Organometallics
Article
and providing the opportunity to expand the structural
diversity of chiral-at-metal catalysts. Through an established
auxiliary-mediated approach, the final catalyst can be obtained
conveniently in a nonracemic fashion with high enantiomeric
purities (>99% ee for each enantiomer). A high catalytic
performance was demonstrated for asymmetric [2 + 2]
photocycloadditions. Further applications of the newly
synthesized catalyst and the development of bifunctional
versions thereof are currently under investigation in our
laboratory.
(w), 2332 (w), 1621 (w), 1584 (w), 1523 (w), 1485 (w), 1428 (w),
1
366 (w), 1335 (w), 1295 (w), 1262 (w), 1161 (w), 1027 (w), 831
−
1
(s), 769 (w), 733 (w), 695 (w), 651 (w), 556 (m), 453 (w) cm .
+
HRMS (APCI) m/z calcd. for C H N Rh [M-HCl] : 488.1316,
2
4
27
5
found: 488.1329. HRMS (APCI): m/z calcd for C H ClN Rh [M −
2
2
25
4
+
MeCN] 483.0817, found 483.0830. Mp: 214 °C dec (CH Cl ).
2
2
Synthesis of rac-RhNS. A suspension of complex 2 (100 mg, 0.15
mmol, 1.00 equiv) and 5-tert-butyl-2-phenylbenzothiazole (3; 40.0
mg, 0.15 mmol, 1.00 equiv) in 2-ethoxyethanol (2.25 mL) and H O
2
(0.75 mL) was stirred at 80 °C for 72 h in the dark. The reaction
mixture was cooled to room temperature, and the solvent was
removed thoroughly under reduced pressure (water bath of rotary
evaporator set to 60 °C). AgPF (68 mg, 0.27 mmol, 1.80 equiv) and
6
EXPERIMENTAL SECTION
■
MeCN (3.00 mL) were added, successively, and the resulting
suspension was stirred at 60 °C for 14 h in the dark. The mixture was
cooled to room temperature, and then it was filtered over a short plug
of Celite and rinsed with MeCN. The solvent was removed under
reduced pressure, and the obtained residue was purified by column
General Methods and Materials. All reactions were carried out
under a nitrogen atmosphere in oven-dried glassware unless noted
otherwise. Solvents were distilled under nitrogen from calcium
hydride (MeCN, CH Cl ), sodium/benzophenone (THF), or sodium
2
2
(
toluene) prior to use. Reagents that were purchased from
chromatography (CH
rac-RhNS (72 mg, 0.08 mmol, 56%, ratio of rac-RhNS to RhS/RhN
5.6:1) as a pale yellow solid. TLC (CH Cl /MeCN 20/1): R = 0.40.
): δ 8.52 (d, J = 1.6 Hz, 1H), 8.03 (d, J =
2 2
Cl /MeCN 60/1 → 40/1 → 20/1) to afford
commercial suppliers were used without further purification. Flash
column chromatography was performed with silica gel 60 M from
Macherey-Nagel (irregularly shaped, 230−400 mesh, pH 6.8, pore
2
2
f
1
H NMR (300 MHz, CD Cl
2 2
−
1
2
volume 0.81 mL × g , mean pore size 66 Å, specific surface 492 m ×
8.6 Hz, 1H), 7.99 (d, J = 1.1 Hz, 1H), 7.82 (dd, J = 7.8, 1.1 Hz, 1H),
7.72 (dd, J = 8.6, 1.8 Hz, 1H), 7.68−7.64 (m, 2H), 7.61 (d, J = 8.7
Hz, 1H), 7.05 (dt, J = 7.6, 1.0 Hz, 1H), 7.00 (dt, J = 7.5, 1.0 Hz, 1H),
6.84−6.78 (m, 2H), 6.27 (d, J = 7.8 Hz, 1H), 6.15 (d, J = 7.8 Hz,
−
1
g , particle size distribution 0.5% < 25 μm and 1.7% > 71 μm, water
1
13
1
19
1
content 1.6%). H NMR, C{ H} NMR, and F{ H} NMR spectra
were recorded on a Bruker AV II 300 MHz, AV III HD 250 MHz, AV
III 500 MHz, AV III HD 500 MHz, or AV II 600 MHz spectrometer
at ambient temperature. Chemical shift values δ are reported in ppm
1
3
1H), 4.29 (s, 3H), 2.18 (brs, 6H), 1.47 (s, 9H), 1.46 (s, 9H) ppm.
