Stereoselective synthesis of 1,2,3,4-tetrasubstituted dienes from allenoates and aldehydes: An observation of phosphine-induced chemoselectivity

Article abstract of DOI:10.1021/ol101429z

(Equation Presented). Phosphine-mediated olefination between α-substituted allenoates and aldehydes to form 1,2,3,4-tetrasubstituted 1,3-dienes is presented. High levels of chemo-and diastereoselectivity and yield are obtained for a wide scope of substrates with the choice of appropriate phosphines. This reaction evidences the capacity of phosphines in the control of reaction pathways and provides a highly efficient synthetic method for tetrasubstituted conjugated dienes.

Full text of DOI:10.1021/ol101429z

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3556-3559
Stereoselective Synthesis of
1,2,3,4-Tetrasubstituted Dienes from
Allenoates and Aldehydes: An
Observation of Phosphine-Induced
Chemoselectivity
Silong Xu, Wen Zou, Guiping Wu, Haibin Song, and Zhengjie He*
The State Key Laboratory of Elemento-Organic Chemistry and Department of
Chemistry, Nankai UniVersity, 94 Weijin Road, Tianjin 300071, P.R. China
zhengjiehe@nankai.edu.cn
Received June 21, 2010
ABSTRACT
Phosphine-mediated olefination between r-substituted allenoates and aldehydes to form 1,2,3,4-tetrasubstituted 1,3-dienes is presented. High
levels of chemo- and diastereoselectivity and yield are obtained for a wide scope of substrates with the choice of appropriate phosphines.
This reaction evidences the capacity of phosphines in the control of reaction pathways and provides a highly efficient synthetic method for
tetrasubstituted conjugated dienes.
Conjugated dienes are of great importance since they are
present as substructures in a large number of naturally
occurring and medicinally relevant substances1 and also serve
as versatile intermediates in many important organic trans-
formations.2 Classical procedures for the syntheses of 1,3-
dienes mainly include the conventional elimination reactions
from allyl bromide, allyl alcohol or dihalogenated com-
pound,3 the P-, S-, and Si-based carbonyl olefinations,4 and
the transition-metal-catalyzed diene formations.5 Despite the
effectiveness of the existing methods, the development of
new strategies particularly aiming at the stereoselective
synthesis of polysubstituted conjugated dienes remains
challenging and highly desirable.6 Recently, much effort has
been devoted to this area.7
During the past decade, chemical transformations involv-
ing allenoates and a wide range of electron-poor CdC bonds
(3) (a) Normant, J. F. Modern Synthetic Methods; Scheffold, R., Ed.;
Wiley: Chichester, 1983; Vol. 3, pp 139-171. (b) Larock, R. C. Compre-
hensiVe Organic Transformations; VCH Publishers, Inc.: New York, 1989;
pp 241-262.
(1) (a) Glasby, J. S. Encyclopaedia of the Terpenoids; Wiley: Chichester,
UK, 1982. (b) Devon, T. K.; Scott, A. I. Handbook of Naturally Occurring
Compounds; Academic: New York, NY, 1972; Vol. II. (c) Pereira, A. R.;
Cabezas, J. A. J. Org. Chem. 2005, 70, 2594–2597. (d) de Figueiredo, R. M.;
Berner, R.; Julis, J.; Liu, T.; Turp, D.; Christmann, M. J. Org. Chem. 2007,
72, 640–642.
(4) (a) Vedejs, E.; Peterson, M. J. AdVances in Carbanion Chemistry;
Snieckus, V., Ed.; Jai Press Inc.: Greenwich, CT, 1996; Vol. 2. (b)
Maryanoff, B. E.; Reitz, A. B. Chem. ReV. 1989, 89, 863–927. (c) Ager,
D. J. Org. React. 1990, 38, 1–223. (d) Blakemore, P. R. J. Chem. Soc.,
Perkin Trans. 1 2002, 2563–2585.
(5) For leading reviews, see: (a) Ref 2c. (b) Trost, B. M.; Toste, F. D.;
Pinkerton, A. B. Chem. ReV. 2001, 101, 2067. (c) Aubert, C.; Buisine, O.;
Malacria, M. Chem. ReV. 2002, 102, 813. (d) Diver, S. T.; Giessert, A. J.
Chem. ReV. 2004, 104, 1317. (e) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D.
