Pd(II)-catalyzed Ph2(O)P-directed C-H olefination toward phosphine-alkene ligands

Article abstract of DOI:10.1021/ol402577p

The Pd(II)-catalyzed Ph₂(O)P-directed C-H olefination to synthesize alkene-phosphine compounds is reported. In contrast to previous examples of various directing groups that guide selective C-H activation, the Ph₂(O)P group not only acts as the directing group but also serves to construct the alkene-phosphine ligands. The monoprotected amino acid (MPAA) ligand Ac-Leu-OH is found to promote this reaction in a significant manner.

Full text of DOI:10.1021/ol402577p

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
Pd(II)-Catalyzed Ph (O)P-Directed CꢀH
2
000–000
Olefination toward PhosphineꢀAlkene
Ligands
†
Hong-Li Wang, Rong-Bin Hu, Heng Zhang, An-Xi Zhou, and Shang-Dong Yang*
†
†
†
,†,‡
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
730000, P. R. China, and State Key Laboratory for Oxo Synthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics, Lanzhou 730000, P. R. China
yangshd@lzu.edu.cn
Received September 3, 2013
ABSTRACT
The Pd(II)-catalyzed Ph
of various directing groups that guide selective CꢀH activation, the Ph (O)P group not only acts as the directing group but also serves to
2
(O)P-directed CꢀH olefination to synthesize alkeneꢀphosphine compounds is reported. In contrast to previous examples
2
construct the alkeneꢀphosphine ligands. The monoprotected amino acid (MPAA) ligand Ac-Leu-OH is found to promote this reaction in a
significant manner.
Phosphine-based ligands have found widespread use in
homogeneous catalysis and often have been applied to
enhance the metal catalyst efficiency or control chiral
proven to be highly effective for asymmetric conjugate
3
additions and allylic substitutions. Atpresent, the strategy
for the synthesis of biaryl-based alkene-phosphine hybrid
ligands involves some traditional procedures such as bro-
mination, lithiation, and so on, which lead to tedious
workup procedures and complicated operations. Further-
more, tediously long synthetic steps also greatly increase
1
induction. In general, the choice of ligand is very crucial
to the success of a reaction. In the past decades, many
metal complexes using various phosphine ligands have
been found to catalyze various reactions, especially the
2
cross-coupling reactions and asymmetric catalysis. How-
(
3) For leading references on chiral phosphineꢀalkene hybrid
ever, despite impressive progress in this area, the design
and synthesis of new ligands that possess novel electronic
and steric properties and functional groups for a particular
application remain a formidable task. Recently, one novel
type of biaryl-based chiral alkeneꢀphosphine hybrid
ligands have received considerable attention and have
ligands, see: (a) Maire, P.; Deblon, S.; Breher, F.; Geier, J.; B o€ hler, C.;
R u€ egger, H.; Sch o€ nberg, H.; Gr u€ tzmacher, H. Chem.;Eur. J. 2004, 10,
4198. (b) Shintani, R.; Duan, W.-L.; Nagano, T.; Okada, A.; Hayashi, T.
Angew. Chem., Int. Ed. 2005, 44, 4611. (c) Duan, W.-L.; Iwamura, H.;
Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 2130. (d) Kasak,
P.; Arion, V. B.; Widhalm, M. Tetrahedron: Asymmetry 2006, 17, 3084.
(
e) Stemmler, R. T.; Bolm, C. Synlett 2007, 1365. (f) Pongr ꢀa cz, P.;
Pet o€ cz, G.; Shawb, M.; Williams, D. B. G.; Koll ꢀa r, L. J. Organomet.
Chem. 2010, 695, 2381. (g) Liu, Z.; Du, H. Org. Lett. 2010, 12, 3054. (h)
Cao, Z.; Liu, Y.; Liu, Z.; Feng, X.; Zhuang, M.; Du, H. Org. Lett. 2011,
13, 2164.
†
Lanzhou University.
‡
Lanzhou Institute of Chemical Physics.
