Enantioselective addition of diethylzinc to benzaldehyde in the presence of sulfur-containing pyridine ligands

Article abstract of DOI:10.1016/S0957-4166(98)00167-0

Diastereomerically pure hydroxy and thiol derivatives of (5S,7S)-5,7- methane-6,6-dimethyl-2-phenyl-5,6,7,8-tetrahydroquinoline were prepared and assessed in the enantioselective addition of diethylzinc to benzaldehyde: enantioselectivities up to 62% were

Full text of DOI:10.1016/S0957-4166(98)00167-0

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 1933–1940
Enantioselective addition of diethylzinc to benzaldehyde in the
presence of sulfur-containing pyridine ligands
Giorgio Chelucci,a,∗ Daniela Berta,a Davide Fabbri,b Gerard A. Pinna,c Antonio Saba a and
Fausta Ulgheri a
aDipartimento di Chimica, Università di Sassari, via Vienna 2, I-07100 Sassari, Italy
bIstituto A.T.C.A.P.A., C.N.R., via Vienna 2, I-07100 Sassari, Italy
cDipartimento Farmaco Chimico Tossicologico, Università di Sassari, Via Muroni 23, I-07100 Sassari, Italy
Received 2 April 1998; accepted 27 April 1998
Abstract
Diastereomerically pure hydroxy and thiol derivatives of (5S,7S)-5,7-methane-6,6-dimethyl-2-phenyl-5,6,7,8-
tetrahydroquinoline were prepared and assessed in the enantioselective addition of diethylzinc to benzaldehyde:
enantioselectivities up to 62% were obtained. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
Chiral ligands with pyridine sp2-nitrogen donors are gaining an important role in homogeneous
catalytic asymmetric reactions.1 In this contest only few examples of sulphur-containing pyridine ligands
have been described2 and to our knowledge no data has been reported on the use of chiral pyridine-thiols.3
Continuing our interest in the synthesis and in the application in asymmetric synthesis of chiral pyridine
derivatives,4 we have been evaluating the potential utility of chiral pyridine-thiols as ligands for metal
complexes in enantioselective catalysis, also on account of the very good results obtained by the related
amino thiols.5
Recently, it has been reported that the aminopyridines 16 and the related alcohols 27 gave mod-
erate to excellent enantioselectivity in the addition of diethylzinc to benzaldehyde. Moreover, the
phenylthiopyridines 3 were effective ligands in palladium catalysed allylic substitutions.2a Since these
compounds were prepared from the same intermediate, 5,7-methane-6,6-dimethyl-2-phenyl-5,6,7,8-
tetrahydroquinoline 4, we decided to use this compound as a starting point for the synthesis of chiral
pyridine thiols.
∗
Corresponding author. E-mail: chelucci@ssmain.uniss.it
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00167-0

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G. Chelucci et al. / Tetrahedron: Asymmetry 9 (1998) 1933–1940
This paper is concerned with the preparation of some diastereomerically pure thiol derivatives
of the tetrahydroquinoline 4 and their application in the enantioselective addition of diethylzinc to
benzaldehyde.8
2. Results and discussion
It is appropriate to note at this point that the ligands 1–3 provided a level of stereodifferentiation
depending on the configuration at the C8 carbon. In the enantioselective addition of diethylzinc to
benzaldehyde, the cis-alcohol 2a gave a better enantioselectivity than its trans-epimer,6 whereas a reverse
result was obtained in the palladium catalysed allylic substitution. In this case, the best stereochemical
outcome was obtained by the trans-phenylthiopyridine 3b.2a Therefore, we devoted our efforts to
obtaining both epimers of the thiol derivatives of 4.
Initially, we decided to prepare the 8-thiol-5,6,7,8-tetrahydroquinolines 5 and 6 which are the cor-
responding thiol derivatives of pyridyl carbinol 2. These compounds would be expected to be very
promising ligands since amino thiols gave improved enantioselectivity over the corresponding amino
alcohol counterparts in the addition of diethylzinc to benzaldehyde9 and in the nickel(II) catalysed
conjugate addition of diethylzinc to chalcone.10 Moreover, they appear to be more readily accessible
from 4 than 1 and 2.
