2426 Organometallics, Vol. 27, No. 11, 2008
Serra-Muns et al.
and chronoamperometry were performed at a gold disk electrode
with a homemade potentiostat and a Tacussel GSTP4 waveform
generator. The voltammograms were recorded on a Nicolet 301
oscilloscope. All experiments were performed under an argon
atmosphere.
where the Pd(0) moieties are still ligated to one CdC bond of
1a and to one P of dppf, which behaved as a monodentate ligand,
is not excluded. Upon addition of PhI (1 equiv), the signals
disappeared, leading to two new doublets at 25.53 ppm (d, JPP
) 34 Hz, 1P) and +7.50 ppm (d, JPP ) 34 Hz, 1P), which
characterized the complex PhPdI(dppf) formed in oxidative
additions.20 Consequently, a mixture of undefined Pd(0) ligated
by dppf and by 1a was formed and reacted with PhI. Even in
the absence of clear structural data for these Pd(0) complexes,
the reaction of PhI (2 mM) with the Pd(0) complexes generated
in situ from Pd0(1a) (2 mM) and dppf (2 mM) in THF was
monitored by chronoamperometry performed at a rotating
electrode polarized at +0.45 V (Figure 7c). From the half-
reaction time of the reaction, it emerges that the oxidative
addition of PhI is slower than that involving PPh3 as ligand in
THF (Figure 7b,c; Table 1, Figure S12).
Chemicals. THF was distilled from Na/benzophenone. DMF was
distilled from calcium hydride under vacuum and kept under argon.
The ligands PPh3, PnBu3, and dppf were commercial. The ligand
1a was synthesized as indicated in the Supporting Information.
PhPdI(PPh3)2 was prepared as described in the literature.11
Synthesis of Pd0(1a). A 0.911 g (1.01 mmol) amount of the
macrocycle 1a and 1.52 g (1.32 mmol) of Pd(PPh3)4 in 50 mL of
THF were stirred at reflux for 15 h. After filtration, the filtrate was
evaporated under vacuum to give a residue that was purified through
a silica gel column (hexane/ethyl acetate) to afford 720 mg of Pd(1a)
as a white solid (71% yield). IR (ATR): ν 2955, 2867, 1593, 1311,
1148 cm-1. 1H NMR (250 MHz, TMS, CDCl3): δ 1.28 (m, 36H),
1.77 (m, 2H), 2.12 (m, 3H), 2.91 (sept. J ) 6.9 Hz, 2H), 3.21 (m,
3H), 3.85 (m, 2H), 4.17 (m, 6H), 4.64 (m, 6H), 7.18 (m, 6H), 7.81
(m, 2H) ppm. 19F NMR (235.36 MHz, CCl3F, CDCl3): δ -108.48
(m, 1F). Anal. Calcd for C48H68FN3O6S3Pd: C, 57.38; H, 6.82; N,
4.18; S, 9.57. Found: C, 57.27; H, 6.84; N, 4.10 ; S, 9.30.
General Procedure for Cyclic Voltammetry. Experiments were
carried out in a three-electrode thermostated cell (25 °C) connected
to a Schlenk line. The reference was a saturated calomel electrode
(Radiometer) separated from the solution by a bridge filled with
1.5 mL of THF containing nBu4NBF4 (0.3 M). The counter
electrode was a platinum wire of ca. 1 cm2 apparent surface area.
Fifteen milliliters of THF containing nBu4NBF4 (0.3 M) was
introduced into the cell followed by 15.9 mg (0.06 mmol) of PPh3
and 30.1 mg (0.03 mmol) of Pd0(1a). Cyclic voltammetry was
performed with time at a steady gold disk electrode (d ) 0.5 mm)
at a scan rate of 0.5 V s-1. Increasing amounts of PPh3 were then
added to the cell, and cyclic voltammetry was performed with time
at a scan rate of 0.5 V s-1. Similar experiments were performed
from 30.1 mg (0.03 mmol) of Pd0(1a) in the presence of 16.8 mg
(0.03 mmol) of dppf in THF or 15.7 µL (0.06 mmol) of PnBu3 in
DMF.
