Angewandte
Chemie
the waxy yellow solid was washed with hexane and dried in vacuo.
of the bicarbonate salt with CO2. Thus, CO2 has two roles: it
affects the protonation of the amidine groups through the
formation of carbonic acid by reaction with water, and it also
removes CO from the system, which allows 1 to coordinate to
the metal atom. When 1 and 2 are both present in the
coordination sphere, there are seven amidines per rhodium
atom, which is sufficient to allow efficient phase switching
upon protonation of the amidines. The lower rate of hydro-
formylation of 1-octene when both ligands are present
relative to when 2 is used alone, may arise because 1 can
compete with the alkene for the vacant coordination site once
CO is lost from [RhH(CO)2(2)] to form the active species in
the catalytic cycle. Some support for this suggestion comes
from the broad PPh3 resonance that was detected in the
31P NMR spectrum under CO/H2 (Figure 2a).
The product was purified by column chromatography (9:1 CH2Cl2:
MeOH, 2% v/v NEt3) to give a yellow foam. Yield; 0.216 g, 30%.
Rf = 0.64 (9:1 CH2Cl2: MeOH, 2% v/v NEt3); 1H NMR
(400 MHz, CDCl3): d = 7.29 (dd, 2H, J = 7.9, 1.3 Hz, Ar-H, xant),
7.09 (t, 4H, J = 7.6 Hz, Ar-H, Ph), 6.85 (t, 2H, J = 7.6 Hz, Ar-H, xant),
6.8 (m, 4H, Ar-H, Ph), 6.62 (d, 4H, J = 8.1 Hz, Ar-H, Ph), 6.57 (bdq,
2H, J = 7.9, 1.65 Hz, Ar-H, xant), 6.51 (bdq, 4H, J = 5.9,1.665 Hz, Ar-
H, Ph), 2.91 (s, 24H, NC(CH3)N(CH3)2), 1.66 (s, 12H, NC(CH3)N-
(CH3)2), 1.58 ppm (s, 6H, C(CH3)2).13C NMR (100 MHz, CDCl3): d =
157.5 (s, 1C, NC(CH3)N(CH3)2), 152.0 (t, J = 9.3 Hz, Ar-C), 151.8 (t,
J = 3.3 Hz, Ar-C), 137.7 (dd, J = 7.3, 6.0 Hz, Ar-C), 132.2 (s, Ar-C),
129.0 (s, Ar-C), 128.3 (t, J = 4.1 Hz, Ar-C), 127.7 (dd, J = 11.5, 9.3 Hz,
Ar-C), 126.7 (dd, J = 12.6 Hz, J = 8.8 Hz, Ar-C), 125.9 (s, Ar-C), 122.9
(s, Ar-C), 122.6 (s, Ar-C) 37.9 (s, 8C, NC(CH3)N(CH3)2), 34.3 (s, 1C,
C(CH3)2), 32.1 (s, 2C, C(CH3)2, 14.9 ppm (s, 4C, NC(CH3)N(CH3)2).
31P NMR (121 MHz, CDCl3): d = À16.5 ppm (s, 2P). HRMS (ESI): m/
z: calcd for C55H65N8OP2: 915.4712 [M+H]+; found: 915.4714.
Hydroformylation reactions were carried out as described
previously,[6] but by using [Rh(acac)(CO)3] (1 mmoldmÀ3) and 2
(2.2 mmol) with or without 1 (2.2 mmol) in toluene (8 mL). Once the
catalytic system had equilibrated at 1008C under CO/H2 (1:1, 15 bar),
1-octene (2 mL) was injected by using a substrate injector and the
pressure was adjusted to 20 bar. Gas was fed from a ballast vessel to
maintain the pressure at 20 bar. The pressure in the ballast vessel was
recorded electronically. After the reactions, the autoclave was cooled
and depressurized. It was then opened and the contents were
analyzed by GC-FID (HP-1 column, 30 m, I.D 0.25 mm, 0.25 mm
thick film) that separated the compounds on the basis of boiling
points. The recycling procedure was carried out as described
previously.[6]
Experimental Section
All manipulations were carried out under dry N2 by using standard
Schlenk line and catheter tubing techniques.
4,5-Bis(dichlorophosphino-9,9-dimethylxanthene) was prepared
by a literature procedure.[8]
4,5-Bis-(bis(3-aminophenyl)phosphino)-9,9-dimethylxanthene:
9,9-Dimethyl-4,5-bis-(dichlorophosphino)xanthene
(2.00 g,
4.854 mmol, 1 equiv) was dissolved in THF (30 mL) and [bis(trime-
thylsilyl)amino]phenylmagnesium chloride (4.854 mL, 1m solution in
THF) was slowly added at À408C. The reaction mixture was allowed
to warm to room temperature and stirred for 12 h, then the reaction
was stirred for an additional 12 h at 558C. The solvent was removed
in vacuo to give a pale yellow oil. The crude product was dissolved in
degassed methanol in a separate round-bottomed flask that was
equipped with a reflux condenser. The mixture was heated at reflux
for 18 h and the product was isolated by removing the solvent
in vacuo. The crude product was recrystallized from methanol and
dried in vacuo to give a fine white powder. Yield: 2.184 mmol, 45%.