(
1) (a) Comprehensive Asymmetric Catalysis IꢀIII; Jacobsen, E. N.,
(4) For selected reviews of CꢀH activation: (a) Ritleng, V.; Sirlin, C.;
Pfeffer, M. Chem. Rev. 2002, 102, 1731. (b) Chen, X.; Engle, K. M.;
Wang, D.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48, 5094. (c) Lyons,
T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147. (d) Bras, J. L.;
Muzart, J. Chem. Rev. 2011, 111, 1170. (e) Charles, S. Y.; Dong, V. M.
Chem. Rev. 2011, 111, 1215. (f) Cho, S. H.; Kim, J. Y.; Kwak, J.; Chang,
S. Chem. Soc. Rev. 2011, 40, 5068. (g) Arockiam, P. B.; Bruneau, C.;
Dixneuf, P. H. Chem. Rev. 2012, 112, 5879. (h) Song, G.; Wang, F.; Li,
X. Chem. Soc. Rev. 2012, 41, 3651. (i) Colby, D. A.; Tsai, A. S.; Bergman,
R. G.; Ellman, J. A. Acc. Chem. Res. 2012, 45, 814. (j) Li, B.-J.; Shi, Z.-J.
Chem. Soc. Rev. 2012, 41, 5588. (k) Yu, D.-G.; Li, B.-J.; Shi, Z.-J.
Tetrahedron 2012, 68, 5130.
Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999. (b) Fern ꢀa ndez-P ꢀe rez,
H.; Etayo, P.; Panossian, A.; Vidal-Ferran, A. Chem. Rev. 2011, 111, 2119.
(
c) Kamer, P. C. J.; van Leeuwen, P. W. N. M. Phosphorus (III) Ligands
in Homogeneous Catalysis: Design and Synthesis; Wiley-VCH: Chichester,
012.
2) (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley:
New York, 1994; (b) Tietze, L. F.; Ila, H.; Bell, H. P. Chem. Rev. 2004, 104,
453. (c) de Meijere, A.; Diederich, F. Metal-Catalyzed Cross-Coupling
2
(
3
Reactions; Wiley-VCH: Weinheim, 2004. (d) van Leeuwen, P. W. N. M.;
Kamer, P. C. J.; Claver, C.; P ꢁa mies, O.; Di ꢀe guez, M. Chem. Rev. 2011, 111,
077.
2
1
0.1021/ol402577p r XXXX American Chemical Society

a,b
Scheme 1. Different Pathways for Synthesis of
PhosphineꢀAlkene Ligand
Table 1. Reaction Conditions Screening
c
entry
cat. (mol %)
oxidant (equiv) ligand (mol %) yield (%)
preparation costs. Therefore, the development of concise
and highly efficient methods is still extremely attractive
and a significant challenge. Combining a directing group
with transition-metal-catalyzed direct CꢀH olefination
provides one straightforward and atom-efficient protocol
for the construction of alkeneꢀphosphine hybrid com-
1
2
3
4
5
6
7
8
9
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
2
2
2
2
2
2
2
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
Cu(OAc)
AgOAc (3.0)
AgNO (3.0)
Phl(OAc) (3.0)
Selectfluor (3.0)
2
(3.0)
52
56
44
51
63
42
3
2
K
O
2
S
2
O
8
(3.0)
(1.0 atm)
(3.0) L-Ac-Leu-OH (20) 74
2
n. r.
4
,5
Cu(OAc)
AgOAc (3.0)
AgNO (3.0)
Phl(OAc)
Selectfluor (3.0) L-Ac-Leu-OH (20) 43
(3.0) L-Ac-Leu-OH (20) 44
2
pounds. Over the past several years, we have trained our
focus on the development of new and efficient protocols
L-Ac-Leu-OH (20) 83
L-Ac-Leu-OH (20) 61
10
3
6
for the transition metal-catalyzed CꢀP bond formation.