Angew. Chem., Int. Ed. 2005, 44, 4442. (f) Hansen, E. C.; Lee, D. Acc.
Chem. Res. 2006, 39, 509.
(2) (a) Rappoport, Z. The Chemistry of Dienes and Polyenes; John Wiley
& Sons: Chichester, 1997; Vol. 1 and 2001; Vol. 2. (b) Nicolaou, K. C.;
Snyder, S. S.; Montagnon, T.; Vassilikogiannakis, G. Angew. Chem., Int.
Ed. 2002, 41, 1668–1698. (c) Negishi, E.-I.; Huang, Z.; Wang, G.; Mohan,
S.; Wang, C.; Hattori, H. Acc. Chem. Res. 2008, 41, 1474.
10.1021/ol101429z  2010 American Chemical Society
Published on Web 07/15/2010

Recently, we reported a novel PPh3-mediated reductive
cyclopropanation between R-substituted allenoates 1 and alde-
hydes 2 to afford highly functionalized cyclopropanes 3, in
which PPh3 acted as both a nucleophilic trigger and a deoxy-
genating agent (Scheme 1, bottom left).11 Further survey on
this reaction revealed that the choice of phosphines had a
dramatic impact on the reaction chemoselectivity, leading to a
new phosphine-mediated olefination reaction to form 1,2,3,4-
tetrasubstituted 1,3-dienes 4 (Scheme 1, bottom right). Herein
we wish to communicate the results from such a survey.
Scheme 1. Phosphine-Induced Chemoselectivity of Allenoates
Table 1. Survey on Conditions for Formation of 4aa
or polarized CdX bonds (X ) N, O) under the mediation
of phosphines have been extensively studied.8 A number of
new reactions with high synthetic potentials have emerged,
including many cycloaddition reactions,9 olefination,10 and
cyclopropanation.11 Relevant investigations have revealed
that the electronic and steric properties of phosphines can
exert significant influence on the reaction chemoselectivity.
For example, Kwon et al.12 reported that the use of small-
size phosphine catalyst PMe3 led to the formation of dioxane
products, while bulky phosphines like PCyp3 (Cyp )
cyclopentyl) produced pyrones exclusively in the phosphine-
catalyzed reactions of 2,3-butadienoates with aldehydes
(Scheme 1, top right). Shi and co-workers13 demonstrated
that under the catalysis of PPh2Me the reaction of 2,3-
butadienoates with salicylic imines afforded the normal [3
+ 2] cycloadducts, whereas Huang’s investigations14 un-
veiled that the difunctional catalyst (2′-hydroxybiphenyl-2-
yl)-diethylphosphine was capable of tuning the reaction to a
novel [4 + 1] annulation to form dihydrobenzofurans
(Scheme 1, top left).
time 3a: yield 4a: yield
(h)
entry
PR3
PBu3
PPhMe2
PPh2Me
PTA
Ph3P
P(OMe)3
P(NMe2)3 CH2Cl2
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
PBu3
solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
(%)b
(%),b drc
1
2
3
4
5
6
7
8
8
48
48
54
48
48
48
23
30
6
27
33
23
30
23
24
7
5
84, 20:1
68, 20:1
70, 6:1
66, 20:1
/
trace
trace
8
48
/
/
/
/
CH3CN
1,4-dioxane
DMSO
DMF
EtOH
toluene
xylene
THF
CHCl3
ethyl acetate
CHCl3
CHCl3
44
7
25
55
/
31, 20:1
40, 20:1
35, 20:1
25, 20:1
14, 20:1
81, 20:1
64, 20:1
84, 20:1
87, 20:1
76, 20:1
57, 20:1
96, 20:1
9
10
11
12
13
14
15
16
17
18d
19e
6
17
15
3
17
10
3
24
24
(6) (a) Ref 2c. (b) Vasil’ev, A. A.; Sterebryakov, E. P. Russ. Chem.