Indeed, ligands 5 and 6 were obtained in a one-step procedure (Scheme 1). The red solution of lithiated
4, obtained by treatment with butyllithium containing TMEDA at −78°C for 1 h and then 2 h at −30°C,
was quenched with elemental sulphur at −78°C to give a 1:3 mixture of 5 and 6 which were separated
by flash chromatography.
Scheme 1.
The ability of the new ligands to provide asymmetric induction in the enantioselective addition of
diethylzinc to benzaldehyde was examined. All reactions were carried out in hexane–toluene solution in
the presence of 6 mol% of the ligands at 25°C (Table 1). Disappointing results were obtained with both
epimers 5,6 (vide infra). Next, we decided to examine the possibility of preparing 1–3 SH–N ligands so
that the zinc derivative can form a six-membered chelating ring rather than a five-membered one as in
the case of that formed by 5 or 6. Therefore the synthesis of 12 and 15 was planned.
The lithium derivative of 4 was treated with DMF at −60°C to give the enol 7 which was reduced
with sodium borohydride to give a 3:7 mixture of diastereomeric alcohols 8 and 9 (Scheme 2). These

G. Chelucci et al. / Tetrahedron: Asymmetry 9 (1998) 1933–1940
1935
Table 1
Enantioselective addition of diethylzinc to benzaldehydea
epimers were separated by chromatography on a long column and then converted into the corresponding
mesylates 10 and 13, which by treatment with five equivalents of potassium thioacetate in ethanol at
reflux temperature gave the thiol acetates 11 and 14, respectively. Finally, lithium aluminium hydride
reduction afforded the methanethiols 12 and 15. With the primary thiols in hand, we examined the
possibility of preparing the corresponding tertiary ones. Thus, the lithium derivative of 4 was treated
with benzophenone to give the carbinol 16 as a single diastereomer. Initially, direct displacement of the
hydroxy with the thiol group was attempted using the Lawesson’s methodology11 but only the dehydrated
product 18 was obtained. Therefore, we decided to prepare the chloride from the parent compound 16 as
an intermediate with the aim of following the above protocol and we chose thionylchloride/triethylamine
as the reagent for the OH/Cl replacement reaction.12 Once again, only the alkene 18 was obtained in this
case. In order to reduce the dehydration reaction, the phenyl group was replaced by the methyl group.
Thus, the carbinol 17 was prepared as a single diastereomer by treatment of the lithium derivative of 4
with acetone. In this case, the attempted conversion to the corresponding chloride gave the unsaturated
compound 19. Next, we attempted to obtain the tertiary thiol by addition of thiolacetic acid to 19. Several
13
14
experiments were carried out using TiCl4 or HClO4 as catalysts but all these methods proved to be
unsuccessful. Finally, on account of a very recent report which describes the preparation of pyridine
dithiols by base-induced addition of 2,6-lutidine to non-enolisable thioketones,3 lithiated 4 was allowed
to react with adamantanethione, but the reaction also failed in this case.
Having prepared these ligands, we studied the properties as catalysts not only of pyridine-thiols but
also of the related pyridine-alcohols. The results are reported in Table 1. As mentioned before, ligands 5
and 6 gave a very low asymmetric induction and the same sense of enantioselection. It was not possible
to give any sure explanation for this disappointing result (the corresponding alcohols 2a and 2b gave
91 and 28% ee, respectively7), however, this stereochemical outcome implies a probable unsuccessful
coordination of the zinc atom of the initial adduct Zn–S to the pyridine nitrogen. This is not the case
for ligands 12 and 15 whose zinc derivatives form a six-membered chelating ring rather than a five-
membered one as in the case of 5 and 6. Moreover, thiols 12 and 15 proved to be better catalysts than the
corresponding hydroxy derivatives 8 and 9, respectively.
It should be noted that both epimers 8 and 12, and 9 and 15 which differ only in the C8 carbon
configuration, gave the opposite configuration of 1-phenylpropanol. But, whereas the epimeric alcohols

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G. Chelucci et al. / Tetrahedron: Asymmetry 9 (1998) 1933–1940
Scheme 2.