11 Pd(OAc)
2
(3.0) L-Ac-Leu-OH (20) 53
1
1
1
2
3
4
Pd(OAc)
Pd(OAc)
Pd(OAc)
Herein, by using the P(O)Ph as a new directing group, we
2
2 2 8
K S O
first disclose a novel protocol of palladium-catalyzed CꢀH
olefination to synthesize a series of alkeneꢀphosphine
hybrid compounds (Scheme 1). In contrast to the former
reports that involve Pd(II)- and Rh(III)-catalyzed phos-
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
(10) AgOAc (3.0)
Bipy (20)
nr
nr
78
85
76
70
30
nr
78
63
76
75
78
87
77
71
15 Pd(OAc) (10)
16 Pd(OAc)
2
1,10-Phen (20)
L-Ac-Ala-OH (20)
Ac-Gly-OH (20)
L-Ac-Val-OH (20)
L-Ac-lle-OH (20)
2
2
2
2
(10)
(10)
(10)
(10)
17
18
19
Pd(OAc)
Pd(OAc)
Pd(OAc)
phorous acid or phosphate ester directed CꢀH func-
7
tionalizations, diphenylphosphine oxide (P(O)Ph ) not
2
20 Pd(OAc) (10)
2
PPh (20)
3
21
22
23
24
Pd(OAc)
PdCl (10)
Pd(PPh
Pd(TFA)
2
(10)
dppe (20)
only acts as the directing group to direct CꢀH activation
to make a useful ligand but also incorporates broadly
useful ligands in the reactions.
2
Ac-Gly-OH (20)
Ac-Gly-OH (200
Ac-Gly-OH (20)
Ac-Gly-OH (20)
Ac-Gly-OH (20)
Ac-Gly-OH (10)
Ac-Gly-OH (5)
Ac-Gly-OH (10)
Ac-Gly-OH (10)
3
)
4
(10)
(10)
2
We began our exploration with 2-diphenylphosphino-
-methylbiphenyl (1a) and ethyl acrylate as the model
2 2
25 Pd(PhCN) Cl
0
26 Pd(PPh ) Cl (10) AgOAc (3.0)
2
3 2
2
27
28
29
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
(5)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (2.0)
AgOAc (3.0)
substrates with which to identify suitable reaction con-
ditions (Table 1). First, we evaluated various oxidants in
the presence of Pd(OAc) (10 mol %) as a catalyst in
2
2
2
(2.5)
(5)
(5)
d
30 Pd(OAc)
76
2
a
All reactions were carried out in the presence of 0.2 mmol of 1a
(5) For leading references of various directing groups oriented CꢀH
olefination, see: (a) Wasa, M.; Engle, K. M.; Yu, J.-Q. J. Am. Chem. Soc.
b
in 2.0 mL of different solvents at 100 °C under air atmosphere. 1,
0
1
2
0-Phen =1, 10-phenanthrline, Bipy =2, 2 -bipyridine, dppe = 1,
2010, 132, 3680. (b) Patureau, F. W.; Glorius, F. J. Am. Chem. Soc. 2010,
c
-bis(diphenylphosphino)ethane. Yield of isolated product. Reaction
d
1
32, 9982. (c) Huang, C.; Chattopadhyay, B.; Gevorgyan, V. J. Am.
temperature: 110 °C.
Chem. Soc. 2011, 133, 12406. (d) Patureau, F. W.; Besset, T.; Glorius, F.
Angew. Chem., Int. Ed. 2011, 50, 1064. (e) Rakshit, S.; Grohmann, C.;
Besset, T.; Glorius, F. J. Am. Chem. Soc. 2011, 133, 2350. (f) Gong, T.-J.;
Xiao, B.; Liu, Z.-J.; Wan, J.; Xu, J.; Luo, D.-F.; Fu, Y.; Liu, L. Org. Lett.
CF CH OH at 100 °C. To our delight, the use of Cu(OAc) ,
3
2
2
2
011, 13, 3235. (g) Li, H.; Li, Y.; Zhang, X.-S.; Chen, K.; Wang, X.; Shi,
Z.-J. J. Am. Chem. Soc. 2011, 133, 15244. (h) Zhao, P.; Niu, R.; Wang,
F.; Han, K.; Li, X. Org. Lett. 2012, 14, 4166. (i) Leow, D.; Li, G.; Mei,
T.-S.; Yu, J.-Q. Nature 2012, 486, 518. (j) Zhao, P.; Wang, F.; Han, K.;
Li, X. Org. Lett. 2012, 14, 3400. (k) Baxter, R. D.; Sale, D.; Engle, K. M.;
Yu, J.-Q.; Blackmond, D. G. J. Am. Chem. Soc. 2012, 134, 4600. (l)
Wang, C.; Chen, H.; Wang, Z.; Chen, J.; Huang, Y. Angew. Chem., Int.
