Palladium-catalyzed synthesis of 1-alkylphosphonium salts from 1-alkenes

Article abstract of DOI:10.1021/ja055775s

A palladium(0) complex catalyzes the addition reaction of a triarylphosphine and a protic acid to a 1-alkene, giving a 1-alkylphosphonium salt. The treatment of atmospheric ethylene, triphenylphosphine, and (CF₃SO₂)₂NH in

Full text of DOI:10.1021/ja055775s

Published on Web 12/14/2005
Palladium-Catalyzed Synthesis of 1-Alkylphosphonium Salts from 1-Alkenes
Mieko Arisawa and Masahiko Yamaguchi*,†
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku UniVersity,
Aoba, Sendai 980-8578, Japan
Received August 23, 2005; E-mail: yama@mail.pharm.tohoku.ac.jp
Scheme 1
Quaternary phosphonium salts are organophosphorus compounds
widely used for the Wittig olefination reagent, catalyst, electrolyte,
ionic liquid, and surface active reagent and are generally prepared
by the substitution reaction of organohalogen compounds with
tertiary phosphines.1 The addition reaction to unsaturated com-
pounds, however, is preferable from economical and environmental
standpoints, since the substrates are readily available and the
Table 1. Metal-Catalyzed Addition of Triphenylphosphine to
Ethylene
synthesis is straightforward (Scheme 1). Although the addition of
tertiary phosphines to electron-deficient alkenes readily occurs,2,3
a catalyzed reaction with unactivated alkene was not known, and
only some stoichiometric reactions have been reported.4
Previously, we reported the palladium- and rhodium-catalyzed
addition reaction of triphenylphosphine and sulfonic acid to alkynes
to form alkenylphosphonium salts.5 The addition of these com-
pounds to 1-alkenes giving alkylphosphonium salts, however, did
entry
cat (mol %)
HX
equiva
yield%b
not proceed under the conditions, and to extend this methodology
to the less reactive substrate, the reactions of activated olefins,
allenes,6 and 1,3-dienes7 were examined. On the basis of the results,
we report here the palladium-catalyzed anti-Markovnikov addition
reaction of triarylphosphines to unactivated 1-alkenes in the
presence of bis(trifluoromethanesulfonyl)imide (Tf2NH). This is a
novel carbon-phosphorus bond-forming reaction, in which the
ligand coupling of a simple alkene and a tertiary phosphine on
palladium metal occurs.
1
2
3
4
5
6
Pd2(dba)3‚CHCl3 (1.25)
Tf2NH
1.1
1.0
1.2
1.1
1.1
1.1
99
(30)
n.r.
(34)
98
TfOH
Tf2NH
Tf2NH
Pd2(dba)3‚CHCl3 (0.1)
RhH(PPh3)4 (1.25)
n.r.
a Amount of HX to triphenylphosphine. b Isolated yields. Shown in
parentheses are yields determined by 1H NMR based on triphenylphosphine.
n.r. ) no reaction.
An atmospheric ethylene (balloon) was treated with triph-
enylphosphine and Tf2NH (1.1 equiv) in the presence of Pd2(dba)3‚
CHCl3 (dba ) dibenzylideneacetone) (1.25 mol %) in chloroben-
zene at 65 °C for 5 h, and ethyltriphenylphosphonium bis(trifluoro-
methanesulfonyl)imidate was obtained after recrystallization in 99%
yield (Table 1, entry 1). Unlike the reaction of 1,3-dienes,7 RhH-
(PPh3)4 was not an effective catalyst (entry 6), and Tf2NH gave
better results compared with trifluoromethanesulfonic acid (TfOH)
(entry 4). The catalyst loading could be reduced to 0.1 mol %
without affecting the yield of the product (entry 5). The use of a
slight excess of Tf2NH over phosphine (molar ratio ) 1.1:1) was
critical, and the yields of the product decreased when the ratio was
larger (molar ratio ) 1.2:1) or smaller (molar ratio ) 1:1) (entries
2 and 3). This suggested that the acid is not only the hydrogen
source required to add to 1-alkenes but is also an activator of the
palladium complex.8 The presence of a larger amount of acid,
however, might have inactivated the phosphine by shifting the
equilibrium toward protonation.
The reaction was applicable to substituted 1-alkenes (Table 2).
When atmospheric propene (balloon) was treated with triph-
enylphosphine and Tf2NH (1.1 equiv) in the presence of Pd2(dba)3‚
