Green Chemistry
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ture and treated with water (100 mL). The crude product was 4.54 (dd, J = 10.8, 4.4 Hz, 1H), 4.43 (dd, J = 10.0, 7.2 Hz, 1H),
extracted with EtOAc (2 100 mL), rinsed with water 3.98–3.79 (m, 3H), 3.01 (s, 3H); 1.43 (s, 9H), 1.42 (s, 9H), 1.28
×
(2 × 100 mL), dried over Na2SO4 and concentrated to afford a (s, 18H); 13C-NMR (100 MHz, CDCl3): δ 169.0, 168.8, 157.9,
mixture of diazide 9 and 2-bromoprop-2-en-1-ol 1021 (10/1) as a 157.9, 140.5, 140.3, 136.8, 136.7, 127.9, 127.5, 126.6, 126.3,
light brown oil which was used in the next step without 117.6, 117.5, 70.4, 68.3, 60.8., 37.5, 35.0, 34.1, 34.1, 31.6, 31.4,
further purification; 2,3-diazidopropan-1-ol 9: 1H-NMR 31.4, 29.4; HRMS (DART-TOF) Calcd for C34H52N2O5S (M +
(400 MHz, CDCl3): 3.77–3.62 (m, 3H), 3.47 (dq, J = 18.1, 4.4 Hz, H)+: 601.36752; found 601.36934.
2H); 13C-NMR (100 MHz, CDCl3): δ 62.8, 62.4, 51.7. When non-
Schiff base 16a. To a solution of mesylate 14 (510.8 mg,
dry DMF was used as solvent, 1,3-diazidopropan-2-ol was iso- 0.85 mmol) in dry MeCN (8 mL) was added successively DIPEA
lated as the major product: 1H-NMR (400 MHz, CDCl3): (593 μL, 3.4 mmol), 1-benzylpiperazine 15a22 (450.1 mg,
3.95–3.91 (m, 1H), 3.42–3.36 (dd, J = 7.8 Hz, 4H): 13C-NMR 2.55 mmol) and potassium iodide (30.0 mg, 0.18 mmol) at
(100 MHz, CDCl3): δ 69.8, 54.1.
room temperature. The reaction mixture was heated at reflux
To a solution of diazide 9 obtained above in THF–H2O for 24 hours (dark orange), quenched by addition of water
(50 mL, 4/1) was added Ph3P (5.246 g, 20 mmol) portionwise at (30 mL) and extracted with CH2Cl2 (2 × 50 mL). The organic
room temperature (N2 evolution was observed). The reaction fractions were combined, dried over Na2SO4, concentrated
mixture was heated at reflux overnight, diluted with water under vacuum and the crude product purified by flash chrom-
(20 mL) and THF was evaporated under vacuum. The mixture atography (gradient; 0–2% MeOH–CH2Cl2) to give the product
was triturated, the white solid formed was filtered, rinsed with as a thick bright yellow oil (376.3 mg, 65%) which slowly solidi-
H2O (10 mL) and the aqueous solution was concentrated fied over time; mp 91–93 °C; Rf = 0.10 (100% CH2Cl2); IR (neat)
under vacuum (60 °C) to give 2,3-diaminopropan-1-ol 11 as a 2952, 2866, 2807, 1630, 1460, 1443, 1360, 1252, 1176, 830, 736,
1
yellow oil which was used in the next step without further puri- 698 cm−1; H-NMR (400 MHz, CDCl3): δ 13.68 (bs, 1H), 13.56
fication. Presence of residual water or DMF in the crude (bs, 1H), 8.34 (s, 1H), 8.33 (s, 1H), 7.34–7.04 (m, 7H), 7.04 (d,
material does not affect the yield of the next reaction.
J = 2.4 Hz, 1H), 7.02 (d, J = 2.4 Hz, 1H), 4.09 (dd, J = 12.0,
To a solution of 11 obtained above in MeOH (50 mL) was 2.4 Hz, 1H), 3.75–3.60 (m, 2H), 3.49 (s, 2H), 2.73 (dd, J = 13.2,
added 3,5-di-tert-butyl-2-hydroxybenzaldehyde 12 (2.340 g, 6.8 Hz, 1H), 2.60–2.46 (m, 5H), 1.42 (s, 9H), 1.41 (s, 9H), 1.27
10 mmol) in one portion. The reaction mixture was heated at (s, 9H), 1.26 (s, 9H); 13C-NMR (100 MHz, CDCl3): δ 167.8,
reflux for 6 hours, concentrated under vacuum and the 167.1, 158.3, 158.2, 140.2, 140.2, 138.4, 136.8, 136.7, 129.4,
crude material purified by flash chromatography (gradient; 128.4, 127.2, 127.2, 127.1, 126.4, 126.2, 118.1, 118.0, 67.5,
0–50–100% CH2Cl2–hexanes) to give 13 as a thick bright yellow 63.3, 63.0, 62.1, 54.0, 53.4, 35.2, 34.3, 31.7, 29.6; HRMS
oil (2.103 g, 41%) which slowly solidified over time; mp (DART-TOF) Calcd for C44H64N4O2 (M + H)+: 681.51075; found
90–93 °C; Rf = 0.25 (50% CH2Cl2–hexanes + 1% Et3N); IR (neat) 681.50843.