ReV. 2001, 70, 735. (c) Clark, D. A.; Kulkarni, A. A.; Kalbarczyk, K.;
Schertzer, B.; Diver, S. T. J. Am. Chem. Soc. 2006, 128, 15632. (d) Perez,
L. J.; Shimp, H. L.; Micalizio, G. C. J. Org. Chem. 2009, 74, 7211.
(7) (a) Zhang, X.; Larock, R. C. Org. Lett. 2003, 5, 2993. (b) Fu, C.;
Ma, S. Org. Lett. 2005, 7, 1707. (c) Horiguchi, H.; Tsurgui, H.; Satoh, T.;
Miura, M. AdV. Synth. Catal. 2008, 350, 509. (d) Aponte, J. C.; Hammond,
G. B.; Xu, B. J. Org. Chem. 2009, 74, 4623. (e) Mundal, D. A.; Lutz,
K. E.; Thomson, R. J. Org. Lett. 2009, 11, 465. (f) Zhang, X.; Larock,
R. C. Tetrahedron 2010, 66, 4265. (g) Paih, J. L.; Bray, C. C.; De´rien, S.;
Dixneuf, P. H. J. Am. Chem. Soc. 2010, 132, 7391.
a Typical conditions: under N2 atmosphere and at room temperature, to
a stirred solution of aldehyde 2a (0.2 mmol) and allenoate 1a (0.3 mmol)
in solvent (2 mL) was added phosphorus reagent (0.3 mmol). b Isolated
yield based on 2a. Based on the H NMR assay of crude product with
(Z,E)-4a being the major. d 1.0 equiv of water was added. e 5 mL of solvent
was used.
c
1
(8) For recent reviews, see: (a) Lu, X.; Zhang, C.; Xu, Z. Acc. Chem.
Res. 2001, 34, 535. (b) Methot, J. L.; Roush, W. R. AdV. Synth. Catal.
2004, 346, 1035. (c) Nair, V.; Menon, R. S.; Sreekanth, A. R.; Abhilash,
N.; Biju, A. T. Acc. Chem. Res. 2006, 39, 520. (d) Ma, S. Chem. ReV.
2005, 105, 2829. (e) Ma, S. Aldrichimica Acta 2007, 40, 91. (f) Ye, L.-W.;
Zhou, J.; Tang, Y. Chem. Soc. ReV. 2008, 37, 1140. (g) Cowen, B. J.; Miller,
S. J. Chem. Soc. ReV. 2009, 38, 3102.
Initial investigation showed that employing more nucleo-
philic PBu3 instead of PPh3 in the reaction of allenoate 1a
and benzaldehyde (2a) in CH2Cl2 at room temperature led
to the formation of diene 4a in 84% yield and excellent
diastereoselectivity (dr 20:1), along with only a small amount
of cyclopropane 3a (5% yield) (Table 1, entry 1). Several
other nucleophilic phosphorus reagents were also examined.
Ph2PMe, PhPMe2, and 1,3,5-triaza-7-phosphaadamantane
(PTA)15 also readily produced the diene 4a, but in lower
yields or diastereoselectivity compared with PBu3 (entries
2-4). PPh3 only gave cyclopropane 3a in 48% yield under
similar conditions (entry 5), while P(OMe)3 and P(NMe2)3
failed to mediate either cyclopropanation or olefination
reaction (entries 6, 7). With the choice of PBu3 as the
(9) For selected examples, see: (a) Zhang, C.; Lu, X. J. Org. Chem.
1995, 60, 2906. (b) Xu, Z.; Lu, X. Tetrahedron Lett. 1997, 38, 3461. (c)
Xu, S.; Zhou, L.; Ma, R.; Song, H.; He, Z. Chem.sEur. J. 2009, 15, 8698.
(d) Tran, Y. S.; Kwon, O. J. Am. Chem. Soc. 2007, 129, 12632. (e) Zhu,
X.-F.; Lan, J.; Kwon, O. J. Am. Chem. Soc. 2003, 125, 4716.
(10) Xu, S.; Zhou, L.; Zeng, S.; Ma, R.; Wang, Z.; He, Z. Org. Lett.
2009, 11, 3498.
(11) Xu, S.; Zhou, L.; Ma, R.; Song, H.; He, Z. Org. Lett. 2010, 12,
544.
(12) (a) Zhu, X.-F.; Henry, C. E.; Wang, J.; Dudding, T.; Kwon, O.
Org. Lett. 2005, 7, 1387. (b) Zhu, X.-F.; Schaffner, A.-P.; Li, R. C.; Kwon,
O. Org. Lett. 2005, 7, 2977. (c) Creech, G. S.; Kwon, O. Org. Lett. 2008,
10, 429.
(13) Shi, Y.-L.; Shi, M. Org. Lett. 2005, 7, 3057.