Ed. 2012, 51, 7242. (m) Shen, Y.; Liu, G.; Zhou, Z.; Lu, X. Org. Lett.
AgOAc, AgNO , K S O , PhI(OAc) , and Selectfluor as
3 2 2 8 2
oxidants yielded the desired product of 2a; AgOAc was
most effective (entries 1ꢀ6, Table 1). Notably, the reaction
failed with oxygen as the oxidant (entry 7, Table 1).
Furthermore, the control experiment showed that the
palladium as catalyst is necessary to the success of the
transformation. Results of solvents screening indicated
2
013, 15, 3366. (n) Brasse, M.; C ꢀa mpora, J.; Ellman, J. A.; Bergman,
R. G. J. Am. Chem. Soc. 2013, 135, 6427. (o) Dai, H.-X.; Li, G.; Zhang,
X.-G.; Stepan, A. F.; Yu, J.-Q. J. Am. Chem. Soc. 2013, 135, 7567.
that CF CH OH was still the best choice. Recently, in
2
(6) (a) Li, Y.-M.; Sun, M.; Wang, H.-L.; Tian, Q.-P.; Yang, S.-D.
3
Angew. Chem., Int. Ed. 2013, 52, 3972. (b) Zhang, H.-Y.; Sun, M.; Ma,
Y.-N.; Tian, Q.-P.; Yang, S.-D. Org. Biomol. Chem. 2012, 10, 9627. (c)
Sun, M.; Zhang, H.-Y.; Han, Q.; Yang, K.; Yang, S.-D. Chem.;Eur. J.
the field of Pd(II)-catalyzed carboxylate-directed CꢀH
olefination reactions, Yu and co-workers reported a highly
significant discovery: the addition of mono-N-protected
2
011, 17, 9566. (d) Hu, J.; Zhao, N.; Yang, B.; Wang, G.; Guo, L.-N.;
Liang, Y.-M.; Yang, S.-D. Chem.;Eur. J. 2011, 17, 5516.
7) (a) Meng, X.; Kim, S. Org. Lett. 2013, 15, 1910. (b) Chan, L. Y.;
(
Kim, S.; Ryu, T.; Lee, P. H. Chem. Commun. 2013, 49, 4682. (c) Chan, L.;
Cheong, L.; Kim, S. Org. Lett. 2013, 15, 2186. (d) Seo, J.; Park, Y.; Jeon,
I.; Ryu, T.; Park, S.; Lee, P. H. Org. Lett. 2013, 15, 3358. (e) Chary, B. C.;
Kim, S.; Park, Y.; Kim, J.; Lee, P. H. Org. Lett. 2013, 15, 2692. (f) Unoh,
Y.; Hashimoto, Y.; Takeda, D.; Hirano, K.; Satoh, T.; Miura, M. Org.
Lett. 2013, 15, 3258. (g) Zhao, D.; Nimphius, C.; Lindale, M.; Glorius,
F. Org. Lett. 2013, 15, 4504.
(8) (a) Wang, D.-H.; Engle, K. M.; Shi, B.-F.; Yu, J.-Q. Science 2010,
327, 315. (b) Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Angew. Chem., Int.
Ed. 2010, 49, 6169. (c) Lu, Y.; Leow, D.; Wang, X.; Engle, K. M.; Yu,
J.-Q. Chem. Sci. 2011, 2, 967. (d) Engle, K. M.; Thuy-Boun, P. S.; Dang,
M.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 18183. (e) Engle, K. M.; Mei,
T.-S.; Wasa, M.; Yu, J.-Q. Acc. Chem. Res. 2012, 45, 788. (f) Li, G.;
Leow, D.; Wan, L.; Yu, J.-Q. Angew. Chem., Int. Ed. 2013, 52, 1245.