1H NMR (300 MHz, CDCl3): d = 7.50 (d, 2H, J = 7.5 Hz, Ar-H,
xant), 7.01 (t, 2H, J = 7.5 Hz, Ar-H, xant), 6.93 (t, 4H, J = 7.3 Hz, Ar-
H, Ph), 6.67 (d, 2H, J = 7.0 Hz, Ar-H, xant), 6.50–6.46 (m, 8H, Ar-H
Ph), 6.28 (bt, 4H, Ar-H, Ph), 5.00 (s, 8H, NH2), 1.57 ppm (s, 6H,
CH3); 13C NMR (75 MHz, CDCl3): d = 152.4 (d, J = 9.6 Hz, Ar-C),
148.7 (t, J = 4.1 Hz, Ar-C), 138.1 (t, J = 6.0 Hz, Ar-C), 132.6 (s, Ar-C),
130.0 (s, Ar-C), 129.1 (bs, Ar-C), 127.3 (s, Ar-C), 123.7 (s, Ar-C), 121.7
(t, J = 10.0 Hz, Ar-C), 119.5 (t, J = 11.8 Hz, Ar-C), 114.7 (s, Ar-C),
34.0 (s, 1C, C(CH3)2), 32.2 ppm (s, 2C, C(CH3)2); 31P NMR(121 MHz,
CDCl3): d = À19.18 ppm (2P); MS (ESI): m/z: 661, 639 [M+H]+;
HRMS (ESI): m/z: calcd for C39H37N4OP2: 639.2446 [M+H]+; found:
639.2457. Despite some contamination with solvents, this compound
was used for the next step of the synthesis.
High-pressure 1H NMR studies were carried out in a sapphire
tube that was fitted with a dip-tube for bubbling gases into the
reaction (Hydraulik und Industrie-Technic GmbH, Rostock). [Rh-
(acac)(CO)2] (0.023 g, 0.089 mmol), PPh3 (0.047 g, 0.18 mmol), and
xantphos (0.18 mmol) were dissolved in [D8]toluene (3 mL), which
resulted in effervescence and an orange solution. The solution was
heated to boiling and CO/H2 (1:1) was bubbled through it for 2 min.
This solution (2 mL) was transferred under CO/H2 into the NMR
tube, which was then pressurized with CO/H2 (15 bar). 31P{1H} NMR
and 1H NMR spectra were recorded at room temperature, À708C,
and 1308C.
1H NMR (400 MHz, [D8]toluene, room temperature): d =
À8.8 ppm (dt, JH,Rh = 6.4 Hz, JH,P = 14.8 Hz [RhH(CO)2(xantphos)]),
lit. d = À8.5 ppm (dt, JH,Rh = 6.4 Hz, JH,P = 10 Hz).[2] 31P{1H} NMR
(121 MHz, [D8]toluene, room temperature): d = 20.7 ppm (d, JP, Rh
128 Hz, [RhH(CO)2(xantphos)]), lit. d = 21.1 ppm (d, JP, Rh
=
=
127 Hz),[2] À0.7 (br. s, PPh3), À17.6 ppm (s, xantphos), lit. d =
À17.5 ppm.[2]
After cooling, the pressure was released and N2 was bubbled
through the solution to remove CO/H2. CO2 was then bubbled
through the solution for 15 min before the NMR spectra were
recorded again at room temperature, À808C, and 1008C.
1H NMR (400 MHz, [D8]toluene, room temperature): d =
4,5-Bis(-[3-(1-dimethylaminoethylideneamino)
phenyl]phos-
phino}-9,9-dimethyl-9H-xanthene (2): 4,5-Bis-(bis(3-aminophenyl)-
phosphino)-9,9-dimethylxanthene (0.5 g, 0.785 mmol, 1 equiv) was
placed into a microwave reaction vessel that was equipped with a
magnetic stirrer. The tube was sealed with a septum and purged with
alternate vacuum and N2 (three times). Dimethylacetamide dime-
thylacetal (2.14 g, 2.35 mL, 20 equiv) was added and the tube was
heated at 1608C for 1 h in a microwave oven (400 W at 2.45 GHz).
The crude mixture was transferred to a Schlenk tube. The excess
dimethylacetamide dimethylacetal and the methanol that was pro-
duced were removed in vacuo and the residue was redissolved in
toluene. The toluene phase was washed with water (10 mL). After the
water layer was removed, a further aliquot of water (15 mL) was
added to the toluene phase and CO2 was bubbled through the stirred,
biphasic mixture for 1.5 h. The toluene phase was discharged and
fresh toluene (15 mL) was added. N2 was bubbled through the stirred,
biphasic mixture for 1.5 h at 608C. The toluene was evaporated, then
À9.4 ppm (ddt, JH,Rh = 1.0 Hz, J
= 20 Hz, JH,P(xantphos) = 12 Hz
H,P(PPh3)
[RhH(CO)(PPh3)(xantphos)]), lit. d = À9.1 ppm (ddt, JH,Rh = 1.7 Hz,
H,P(PPh ) = 18,
H,P(xantphos) = 12 Hz).[2] 31P{1H} NMR (121 MHz,
[D8]toluene, room temperature): d = 42.3 ppm (ddd, JP, Rh = 167 Hz,
J
J
3
J
J
P, P = 130 Hz) [RhH(CO)(PPh3)(xantphos)]), lit. d = 42.7 ppm (ddd,
P, Rh = 151 Hz, JP, P = 119 Hz, ddd as a result of second-order effects),[2]
25.2 ppm (dd,
JP, Rh = 147 Hz, J
P, P = 130 Hz)[2] [RhH(CO)(PPh3)-
(xantphos)]), lit. d = 25.7 ppm (dd, JP, Rh = 128 Hz, JP, P = 119 Hz),[2]
À4.7 (br s, PPh3), À17.6 ppm (s, xantphos).
Degassed water (1 mL) was added to the solution (1 mL) that
remained once the NMR tube had been filled and CO2 was bubbled
through this mixture for 5 min. The orange organic phase was studied
by 1H NMR and 31P{1H} NMR spectroscopy. The NMR spectra of the
Angew. Chem. Int. Ed. 2012, 51, 1648 –1652
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1651