3354, 2953, 2911, 2857, 1626, 1439, 1249, 1172, 877, 730 cm−1
;
Cobalt-Schiff base complex 17a. To a solution of Schiff base
1H-NMR (400 MHz, CDCl3): δ 13.48 (s, 1H), 13.33 (bs, 1H), 8.45 16a (121.3 mg, 0.18 mmol) in i-propanol (2 mL) was added a
(s, 1H), 8.36 (s, 1H), 7.38 (d, J = 2.4 Hz, 1H), 7.36 (d. J = 2.4, solution of Co(OAc)2·4H2O (44.3 mg, 0.18 mmol) in methanol
1H), 7.09 (d, J = 2.4 Hz, 1H, 7.05 (d, J = 2.4 Hz, 1H), 3.96–3.93 (1 mL). The mixture was heated at reflux for 3 h under argon.
(m, 2H), 3.88–3.83 (m, 1H), 3.76–3.66 (m, 2H), 1.43 (s, 9H), The dark brown solution was concentrated and the product
1.42 (s, 9H), 1.28 (s, 18H); 13C-NMR (100 MHz, CDCl3): δ 168.3, was precipitated in 15 mL of hexanes. The green brown solid
168.0, 140.4, 140.1, 136.6, 127.4, 127.2, 126.4, 126.1, 117.8, was recovered by filtration, rinsed with hexanes and then dried
117.7, 71.6, 64.6, 61.1., 35.0, 34.1, 31.5, 29.5, 29.4; HRMS under vacuum for 16 h at 80 °C to give 17a (102.1 mg, 78%) as
(DART-TOF) Calcd for C33H50N2O3 (M + H)+: 523.38997; found a red/light brown solid. IR (neat) 2952, 2911, 2866, 1595, 1529,
523.38990.
1363, 1315, 1252, 1169, 788, 702 cm−1; HRMS (DART-TOF)
2,3-Bis((E)-(3,5-di-tert-butyl-2-hydroxybenzylidene)amino)- Calcd for C44H62CoN4O2 (M
propyl methanesulfonate (14). To a solution of alcohol 13 738.42596.
(1.000 g, 1.92 mmol) and Et3N (517 μL, 3.84 mmol) in CH2Cl2
(10 mL) was added methanesulfonyl chloride (221 μL,
2.87 mmol) dropwise at 0 °C. The reaction mixture was stirred
+
H)+: 738.42830; found
General procedures for the oxidation of p-phenols to
benzoquinones
at the same temperature for 1 hour, quenched by addition of In a Fisher-Porter bottle, p-phenol substrate (1 mmol) and
NaHCO3 (20 mL of a saturated aqueous solution) and diluted Co-Schiff base complex 17a–f (0.05 mmol) were combined in
with CH2Cl2 (20 mL). The organic fraction was separated, 5 mL of MeOH or 5 mL of MeOH–CH2Cl2 (4/1) for catalyst 22.
dried over Na2SO4 and concentrated to give 14 as a thick The bottle was flushed with oxygen three times and then press-
yellow oil (1.094 g, 95%) which slowly solidified over time; mp urized with oxygen to 50 psi. After 16 hours under vigorous
166–169 °C; Rf = 0.25 (50% CH2Cl2–hexanes + 1% Et3N); IR stirring, the reaction mixture is concentrated under vacuum at
(neat) 2959, 2869, 1630, 1439, 1353, 1332, 1180, 996, 979, room temperature and the crude material purified by flash
826 cm−1 1H-NMR (400 MHz, CDCl3): δ 13.26 (s, 1H), 13.06 chromatography (eluent: gradient 0–5–10% EtOAc–CH2Cl2).
;
(bs, 1H), 8.46 (s, 1H), 8.38 (s, 1H), 7.40 (d, J = 2.4 Hz, 1H), 7.38 2,6-Dimethoxybenzoquinone 4 (bright yellow) was recovered by
(d. J = 2.4, 1H), 7.09 (d, J = 2.4 Hz, 1H), 7.06 (d, J = 2.4 Hz, 1H), filtration at the end of the reaction and satisfactorily matched
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Green Chem.