(15) An air-stable and water-soluble phosphine, often used as a
nucleophilic trialkylphosphine. See ref 10 and references cited therein.
(14) Meng, X.; Huang, Y.; Chen, R. Org. Lett. 2009, 11, 137.
Org. Lett., Vol. 12, No. 15, 2010
3557

phosphine agent, further survey on conditions indicated such
polar solvents as CH3CN, 1,4-dioxane, DMSO, DMF, and
ethanol were detrimental to the olefination reaction with
regard to yield and chemoselectivity (entries 8-12). Among
other tested solvents (entries 13-17), CHCl3 emerged as the
best, giving diene 4a in 87% yield and excellent stereose-
lectivity (entry 16). Addition of water as a protic additive
resulted in substantial decrease in both yield and chemose-
lectivity (entry 18). To our surprise, a relatively dilute
reactant concentration was found to be good for the olefi-
nation, which gave 4a in almost quantitative yield when run
at 0.04 M concentration of 2a (entry 19). Thus, the optimal
conditions for the olefination were established.
yields (entries 4, 9, 17) with an exception of salicylaldehyde,
which underwent the olefination to give the corresponding
diene 4d in 72% yield (entry 5).16 Relatively electron-rich
benzaldehydes exhibited better chemoselectivity for the
olefination, predominantly affording the dienes 4 in good to
excellent yields (entries 1-3, 5-6). Halo-benzaldehydes
produced the corresponding dienes 4 as the major associated
with the cyclopropanation products as the minor (entries 7,
8, 10-12). For the more reactive benzaldehydes bearing
electron-withdrawing substituents, their olefinations with the
allenoate 1a were better mediated with the less nucleophilic
phosphine PPh2Me, readily giving the dienes in fair to
excellent yields (entries 13-16). Heteroaromatic aldehydes
like furylaldehyde or thiofurylaldehyde also proved effective
in the olefination with 1a, giving the dienes 4o and 4p in
good yields (entries 18, 19).
An R,ꢀ-unsaturated aldehyde such as (E)-cinnamaldehyde
was also effective, giving (Z,E,E)-triene 4q in 80% yield
(Table 2, entry 20). Aliphatic aldehydes, in sharp contrast
with their failure in the reported cyclopropanation,11 worked
well in the PBu3-mediated olefination with allenoate 1a,
providing the corresponding dienes 4 in excellent yields with
high chemo- and stereoselectivities (entries 21-24). Notably,
dialdehydes like glutaraldehyde and terephthalic aldehyde
readily incorporated with two molecules of the allenoate 1a,
forming tetraenes 5a and 5b, respectively, in moderate yield
and high stereoselectivity (Scheme 2).
Table 2. Synthesis of 4 from Allenoates 1 and Aldehydes 2a
entry
1
R2 in 2
time (h) 3 (%)b
4 (%)b, drc
1
2
3
4
5
6
7
8
1a C6H5
24
24
24
23
1
3a, 3
3b, 3
\
3c, 99
\
\
3d, 9
3e, 12 4g, 62, 9:1
3f, 83
3g, 13 4h, 69, 12:1
3h, 14 4i, 72, 10:1
4a, 96, 20:1
4b, 85, 20:1
4c, 87, 20:1
\
4d, 72, 20:1
4e, 88, 20:1
4f, 90, 20:1
1a 4-CH3C6H4
1a 4-CH3OC6H4
1a 2-CH3OC6H4
1a 2-OHC6H4
1a 3-Br-4-CH3OC6H3
1a 4-ClC6H4
1a 3-ClC6H4
1a 2-ClC6H4
1a 4-IC6H4
4
Scheme 2. Synthesis of Tetraenes 5 from Dialdehydes and 1a
24
24
24
3
1
2.5
1
1.2
6
6
4
4
1
24
40
24
24
24
24
24
4
9
\
10
11
12
13d
14d
15d
16d
17d
18
19
20
21
22
23
24
1a 3-BrC6H4
1a 4-FC6H4
3i, 8
3j, 2
\
3k, 4
4j, 84, 11:1
4k, 63, 8:1
4l, 79, 12:1
4m, 95, 20:1
4n, 96, 20:1
\
4o, 88, 20:1
4p, 85, 20:1
4q, 80, 20:1
4r, 98, 20:1
4s, 99, 20:1
4t, 97, 20:1
4u, 87, 20:1
4v, 63, 12:1
4w, 91, 17:1
4x, 79, 20:1
1a 4-NO2C6H4
1a 3-NO2C6H4
1a 4-CF3C6H4
1a 3-CF3C6H4
1a 2-CF3C6H4
1a 2-furyl
1a 2-thiofuryl
1a E-styryl
1a C2H5
\
Several structurally similar R-substituted allenoates 1 (R1
) CN, 1b; Ph, 1c; E-styryl, 1d; n-Pr, 1e; H, 1f) were also
investigated in the olefination with benzaldehyde. The
allenoates 1b, 1c, and 1d all exhibited reactivity similar to
that of 1a and readily underwent the olefination under the
mediation of PBu3 or PPh2Me, giving the corresponding
dienes 4v and 4w or the triene 4x in 63-91% yields and
high stereoselectivity (Table 2, entries 25-27). However,
R-butyl allenoate 1e and R-methyl allenoate 1f both failed
to afford the corresponding olefination products.17 This fact
implies that an unsaturated conjugative group R1 in 1 is
required to effect the phosphine-mediated olefination.
