New 1-amino-1,2-diphenylethanols as ligands for the enantioselective addition of alkyllithiums to benzaldehyde

Article abstract of DOI:10.1016/S0957-4166(98)00473-X

In the presence of equimolar amounts of lithium alkoxides derived from N-substituted 2-amino-1,2-diphenylethanols, alkyllithium reagents add to benzaldehyde to furnish optically active secondary alcohols with enantiomeric excesses of up to 86%. The best results were obtained using the N-isopropyl- N-methyl substituted amino-alcohol.

Full text of DOI:10.1016/S0957-4166(98)00473-X

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 10 (1999) 169–176
New 1-amino-1,2-diphenylethanols as ligands for the
enantioselective addition of alkyllithiums to benzaldehyde
Michael Schön and Reto Naef ∗
Novartis Pharma AG, CH-4002, Basel, Switzerland
Received 9 November 1998; accepted 23 November 1998
Abstract
In the presence of equimolar amounts of lithium alkoxides derived from N-substituted 2-amino-1,2-diphenyl-
ethanols, alkyllithium reagents add to benzaldehyde to furnish optically active secondary alcohols with enantio-
meric excesses of up to 86%. The best results were obtained using the N-isopropyl-N-methyl substituted amino-
alcohol. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
Without any doubt, the enantioselective addition of organometallic reagents to aldehydes and ketones
are among the most powerful reactions in organic synthesis and consequently have been extensively
studied in the past 20 years.1–3 Despite spectacular successes, the methods developed often display
some major disadvantages. In almost every case substantial excesses of the ligand and temperatures
below −100°C1,2 are required in order to obtain good selectivities, or the reagent has to be prepared
independently.3 Two notable exceptions are the TADDOL-mediated addition of Grignard reagents
developed by Seebach4 and the addition of diorganozincs to aldehydes5 and ketones6 under amino-
alcohol catalysis; although the scope of the former reaction is strictly limited to the availability of the
organozinc reagents.
We therefore set ourselves the goal of developing new ligands which could be easily used by adding
an equimolar amount of the ligand to the solution of the organolithium reagent followed by addition of
the appropriate aldehyde or ketone at −78°C. We report herein our preliminary studies concerning the
asymmetric addition of butyllithium to benzaldehyde in THF† at −78°C as a standard reaction.
∗
†
Corresponding author. E-mail: reto.naef@pharma.novartis.com
In accordance with Jackman2e and coworkers THF turned out to be the solvent of choice.
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00473-X

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M. Schön, R. Naef / Tetrahedron: Asymmetry 10 (1999) 169–176
2. Results and discussion
We have surveyed various readily available amino-alcohols in order to investigate their struc-
ture–selectivity relationships. 2-Dimethylamino-1,2-diphenylethanol 17 (entry 1 in Table 1) provided
encouraging results and represents (to the best of our knowledge) the best literature ligand under the
reaction conditions applied {71% ee favouring the (S)-enantiomer, see entry 1 in Table 1}.2e It is
noteworthy that other known ligands for the enantioselective addition of organolithium reagents to
aldehydes, e.g. Mukaiyama’s prolinol,2a gave disappointing results using our standard reaction conditions
{17% ee favouring the (S)-enantiomer}.
Table 1
Reaction of benzaldehyde with butyllithium in the presence of ligands 1–97–9
Since the work of Mukaiyama,2a Soai10 and Thompson11 clearly demonstrated that changing the steric
and electronic nature of the amino group can have a significant influence on the enantioselectivity of the
addition reaction, we chose ligand 1 as the lead structure. The derivatives, prepared according to standard
alkylation and reductive amination procedures,‡ are summarised in Table 1.
We considered that the introduction of two different substituents on the nitrogen could improve the
enantioselectivity of the addition reaction by the formation of an additional stereocentre during the
chelation process. Although it is at present not clear whether the coordination is stereoselective, the
effect of the additional stereocentre is apparent (Table 1, entries 4–9 compared with 1–3). On the one
hand, branched alkyl substituents were shown to improve the (S)-selectivity (entries 1 and 5); whilst on
‡
For further details refer to the experimental section.