8 and 9 gave a different level of stereodifferentiation, the epimeric thiols 12 and 15 showed the same
stereodifferentiating ability. Therefore, in this case the steric course of the reaction depends on the
stereogenic centre on the C8 carbon and it is surprisingly insensitive to the other stereocentres. The two
epimers behave as pseudoenantiomers. Tertiary alcohols 16 and 17 were more effective than the primary
alcohol 9 with the same C8 carbon configuration, but surprisingly 17 induced the opposite configuration
in the product compared to 16 and 9 indicating a very different steric course of the reaction.
In conclusion, we have reported the synthesis of new pyridine-thiols and the first use of these type of
compounds as ligands for asymmetric catalysis. Further studies aimed at the synthesis of new pyridine-
thiols are in progress.
3. Experimental
Boiling points are uncorrected. Melting points were determined on a Buchi 510 capillary apparatus and
are uncorrected. The 1H NMR (300 MHz) spectra were obtained with a Varian VXR-300 spectrometer.
Optical rotations were measured with a Perkin–Elmer 241 polarimeter in a 1 dm tube. Elemental
analyses were performed on a Perkin–Elmer 240 B analyser. A 30 m Beta Dex-120 column (Supelco)
was used to determine the enantiomeric excess of (S)- and (R)-1-phenylpropanol: 100°C (5 min),
5°C/min, 170°C (20 min). (5S,7S)-5,6,7,8-Tetrahydro-6,6-dimethyl-2-phenyl-5,7-methanoquinoline 4

G. Chelucci et al. / Tetrahedron: Asymmetry 9 (1998) 1933–1940
1937
([α]D25 +90.5 (c 1.8, CHCl3)) was prepared starting from (−)-pinocarvone (obtained by MnO2 oxidation
of commercial (−)-trans-pinocarveol [(1S,3R,5S)-6,6-dimethyl-2-methylenebicyclo[3.1.1]heptan-3-ol]
(Fluka A.G.)) according to reported procedures.6,7
3.1. (5S,7S,8S)- and (5S,7S,8R)-5,7-Methano-6,6-dimethyl-2-phenyl-8-thiol-5,6,7,8-tetrahydroquino-
line 5 and 6
A solution of 4 (0.498 g, 2 mmol) in THF (2 ml) was added dropwise under an argon atmosphere to a
solution of butyllithium (2.2 mmol, 1.37 ml of a 1.6 M solution in hexane) and tetramethylethylendiamine
(0.31 ml, 2 mmol) in THF (16 ml) at −78°C. The resulting solution was stirred for 1 h at −78°C and for
2 h at −30°C, then a solution of sulphur (0.064 g, 2 mmol) in THF (5 ml) was added dropwise at −78°C.
The solution was allowed to reach room temperature slowly, H2O was added and the organic phase
separated. The aqueous phase was extracted with ether, the combined organic phases were dried over
anhydrous Na2SO4, the solvent was evaporated and the residue was purified by flash chromatography
(petroleum ether:ethyl acetate=95:5) to give pure thiols 5 and 6. Compound 5: 0.123 g (22%); m.p.
25
79–80°C; [α]D +35.4 (c 1.4, CHCl3); 1H NMR (CDCl3) δ 8.08 (m, 2H, Ph–H2 and H6), 7.50 (d, 1H,
J=7.8 Hz, H4), 7.46–7.32 (m, 3H, Ph), 7.32 (d, 1H, J=7.8 Hz, H3), 4.83 (d, 1H, J=3.0 Hz, H8), 3.04 (m,
1H), 2.82 (t, 1H, J=5.4 Hz), 2.64 (m, 1H), 1.67 (d, 1H, J=10.5 Hz), 1.49 (s, 3H), 1.25 (s broad, 1H, SH),
0.64 (s, 3H). Anal. calcd for C18H19NS: C, 76.83; H, 6.81; N, 4.98. Found: C, 76.58; H, 6.99; N, 5.19.
