A procedure for Appel halogenations and dehydrations using a polystyrene supported phosphine oxide

Article abstract of DOI:10.1016/j.tetlet.2013.11.098

The conversion of a commercially available polystyrene supported phosphine oxide into synthetically useful polymeric halophosphonium salts using oxalyl chloride/bromide takes place at room temperature in 5 min and generates only CO and CO₂ as by-products. The polymeric halophosphonium salts so obtained are useful reagents for Appel halogenations and other dehydrative coupling reactions. This gives rise to a simple three-step synthesis cycle for Appel and related reactions using a commercially available polymeric phosphine oxide with very simple purification and no phosphorus waste.

Full text of DOI:10.1016/j.tetlet.2013.11.098

Tetrahedron Letters 55 (2014) 799–802
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A procedure for Appel halogenations and dehydrations using
a polystyrene supported phosphine oxide
⇑
Xiaoping Tang, Jie An, Ross M. Denton
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
The conversion of a commercially available polystyrene supported phosphine oxide into synthetically
useful polymeric halophosphonium salts using oxalyl chloride/bromide takes place at room temperature
in 5 min and generates only CO and CO2 as by-products. The polymeric halophosphonium salts so
obtained are useful reagents for Appel halogenations and other dehydrative coupling reactions. This gives
rise to a simple three-step synthesis cycle for Appel and related reactions using a commercially available
polymeric phosphine oxide with very simple purification and no phosphorus waste.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 3 September 2013
Revised 9 October 2013
Accepted 26 November 2013
Available online 2 December 2013
Keywords:
Phosphine oxides
Halogenation
Polymeric reagents
Chlorophosphonium salts
Phosphorus(V) reagents have been employed extensively in
chemical synthesis since the 1960s.1 For example, one particularly
effective reagent system that has found a very widespread applica-
tion is the phosphine/carbon tetrahalide system, usually referred
to as the ‘‘Appel conditions’’. This reagent combination has been
used to effect halogenations of alcohols and carboxylic acids as
well as a range of other dehydrative processes with wide substrate
scope (Scheme 1A).2 An alternative to the carbon tetrahalide sys-
tem involves the use of chlorotriphenylphosphonium chloride
(Scheme 1A) to carry out analogous transformations.3 Although
this reagent is commercially available it is prone to hydrolysis
and, therefore, is often prepared in situ by the treatment of tri-
phenylphosphine with chlorine, phosgene, or more conveniently,
with hexachloroethane or triphosgene.4
includes Ley’s bipyridyl tagged phosphine reagent9 and recent
work using monolithic phosphines,10 Porco’s anthracene tagged
reagent,11 Curran’s fluorous phosphines,12 Charette’s tetraaryl sup-
ported phosphines,13 Janda’s PEG-based polymers,14 Toy’s Rasta
Resin,15 and Barrett’s strategy of impurity annihilation involving
a tagged diazodicarboxylate and a polymeric phosphine.16 While
these systems greatly simplify purification the subsequent
reductive recycling of the phosphorus reagent can be less straight-
forward because the reduction of triarylphosphine oxides17 re-
quires relatively harsh reaction conditions using metal hydrides18
or silane reagents (Scheme 1B).19 While some recent methods have
greatly improved this process, particularly with regard to
functional group tolerance,20 the reduction of polymeric phosphine
oxides is more difficult still and has received much less attention.21
Regardless of which particular protocol is employed, once the
desired reaction is complete, the phosphine oxide by-product must
be removed to obtain pure material and this purification is often
difficult to achieve. Indeed, a wide variety of methods have been
developed over the years to alleviate purification difficulties. Some
of the most effective methods involve the use of polymeric5 or
tagged phosphine reagents,6 which can either be removed by
filtration or undergo phase switching (e.g. to facilitate aqueous
extraction) once the reaction is complete.7 Since Hodge’s report
on the use of 1% cross-linked polystyrene-containing phosphine
residues for reactions under Appel conditions appeared in 1983,8
there has been a great deal of progress in the development of
supported or tagged phosphine reagents. Notable work in this area
(A) Two protocols for Appel reactions
/CX4
Ph
P
Ph
O
Ph
starting
material
product
P
Ph
HCX3
HCl
Ph
Ph
Ph
Ph
Cl
P
Ph
Ph
PhCl
O
starting
material
product
P
Ph
(B) Recycling of the polymeric phosphine oxide by-product
O
HSiCl3
Al salts or siloxane
P
Ph
P
Ph
Ph
Ph
or AlH3 THF
⇑
Corresponding author. Tel.: +44 115 951 4194; fax: +44 115 951 3564.