M. Schön, R. Naef / Tetrahedron: Asymmetry 10 (1999) 169–176
171
the other hand, the introduction of an additional chelating group (entry 9) reduces the (S)-selectivity.§
N-Isopropylamine 5 turned out to be the most effective ligand and was chosen for further investigations.
It should be noted that in the case of the N-isopropyl substituted ligand 5, a change in the molar
ratio of ligand alkoxide Li-5:BuLi:PhCHO from 1:1:1 to 4:1:1 has no significant influence on the
enantioselectivity (Table 2, entries 3–5). These results are in strong contrast to those reported for the
addition of methyllithium to benzaldehyde in the presence of ligand 1.2e Further investigations are in
progress, but it should be stressed that these ratios do not necessarily define the stoichiometry of the
reactant since alkyllithium, lithium alkoxide and several mixed aggregates may co-exist in equilibrium.
The enantioselectivities obtained using a catalytic amount of the ligand alkoxide Li-5 indicate that there
is no significant enhancement in the reactivity.
Table 2
Effect of the concentration of lithium alkoxide Li-5 on the enantioselectivity of butyllithium addition
to benzaldehyde in THF at −78°C
To elucidate the potential of the 2-amino-1,2-diphenylethanol mediated enantioselective addition to
benzaldehyde we also surveyed a series of organolithium reagents. The results observed are summarised
in Table 3.
The results presented in Table 3 clearly demonstrate the potential of the known ligand 12e in the
t
enantioselective addition of a variety of different organolithium reagents (except BuLi¶) to aldehydes
using convenient reaction conditions. However, the facial selectivity can be improved significantly by
using the corresponding N-isopropylamine 5. Although small in terms of energy — a change from 76:24
to 93:=7 (Table 3, entries 1 and 2) corresponds to a difference of only 0.5 kcal/mol in free activation energy
∆∆G195 — this effect might be of great practical significance.
The stereochemical outcome of the addition reaction may be rationalised by the transition state model
shown in Fig. 1. Lithium alkoxide Li-5 and the organolithium reagent form a rigid, mixed dimer|| whose
conformation is fixed by the additional stereocentre at the chelating nitrogen atom.†† Benzaldehyde
is coordinated via an LiX-bridge and its phenyl moiety is placed in a pseudo-equatorial position
§
In the case of α,α-diphenylprolinol-derived ligands, the 2-morpholinoethyl substituent was shown to significantly improve
the enantioselectivity of the addition of butyllithium to benzaldehyde, compared with the results obtained for DPMPM12 {24%
(R) obtained for the N-methyl compound vs 55% ee (R) for the diamine}. These results will be described elsewhere.
¶
As a result of steric crowding, the formation of a chiral mixed dimer (see Fig. 1) seems to be impossible. It is also known
from the literature that tBuLi forms no dimers or higher aggregates in ethereal solvents.13
||
Garrity et al. were able to show by rapid-injection NMR studies that dimers are more reactive than the corresponding
tetramers.14
†† For a review on chelation of β-amino-lithoxides see Nichols et al.15

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M. Schön, R. Naef / Tetrahedron: Asymmetry 10 (1999) 169–176
Table 3
Reaction of benzaldehyde with organolithium reagents in the presence of ligands 1 and 5
Figure 1.
avoiding steric interaction with the coordinated tetrahydrofuran molecule. The nucleophilic addition to
benzaldehyde occurs from the Si-face.
In conclusion, we have demonstrated that 2-N,N-dialkylamino-1,2-diphenylethanols are effective
ligands for the enantioselective addition of organolithium reagents to aldehydes. In addition, we have
developed an operationally simple procedure for the synthesis of optically active secondary alcohols
using commercially available organolithium reagents. Considering the high reactivity14 of the reaction
partners involved in this reaction, the observed enantioselectivity using these simple ligands is unexpected
and quite remarkable. Our current efforts focus on further improvement of the ligands and elucidation of
the scope and the limitation of this method, especially with regard to other electrophiles.