Compound 6: 0.287 g (51%); m.p. 89–90°C; [α]D25 +90.2 (c 1.6, CHCl3); 1H NMR (CDCl3) δ: 8.05 (m,
2H, Ph–H2 and H6), 7.50–7.34 (m, 4H, Ph and H4), 7.26 (d, 1H, J=7.8 Hz, H3), 4.64 (s, 1H, H8), 2.77 (t,
1H, J=6.0 Hz), 2.65 (dt, 1H, J=10.5, 6.0 Hz), 2.57 (d, 1H, J=6.0 Hz), 2.42 (m, 1H), 1.66 (d, 1H, J=10.5
Hz), 1.42 (s, 3H), 0.68 (s, 3H). Anal. calcd for C18H19NS: C, 76.83; H, 6.81; N, 4.98. Found: C, 76.66;
H, 6.56; N, 5.12.
3.2. (5S,7S,8S)-
and
(5S,7S,8R)-8-(Hydroxymethyl)-5,7-methane-6,6-dimethyl-2-phenyl-5,6,7,8-
tetrahydroquinoline 8 and 9
A solution of N,N-dimethylformamide (0.35 g, 4.8 mmol) in THF (5 ml) was added dropwise at −60°C
to a solution of lithiated 4 (0.996 g, 4 mmol) under an argon atmosphere. The solution was allowed to
reach room temperature slowly, treated with water and then extracted with ether. The organic phase was
dried over anhydrous Na2SO4 and the solvent evaporated. The residue (1 g) was used in the next step
without further purification (alternatively the residue was purified by flash chromatography eluting with
petroleum ether:ethyl acetate=9:1). 1H NMR (CDCl3) δ: 7.88 (m, 2H), 7.52–7.35 (m, 5H), 6.85 (d, 1H),
2.85 (t, 1H), 2.77 (m, 1H), 2.66 (t, 1H), 1.44 (s, 3H), 1.40 (d, 1H), 0.66 (s, 3H).
Sodium borohydride (0.6 g, 16 mmol) was added portionwise to a mixture of the crude aldehyde 7
(1 g) in methanol (10 ml) and an aqueous saturated solution of Na2CO3. After 5 h stirring, H2O was
added, the methanol was evaporated in vacuo and the aqueous mixture was extracted with ether. The
organic phase was dried over anhydrous Na2SO4, the solvent was evaporated and the residue purified
by chromatography on a column (3×200 cm) of silica gel (32–63 microns) eluted with a mixture of
methylene chloride:petroleum ether:ethyl ether=7:3:0.5 to give pure alcohols 8 and 9. Compound 8: 234
mg (21%); [α]D −25.5 (c 1.3, CHCl3); 1H NMR (CDCl3) δ 7.92 (m, 2H, Ph–H2 and H6), 7.53–7.36
25
(m, 4H, Ph and H4), 7.34 (d, 1H, J=7.8 Hz, H3), 5.91 (s broad, 1H, OH), 4.24 (t, 1H, J=10.5 Hz), 3.74
(dd, 1H, J=10.2, 4.2 Hz), 3.59 (m, 1H), 2.79 (m, 2H), 2.30 (dt, 1H, J=6.3, 2.1 Hz), 1.46 (d, 1H, J=8.7
Hz), 1.42 (s, 3H), 0.66 (s, 3H). Anal. calcd for C19H21NO: C, 81.68; H, 7.58; N, 5.01. Found: C, 81.73;
H, 7.41; N, 5.12. Compound 9: 558 mg (50%); [α]D25 +150.3 (c 1.0, CHCl3); 1H NMR (CDCl3) δ: 7.92

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G. Chelucci et al. / Tetrahedron: Asymmetry 9 (1998) 1933–1940
(m, 2H, Ph–H2 and H6), 7.55–7.36 (m, 4H, Ph and H4), 7.34 (d, 1H, J=7.8 Hz, H3), 5.80 (s broad, 1H,
OH), 4.01 (t, 1H, J=10.2 Hz), 3.82 (dd, 1H, J=10.2, 4.5 Hz), 3.34 (m, 1H), 2.81 (t, 1H, J=5.7 Hz), 2.61
(dt, 1H, J=11.4, 5.7 Hz), 2.18 (dt, 1H, J=6, 2.4 Hz), 1.42 (s, 3H), 1.27 (d, 1H, J=9.0 Hz), 0.74 (s, 3H).
Anal. calcd for C19H21NO: C, 81.68; H, 7.58; N, 5.01. Found: C, 81.79; H, 7.61; N, 4.97.
3.3. (5S,7S,8S)-5,7-Methane-6,6-dimethyl-8-(mercaptomethyl)-2-phenyl-5,6,7,8-tetrahydroquinoline
12
Methanesulfonyl chloride (0.2 ml, 0.258 mmol) in anhydrous CH2Cl2 (2 ml) was added at 0°C to
a solution of 8 (0.48 g, 172 mmol), Et3N (0.65 ml), 4-(dimethylamino)pyridine (20 mg) in anhydrous
CH2Cl2 (10 ml). The resulting solution was stirred at 0°C for 10 min and then at room temperature for
2 h. The reaction mixture was poured into a 10% NaHCO3 solution, the organic phase was separated,
washed with H2O, dried over Na2SO4 and the solvent was removed under reduced pressure. The residue
(0.65 g) was used in the next step without further purification. A solution of the crude mesylate (0.65
g) and potassium thioacetate (1.08 g, 9.45 mmol) in ethanol (13 ml) was heated under reflux for 15 h.
The solvent was evaporated and the residue was purified by flash chromatography (petroleum ether:ethyl
acetate=9:1) to give the thiol acetate 11 (0.339 g, 52%) as a single spot on TLC (Rf=0.6; petroleum
ether:ethyl acetate=9:1).
A 1 M solution of LiAlH4 in Et2O (10 ml, 10 mmol) was added dropwise under an argon atmosphere
to a solution of 11 (0.339 g, 0.95 mmol) in Et2O (5 ml) at 0°C. After stirring at room temperature for
12 h, water was added and the organic phase separated. The aqueous phase was extracted with ether, the
combined organic phases dried over anhydrous Na2SO4, the solvent was evaporated and the residue was
purified by flash chromatography (petroleum ether:ethyl acetate=95:5) to give pure 12: 0.21 g (75%);
25
1
[α]D −136.8 (c 0.6, CHCl3); H NMR (CDCl3) δ: 7.96 (m, 2H, Ph–H2 and H6), 7.43–7.24 (m, 4H,
Ph and H4), 7.22 (d, 1H, J=7.8 Hz, H3), 3.73 (dd, 1H, J=14.4, 4.1 Hz), 2.25 (dd, 1H, J=10.3, 4.1 Hz),
2.75–2.72 (m, 4H), 1.38 (s, 3H), 1.30 (d, 1H, J=8.2 Hz), 1.16 (s broad, 1H, OH), 0.60 (s, 3H). Anal. calcd
for C19H21NS: C, 77.25; H, 7.17; N, 4.74. Found: C, 77.35; H, 7.19; N, 4.78.
3.4. (5S,7S,8R)-5,7-Methane-6,6-dimethyl-8-(mercaptomethyl)-2-phenyl-5,6,7,8-tetrahydroquinoline
15
The procedure reported for the preparation of compound 12 was followed. Starting from compound 9
(0.208 g, 0.748 mmol) the thiol acetate 14 (0.16 g, 63%) was obtained. From 14 (0.16 g), the thiol 15
25
1
was obtained: 0.108 g (77%); [α]D +36.8 (c 0.7, CHCl3); H NMR (CDCl3) δ: 7.94 (m, 2H, Ph–H2
and H6), 7.43–7.24 (m, 4H, Ph and H4), 7.20 (d, 1H, J=7.8 Hz, H3), 3.62 (dd, 1H, J=12.2, 4.1 Hz), 3.14
(m, 1H), 2.70 (s, 1H), 2.62–2.45 (m, 4H), 1.39 (s, 3H), 1.20 (d, 1H, J=8.2 Hz,), 0.61 (s, 3H). Anal. calcd
for C19H21NS: C, 77.25; H, 7.17; N, 4.74. Found: C, 77.41; H, 7.14; N, 4.69.