B
Org. Lett., Vol. XX, No. XX, XXXX

a,b
Table 2. Evaluation of Different Directing Groups
Table 3. Pd(II)-Catalyzed CꢀH Olefination with Different
-Diphenylphosphino Oxide Derivatives and Various
Acrylates
2
a,b
a
All reactions were carried out under the optimal conditions
b
c
2
reported in the text. Isolated yields. 10 mol % Pd(OAc) was used.
amino acids as ligand could enhance the reactivity of the
8
catalytically active Pd(II) species. With this research in
mind, we selected the amino acid L-Ac-Leu-OH as the
ligand and tested the reactions under different oxidant
conditions (entries 8ꢀ13, Table 1). Use of L-Ac-Leu-OH as
the ligand was very effective; indeed, the highest yield of 2a
was afforded in an 83% yield when AgOAc was used as the
oxidant. Subsequently, we carried out evaluations of other
amino acids and some phosphine or nitrogen ligands (e.g.,
Bipy, 1, 10-Phen, dppe and PPh ) withencouraging results:
3
the amino acids of Ac-Gly-OH were revealed as the best
choice, and the yield of 2a was improved to 85% (entries
1
Pd(PPh ) , Pd(TFA) , and Pd(PhCN) Cl , could also
4ꢀ21, Table 1). Other Pd catalysts, such as PdCl ,
2
3 4 2 2 2
prompt this reaction, but with relatively lower yields
entries 22ꢀ26, Table 1). Decreasing the load of Pd(OAc)2
to 5 mol % was helpful: 2a was obtained in 87% yield
(
(
entries 27ꢀ28, Table 1). The equivalent of the AgOAc
evaluation demonstrated that 3.0 equiv was best (entry 29,
Table 1). Moreover, reductions in temperature also de-
creased the yield (entry 30, Table 1).
a
All reactions were carried out in the presence of 0.2 mmol of 1aꢀt in
b
c
OH. Isolated yield. 10 mol % Pd(OAc)
2
.0 mL of CF
3
CH
2
2
was used.
Under optimized reaction conditions (entry 27, Table 1),
we evaluated various phosphate directing groups. In addi-
tion to P(O)Ph , the P(O)(OEt) and P(O)(i-Pr) groups
2
2
2
Table 4. Pd(II)-Catalyzed CꢀH Olefination with Different
a,b
were also effective in this transformation with correspond-
ing products afforded in good yields (Table 2, 2aꢀc).
However, use of other phosphates, including triphenyl-
phosphine oxide and R-diethyl naphthalenyl phosphonate,
failed to induce CꢀH olefination (Table 2, 2d,e). Notably,
these results demonstrate the significance of the seven-
member cyclopalladium pretransition state as the key to
this reaction.
Styrenes
b
b
entry
R
Ar
yield (%) entry
R
Ar
yield (%)
We next examined the scope of different substituted
diphenylphosphine oxide derivatives and various acry-
lates (Table 3). The structure of 3f was confirmed by
single X-ray diffractions analysis (see the Supporting
5a Ph Ph
58
52
45
5e
Ph p-FPh
52
56
i
5b
5c
-Pr Ph
5d Ph m-NO Ph
2
Ph o-CIPh
a
All reactions were carried out in the presence of 0.2 mmol of 1aꢀr in
b
OH. Isolated yield.
