined the recyclability of 18 in the halogenation of
according to both 1H and13C NMR analysis. Reactions with epoxide 10C (Table 7). As was the case for 16, bifunctional
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
Table 6: Halogenation reactions of epoxides using 18.a
Entry
1
Substrate
Product
Yield (%)
11Aa, X = Cl
11Ab, X = Br
95
93
10A
2
3
4
5
11Ba, X = Cl
11Bb, X = Br
11Ca, X = Cl
89
92
95
10B
10C
6
7
8
11Cb, X = Br
11Da, X = Cl
11Db, X = Br
96
98
92
10D
aReaction conditions: 1.2 mmol 18, 1.1 mmol oxalyl halide, 0.5 mmol 10 in 5 mL CHCl3, reflux.
Scheme 5: Synthesis of bifuncitonal rasta resin 18.
polymer 18 could be repeatedly recovered and reused without
Table 7: Recycling of 18 in the chlorination of 10c.a
an observable decrease in its reactivity. However, whereas 16
could be reused directly after recover, reusing 18 required
washing it with an aqueous solution of Na2CO3 after each reac-
tion.
Run
1
2
3
4
5
6
7
Conclusion
In summary, we have designed and synthesized recyclable
heterogeneous rasta resin-supported triphenylphosphine oxide
16, and have applied it as a phosphonium halide salt precursor
in a wide range of halogenation reactions from which it is
readily recovered and reused. The reusability of this polymer
Yield of 11Ca (%)
Recovery of 18 (%)
94 91 93 94 93 92 90
98 98 97 98 95 93 90
aReaction conditions: 1.2 mmol 18, 1.1 mmol oxalyl chloride, 0.5 mmol
10C in 5 mL CHCl3, reflux.
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Beilstein J. Org. Chem. 2014, 10, 1397–1405.
was demonstrated by the fact that all of the reactions reported removed under reduced pressure to afford the desired product 9
herein involving 16 were performed by repeatedly reusing a in an essentially pure state based on 1H and 13C NMR spectro-
single batch of it. We also prepared bifunctional rasta resin 18 scopic analyses.
which bears amine groups in addition to the phosphine oxide
moieties for use in epoxide halogenation reactions. The prod- General procedure for epoxide halogenation reactions using
ucts of all of these reactions, except for the aldehyde halogena- 18: Polymer 18 (1.3 g, 1.2 mmol) was suspended in chloroform
tion reactions, can be easily separated from the phosphine oxide (10 mL) and the oxalyl halide was added (1.1 mmol). Upon
functionalized polymer simply by filtration, and isolated cessation of gas evolution, epoxide 10 (0.5 mmol) was added,
directly in high purity. Thus, the use of polymers 16 or 18 as and the reaction was magnetically stirred and heated to reflux.
reagent precursors represents a convenient alternative to the After the reaction was completed as monitored by TLC, the
“phosphine oxide-catalyzed” methods of Denton and mixture was cooled to room temperature and filtered. The solid
co-workers, which generally require slow addition of the oxalyl on funnel was washed with dichloromethane (10 mL × 3). The
halide and chromatographic purification of the product. In order solvent of filtrate was removed under reduced pressure to afford
to assess the overall utility of 16 and 18, we are currently exam- the desired product 11 in an essentially pure state based on 1H
ining their applications as organocatalysts [39-43], and will and 13C NMR spectroscopic analyses.
report results of these studies in due course.
General procedure for recovery and reuse of 16 and 18:
Experimental
After being separated from the reaction mixture by filtration,
General procedure for Appel reactions using 16: Polymer 16 the polymer, 16 or 18, was rinsed sequentially using deionized
(0.6 g, 0.6 mmol) was suspended in dichloromethane (5 mL), water (30 mL), dichloromethane (50 mL), MeOH (50 mL), THF
and the oxalyl halide (0.6 mmol) was added. Upon cessation of (50 mL), diethyl ether (50 mL), hexane (50 mL). It was then
gas evolution, alcohol 4 (0.5 mmol) was added, and the reac- dried under vacuum at 60 °C prior to use in the next reaction
tion mixture magnetically stirred and heated to reflux. After the cycle. Furthermore, 18 was initially washed with a saturated
reaction was completed as monitored by TLC, the mixture was aqueous solution of Na2CO3 in order to ensure that it was
cooled to room temperature and filtered. The solid on funnel deprotonated and ready for use in the next reaction cycle.
was washed with dichloromethane (10 mL × 3). The solvent of
filtrate was removed under reduced pressure to afford the Note Added in Proof
desired product 5 in an essentially pure state based on 1H and After the initial submission of our manuscript we became aware
13C NMR spectroscopic analyses.
of a recent report by Denton and co-workers regarding similar
work using polystyrene-supported halophosphonium salts in
General procedure for aldehyde halogenation reactions Appel and dehydration reactions [44]. This work utilized a
using 16: Polymer 16 (0.3 g, 0.3 mmol) was suspended in chlo- commercially available polymer-supported phosphine oxide
roform (3 mL), and the oxalyl halide was added (0.3 mmol). based on the Merrifield resin architecture, and it is noteworthy
Upon cessation of gas evolution, aldehyde 6 (0.1 mmol) was that the reactions reported in this manuscript required a 6-fold
added, and the reaction mixture was magnetically stirred and excess of the halophosphonium salt compared to the substrate.
heated to reflux. After 72 hours, the reaction was cooled to Use of our rasta resins 16 and 18 required only a 20 mol %
room temperature and filtered. The solid on funnel was washed excess.
with dichloromethane (5 mL × 3). The solvent of filtrate was
removed under reduced pressure. The resulting crude product 7
Supporting Information
was purified by flash silica gel column chromatography using
ethyl acetate and hexane as eluent mixture.
Supporting Information File 1
Additional experimental details and characterization data of
General procedure for aziridine halogenation reactions of
using 16: Polymer 16 (0.6 g, 0.6 mmol) was suspended in
dichloromethane (5 mL), and the oxalyl halide was added
(0.6 mmol). Upon cessation of gas evolution, aziridine 8
(0.5 mmol) was added, and the reaction mixture was magneti-
cally stirred and heated to reflux. After the reaction was
completed as monitored by TLC, the mixture was cooled to
room temperature and filtered. The solid on funnel was washed
with dichloromethane (10 mL × 3). The solvent of filtrate was
synthesized compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-143-S1.pdf]
Acknowledgements
This research was supported financially by the University of
Hong Kong and the Research Grants Council of the Hong Kong
S. A. R., P. R. of China (Project No. HKU 705209P).
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26.Chen, G.; Tao, L.; Mantovani, G.; Geng, J.; Nystroem, D.;
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