M. Schön, R. Naef / Tetrahedron: Asymmetry 10 (1999) 169–176
173
3. Experimental
3.1. General
Solvents and reagents were purified and dried according to standard procedures. TLC analyses were
done on Si60 F254-coated plates (E. Merck); detection was by UV at 254 nm or phosphomolybdic acid
(10% in EtOH). Silica 0.040–0.063 mm (E. Merck) was used for flash chromatography. Melting points
were determined on a Büchi 512 apparatus and are uncorrected. IR spectra were recorded on a Bruker IFS
66 spectrometer. NMR spectra were obtained from a Bruker AMX 400 spectrometer. Specific rotation
was measured on a Perkin–Elmer polarimeter 241 MC.
3.2. Preparation of ligands 4–9
3.2.1. (1R,2S)-N-Butyl-N-methyl-2-amino-1,2-diphenylethanol 4
A suspension of commercially available (1R,2S)-2-amino-1,2-diphenylethanol (1.77 g, 8.30 mmol)
and K2CO3 (1.15 g, 8.30 mmol) in butyl bromide (2.27 g, 16.6 mmol) and EtOH (16 ml) was heated
under reflux for 1 day. The mixture was filtered and concentrated in vacuo to leave a white solid (2.1
g). Column chromatography (hexane:EtOAc, 1:2) furnished the pure N-butylamine (1.32 g, 59%) and
N,N-dibutylamine (0.64 g, 27%) with spectroscopic data identical to those reported,9 both as colourless
crystals: mp 135–136°C (N-butylamine), 141–142°C (N,N-dibutylamine).
To a solution of the N-butylamine in HCOOH/H2O (1.4 ml+0.33 ml) was added formaldehyde at
100°C (37% aqueous solution, 1.05 ml) and stirring was continued for an additional 4 h. The mixture
was cooled and made alkaline with 2 N NaOH followed by extraction with EtOAc (3×10 ml). The
combined organic layers were dried over Na2SO4 and evaporated. Purification of the crude product by
column chromatography (hexane:EtOAc, 1:1) afforded ligand 4 (1.35 g, 57%) as a colourless oil. [α]20
D
−58.0 (c=0.39, CHCl3). HRMS (EI) calcd for C19H25NO 284.2016 (MH+), found 284.2009 (MH+). IR
(neat) 3200–3600, 3062, 3028, 2957, 2931, 2861, 2796, 1495, 1453, 1198, 1090, 1059, 1029, 775, 750,
1
702 cm−1. H NMR (CDCl3) δ 0.88 (t, J=7 Hz), 1.15–1.35 (m, 2H), 1.4–1.6 (m, 2H), 2.36 (s, 3H),
2.4–2.6 (m, 2H), 3.53 (d, J=6 Hz, 1H), 5.34 (d, J=6 Hz, 1H), 7.0–7.3 (m, 10H). 13C NMR (CDCl3) δ
14.4, 20.7, 29.1, 40.0, 55.1, 72.9, 75.8, 126.6, 127.2, 127.6, 127.9, 130.0, 137.0, 141.8.
3.2.2. (1R,2S)-N-Isopropyl-N-methyl-2-amino-1,2-diphenylethanol 5
Typical procedure: According to the procedure of Shioiri,16 acetone (2.96 g, 50.9 mmol) and sodium
cyanoborohydride (998 mg, 12.7 mmol) were added to a solution of commercially available (1R,2S)-2-
amino-1,2-diphenylethanol (1.81 g, 8.49 mmol) in MeOH (34 ml). The pH of the solution was adjusted to
ca. 4 by adding acetic acid and the resulting mixture was stirred for 5 h. The mixture was made alkaline
by adding 2 N NaOH and extracted with EtOAc (3×50 ml). The combined organic layers were dried
over Na2SO4 and evaporated to leave 2.08 g (96%) of the crude N-isopropylamine which was used for
the next step without further purification.
To a solution of the crude product in HCOOH/H2O (2.4 ml+0.6 ml) was added formaldehyde at
100°C (37% aqueous solution, 1.8 ml) and stirring was continued for an additional 4 h. The mixture
was cooled and made alkaline with 2 N NaOH followed by extraction with EtOAc (3×10 ml). The
combined organic layers were dried over Na2SO4 and evaporated. Purification of the crude product by
column chromatography (hexane:EtOAc, 2:1) afforded ligand 5 (1.99 g, 87%) as colourless crystals: mp
67–68°C. [α]20 −105 (c=0.52, CHCl3). Anal. calcd for C18H23NO: C, 80.26; H, 8.61; N, 5.20. Found:
D
C, 80.16; H, 8.65; N, 5.26. IR (KBr) 3200–3700, 2971, 2882, 2783, 1453, 1388, 1367, 1200, 1184, 1060,

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M. Schön, R. Naef / Tetrahedron: Asymmetry 10 (1999) 169–176
1047, 791, 764, 706, 696, 589 cm−1. 1H NMR (CDCl3) δ 0.89, 1.00 (2d, J=6 Hz, 6H), 2.36 (s, 3H), 3.10
(h, J=6 Hz, 1H), 3.57 (d, J=4 Hz, 1H), 5.24 (d, J=4 Hz, 1H), 6.9–7.2 (m, 10H). 13C NMR (CDCl3) δ
15.5, 20.1, 32.3, 49.5, 72.6, 74.4, 126.5, 127.0, 17.4, 127.8, 129.9, 137.5, 141.6.
3.2.3. (1R,2S)-N-(3-Pentyl)-N-methyl-2-amino-1,2-diphenylethanol 6
According to the typical procedure given for 5; 1.18 g (5.53 mmol) (1R,2S)-2-amino-1,2-diphenyl-
ethanol; 651 mg (8.30 mmol) sodium cyanoborohydride; 476 mg (5.53 mmol) 3-pentanone; 22 ml
MeOH; reaction time 3 days at r.t.; column chromatography (hexane:EtOAc, 1:1) of the crude product
afforded the pure secondary amine (1.14 g, 73%) as a colourless oil; 0.75 ml HCOOH, 0.19 ml H2O, 0.56
ml 37% aq. H2CO; reaction time 6 h at 100°C; column chromatography (hexane:EtOAc, 9:1) of crude
product afforded ligand 6 (1.02 g, 85%) as a colourless oil. [α]20 −83.0 (c=1.71, CHCl3). Anal. calcd for
D
C20H27NO: C, 80.76; H, 9.15; N, 4.71. Found: C, 80.64; H, 9.28; N, 4.73. IR (neat) 3200–3700, 2962,
1
2932, 2873, 1495, 1453, 1200, 1089, 1054, 1030, 999, 922, 777, 754, 701 cm−1. H NMR (CDCl3) δ
0.69, 0.99 (2t, J=7 Hz, 6H), 1.2–1.6 (m, 4H), 2.36 (s, 3H), 2.45–2.55 (m, 1H), 3.74 (d, J=4 Hz, 1H), 5.33
(d, J=4 Hz, 1H), 6.9–7.2 (m, 10H). 13C NMR (CDCl3) δ 11.9, 12.0, 21.8, 23.6, 32.5, 61.8, 72.7, 74.3,
126.4, 126.9, 127.4, 127.8, 130.2, 132.3, 137.3, 141.5.
3.2.4. (1R,2S)-N-Cyclohexyl-N-methyl-2-amino-1,2-diphenylethanol 7
According to the typical procedure given for 5; 1.18 g (5.53 mmol) (1R,2S)-2-amino-1,2-diphenyl-
ethanol; 651 mg (8.30 mmol) sodium cyanoborohydride; 815 mg (8.30 mmol) cyclohexanone; 22 ml
MeOH; reaction time 3 days at r.t.; column chromatography (hexane:EtOAc, 1:1) of the crude product
afforded the pure secondary amine (860 mg, 52%) as a colourless oil; 1.4 ml HCOOH, 0.35 ml H2O, 1.0
ml 37% aq. H2CO; reaction time 6 h at 100°C; column chromatography (hexane:EtOAc, 2:1) of crude
product afforded ligand 7 (770 mg, 45%) as colourless crystals: mp 87–88°C. [α]20 −74.3 (c=0.44,
D
CHCl3). Anal. calcd for C21H27NO: C, 81.51; H, 8.79; N, 4.53. Found: C, 81.43; H, 8.80; N, 4.68. IR
(KBr) 3200–3700, 2937, 2857, 2783, 1451, 1339, 1198, 1088, 1061, 1052, 1030, 1020, 771, 751, 705
1
cm−1. H NMR (CDCl3) δ 0.9–1.8 (m, 10H), 2.40 (s, 3H), 2,65 (tt, J=12 Hz, 3 Hz, 1H), 2.74 (d, J=5
Hz, 1H), 5.29 (d, J=5 Hz, 1H), 6.9–7.2 (m, 10H). 13C NMR (CDCl3) δ 26.3, 26.6, 26.7, 27.4, 34.2, 59.0,
72.6, 73.7, 126.5, 126.9, 127.4, 127.8, 129.9, 137.6, 141.8.
3.2.5. (1R,2S)-N-Cyclopentyl-N-methyl-2-amino-1,2-diphenylethanol 8
According to the typical procedure given for 5; 1.18 g (5.53 mmol) (1R,2S)-2-amino-1,2-diphenyl-
ethanol; 651 mg (8.30 mmol) sodium cyanoborohydride; 698 mg (8.30 mmol) cyclopentanone; 22 ml
MeOH; reaction time 3 days at r.t.; column chromatography (hexane:EtOAc, 1:1) of the crude product
afforded the pure secondary amine (1.02 g, 62%) as a colourless oil; 0.8 ml HCOOH, 0.2 ml H2O, 0.6
ml 37% aq. H2CO; reaction time 6 h at 100°C; column chromatography (hexane:EtOAc, 2:1) of crude
product afforded ligand 8 (652 mg, 40%) as colourless crystals: mp 84–85°C. [α]20 −36.1 (c=1.46,
D
CHCl3). Anal. calcd for C20H25NO: C, 81.31; H, 8.53; N, 4.74. Found: C, 81.02; H, 8.84; N, 4.85. IR
(KBr) 3000–3600, 3029, 3088, 2964, 2866, 2804, 1492, 1453, 1088, 1060, 1019, 781, 753, 710, 699, 608
1
cm−1. H NMR (CDCl3) δ 1.3–1.8 (m, 8H), 2.28 (s, 3H), 3.1–3.2 (m, 1H), 3.69 (d, J=6 Hz, 1H), 5.33
(d, J=6 Hz, 1H), 7.0–7.2 (m, 10H). 13C NMR (CDCl3) δ 24.6, 24.7, 26.7, 34.4, 62.6, 73.1, 74.2, 126.8,
127.2, 127.6, 127.9, 128.0, 128.2, 130.0, 137.0, 142.3.
3.2.6. (1R,2S)-N-(2-Morpholinoethyl)-N-methyl-2-amino-1,2-diphenylethanol 9
A suspension of commercially available (1R,2S)-2-amino-1,2-diphenylethanol (799 mg, 3.75 mmol),
N-(2-chloroethyl)-morpholine hydrochloride (837 mg, 4.50 mmol) and K2CO3 (1.04 g, 7.50 mmol) in

M. Schön, R. Naef / Tetrahedron: Asymmetry 10 (1999) 169–176
175
EtOH (10 ml) was heated under reflux for 18 h. The mixture was filtered and concentrated in vacuo to
leave 2.33 g of a colourless oil which was purified by column chromatography (EtOAc+5% NEt3). The
pure secondary amine (1.01 g, 83%) was dissolved in 98% HCOOH (8.0 ml) and H2O (1.8 ml). The
reaction mixture was heated under reflux, 37% aq. H2CO was added (6.0 ml) and stirring was continued
for 18 h. The mixture was cooled and made alkaline with 2 N NaOH followed by extraction with EtOAc
(3×10 ml). The combined organic layers were dried over Na2SO4 and evaporated. Purification of the
crude product by column chromatography (EtOAc+5% NEt3) afforded ligand 9 (676 mg, 54%) as a
colourless oil. [α]20 −20.6 (c=0.33, CHCl3). Anal. calcd for C21H28N2O2: C, 74.08; H, 8.29; N, 8.23.
D
Found: C, 73.80; H, 8.12; N, 8.21. IR (KBr) 3200–3400, 2967, 2948, 2852, 2812, 1448, 1385, 1118,
1032, 1010, 867, 766, 749, 707, 700, 652 cm−1. 1H NMR (CDCl3) δ 2.34 (s, 3H), 2.4–2.9 (m, 8H), 3.56
(d, J=5 Hz, 1H), 3.65–3.75 (m, 4H), 5.36 (d, J=5 Hz, 1H), 7.0–7.2 (m, 10H). 13C NMR (CDCl3) δ 35.6,
53.6, 54.1, 58.1 67.5, 73.3, 74.5, 126.9, 127.5, 127.6, 127.9, 128.1, 128.3, 130.4, 136.8, 144.0.
3.3. General procedure for the enantioselective addition of organolithium reagents to aldehydes
A 25 ml flask, purged with Ar, was charged with a solution of the ligand (0.5 mmol) in dry THF (10
ml). At 0°C a solution of the organolithium reagent (1.0 mmol) was added and the resulting solution
was cooled down to −78°C. Afterwards, the aldehyde (0.5 mmol) was added via syringe and stirring
was continued for 1 h. The reaction mixture was quenched by addition of 1 M HCl (5 ml) and EtOAc
(10 ml). Afterwards the organic layer was separated and the aqueous layer was extracted with EtOAc
(3×10 ml). The extracts were dried over Na2SO4 and concentrated under reduced pressure. Preparative
TLC (hexane:EtOAc, 9:1) of the residue afforded the sec-alcohol as chromatographically homogenous
material. The enantiomeric composition was determined on a chiral HPLC column (Chiracel OD).8
Acknowledgements
We are grateful to Dr. N. Djordjevic and Mr. F. Houdière for carrying out the HPLC analysis.
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