E-mail address: ross.denton@nottingham.ac.uk (R.M. Denton).
Scheme 1. (A) Appel reactions using a phosphine/CX4 combination and a chloro-
phosphonium salt. (B) Reductive recycling of polymeric phosphine oxides.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.11.098

800
X. Tang et al. / Tetrahedron Letters 55 (2014) 799–802
Scheme 3. Conversion of polystyrene-supported phosphine oxide into polymeric
halophosphonium salts.33
Scheme 2. A three-step process for Appel reactions using a polymeric phosphine
oxide.
The present study is based on the transformation of phosphine
oxides into chlorophosphonium salts using oxalyl chloride at room
temperature, as originally reported in 1977 by Fukui.26 This under-
appreciated reaction has been recently exploited by us in catalytic
Appel reactions,24 and in other contexts by Tanaka27 and Gilheany
to achieve phosphine oxide reductions.28 However, to the best of
our knowledge, the conversion of polystyrene-supported phos-
phine oxides into chlorophosphonium salts using this reagent
has not been reported. We began by establishing optimum condi-
tions for the conversion of a commercial polystyrene-supported
phosphine oxide29 (Scheme 3). It should be noted that although
we opted to use a commercial phosphine oxide the methods of
Toy15 and Charette30 can be used to prepare a variety of polysty-
rene-supported phosphines from which the corresponding oxides
In this Letter we describe a different approach to the use of
polymeric phosphorus(V) reagents, which involves the use of a
polystyrene-bound phosphine oxide for Appel and related transfor-
mations (Scheme 2). This redox-neutral approach,22 complements
existing catalytic phosphorus-mediated reactions,23,24 and differs
from previous work using supported or polymeric phosphorus
reagents because it obviates the need for difficult reductive recy-
cling of the polymeric phosphorus reagent.25 A further significant
advantage of the new three-step protocol is that the only by-prod-
ucts associated with the activation step are carbon dioxide and
carbon monoxide.
Table 1
Representative reactions with polystyrene-supported halophosphonium salts34
Entry
1
Starting material
PPS reagent
PPS Cl
Conditions
Product
Yield
CHCl3
60 °C
18 h
>95%, run 1
>95%, run 2
>95%, run 3
CHCl3
60 °C
18 h
2
3
PPS Br
PPS Cl
90%
OH
Cl
DCE
75 °C
18 h
Me
Me
90%
84%
Br
Br
DCE
75 °C
18 h
4
5
PPS Cl
PPS Cl
CHCl3
60 °C
18 h
>95%, run 1
>95%, run 2
>95%, run 3
DCE
75ꢀC
18 h
6
7
PPS Cl
PPS Cl
57%
CHCl3
60 °C
18 h
>95%
CHCl3
60 °C
>95%, run 1
>95%, run 2
8
PPS Cl
PPS Cl
PPS Cl
18 h
>95%, run 3
CHCl3
60 °C
18 h
9
>95%
CHCl3
60ꢀC
18 h
10
86%
85%
CHCl3
60 °C
18 h
11
PPS Cl

X. Tang et al. / Tetrahedron Letters 55 (2014) 799–802
801
5. For reviews, see: (a) Ley, S. V.; Baxendale, I. R.; Bream, R. N.; Jackson, P. S.;
Leach, A. G.; Longbottom, D. A.; Nesi, M.; Scott, J. S.; Storer, R. I. S. J. J. Chem. Soc.,
Perkin Trans. 2000, 1, 3815; (b) Ley, S. V.; Baxendale, I. R.; Brusotti, G.;
Caldarelli, M.; Massi, A.; Nesic, M. Il Farmaco 2002, 57, 321; (c) Lu, J.; Toy, P. H.
Chem. Rev. 2009, 109, 815.
6. (a) Camp, D.; Jenkins, I. D. Aust. J. Chem. 1988, 44, 1835; (b) Starkey, G. W.;
Parlow, J. J.; Flynn, D. L. Bioorg. Med. Chem. Lett. 1998, 8, 2385; (c) Kiankarimi,
M.; Lowe, R.; McCarthy, J. R.; Whitten, J. P. Tetrahedron Lett. 1999, 40, 4497.
7. O’Brien, M.; Denton, R. M.; Ley, S. V. Synthesis 2011, 1157.
could be obtained. We were pleased to observe that, upon
suspension of the polymeric phosphine oxide in chloroform and
treatment with two equivalents of either oxalyl chloride, oxalyl
bromide, or sulfuryl chloride (Scheme 3) at room temperature,
the corresponding halophosphonium salts were formed.31 Subse-
quent analysis of the polymeric phosphine oxide via ATR-IR
indicated a phosphoryl stretch at 1197 cmÀ1, which was absent
in the halophosphonium salts.
8. Harrison, C. R.; Hodge, P.; Hunt, B. J.; Khoshdel, E.; Richardson, G. J. Org. Chem.
1983, 48, 3721.
While the utility of polymeric halophosphonium salts has
been established4 we carried out a number of representative
halogenation and dehydration reactions in order to exemplify the
simplicity and practicality of the protocol for mmol-scale reactions
(Table 1). We first examined chlorination of decanol (entry 1). This
was accomplished in excellent yield and the decyl chloride product
isolated via filtration and evaporation was judged to be pure by 1H
NMR. The recovered polymer was washed with dichloromethane,
reactivated, and used for a second chlorination reaction, which
took place with no loss in yield. This process was then repeated
for a third time, again with no loss in yield of the alkyl chloride.
The analogous bromination reaction (entry 2) was carried out
using PPS Br under identical conditions. A second chlorination
reaction (entry 3) also proceeded in excellent yield. A deoxydichlo-
rination32 reaction was next investigated (entry 4) and provided
the expected syn-1,2-dichloride with retention of the silicon
protecting group. The conversion of aldehydes into synthetically
useful 1,1-dichlorides was next established (entries 5–7).
In the case of entry 5, the reaction was performed three times
with the same batch of polymer with no loss in yield. The conver-
sion of oximes into nitriles also proceeded without incident
(entries 8–10). Finally, amide coupling was also shown to be viable
in good yield (entry 11). In most instances the polymeric halophos-
phonium salts were used immediately after the evaporation of the
solvent. However, a sample of PPS Cl retained its activity for the
chlorination of decanol after storage in a glass-stopped flask under
nitrogen for 16 h.
In summary we have developed a simple protocol for the gener-
ation of polymeric halophosphonium salts from phosphine oxides
and demonstrated the applicability of the polymeric reagents so
obtained for halogenation and dehydration reactions under Appel
conditions. The new three-step protocol is simple, generates only
gaseous by-products, and allows expensive,29 polystyrene-sup-
ported phosphine oxides to be very easily reactivated and reused.
The convenient nature of the activation step and the recent
documentation of Appel halogenation reactions in-flow,10 opens
up the possibility of flow-based reactions with integrated regener-
ation of polymeric phosphorus reagents. The development of such
processes is currently underway.
9. Smith, C. D.; Baxendale, I. R.; Tranmer, G. K.; Bauman, M.; Smith, S. C.;
Lewthwaite, R. A.; Ley, S. V. Org. Biomol. Chem. 2007, 5, 1562.
10. Roper, K. A.; Lange, H.; Polyzos, A.; Berry, M. B.; Baxendale, I. R.; Ley, S. V.
Beilstein J. Org. Chem. 2011, 7, 1648.
11. Lan, P.; Porco, J. A., Jr.; South, M. S.; Parlow, J. J. J. Comb. Chem. 2003, 5, 660.
12. (a) Dandapani, S.; Curran, D. P. Tetrahedron 2002, 58, 3855; for a review on the
use of fluorous reagents, see: (b) Curran, D. P. Angew. Chem., Int. Ed. 1998, 37,
1174; for a more recent review, see: (c) Dobbs, A. P.; Kimberly, M. R. J. Fluorine
Chem. 2002, 118, 3.
13. Poupon, J.-C. A.; Boezio, A.; Charette, A. B. Angew. Chem. Int. Ed. 2006, 45, 1415.
14. (a) Sieber, F.; Wentworth, P., Jr.; Toker, J. D.; Wentworth, A. D.; Metz, W. A.;
Reed, N. N.; Janda, K. D. J. Org. Chem. 1999, 64, 5188; (b) Kollhofer, A.; Plenio, H.
Chem. Eur. J. 2003, 9, 1416.
15. (a) Choi, M. K. W.; He, H. S.; Toy, P. H. J. Org. Chem. 2003, 68, 9831; (b) Leung, P.-
S. W.; Teng, Y; Toy, P. H. Synlett 2010, 1997.
16. Barrett, A. G. M.; Roberts, R. S.; Schröder, J. Org. Lett. 2000, 2, 2999.
17. a Maier, L. In Organic Phosphorus Compounds; Maier, L., Kosolapoff, G. M., Eds.;
Wiley-Interscience: New York, 1972; pp 1–226. vol. 1, chapter 1; (b) Beck, P. In
Organic Phosphorus Compounds; Maier, L., Kosolapoff, G. M., Eds.; Wiley-
Interscience: New York, 1972; pp 189–508. vol. 2, chapter 4; (c) Gilheany, D.
G.; Mitchell, C. M. In The Chemistry of Organophosphorus Compounds; Hartley, F.
R., Ed.; Chicester: Wiley-Interscience, 1990; pp 151–190. vol. 1, chapter 7; (d)
Gallagher, M. J. In The Chemistry of Organophosphorus Compounds; Hartley, F. R.,
Ed.; Chichester: Wiley-Interscience, 1992; pp 53–76. vol. 2, chapter 2.
18. (a) Henson, P. D.; Ockrymiek, S. B.; Raymond, J.; Markham, E. J. Org. Chem. 1974,
39, 2296; (b) Imamoto, T.; Takeyama, T.; Kusumoto, T. Chem. Lett. 1985, 1491;
(c) Imamoto, T.; Kusumoto, T.; Suzuki, N.; Sato, K. J. Am. Chem. Soc. 1985, 107,
5301; (d) Imamoto, T.; Oshiki, T.; Onozawa, T.; Kusumoto, T.; Sato, K. J. Am.
Chem. Soc. 1990, 112, 5244; (e) Griffin, S.; Heath, L.; Wyatt, P. Tetrahedron Lett.
1998, 39, 4405; (f) Imamoto, T.; Kikuchi, S.; Miura, T.; Wada, Y. Org. Lett. 2000,
3, 87; (g) Keglevich, G.; Fekete, M.; Chuluunbaatar, T.; Dobó, A.; Harmatc, V.;
Toke, L. J. Chem. Soc., Perkin Trans. 2000, 1, 4451; (h) Keglevich, G. T.;
Chuluunbaatar, T.; Ludanyib, K.; Tokec, L. Tetrahedron 2000, 56, 1; (i) Stankevic,
M.; Pietrusiewicz, K. M. Synlett 2003, 1012; (j) Busacca, C. A.; Lorenz, J. C.;
Grinberg, N.; Haddad, N.; Hrapchak, M.; Latli, B.; Lee, H.; Sabila, P.; Saha, A.;
Sarvestani, M.; Shen, S.; Varsolona, R.; Wei, X.; Senanayake, C. H. Org. Lett.
2005, 7, 4277; (k) Busacca, C. A.; Raju, R.; Grinberg, N.; Haddad, N.; James-
Jones, P.; Lee, H.; Lorenz, J. C.; Saha, A.; Senanayake, C. H. J. Org. Chem. 2008, 73,
1524.
19. (a) Horner, L.; Balzer, W. D. Tetrahedron Lett. 1965, 6, 1157; (b) Bauld, N. L.;
Farr, F. J. Am. Chem. Soc. 1969, 91, 2788; (c) Tang, R.; Mislow, K. J. Am. Chem. Soc.
1969, 91, 5645; (d) Naumann, K.; Zon, G.; Mislow, K. J. Am. Chem. Soc. 1969, 91,
7012; (e) Marsi, K. L. J. Org. Chem. 1974, 39, 265; (f) Coumbe, T.; Lawrence, N. J.;
Muhammad, F. Tetrahedron Lett. 1994, 35, 625; (g) Wu, H.-C.; Yu, J.-Q.; Spencer,
J. B. Org. Lett. 2004, 6, 4675; (h) Berthod, M.; Favre-Réguillon, A.; Mohamad, J.;
Mignani, G.; Docherty, G.; Lemaire, M. Synlett 2007, 1545; (i) Petit, C.; Favre-
Reguillon, A.; Albela, B.; Bonneviot, L.; Mignani, G.; Lemaire, M. Organometallics
2009, 28, 6379.
20. For two recent approaches to silane-based catalytic phosphine oxide reduction,
see: (a) Li, Y.; Das, S.; Zhou, S.; Junge, K.; Beller, M. J. Am. Chem. Soc. 2012, 134,
9727; (b) Li, Y.; Lu, L.-Q.; Das, S.; Pisiewicz, S.; Junge, K.; Beller, M. J. Am. Chem.
Soc. 2012, 134, 18325.
21. (a) Capturo, R.; Ferreri, C.; Noviello, S.; Palumbo, G. Synthesis 1986, 499; (b)
Capturo, R.; Ferreri, C.; Palumbo, G. Synth. Commun. 1986, 16, 1081; (c)
Capturo, R.; Ferreri, C.; Palumbo, G. Synthesis 1987, 386; (d) Capturo, R.; Ferreri,
C.; Palumbo, G. Synth. Commun. 1987, 17, 1669; (e) Regen, S. L.; Lee, D. P. J. Org.
Chem. 1975, 40, 1669; (f) Hon, Y.-S.; Wu, K.-C. Tetrahedron 2003, 59, 493.
22. The concept of accessing phosphorus(V) reagents from phosphine oxides was
Acknowledgements
We wish to acknowledge the University of Nottingham, the
EPSRC (Grant EP/J000868/1) and The Royal Society (Research Grant
to R.M.D.) for funding. We are also grateful to Dr. Adrienne Davis,
Dr. Shazad Aslam and Mr. Kevin Butler for analytical support.
first introduced by Marsden in the context of a catalytic aza-Wittig cyclisation
24e
reaction, see Ref.
.
23. Marsden S.P., The kvnfjnv kjfvn, in: Dunn P.J., Hii, K.K. (Eds.), Sustainable
Catalysis Challenges and Practices for the Pharmaceutical and Fine Chemical
Industries Wiley, New York, 2013, p. 339.
References and notes
24. (a) Denton, R. M.; Jie, A.; Adeniran, B. Chem. Commun. 2010, 46, 3025; (b)
Denton, R. M.; Tang, X.; Przeslak, A. Org. Lett. 2010, 12, 4678; (c) Denton, R. M.;
Jie, A.; Adeniran, B.; Blake, A.; Lewis, W.; Poulton, A. J. Org. Chem. 2011, 76,
6749; (d) Denton, R. M.; Jie, A.; Lindovska, P.; Lewis, W. Tetrahedron 2012, 68,
2899; (e) McGonagle, A. E.; Marsden, S. P.; McKever-Abbas, B. Org. Lett. 2008,
10, 2589; (f) O’Brien, C. J.; Tellez, J. L.; Nixon, Z. S.; Kang, L. J.; Carter, A. L.;
Kunkel, S. R.; Przeworski, K. C.; Chass, C. G. Angew. Chem., Int. Ed. 2009, 48,
6836; (g) van Kalkeren, H. A.; Leenders, S. H. A. M.; Hommersom, C. R. A.;
Rutjes, F. P. J. T.; van Delft, F. L. Chem. Eur. J. 2011, 17, 11290; (h) van Kalkeren,
H. A.; Bruins, J. J.; Rutjes, F. P. J. T.; Rutjes, F. P. J. T.; van Delft, F. L. Adv. Synth.
Catal. 2012, 354, 1417; (i) Kosal, A. D.; Wilson, E. E.; Ashfeld, B. L. Angew. Chem.,
Int. Ed. 2012, 124, 12202; (j) Harris, J. R.; Haynes, M. T., II; Thomas, A. M.;
1. For reviews, see: (a) Appel, R.; Halstenberg, M. In Organophosphorus Reagents in
Organic Synthesis; Cadogan, J. I. G., Ed.; Academic Press: London, 1979. Chapter
9; (b) Appel, R. Angew. Chem. Int. Ed. Engl. 1975, 14, 801.
2. (a) Downie, I. M.; Lee, J. B.; Matough, M. F. S. Chem. Commun. 1968, 1350; (b)
Downie, I. M.; Holmes, J. B.; Lee, J. B. Chem. Ind. (London) 1966, 900; (c) Lee, J. B.;
Nolan, T. J. Can. J. Chem. 1966, 44, 1331; (d) Hooz, J.; Giliani, S. S. H. Can. J. Chem.
1968, 46, 86; (e) Weiss, R. G.; Snyder, E. I. Chem. Commun. 1968, 1358; (f) Lee, J.
B. Am. Chem. Soc. 1966, 88, 3440; (g) Barstow, L. E.; Hruby, V. J. J. Org. Chem.
1971, 6, 1305.
3. Masaki, M.; Fukui, K. Yuki Gosei Kogaku Kyokaishi 1979, 37, 57.
4. Wells, A. Synth. Commun. 1994, 24, 1715.

802
X. Tang et al. / Tetrahedron Letters 55 (2014) 799–802
Woerpel, K. A. J. Org. Chem. 2010, 75, 5083; (k) Dunn, N. L.; Ha, M.; Radosevich,
32. Typical procedure for the synthesis of PPS Cl/Br: Polystyrene-supported
triphenylphosphine oxide (see Ref. 29) (2.50 g, 3.00 mmol) was suspended in
CHCl3 (20 mL) and oxalyl chloride/bromide or sulfuryl chloride (1 equiv) was
then added over 1 min. Vigorous evolution of gas was observed for
approximately 2 min after the addition was complete. After stirring at room
temperature for a further 20 min, the solvent was removed in vacuo to afford
the white free-flowing polymeric phosphonium salts which were used
immediately without further purification.
A. T. J. Am. Chem. Soc. 2012, 134, 11330; (l) O’Brien, C. J.; Lavinge, F.; Coyle, E. E.;
Holahan, A. J.; Doonan, B. J. Chem. Eur. J. 2013, 19, 5854.
25. An exception to this can be found in the work of Jenkins who has developed a
polymeric version of the Hendrickson reagent (triphenylphosphine ditriflate),
see: (a) Elson, K. E.; Jenkins, I. D.; Loughlin, W. A. Tetrahedron Lett. 2004, 45,
2491; (b) Fairfull-Smith, K. E. E.; Jenkins, I. D.; Loughlin, W. A. Org. Biomol.
Chem. 2004, 2, 1979.
26. Masaki, M.; Fukui, K. Chem. Lett. 1977, 151.
27. (a) Yano, T.; Kuroboshi, M.; Tanaka, H. Tetrahedron Lett. 2010, 51, 698; (b) Yano,
T.; Hoshino, M.; Kuroboshi, M.; Tanaka, H. Synlett 2010, 801.
33. (a) Isaacs, N. S.; Kirkpatrick, D. Tetrahedron Lett. 1972, 13, 3869; (b) Sonnet, P.
E.; Oliver, J. E. J. Org. Chem. 1976, 41, 3279; (c) Echigo, Y.; Watanabe, Y.;
Mukaiyama, T. Chem. Lett. 1977, 1013; (d) Oliver, J. E.; Sonnet, P. E. Org. Synth.
1978, 58, 64; (e) Croft, A. P.; Bartsch, R. A. J. Org. Chem. 1983, 48, 3353; (f)
Iranpoor, N.; Firouzabadi, H.; Azadi, R.; Ebrahimzadeh, F. Can. J. Chem. 2006, 84,
69; (g) Yoshimitsu, T.; Fukumoto, N.; Tanaka, T. J. Org. Chem. 2009, 74, 696.
34. Typical procedure for the deoxychlorination and dehydration reactions: To a
suspension of PSP Cl or PSP Br (6 equiv) in CHCl3 or DCE (20 mL) was added
the appropriate substrate(s) e.g. alcohol/aldehyde/epoxide/oxime/carboxylic
acid/amine (1 equiv). The mixture was heated at either 60 °C (CHCl3 reactions)
or 75 °C (DCE reactions) for 18 h. The reaction mixture was filtered and the
filter cake washed with CH2Cl2 (2 Â 10 mL). The solvent was removed in vacuo
to afford the corresponding products, which were either judged pure by 1H
NMR analysis of the crude reaction product or were purified via normal phase
silica gel flash column chromatography eluting with a Et2O/petroleum ether
40–60 solvent mixture. The recovered polymer could be reused without any
further manipulation.
28. (a) Rajendran, K. V.; Gilheany, D. G. Chem. Commun. 2011, 817; (b) Rajendran,
K. V.; Gilheany, D. G. Chem. Commun. 2012, 10040; (c) Rajendran, K. V.;
Kudavalli, J. S.; Dunne, K. S.; Gilheany, D. G. Eur. J. Org. Chem. 2012, 14, 2720; (d)
Byrne, P. A.; Rajendran, K. V.; Muldoon, J.; Gilheany, D. G. Org. Biomol. Chem.
2012, 10, 3531; (e) Nikitin, K.; Helger, M.-B.; Gilheany, D. G. Chem. Commun.
2013, 1434.
29. Alfa Aesar 1.2–1.8 mmol/g on polystyrene (L19474-09, price £25.74/g) was
used in this work. Assuming a loading of 1.8 mmol/g, this equates to £14.30/
mmol. This should be contrasted with triphenylphosphine, which is £0.10/
mmol based on a price of £9.09 for 25 g (from the same supplier).
30. Charette, A. B.; Boezio, A. A.; Janes, M. K. Org. Lett. 2000, 2, 3777.
31. Samples of the polymeric phosphine oxide and halophosphonium salts were
prepared by grinding in a mortar with a pestle before being analysed via
ATR-IR.