C
NMR (126 MHz, CD Cl ): δ 176.6, 161.7, 158.3, 158.2, 152.5, 150.5,
2
2
1
3
with the solvent resonance as internal standard. All C NMR signals
are singlets unless noted otherwise. IR spectra were recorded on a
Bruker Alpha FT-IR spectrometer. CD spectra were aquired with a
JASCO J-810 CD spectropolarimeter (600−200 nm, data pitch 0.5
nm, bandwidth 1 nm, response 1 s, sensitivity standard, scanning
speed 50 nm/min, accumulation of three scans). High-resolution
mass spectrometry was performed on a Finnigan LTQ-FT Ultra mass
spectrometer (Thermo Fischer Scientific) using ESI or APCI as the
ionization source. EI mass spectra were recorded on an AccuTOF
GCv instrument (JEOL). Melting points were determined on a
Mettler Toledo MP70 apparatus using capillary tubes closed on one
end. Chiral HPLC was performed on an Agilent 1200 or 1260
instrument or on an Agilent 1200 instrument with an Agilent 6120
Series Quadrupole LC/MS System with multimode source.
148.5, 140.4, 139.7, 135.1, 134.1, 134.0, 133.5, 131.0, 129.9, 129.3,
126.1, 125.3, 125.0, 124.1, 124.0, 123.1, 122.8, 121.4 (2C), 117.1,
112.6, 110.4, 35.6, 35.4, 32.8, 31.8 (3C), 31.6 (3C), 3.64 (2C) ppm.
1
9
F NMR (282 MHz, CD
Cl ): δ −74.0 (d, JP−F = 711 Hz, 6F) ppm.
2
2
IR (neat): ν 2957 (w), 2868 (w), 2282 (w), 1580 (w), 1555 (w),
̃
1513 (w), 1480 (w), 1440 (w), 1419 (w), 1364 (w), 1330 (w), 1294
(w), 1257 (w), 1160 (w), 1122 (w), 1026 (w), 993 (w), 933 (w), 835
(s), 760 (w), 729 (w), 699 (w), 671 (w), 651 (w), 611 (w), 555 (m),
−
1
+
458 (w) cm . HRMS (APCI): m/z calcd for C H N RhS [M]
3
9
41
5
714.2132, found 714.2135. Mp: 213 °C (CH Cl ).
2
2
Synthesis of Λ- and Δ-(S)-5. rac-RhNS (93 mg, 0.11 mmol, 1.00
equiv), K CO (45 mg, 0.33 mg, 3.00 equiv), and chiral auxiliary (S)-
2
3
4 (31 mg, 0.12 mmol, 1.10 equiv) were dissolved in EtOH (4.60 mL,
absolute) and stirred for 5 h at 70 °C. The reaction mixture was
cooled to room temperature, diluted with CH Cl , and filtered over a
Synthesis of Complex 2. The synthesis was related to a
2
2
procedure reported by our group but was performed with
modifications. A suspension of RhCl3 hydrate (100 mg, 0.39
mmol, 1.00 equiv) and 5-tert-butyl-1-methyl-2-phenylbenzimidazole
short plug of Celite. The solvent was removed under reduced
pressure, and the mixture of four diastereomers was purified by
20
column chromatography (n-pentane/EtOAc + 1% Et N 10/1 → 8/1
3
(
1; 113 mg, 0.43 mmol, 1.10 equiv) in 2-ethoxyethanol (5.85 mL)
→ 6/1 → 5/1 → 4/1 → 3/1) to give Λ-(S)-5a (25 mg, 0.03 mmol,
26%), Λ-(S)-5b (12 mg, 0.01 mmol, 13%), and Δ-(S)-5a (42 mg,
0.05 mmol, 43%) as yellow solids. Complex Δ-(S)-5b could not be
isolated, most likely due to formation only in trace amounts. If