In all cases of the olefination listed in Table 2, the
diastereoselectivity of the olefination products 4 remained
on a modest to high level (8:1 to 20:1) with the illustrated
isomer 4 as the major product. The structures and relative
stereochemistry of the olefination products 4 and 5 were well
3l, 51
\
\
\
\
\
\
\
\
\
\
1a n-C3H7
1a iso-C4H9
1a n-C5H11
25d,e 1b C6H5
26
27
1c C6H5
1d C6H5
a For details, see Supporting Information. b Isolated yield based on 2.
c Based on the1H NMR assay of crude product. d PPh2Me was used instead
of PBu3. e 2.0 equiv of both 1b and phosphine was used.
Under the optimized conditions, the substrate scope of this
olefination was examined (Table 2). A variety of aldehydes
2 were explored with the allenoate 1a. Except some
o-substituted ones, aromatic aldehydes with electron-donating
or -withdrawing groups readily gave dienes 4 in fair to
excellent yields with good to high levels of diastereoselec-
tivity (entries 1-17). o-Substituted benzaldehydes exclu-
sively afforded cyclopropanation products 3 in 51-99%
(16) The exceptional reactivity of o-substituted benzaldehydes, which
presumably results from the increased steric hindrance, in the reactions with
allenoates was also observed in ref 12a.
(17) A distinct Wittig olefination of R-methyl allenoate 1f with salicy-
laldehydes or aromatic aldehydes to form trisubstituted dienes was observed:
(a) He, Z.; Tang, X.; He, Z. Phosphorus Sulfur Silicon Relat. Elem. 2008,
183, 1518. (b) Khong, S. N.; Tran, Y. S.; Kwon, O. Tetrahedron 2010, 66,
4760.
3558
Org. Lett., Vol. 12, No. 15, 2010

1
identified by H and 13C NMR spectra and in some cases
also by NOESY and X-ray crystallographic analyses (for
characterization data, see Supporting Information).
Scheme 3. Proposed Mechanism for Formation of 4
The mechanism of the phosphine-mediated olefination was
investigated through 31P NMR tracking and deuterium-
labeling experiments (for details, see Supporting Informa-
tion). The reaction between the allenoate 1a (0.075 mmol),
p-chlorobenzaldehyde (0.05 mmol), and PBu3 (0.075 mmol)
in CDCl3 (0.5 mL) was run in an NMR tube and monitored
by 31P NMR spectroscopy. An appreciable signal at δ 21.7
ppm appeared in the course of the reaction, along with two
signals (δ 48.6 and -30.8 ppm) which corresponded to
Bu3PO and PBu3, respectively. This signal could disappear
in 5 min upon addition of more p-chlorobenzaldehyde (up
to 0.025 mmol) into the sample. This result, in combination
with the 31P NMR chemical shift (δ 21.7 ppm), strongly
implied that the emerging signal most likely came from a
tetracoordinate phosphorus ylide intermediate.18 In the
deuterium-labeling experiments, a deuterated allenoate 1a-
d2 (80% D) was subjected to the PBu3-mediated olefination
with benzaldehyde, resulting in a deuterated diene 4a-d4 in
84% yield with 40% and 10% deuterium at the γ- and ꢀ′-
carbons, respectively (eq 1). In contrast with this result,
The high chemoselectivity in the olefination between
allenoates 1 and aldehydes 2 may be ascribed to the increased
reactivity of the in situ formed phosphorus ylide 7 for the
Wittig reaction by employing electron-rich phosphines like
PBu3.4b Also, the high stereoselectivity of the olefination in
this study could be well rationalized in terms of the Wittig
reaction of the stabilized allylic phosphorus ylide 7 under
neutral and salt-free conditions.20
In conclusion, a novel phosphine-mediated olefination of
R-substituted allenoates with aldehydes has been successfully
realized by choosing more nucleophilic phosphines. This
olefination provides an efficient method for stereoselective
synthesis of 1,2,3,4-tetrasubstituted 1,3-dienes from readily
available starting materials. On the basis of the results in this
work, the in situ generated phosphorus ylide 7 is believed to
be the key intermediate which is responsible for the subsequent
Wittig reaction with the aldehyde. Recently, in situ formed
phosphorus ylides have proven to be powerful in construction
of carbon carbon double bonds.21 Future efforts on this
olefination will be directed toward broadening its application
in the synthesis of conjugated dienes and polyenes.