Conclusion
Besides its ability to recover and recycle Pd(0) catalysts in
catalytic reactions, the macrocyclic ligand 1a plays an important
role in the kinetics of the oxidative addition of aryl halides
performed from Pd0(1a) associated with phosphine ligands
(PPh3, PnBu3, dppf) in THF or DMF. The following reactivity
order with PhI has been established:
Pd0(1a) + 2 PnBu3 > Pd0(1a) + 2 PPh3 > Pd0(PPh3)4 >
DMF
THF
THF
Pd0(1a) + 4 PPh3 > Pd0(1a) + dppf (2)
THF
THF
The ligand 1a is in equilibrium with the reactive Pd0(PPh3)2
complex via Pd0(PPh3)3. The concentration and reactivity of
Pd0(PPh3)2 is controlled by the concentration of 1a, and the
oxidative addition of PhI is slower than that performed from
1a-free precursors at equal ligand loading and Pd concentration
(e.g., {Pd0(1a) + 4 PPh3} less reactive than Pd0(PPh3)4). In
contrast to PPh3, 1a is also a ligand in Pd0(η2-1a)(PnBu3)2 which
is in equilibrium with the most reactive Pd0(PnBu3)2 complex.
Once more, the ligand 1a plays an important role since the rate
of the oxidative addition of PhBr is found to be limited by the
dissociation of Pd0(PnBu3)2 from Pd0(η2-1a)(PnBu3)2 at high
PhBr concentrations. The reaction of PhI also involves Pd0(η2-
1a)(PnBu3)2 as the reactive species. The decelerating effect
induced by the ligand 1a can be of interest if the fast oxidative
addition has to be slowed down so that its rate becomes closer
to the rate of the following slower step (e.g., transmetalation or
carbopalladation). This emphasizes the key role played by
unsaturated CdC ligands of Pd(0) precursors in oxidative
addition, as reported by some of us in the case of dba16,21 or
with extra olefins.22
General Procedure for the Kinetics of the Oxidative
Addition of PhI to Pd(0) Complexes Generated in Situ from
Pd0(1a) Associated with PPh3 or dppf. Experiments were carried
out in the same cell as used for cyclic voltammetry (see above).
Fifteen milliliters of THF containing nBu4NBF4 (0.3 M) was
introduced into the cell followed by 15.9 mg (0.06 mmol) of PPh3
and 30.1 mg (0.03 mmol) of Pd0(1a). After 1 h, the kinetic
measurement for the oxidative addition of PhI was performed at a
rotating gold disk electrode (Radiometer, EDI 65109, d ) 2 mm,
angular velocity ω ) 105 rad s-1) polarized at +0.5 V vs SCE.
The decrease of the oxidation current of the Pd(0) complex was
recorded with time after addition of 3.4 µL (0.03 mmol) of PhI
until total conversion. Similar experiments were done in the
presence of 32 mg (0.12 mmol) of PPh3 or 16.8 mg (0.03 mmol)
of dppf.
Experimental Part
31P NMR spectra were recorded on a Bruker spectrometer (101
MHz) with H3PO4 as an external reference. Cyclic voltammetry
General Procedure for the Kinetics of the Oxidative
Addition of PhBr to Pd(0) Complexes Generated in Situ
from Pd0(1a) Associated with PnBu3. The kinetics of the oxidative
addition of PhBr (2, 20, 40, 60, and 80 mM) to the Pd(0) complex
generated from 30.1 mg (0.03 mmol) of Pd0(1a) and 15.7 µL (0.06
mmol) of PnBu3 in DMF was monitored at a rotating gold disk
electrode as explained above in THF (see kinetic curves in the
Supporting Information).
General Procedure for 31P NMR (101.3 MHz, H3PO4)
Experiments. To an NMR tube containing 0.5 mL of DMF and
0.1 mL of acetone-d6 were added 9 mg (0.009 mmol) of Pd0(1a)
followed by 4.7 µL (0.018 mmol) of PnBu3. The 31P NMR was
performed from time to time. Similar experiments were performed
(20) Amatore, C.; Broeker, G.; Jutand, A.; Khalil, F. J. Am. Chem. Soc.
1997, 119, 5176–5185.
(21) (a) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F. Org. Lett. 2004, 6,
4435–4438. (b) Mace´, Y.; Kapdi, A. R.; Fairlamb, I. J. S.; Jutand, A.
Organometallics 2006, 25, 1795–1800. (c) Fairlamb, I. J. S.; Kapdi, A. R.;
Lee, A. F.; McGlacken, G. P.; Weissburger, F.; de Vries, A. H. M.;
Schmieder-van de Vondervoort, L. Chem.-Eur. J. 2006, 12, 8750–8761.
(d) Fairlamb, I. J. S.; Lee, A. F. Organometallics 2007, 26, 4087–4089.
(22) For reviews on the effect of olefins on oxidative additions, see: (a)
Jutand, A. Pure Appl. Chem. 2004, 76, 565–576. (b) Johnson, J. B.; Rovis,
T. Angew. Chem., Int. Ed. 2008, 47, 840–871.
Serra-Muns, Anna