necessary, the individual diastereomers have to be purified again by
column chromatography to provide the final catalysts Λ- and Δ-RhNS
and H O (1.95 mL) was stirred at 150 °C for 7 h. The reaction
mixture was cooled to room temperature, and the solvent was
removed thoroughly under reduced pressure (water bath of rotary
2
evaporator set to 60 °C). AgPF (177 mg, 0.70 mmol, 1.80 equiv) and
6
MeCN (7.80 mL) were added, successively, and the resulting
suspension was stirred at 60 °C for 15 h in the dark. The mixture was
cooled to room temperature, and then it was filtered over a short plug
of Celite and rinsed with MeCN. The solvent was removed under
reduced pressure, and then the residue was filtered again over a short
silica pad (ca. 1 cm) using CH Cl /MeCN 50/1 as eluent to remove
1
1
3
1
Λ-(S)-5a. TLC (n-pentane/EtOAc 2/1 + 1% Et N): R = 0.38. H
3
f
NMR (600 MHz, CD Cl ): δ 8.98 (d, J = 1.6 Hz, 1H), 7.73 (dd, J =
2
2
2
2
7.8, 1.1 Hz, 1H), 7.63 (d, J = 8.5 Hz, 1H), 7.59 (d, J = 1.3 Hz, 1H),
7.48−7.46 (m, 2H), 7.39 (d, J = 8.8 Hz, 1H), 7.34 (dd, J = 7.6, 1.1
Hz, 1H), 6.95−6.92 (m, 2H), 6.89−6.85 (m, 1H), 6.82−6.79 (m,
1H), 6.78 (dt, J = 7.4, 1.4 Hz, 1H), 6.72 (dt, J = 7.5, 1.3 Hz, 1H), 6.55
(d, J = 7.8 Hz, 1H), 6.43 (d, J = 8.7 Hz, 1H), 6.28 (d, J = 7.4 Hz, 2H),
6.60 (d, J = 7.9 Hz, 1H), 5.87 (ddd, J = 13.0, 7.8, 1.1 Hz, 1H), 4.84−
4.80 (m, 2H), 4.10 (s, 3H), 4.01 (dd, J = 6.7, 2.3 Hz, 1H), 1.45 (s,
the excess AgPF . Purification of the obtained yellow oil by column
chromatography (CH Cl /MeCN 20/1 → 10/1 → 5/1) afforded
complex 2 (223 mg, 0.33 mmol, 86%) as a yellow solid. TLC
(
6
2
2
1
CH Cl /MeCN 5/1): R = 0.33. H NMR (500 MHz, CD CN): δ
2
2
f
3
8
.13 (dd, J = 1.7, 0.6 Hz, 1H), 8.01 (dd, J = 7.7, 1.5 Hz, 1H), 7.97
(
dd, J = 7.8, 1.1 Hz, 1H), 7.61 (dd, J = 8.8, 1.8 Hz, 1H), 7.57 (d, J =
1
3
8
1
.8 Hz, 1H), 7.40 (dt, J = 7.6, 1.6 Hz, 1H), 7.35 (dt, J = 7.5, 1.2 Hz,
9H), 1.30 (s, 9H) ppm. C NMR (126 MHz, CD Cl ): δ 175.5 (d,
2 2
1
3
H), 4.21 (s, 3H), 2.63 (s, 3H), 2.10 (s, 3H), 1.47 (s, 9H) ppm.
C
JC,Rh = 3.3 Hz), 174.9 (d, JC,F = 3.4 Hz), 171.1 (d, JC,Rh = 30.6 Hz),
169.6 (d, JC,Rh = 32.4 Hz), 165.8 (d, JC,F = 3.4 Hz), 164.0 (d, JC,F
NMR (126 MHz, CD CN): δ 157.9 (d, J
= 2.5 Hz), 157.1 (d, JC,Rh
=
3
C,Rh
25.2 Hz), 148.8, 140.0, 136.7, 135.7, 134.2, 131.1, 126.6, 125.9,
257.2 Hz), 159.6 (d, JC,Rh = 3.3 Hz), 151.7, 151.6, 147.5, 141.9, 141.5,
140.6, 136.2, 134.9, 134.1, 134.0, 132.6 (d, JC,F = 14.1 Hz), 129.7,
129.5, 128.9, 128.0 (2C), 127.5 (2C), 125.8, 124.6, 123.7, 122.5,
122.2, 121.7, 121.3, 121.1 (d, JC,F = 2.4 Hz), 119.8, 112.6, 109.7,
C,Rh
1
9
̃
3
P−F
E
Organometallics XXXX, XXX, XXX−XXX