CHCl3 (0.5 mol %) in chlorobenzene at 65 °C for 5 h, 1-propyl-
triphenylphosphonium salt was obtained in 95% isolated yield (entry
2). The anti-Markovnikov adduct was obtained exclusively, which
indicated the essential role of metal catalysis in this phosphorus-
carbon bond formation rather than acid catalysis. The reaction of
1-butene (20 equiv), triphenylphosphine, and Tf2NH (1.1 equiv)
in the presence of Pd2(dba)3‚CHCl3 (1.0 mol %) in chlorobenzene
at 65 °C for 8 h gave 1-butyltriphenylphosphonium salt in 92%
yield (entry 3). The recovered butenes contained mainly 2-butene
and a very small amount of 1-butene (2-butene:1-butene > 20:1
by 1H NMR). Such equilibration of butenes was attained within 1
h under the conditions, and a small amount of 1-butene must have
reacted with triphenylphosphine, which indicated a high activity
of the catalyst. The reactions of 1-pentene and 1-hexene did not
proceed to completion and gave modest yields of the phosphonium
salts (entries 4 and 5).
The examination of the reactions for the higher 1-alkenes
revealed tris(p-chlorophenyl)phosphine to be a better substrate,
while tris(p-tolyl)phosphine gave a comparable result with triph-
enylphosphine. The reaction of 1-pentene (5 equiv), Tf2NH (1.1
equiv), and tris(p-chlorophenyl)phosphine in the presence of the
palladium complex (1.0 mol %) gave the corresponding 1-pen-
tylphosphonium salt in 91% yield (entry 9). The amount of
1-pentene could be decreased to 5 equiv for this reaction, despite
its volatile nature (bp, 30 °C) and the rapid olefin migration to
form very a small amount of 1-pentene. The reaction of 1-hexene
also proceeded effectively, giving 1-hexylphosphonium salt in 88%
yield (entry 10). The improvement of the yield using tris(p-
chlorophenyl)phosphine may be either due to the higher stability
of the catalyst possessing this ligand or to the less basic nature of
the phosphine to be less protonated. Analogously, the reactions of
† Tohoku University 21st Century COE Program CRESCENDO.
9
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J. AM. CHEM. SOC. 2006, 128, 50-51
10.1021/ja055775s CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Palladium-Catalyzed Synthesis of 1-Alkylphosphonium
Salts from 1-Alkenes
To summarize, 1-alkylphosphonium salts are synthesized from
1-alkenes, a triarylphosphine, and Tf2NH in the presence of a
palladium catalyst. Carbon-phosphorus bond formation by the
addition of phosphorus reagents to alkenes catalyzed by a transition-
metal complex have high synthetic potential.
Acknowledgment. This work was supported by JSPS (Nos.
16109001 and 17689001). M.A. expresses her thanks for financial
supports from NEDO of Japan (No. 02A44003d) and the Asahi
Glass Foundation.
entry
X
R
mol %
time/h
yield/%a
1
2
3
4
5
6
7
8
9
H
H
0.1
0.5
1.0
1.0
1.0
0.25
0.5
1.0
1.0
1.0
5
5
8
8
14
5
5
8
24
24
99
95
92
47
40
96
96
90
91
88
CH3
C2H5
n-C3H7
n-C4H9
H
Supporting Information Available: Experimental procedures and
compound characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
Cl
CH3
C2H5
n-C3H7
n-C4H9
References
10
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a Isolated yields.
Scheme 2
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ethylene, propylene, and 1-butene with tris(p-chlorophenyl)phos-
phine gave the corresponding products in high yields (entries 6, 7,
and 8).
Since olefin migration is very rapid under the conditions, inner
alkenes can also be used. The treatment of (E)-2-pentene (5 equiv)
or (E)-3-pentene (5 equiv) with Tf2NH (1.1 equiv) and tris(p-
chlorophenyl)phosphine in chlorobenzene at 65 °C for 24 h both
gave 1-pentylphosphonium salt in 91% yields (NMR) (Scheme 2).
The use of mixtures of alkene regioisomers and stereoisomers for
the reaction may have a synthetic advantage.
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J. AM. CHEM. SOC. VOL. 128, NO. 1, 2006 51