Donor-Stabilized Phosphenium Adducts as New Efficient and Immobilizing Ligands
FULL PAPERS
Avance 300 spectrometer. All chemical shifts are relative to
SiMe4 (1H and 13C NMR) and 85% H3PO4 (31P NMR) and
are given in ppm. The elemental analyses were performed in
the first recycling. The deactivation was found to be
due to a protonation reaction of the C!P bond of
the ligand in the presence of the large quantity of pyr-
rolidium salt formed upon the successive recycling.
This observation suggests a certain incompatibility of
such ligands with catalytic reactions performed in
acidic media, or which generate easily transferred
protons, especially when no treatments are done on
the reused phase (water washing, etc.). Conversely,
the catalytic results obtained with the trans-PtCl2(3a)2
complex compared to its related neutral complex
our Institute on Fisons EA 1108 apparatus. The
a
[Pd(allyl)Cl]2, PtCl2ACHTGUNTERN(NUG PPh3)2 5, bromoaryles, phenylacetylene,
R
m-CNB and o-CNB were commercial products and were
used as received. The bis(5-methyl-2-furyl)bromophosphine,
1,3-dimethylimidazolium 2-carboxylate 1, donor-stabilized
phosphenium 3a, PtCl2(3a)2 4, and ionic liquid [BMIM]ACHTUNGTRENNUNG[BF4]
were prepared according to the literature.[10,26–28] The solids
(catalyst and reagents) and the ionic liquid were degassed
under vacuum before use.
trans-PtCl
(PPh3)2 in the hydrogenation of chloro-
[BF4] confirmed the recy-
A
Synthesis of Donor-Stabilized Phosphenium Adduct
3d
cling potential of donor-stabilized phosphenium li-
gands. The simultaneous existence of a strong p-ac-
ceptor character and a positive charge in the phos-
phorus ligand substantially improved the life-time of
the platinum catalyst. The chemical selectivity of the
reaction was also eventually increased by suppressing
the detrimental formation of the dehalogenation
product. Therefore, this cationic platinum complex
represents, to the best of our knowledge, the first ex-
ample of highly selective homogeneous catalyst for
the reduction of nitroarenes in an ionic liquid phase.
The platinum catalytic system was successfully recy-
cled seven times without noticeable metal leaching in
the organic phase and any significant loss of activity.
To a suspension of 1,3-dimethylimidazolium 2-carboxylate 1
(0.216 g, 1.54 mmol) in CH2Cl2 (5 mL) was very slowly
added the bis(5-methyl-2-furyl)bromophosphine (0.488 g,
1.79 mmol). A purple colour appeared immediately which
slowly changed to light brown after 18 h of stirring. The mix-
ture was stirred for two days at room temperature. The sol-
vent was removed under vacuum to afford a light brown
solid which was washed twice with 5 mL of degassed pen-
tane and ether, and dried under vacuum; yield: 0.539 g
1
(95%); mp 1318C. H NMR (300 MHz, CDCl3): d=8.28 (s,
2H, N-CH=CH-N), 6.88 (s, 2H, H3 furyl), 6.09 (m, 2H,
H4 furyl), 3.94 (s, 6H, NCH3), 2.32 (s, 6H, CH3 furyl); 13C{1H}
NMR (75 MHz, CDCl3): d=160.81 (d, JP, C =2.3 Hz, 2C,
C2 furyl), 141.39 (d, JP,C =47.0 Hz, 1C, NCN), 138.24 (s, 2C,
C5 furyl), 127.60 (s, 1C, N-CH=), 127.13 (s, 1C, N-CH=),
3
123.39 (d, 2C, C4 furyl), 108.57 (d, JP,C =8.3 Hz, 2C, C3 furyl),
3
4
37.46 (d, JP, C =8.3 Hz, 2C, NCH3), 13.98 (d, JP, C =19.6 Hz,
2C, CH3 furyl); 31P{1H} NMR (121 MHz, CDCl3): d=ꢀ78.9
(s); anal. calcd. for C15H18N2O2PBr (368.02): C 48.90,
H 4.93, N 7.61; found: C 48.37, H 4.91, N 7.91.
Experimental Section
General Procedures
All reactions were performed in Schlenk-type flasks under
an argon atmosphere except hydrogenation experiments
which were performed in a Parr 300-mL stainless steel auto-
clave equipped with a magnetic drive and an internal glass
vessel. The temperature was controlled by a rigid heating
mantle and by a single loop cooling coil. Solvents were puri-
fied and dried by conventional methods and distilled under
argon. Chromatography was performed on silica gel (230–
400 mesh, Merck Kieselgel 60). GC and GC-MS analyses
were performed in our Laboratories on Shimadzu GC-2014
with a Supelco Equity-5 capillary column (30 mꢄ0.25 mmꢄ
0.25 m), Hewlett–Packard HP-6890 series with a HP-5 capil-
lary column (30 mꢄ0.25 mmꢄ0.25 m) and ThermoFinnigan
Representative Catalytic Experiment for the Heck
Alkynylation Reaction in [BMIM]ACHTUNRGTNEUNG[BF4]
The solid mixture of [PdACTHNUTRGNE(UGN allyl)Cl]2 (6.3 mg, 0.0348 mmol of
Pd), donor-stabilized phosphenium (3a: 32 mg, 0.102 mmol
or 3d: 38 mg, 0.102 mmol) and bromobenzene (0.358 mL,
536 mg, 3.416 mmol) was degassed for 15 min in a 20-mL
Schlenk tube equipped with a magnetic stirrer bar. Under
argon were added 3 mL of [BMIM]ACHTNUTRGNEUNG[BF4] previously de-
gassed under reduced pressure for 10 min. The Schlenk tube
was heated in an oil bath at 908C to give a red-orange solu-
tion. To the ionic liquid solution was added, out of the oil
bath, 0.35 mL pyrrolidine (1.6 equiv., 390 mg, 5.46 mmol,
d=0.87) then 0.60 mL phenylacetylene (1.6 equiv., 558 mg,
5.46 mmol, d=0.93). The resulting mixture was heated at
1258C for 40 h under argon. The product was extracted
from the ionic liquid phase by the addition of diethyl ether
(two times 5 or 10 mL) and decanting off the ether from the
ionic liquid phase (GC yield: 99%). After evaporation the
residue was purified by silica gel chromatography (diethyl
ether/hexane: 1/9) to give (2-phenylethynyl)benzene; isolat-
ed yield: 540 mg (90%).
TRACE GC with
a RTX-5 capillary column (5 mꢄ
0.25 mmꢄ0.25 m) and an UltraFast module. GC yields are
based upon external standard calibration from pure starting
substrates and reaction products. The program used in GC-
MS and GC for experiments was the following: carrier gas
He, gas flow 1.3 mLminꢀ1; injector 3008C, oven temperature
program: 1 min at 608C, 208Cminꢀ1 till 2808C, 5, 10 or
15 min at 2808C, detector FID 3008C or MSD; the program
used in UltraFast GC for experiments was the following:
carrier gas H2, gas flow 0.3 mLminꢀ1; injector 2508C,
column program: 6 sec at 508C, 2008Cminꢀ1 till 2008C, iso-
therm at 2008C, detector FID 2508C. 1H, 31P{1H} and
13C{1H} NMR spectra were recorded at 298 K on a Bruker
The products obtained were analysed by proton NMR,
mass spectrum analysis and elemental analysis, and were
found identical to commercial products.
Adv. Synth. Catal. 2009, 351, 1621 – 1628
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1627