phines.9 Most recently we reported the use of the rasta resin
architecture10 to prepare 1 (Figure 1) that afforded nearly
Scheme 1. Synthesis of 2
Figure 1. Rasta resin concept.
quantitative yields in one-pot Wittig reactions when it was
mixed with an R -halo carbonyl compound, Et3N and an
aldehyde.11,12 The improved efficiency of 1 compared to
other heterogeneous polymer-supported phosphines in Wittig
reactions was attributed to the fact that the phosphine reagent
groups were attached to the flexible grafts of the polymer
that are readily solvent accessible. Thus, 1 is not dependent
upon swelling in the reaction medium as are the other
previously reported or commercially available cross-linked
phosphine functionalized polymers. Additionally, we have
investigated the concept of attaching multiple different
catalytic groups to a single polymer backbone and have
reported examples of such materials bearing combinations
of phosphine and phenol groups.13 Herein we report the
merging of these two areas of research and describe what is
to our knowledge the first example of a bifunctional polymer
bearing two distinct reagent groups and its use in one-pot
Wittig reactions.
in the grafts. Elemental analysis was used to determine the
loading level of 2 to be 1.05 mmol g-1 in phosphine and
1.06 mmol g-1 in amine, and gel-phase 31P NMR analysis
indicated that no oxidation of the phosphine groups occurred.
Scanning electron miscroscopy was performed and both core
3 (Figure 2A) and 2 (Figure 2B) were spherical in shape,
To synthesize a bifunctional rasta resin possessing both
phosphine and amine groups, the synthesis of 1 was modified
to incorporate an amine monomer (Scheme 1). Thus, the
heterogeneous core 3 was prepared as reported,11 and was
then subjected to living polymerization with a 5:1:1 molar
mixture of styrene (4), phosphine monomer 5,9a and amine
monomer 614 to afford 2. 4 was added to impart flexibility
Figure 2. SEM images.
with the later, not surprisingly, being generally larger in
diameter.
After 2 was prepared, its use in one-pot Wittig reactions
of aldehydes A-L with R-halo carbonyl compounds a-c
was examined (Table 1). It was envisioned that the phosphine
groups of 2 would react with a-c to form the corresponding
phosphonium salts and that these in turn would be depro-
tonated intramolecularly by the amine groups to form the
reactive phosphorane moieties (Scheme 2). These reactions
were setup using the aldehyde substrate as the limiting
reagent and 2 as the reagent most in excess. In this way, the
excess of a-c, compared to A-L, would be completely
reacted with 2 and not contaminate the desired products at
the end of the reactions, allowing Aa-Lc to be isolated in
pure form as a mixture of stereoisomers after only filtration
and solvent removal. Thus, a series of aryl aldehydes (entries
1-9), an allylic aldehyde (entry 10), and a pair of aliphatic
aldehydes (entries 11 and 12) were converted into the
corresponding alkenes by a mixture of 2 and a-c in excellent
yields with stereoselectivities similar to what has been
(9) (a) Kwok, M.; Choi, W.; He, H. S.; Toy, P. H. J. Org. Chem. 2003,
68, 9831. (b) Zhao, L. J.; He, H. S.; Shi, M.; Toy, P. H. J. Comb. Chem.
2004, 6, 680. (c) Harned, A. M.; He, H. S.; Toy, P. H.; Flynn, D. L.; Hanson,
P. R. J. Am. Chem. Soc. 2005, 127, 52. (d) Zhao, L.-J.; Kwong, C. K.-W.;
Shi, M.; Toy, P. H. Tetrahedron 2005, 61, 12026. (e) He, H. S.; Yan, J. J.;
Shen, R.; Zhuo, S.; Toy, P. H. Synlett 2006, 563.
(10) (a) Hodges, J. C.; Harikrishnan, L. S.; Ault-Justus, S. J. Comb.
Chem. 2000, 2, 80. (b) Lindsley, C. W.; Hodges, J. C.; Filzen, G. F.; Watson,
B. M.; Geyer, A. G. J. Comb. Chem. 2000, 2, 550. (c) McAlpine, S. R.;
Lindsley, C. W.; Hodges, J. C.; Leonard, D. M.; Filzen, G. F. J. Comb.
Chem. 2001, 3, 1.
(11) Leung, P. S.-W.; Teng, Y.; Toy, P. H. Synlett 2010, 1997
.
(12) For other examples of one-pot Wittig reactions, see: (a) Wu, J.;
Yue, C. Synth. Commun. 2006, 36, 2939, and references cited therein. (b)
Choudary, B. M.; Mahendar, K.; Kantam, M. L.; Ranganath, K. V. S.; Athar,
T. AdV. Synth. Catal. 2006, 348, 1977. (c) El-Batta, A.; Jiang, C.; Zhao,
W.; Anness, R.; Cooksy, A. L.; Bergdahl, M. J. Org. Chem. 2007, 72, 5244.
(d) Liu, D.-N.; Tian, S.-K. Chem.sEur. J. 2009, 15, 4538
.
(13) (a) Kwong, C. K.-W.; Huang, R.; Zhang, M.; Shi, M.; Toy, P. H.
Chem.sEur. J. 2007, 13, 2369. (b) Kwong, C. K.-W.; Fu, M. Y.; Law,
H. C.-H.; Toy, P. H. Synlett 2010, 2617.
Org. Lett., Vol. 12, No. 21, 2010
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