References and Notes
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<A NAME="RU10406ST-7">7</A>
The tert-butyl group of Phebox is important as it increases solubility. The t-Bu-Phebox ligand was prepared from 5-tert-butylisophthalic acid (Aldrich 362514). Using SOCl2, the acid chloride was formed first, followed by reaction with 2-aminoethanol to
give the amide, this was then cyclized using MsCl and Et3N to give t-Bu-Phebox as a colorless solid; mp 135-136 °C; 1H NMR (300 MHz, CDCl3): δ = 8.27 (t, J = 1.5 Hz, 1 H), 8.11 (d, J = 1.5 Hz, 2 H), 4.30 (t, J = 9.0 Hz, 2 H), 4.07 (t, J = 9.0 Hz, 2 H), 1.36 (s, 9 H); 13C NMR (75 MHz, CDCl3): δ = 31.33, 35.06, 55.06, 67.66, 125.1, 127.6, 127.8, 151.5, 164.1. A mixture of
t-Bu-Phebox (817 mg, 3.0 mmol), RhCl3(H2O)3 (870 mg, 3.3 mmol), NaHCO3 (252 mg, 3.0 mmol), MeOH (30 mL) and H2O (1 mL) was heated at 60 °C for 5 h. After concentration, the residue was purified
by column chromatography (EtOAc-hexane) to give the corresponding chloro complex Rh(t-Bu-Phebox)Cl2(H2O) (872 mg, 63%). The chloro complex was treated at r.t. for 15 h with AgOAc (1.25
g) in CH2Cl2 (40 mL) and then purified by column chromatography (EtOAc → EtOAc-MeOH, 10:1) to
give 1 (855 mg, 89%) as a yellow solid; mp 126 °C (dec.); 1H NMR (300 MHz, CDCl3): δ = 7.67 (s, 2 H), 6.51 (br s, 2 H), 4.88 (t, J = 9.6 Hz, 2 H), 4.13 (t, J = 9.6 Hz, 2 H), 1.68 (s, 6 H), 1.41 (s, 9 H); 13C NMR (75 MHz, CDCl3): δ = 24.01, 31.82, 35.18, 50.83, 71.20, 124.9, 130.9, 146.7, 172.7 (J
Rh-C = 4.0 Hz), 182.3, 184.6 (J
Rh-C = 23.9 Hz); IR (KBr): 1720 cm-1.
<A NAME="RU10406ST-8">8</A>
For the synthesis of 2, see ref 1a.
<A NAME="RU10406ST-9">9</A>
Typical procedure (run 6, Table
[1]
): To a solution of cinnamaldehyde (132 mg, 1.00 mmol) and 2 (5.4 mg, 0.010 mmol) in toluene (1.0 mL) at 30 °C, was added diethoxymethylsilane
(0.24 mL, 1.5 mmol). The mixture was stirred for 1 h. After confirmation of complete
consumption of the aldehyde, the mixture was cooled to 0 °C and TBAF (0.4 mL, 1 M
in THF), KF (2 mmol), and MeOH (1 mL) were added. The mixture was stirred for 1 h.
Then aq NaHCO3 was added, and the mixture was extracted with EtOAc. After concentration, the residue
was purified by column chromatography (hexane-EtOAc) to give the dihydrocinnamaldehyde
in 97% (130 mg).
<A NAME="RU10406ST-10">10</A>
(R)-10: [α]D
24.5 -32.6 (c 1.01, EtOH) {Lit. [α]D
22 +32.9° (c 1.00, EtOH)} for 93% ee of S
[6c]
; 1H NMR (300 MHz, CDCl3): δ = 9.72 (t, J = 2.1 Hz, 1 H), 7.28-7.34 (m, 2 H), 7.18-7.23 (m, 3 H), 3.37 (m, 1 H), 2.76 (ddd,
J = 16.5, 6.9, 2.1 Hz, 1 H), 2.66 (ddd, J = 16.5, 7.5, 2.1 Hz, 1 H), 1.32 (d, J = 7.2 Hz, 3 H); 13C NMR (75 MHz, CDCl3): δ = 22.31, 34.39, 51.76, 126.3, 126.5, 128.4, 145.1, 201.3; IR (film): 1723 cm-1; GC: SUPELCO BETA-DEX 325 (30 m), 60 kPa, 100 °C, r.t., 54.9 min for R and 56.2 min for S.
<A NAME="RU10406ST-11">11</A> (S)-12: [α]D
25.4 -15.2 (c 1.00, CHCl3) {Lit. [α]D
24 -16.2 (c 1.00, CHCl3)} for 98% ee of S:
Yamamoto T.
Shimada A.
Ohmoto T.
Matsuda H.
Ogura M.
Kanisawa T.
Flavour Fragr. J.
2004,
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121