Information). We focused first on various acrylates, using
0
2
.0 mL of CF
3
CH
2
2
-diphenylphosphino-2 -methylbiphenyl as a substrate for
Org. Lett., Vol. XX, No. XX, XXXX
C

Scheme 2. Synthesis of Different (R)-PhosphineꢀAlkenes
Scheme 3. Proposed Mechanisms of Pd(OAc)
Olefination
2
-Catalyzed CꢀH
the investigation. We were pleased to find that electron-
deficient olefins such as butyl and benzyl acrylates, alkenyl
phosphite, and sulfone were compatible with the transfor-
mation and that the corresponding products were afforded
in good yields (3aꢀf). Furthermore, a variety of diphenyl-
phosphine oxide derivatives could be converted to the
desired products using these olefins. For example, both
the aromatic ring of 2-diphenylphosphino with an elec-
tron-donating substituent group (1g) and an electron-
withdrawing group (1h) exhibited good reactivity to afford
olefinated products (3g and 3h) in 78% and 83% yields. If
the substrate was altered to 2-diphenylphosphinobiphenyl,
a mixture of mono- and diolefinated products (3i) was
obtained. Interestingly, when the loading of the catalyst
was increased to 10 mol %, only the diolefinated product
1
,2
reactions. Using commercial chiral binaphthyl-based
diphenylphosphine oxide (R)-1 as the substrate, we were
able to synthesize different chiral alkeneꢀphosphine hy-
brid oxide compounds easily without lowering the ee value
via diphenylphosphine oxide directed CꢀH olefination
reactions ((R)-2 and 3, Scheme 2). We selected the product
(
R)-3 carried out the reduction with HSiCl under basic
3
conditions to obtain the pure binaphthyl-based chiral
alkeneꢀphosphine hybrid ligands (R)-3a.
On the basis of the observed experimental results and
8
pioneering reports, we propose a plausible mechanistic
(3j) was obtained in 70% yield. Other electron-donating
substituent groups such as methoxyl and electron-
pathway outlined in Scheme 3. Pd(OAc) first coordinates
2
with the ligand of Ac-Gly-OH to form the activated pal-
ladium catalyst, which reacts with substrate 1a to produce
the cyclopalladium intermediate 1A. This active species
subsequently undergoes coordination with alkenes to form
the complex 1B, which then undergoes insertion to afford
the intermediate 1C. Finally, the product of 2a is afforded
by β-hydride elimination, and the active palladium catalyst
was regenerated in the presence of AgOAc as the oxidant.
withdrawing substituent groups such as F and Cl on the
0
2
-position were also well tolerated in this CꢀH olefina-
tion, but lower yieldsofproducts were observed(3lꢀn). On
the other hand, when different substituent groups were
0
situated on the 3 -position, the electronic effect was very
distinct, and the electron-deficient substrates performed
far better than electron-rich ones (3oꢀq). As expected,
0
0
multisubstituted 2-diphenylphosphino-2 , 3 -dimethylbi-
phenyl can also be converted into the desired product
of 3r with 70% yield. In particular, when we selected
naphthalene as a substrate, the reaction successfully pro-
vided the product of 3s in a 77% yield. Finally, the ethyl
In conclusion, we have developed a Ph (O)P-directed
2
Pd(II)-catalyzed CꢀH olefination to synthesize the highly
important alkene-phosphine compounds. CꢀH olefina-
tion of biaryls is shown to be promoted by the MPAA
ligand Ac-Gly-OH for the first time, providing a new
approach for biaryl ligand synthesis.
2
3
-acetamidoacrylate also afforded the product of 3t in
6% yield.
To our delight, we discovered that less electrophilic
styrenes were also compatible with the CꢀH olefination
reaction, albeit with a higher loading of catalyst. As ex-
pected, different phosphate directing groups such as P(O)-
Ph and P(O)(i-Pr) proved suitable for this transforma-
tion (Table 4, 5a,b). Furthermore, various substituted
styrenes were also investigated with 2-diphenylphosphino-
Acknowledgment. We are grateful for the financial
support of NSFC (Nos. 21272100), FRFCU (lzujbky-
2013-ct02, PCSIRT (IRT1138), and (NCET-11-0215).
We thank S. F. Reichard at the University of Chicago
for editing the manuscript.
2
2
0
2
-methylbiphenyl as the reaction partner (Table 4, 5cꢀe).
Supporting Information Available. Experimental pro-
cedures and full spectroscopic data for all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
We found that the electronic effect was very distinct and
that the electron-deficient styrenes afforded higher yields
than electron-rich styrenes.
Phosphorus-based ligands are key to many metal-catalyed
organic transformations, including many asymmetric
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX