A. Clerici, C. Greco, W. Panzeri, N. Pastori, C. Punta, O. Porta
FULL PAPER
(8ϫCH dl + 8ϫCH meso), 143.6 (meso, 2ϫC-CHOH), 144.3 (dl,
2ϫC-CHOH) ppm. MS (ESI): m/z (%) = 265 (100) [M + H]+.
Experimental Section
General: All materials were purchased from commercial suppliers.
Liquid benzaldehydes were distilled prior to use. All reactions were
performed at room temperature (20 °C) under an atmosphere of
1,2-Bis(4-acetylphenyl)-1,2-ethanediol (meso/dl) (2b): 1H NMR
(400 MHz, [D6]DMSO): δ = 2.52 (meso, 6 H, 2ϫCH3), 2.55 (dl, 6
H, 2ϫCH3), 3.0–3.5 (dl + meso, 4 H, 4ϫOH), 4.70 (s, dl, 2 H,
2ϫCH), 4.77 (s, meso, 2 H, 2ϫCH), 7.29 (d, meso, J = 8.4 Hz, 4
H, ArH), 7.38 (d, dl, J = 8.4 Hz, 4 H, ArH), 7.78 (d, meso, J =
8.4 Hz, 4 H, ArH), 7.85 (d, dl, J = 8.4 Hz, 4 H, ArH) ppm. 13C
NMR (100 MHz, [D6]DMSO): δ = 26.48 (meso, 2ϫCH3), 26.5 (dl,
2ϫCH3), 76.4 (dl, 2ϫCHOH), 76.5 (meso, 2ϫCHOH), 127.2 (dl
+ meso, 4ϫCH), 127.4 (dl + meso, 4ϫCH), 135.4 (meso, 2ϫC),
135.5 (dl, 2ϫC), 147.5 (meso, 2ϫC), 148.3 (dl, 2ϫC), 197.5 (meso,
2ϫCO), 194.6 (dl, 2ϫCO) ppm. MS (ESI): m/z (%) = 299 (100)
[M + H]+.
1
nitrogen. NMR spectra were recorded at 500 or 400 MHz for H
and 125 or 100 MHz for 13C, measured in [D6]DMSO. Chemical
shifts (δ) were presented in ppm, using the DMSO peak (δ
1
=2.50 ppm for H and 39.43 ppm for 13C) as an internal standard.
The ratios of dimers were determined from the intensities of the
1
benzylic protons in the H NMR spectra in which the protons of
the dl isomer appeared at higher field relative to that of the meso
isomer. ESI-MS were performed with an Esquire 3000 plus ion-
trap mass spectrometer equipped with an ESI source. The following
aqueous solutions were used: a 15% acidic solution of TiCl3 and
an 80% solution of TBHP. Flash column chromatography was per-
formed by using 40–63 µm silica gel packing.
1,2-Bis(3-acetylphenyl)-1,2-ethanediol (meso/dl) (2bm): 1H NMR
(400 MHz, [D6]DMSO): δ = 2.46 (s, meso, 6 H, 2ϫCH3), 2.53 (s,
meso, 6 H, 2ϫCH3), 4.70 (d, s after D2O exchange, dl, J = 3.3 Hz,
2 H, 2ϫCH), 4.79 (d, s after D2O exchange, meso, J = 3.3 Hz, 2
H, 2ϫCH), 5.41 (d, D2O exchangeable, dl, 2 H, 2ϫOH), 5.52 (d,
D2O exchangeable, meso, 2 H, 2ϫOH), 7.32–7.83 (m, 8 H dl + 8
H meso, ArH) ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 26.4
(meso, 2ϫCH3), 26.5 (dl, 2ϫCH3), 76.4 (dl, 2ϫCHOH), 76.5
(meso, 2ϫCHOH), 126.4, 126.6, 126.8, 127.4, 127.5, 131.8, 132.1
(8ϫCH meso + 8ϫCH dl), 135.8 (meso, 2ϫC), 136.0 (dl, 2ϫC),
142.6 (meso, 2ϫC), 143.5 (dl, 2ϫC), 197.7 (meso, 2ϫCO), 197.8
(dl, 2ϫCO) ppm. MS (ESI): m/z (%) = 299 (100) [M + H]+, 281
(50) [M – H2O + H]+.
Representative Procedure for Pinacol Reaction with TiCl3/TBHP in
Aqueous Alcoholic Solution by Using an Excess of Peroxide: To a
well-stirred solution of 4-CN-benzaldehyde (1a, 262 mg, 2.0 mmol)
in EtOH (or iPrOH, 10 mL) and TiCl3 (15% aqueous acidic solu-
tion, 8.0 mmol, 8.0 mL), at room temp. and under an atmosphere
of N2 was added dropwise a solution of TBHP (80% solution,
5.0 mmol, 0.62 mL) in EtOH (or iPrOH, 5.0 mL) over ca. 5 min.
The blue color of the reaction mixture discharged and became yel-
low by the end of the TBHP addition. The alcoholic cosolvent was
partially removed in vacuo and the leftover solution was extracted
with EtOAc (3ϫ50 mL). The organic phase was washed with brine,
dried (Na2SO4), and concentrated in vacuo. The crude reaction
mixture (265 mg) contained pinacols dl-2a and meso-2a in a 1:1
Supporting Information (see footnote on the first page of this arti-
cle): Spectroscopic data for diols 2am, 2b, and 2bm.
1
ratio according to H NMR spectroscopic analysis. Pure pinacols
(250 mg, 95%) were obtained by crystallization from EtOAc/hex-
ane (1:1). The same yield was obtained in both EtOH and iPrOH
cosolvents (Table 1).
Acknowledgments
Financial support from MURST (Cofin 2006) is gratefully ac-
knowledged.
Representative Procedure for Pinacol Reaction with TiCl3/TBHP in
Aqueous Alcoholic Solution by Using a 1:1 Stoichiometric Amount
of TBHP/aldehyde: To a well-stirred solution of 4-CN-benzalde-
hyde (1a, 262 mg, 2.0 mmol) in EtOH (or iPrOH, 10 mL) and TiCl3
(15% aqueous acidic solution, 3.0 mmol, 3.0 mL) at room tempera-
ture and under an atmosphere of N2 was added dropwise a solution
of TBHP (80% solution, 2.0 mmol, 0.25 mL) in EtOH (or iPrOH,
5.0 mL) over ca. 5 min. The reaction mixture changed its color
from blue to yellow. However, to ensure the complete decomposi-
tion of the peroxide, additional TiCl3 solution (ca. 0.8 mL,
0.8 mmol) was dropped into the reaction mixture in order to barely
maintain a pale blue color. Work up was as above. The crude reac-
tion residue, added of a weighed amount of a suitable internal stan-
dard (2-CH3-benzyl alcohol) and analyzed by 1H NMR spec-
troscopy, revealed 2a in 77% yield when the reaction was per-
formed in EtOH and 86% yield when iPrOH was used, as a 1:1
isomers mixture (Table 2).
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J. Am. Chem. Soc. 2004, 126, 7578–7584 and references cited
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[6] For various H-abstraction absolute rate constants (kH),
see:J. C. Scaiano in Landolt-Börnstein: Numerical Data and
Functional Relationships in Science and Technology – New
Series: Vol. II/13d: Radical Reactions Rates in Liquid (Ed.: H.
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hedron 2006, 62, 5986–5994.
[8] a) For a recent review on pinacol coupling reaction, see: A.
Chatteerjee, N. N. Joshi, Tetrahedron 2006, 62, 12137–12158;
b) A. Clerici, N. Pastori, O. Porta, Eur. J. Org. Chem. 2002,
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3520; e) A. Clerici, L. Clerici, O. Porta, Tetrahedron Lett. 1996,
37, 3035–3038.
Spectroscopic Data: Diols 2a, 2c, 2d, 2e, 2g, and 2f exhibit spectro-
scopic data consistent with those reported in the literature.[21]
1,2-Bis(3-cyanophenyl)-1,2-ethanediol (meso/dl) (2am): 1H NMR
(500 MHz, [D6]DMSO): δ = 4.66 (d, s after D2O exchange, dl, J =
4.2 Hz, 2 H, 2ϫCH), 4.81 (d, s after D2O exchange, meso, J =
4.2 Hz, 2 H, 2ϫCH), 5.61 (d, D2O exchangeable, dl, 2 H, 2ϫOH),
5.66 (d, D2O exchangeable, meso, 2 H, 2ϫOH), 7.42–7.70 (m, 8 H
meso + 8 H dl, ArH) ppm. 13C NMR (125 MHz, [D6]DMSO): δ =
75.3 (meso, 2ϫCHOH), 75.6 (dl, 2ϫCHOH), 110.1 (dl, 2ϫC-
CN), 110.3 (meso, 2ϫC-CN), 118.8 (meso, 2ϫCN), 118.9 (dl,
2ϫCN), 128.4, 128.5, 130.38, 130.4, 130.5, 130.8, 131.8, 132.1,
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2188–2192; b) V. Jagannadahn, S. Steenken, J. Am. Chem. Soc.
1984, 106, 6542–6551; c) M. Mc Millan, R. O. C. Norman, J.
Chem. Soc. B 1968, 590–597; d) W. E. Griffiths, G. F. Longer,
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