addition of D2O (1.5 equiv) into the reaction of the
nondeuterated allenoate 1a with benzaldehyde also brought
about partially deuterated product 4a-d4 in 52% yield with
similar deuterium incorporations at the same carbons under
otherwise identical conditions (eq 2). These results clearly
indicated that a water-involved hydrogen shift occurred
between the γ- and ꢀ′-hydrogens of the allenoate 1a.
Acknowledgment. Financial support from National Natu-
ral Science Foundation of China (Grant no. 20872063) is
gratefully acknowledged.
Supporting Information Available: Experimental details,
characterization data for new compounds, 1H and 13C NMR
spectra for 3, 4, and 5, as well as the X-ray crystallographical
data (CIF files) for 4b and 4k. This material is available
free of charge via the Internet at http://pubs.acs.org.
A plausible mechanism for the olefination is depicted in
Scheme 3. Initially, nucleophilic attack of the phosphine at
the allenoate 1 forms the resonance-stabilized zwitterionic
intermediate 6. Via a water-assisted hydrogen shift,19
6
OL101429Z
reversibly converts into the allylic phosphorus ylide 7, which
also exists in two resonnance forms 7a and 7b. When R1 in
7 is an unsaturated conjugative group such as CO2Et, CN,
Ph, or styryl, it is believed that the more stable form 7a
should be the major contributor to the phosphorus ylide 7
which is subsequently intercepted by an aldehyde 2 via a
Wittig reaction yielding the olefination product 4.
(19) For representative studies on the water-assisted hydrogen shift in
nucleophilic phosphine catalyses of allenoates, see: (a) refs 9c and 9d. (b)
Guo, H.; Xu, Q.; Kwon, O. J. Am. Chem. Soc. 2009, 131, 6318. (c) Xia,
Y.; Liang, Y.; Chen, Y.; Wang, M.; Jiao, L.; Huang, F.; Liu, S.; Li, Y.;
Yu, Z.-X. J. Am. Chem. Soc. 2007, 129, 3470. (d) Liang, Y.; Liu, S.; Xia,
Y.; Li, Y.; Yu, Z.-X. Chem.sEur. J. 2008, 14, 4361. (e) Liang, Y.; Liu,
S.; Yu, Z.-X. Synlett 2009, 905.
(20) (a) Robiette, R.; Richardson, J.; Aggarwal, V. K.; Harvey, J. N.
J. Am. Chem. Soc. 2006, 128, 2394. (b) Zhou, R.; Wang, C.; Song, H.; He,
Z. Org. Lett. 2010, 12, 976.
(18) Hudson, H. R.; Dillon, K. B.; Walker, B. J. 31P NMR Data of Four
Coordinate Phosphonium Salts and Betaines. In Handbook of Phosphorus-
31 Nuclear Magnetic Resonance Data; Tebby, J. C., Ed.; CRC Press: Boca
Raton, FL, 1991; pp 181-226.
(21) For representative examples, see: (a) Refs 10 and 20b. (b) Low,
K. H.; Magomedov, N. A. Org. Lett. 2005, 7, 2003. (c) McDougal, N. T.;
Schaus, S. E. Angew. Chem., Int. Ed. 2006, 45, 3117.
Org. Lett., Vol. 12, No. 15, 2010
3559