3.5. (5S,7S,8R)-5,7-Methane-6,6-dimethyl-2-phenyl-8-(diphenylhydroxymethyl)-5,6,7,8-tetrahydro-
quinoline 16
A solution of benzophenone (2 mmol) in THF (5 ml) was added dropwise at −30°C to a solution of
lithiated 4 (0.498, 2 mmol). The solution was allowed to reach room temperature slowly, H2O was added
and the organic phase separated. The aqueous phase was extracted with ether, the combined organic
phases were dried over anhydrous Na2SO4, the solvent was evaporated and the residue was purified by
flash chromatography (petroleum ether:ethyl acetate=9:1) to give pure alcohol 16: 0.733 g (85%); m.p.

G. Chelucci et al. / Tetrahedron: Asymmetry 9 (1998) 1933–1940
1939
25
1
105–6°C; [α]D −464.3 (c 1.2, CHCl3); H NMR (CDCl3) δ: 10.25 (s broad, 1H, OH), 8.05 (m, 2H,
2-phenyl H2 and H6), 7.52 (d, 1H, J=7.8, H4), 7.50–7.26 (m, 8H, arom), 7.24 (d, 1H, J=7.8, H3), 7.09 (s
broad, 5H, arom), 4.44 (s, 1H), 2.64 (t, 1H, J=6.0 Hz), 2.55 (t, 1H, J=6.0 Hz), 2.06 (dt, 1H, J=10.5, 6.0
Hz), 1.38 (s, 3H), 0.88 (s, 3H), −0.18 (d, 1H, J=10.5 Hz). Anal. calcd for C31H29NO: C, 86.27; H, 6.77;
N, 3.25. Found: C, 86.44; H, 6.61; N, 3.27.
3.6. (5S,7S,8R)-5,7-Methane-6,6-dimethyl-8-(dimethylhydroxymethyl)-2-phenyl-5,6,7,8-tetrahydro-
quinoline 17
Compound 17 was obtained following the above procedure and using acetone (2 mmol) instead of
25
benzophenone: 0.468 g (77%); m.p. 147–8°C; [α]D −33.0 (c 1.9, CHCl3); 1H NMR (CDCl3) δ: 8.16
(s broad, 1H, OH), 7.91 (m, 2H, Ph–H2 and H6), 7.55 (d, 1H, J=7.8, H4), 7.48–7.34 (m, 3H, Ph), 7.34
(d, 1H, J=7.8, H3), 3.26 (s, 1H, H8), 2.78 (t, 1H, J=6.0 Hz), 2.69 (dt, 1H, J=10.5, 6.0 Hz), 2.36 (dt, 1H,
J=6.0, 1.5 Hz), 1.44 (s, 3H), 1.42 (d, 1H, J=10.5 Hz), 1.34 (s, 3H), 1.13 (s, 3H), 0.72 (s, 3H). Anal. calcd
for C21H25NO: C, 82.03; H, 8.20; N, 4.56. Found: C, 82.24; H, 8.31; N, 4.27.
3.7. Addition of diethylzinc to benzaldehyde: general procedure
A solution of ligand (0.37 mmol) in toluene (5 ml) was cooled at 0°C. A 1 M solution of diethylzinc
in hexane (12.4 ml, 12.4 mmol) was added over a period of 5 min. The mixture was stirred at room
temperature for 20 min, cooled at 0°C, benzaldehyde (0.6 ml, 0.647 g, 6.1 mmol) was added and then
the mixture was stirred at room temperature for the appropriate time (see Table 1). The reaction mixture
was quenched with 10% H2SO4 (10 ml) and extracted with ether. The organic layer was washed with
10% H2SO4, saturated NaHCO3, and dried over anhydrous Na2SO4. The solvent was evaporated and
the residue was purified by flash chromatography to afford pure 1-phenylpropanol. The enantiomeric
excess was determined by capillary GC with a chiral column: (R)-1-phenylpropanol 15.6 min, (S)-1-
phenylpropanol 15.7 min.
Acknowledgements
This work was financially supported by M.U.R.S.T.
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