Chen et al.
washed successively with 0.5 M citric acid, saturated sodium
bicarbonate, and brine and dried. Flash chromatography on
silica gel gave the Boc-protected hybrid dendrimers.
In conclusion, Fre´chet-type core dendritic DPEN ligands
and hybrid dendritic ligands based on Fre´chet’s polyether
dendron and Newkome’s poly(ether-amide) were designed
and synthesized. High catalytic activity and enantio-
selectivity, comparable to the monomeric catalyst, were
achieved in transfer hydrogenation of various unsatur-
ated compounds. Much better recyclability for higher
generation core-functionalized dendritic catalyst was
observed compared with that of the hybrid dendritic
catalysts. This finding supplied strong support for the
stabilizing effect of the bulky polyether dendron on the
catalytically active complex. In particular, to the best of
our knowledge, hybrid dendritic ligands and catalysts
have not been synthesized and used in asymmetric
synthesis.3,9 Future work to develop more efficient recy-
clable dendritic catalysts is in progress.
(R,R)-7a: 81% yield; [R]20 +21.5 (c 0.45, THF); 1H NMR
D
(CD3COCD3, 400 MHz) δ 1.32 (s, 27H), 2.47 (br s, 6H), 3.58-
3.78 (m, 12H), 4.12 (br s, 8H), 4.70-4.72 (m, 3H), 4.93-5.00
(m, 31H), 6.58-6.72 (m, 21H), 6.97-7.53 (m, 82H), 7.92 (br s,
3H), 9.72 (br s, 3H) ppm; partial 13C NMR (CD3COCD3,
50 MHz) δ 28.6, 37.0, 44.6, 60.7, 63.9, 68.2, 69.9, 70.5, 79.5,
102.1, 102.3, 107.3, 107.4, 119.6, 127.9, 128.2, 128.4, 128.6,
128.8, 129.2, 136.8, 138.1, 139.8, 140.3, 140.6, 140.7, 142.6,
142.7, 156.5, 160.7, 160.9, 161.0, 169.2, 173.2 ppm; IR (KBr)
3295, 1687, 1595, 1534, 1452, 1370 cm-1. Anal. Calcd for
C
201H204N14O37S3: C, 68.90; H, 5.87; N, 5.60; S, 2.75. Found:
C, 68.40; H, 5.81; N, 5.54; S, 2.50.
General Procedure for Deprotection of the Boc Group.
Boc-protected dendrimer (0.2 mmol) was dissolved in DCM (1
mL) and the solution was cooled with ice water. TFA (1 mL)
was added and the solution was stirred for 1 h. The solvent
was removed under vacuum and water (10 mL) was added.
The mixture was neutralized with ammonia and stirred at
room temperature for 2 h. The solid was collected, washed with
Experimental Section
General Methods. Melting points were determined in open
capillaries and were uncorrected. NMR spectra were recorded
with tetramethylsilane as the internal standard. Chiral 1,2-
diphenylethylene-diamine was produced in our laboratory
according to Corey’s procedure,18 [R]20D +106.7 (c 1.0, methanol,
R,R-isomer). All other reagents were used without purification
as commercially available.
water, and dried under vacuum.
1
(S,S)-2b: 91% yield; [R]20 +3.5 (c 0.15, CH2Cl2); H NMR
D
(CDCl3, 300 MHz) δ 4.39-4.41 (m, 1H) 4.60-4.61 (m, 1H), 5.08
(s, 4H), 6.79 (s, 1H), 7.48-7.03 (m, 22H), 7.61 (d, J ) 8.1 Hz,
2H), 7.67 (d, J ) 8.7 Hz, 2H) ppm; IR (KBr) 3430, 3195, 1676,
1591, 1323, 1158, 1056 cm-1; ESI HRMS calcd for C41H37N3O5S
+ H 684.2532, obsd 684.2429.
General Procedure for Condensation of Fre´chet-Type
Acid with (S,S)-1. (S,S)-N-Boc-N′-(4-aminophenylsulfonyl)-
1,2-dipheylethylenediamine (1) (467 mg, 1.0 mmol),7 Dendritic
acid (1.05 equiv), and triphenyl phosphite (370 mg, 1.1 mmol)
were stirred in Pyridine (2 mL) at 95 °C for 24 h. The solution
was poured into water (30 mL) and extracted with EtOAc (40
mL). The organic phase was washed successively with 0.5 M
citric acid, sodium bicarbonate, and brine dried. Flash chro-
matography on silica gel gave the Boc-protected dendrimers
2a-5a.
Asymmetric Transfer Hydrogenation Reaction: Gen-
eral Procedure for Asymmetric Transfer Hydrogenation
of Unsaturated Compounds. [RuCl2(cymene)]2 (1.3 mg,
0.002 mmol), Dendritic ligand (1.1 equiv of Ru), and NEt3 (1.2
µL, 0.008 mmol) were stirred in DCM at room temperature
for 2 h. Then unsaturated compound (0.4 mmol) and formic
acid/triethylamine azeotrope (0.2 mL) were added in turn. The
solution was stirred at 28 °C and monitored by TLC. After
completion, the solution was diluted with 5 mL of ethyl acetate,
washed with brine, and dried (Na2SO4). Flash chromatography
gave the pure reduced product.
(S,S)-5a: 51% yield; mp 83-86 °C; [R]20 -2.57 (c 0.43,
D
acetone); 1H NMR (CDCl3, 300 MHz) δ 1.49 (s, 9H), 4.92, 4.98
(br s × 2, 60H), 6.55, 6.65 (br s × 2, 45H), 7.28-7.37 (m, 94H)
ppm, the two NCHs of DPEN are too small to be detected; IR
(KBr) 3415, 3031, 1687, 1449, 1373, 1157, 1051 cm-1; Anal.
Calcd for C242H213N3O35S: C, 77.40; H, 5.72; N, 1.12; S, 0.85.
Found: C, 77.80; H, 5.77; N, 1.12; S, 0.75.
For transfer hydrogenation of activated olefin 12, THF was
used as the solvent.
Acknowledgment. This work was financially sup-
ported by the National Natural Science Foundation of
China (Nos. 203900507 and 20025205). Y.-C.C. is grate-
ful for the financial support of Sichuan University.
General Procedure for the Synthesis of Boc-Protected
Hybrid Dendrimers. Branched ligand (R,R)-6 (192 mg, 0.1
mmol),7b Fre´chet-type acid (1.05 equiv), EDCI (25 mg, 0.13
mmol), and HOBt (14 mg, 0.1 mmol) were stirred in DCM (5
mL) and cooled in ice water. NEt3 (18 µL, 0.13 mmol) was
added and the solution was stirred overnight. The solution was
Supporting Information Available: Characterization
data for compounds 2a-4a, 8a, 3b-5b, 7b, and 8b; NMR and
GC or HPLC data for the reduction products; spectra of
dendritic ligands. This material is available free of charge via
(17) Chen, Y.-C.; Xue, D.; Deng, J.-G.; Cui, X.; Zhu, J.; Jiang, Y.-Z.
Tetrahedron Lett. 2004, 45, 1555.
(18) Pikul, S.; Corey, E. J. Organic Syntheses; Wiley: New York;
Collect Vol. IX, p 387.
JO048317V
1010 J. Org. Chem., Vol. 70, No. 3, 2005