Facile synthesis of enantiomerically pure tert- butyl(methyl)phenylsilanes

Article abstract of DOI:10.1016/S0957-4166(99)00022-1

A method for the preparation of both enantiomers of tert- butyl(methyl)phenylsilane 2 is presented. Racemic tert- butyl(methyl)phenylsilyl chloride 3 was allowed to react with (R)-(-)-2- amino-1-butanol 4 to give hydrochloride 5. Diastereomer separation v

Full text of DOI:10.1016/S0957-4166(99)00022-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 10 (1999) 519–526
Facile synthesis of enantiomerically pure
tert-butyl(methyl)phenylsilanes
Paweł Jankowski,a Ernst Schaumann,b,∗ Jerzy Wicha,a,∗ Andrzej Zarecki a,∗ and
Gunadi Adiwidjajac
aInstitute of Organic Chemistry, Polish Academy of Sciences, PO Box 58, 01-224 Warsaw, Poland
bInstitute of Organic Chemistry, Technical University of Clausthal, D-38678 Clausthal-Zellerfeld, Germany
cInstitute of Mineralogy and Petrography, University of Hamburg, D-20146 Hamburg, Germany
Received 10 December 1998; accepted 19 January 1999
Abstract
A method for the preparation of both enantiomers of tert-butyl(methyl)phenylsilane2 is presented. Racemic tert-
butyl(methyl)phenylsilylchloride 3 was allowed to react with (R)-(−)-2-amino-1-butanol4 to give hydrochloride 5.
Diastereomer separation via treatment of the respective free amine 6 with 0.5 mol equivalent of HCl in hexane-2-
propanol yielded crystalline diastereomerically pure hydrochloride (R)Si-5. The corresponding free amine (R)Si-6
was reduced with LiAlH4 to give (S)-2. The mother liquors obtained after separation of (R)Si-5 on treatment with
oxalic acid provided a crystalline salt that eventually afforded (R)-2. The optical purity of (S)-2 (98% ee) was
documented by its reaction (hydrosilylation) with propargylic alcohol derivative 10 and HPLC analysis of product
11 using a chiral column. © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
Nonracemic organosilicon compounds with an asymmetric silicon atom may potentially serve as
auxiliaries or reagents for enantioselective synthesis. However, chiral silicon derivatives have been
little exploited as compared to organosulfur or organophosphorus compounds in which the heteroatom
provides the stereogenic centre. To date, one of the main reasons for the limited interest in this area of
organosilicon chemistry lies, apparently, in the inaccessibility of appropriate optically active compounds.
In spite of recent efforts towards a practical synthesis of chiral organosilicon compounds,1–3 only
methyl(1-naphthyl)phenylsilane 1 (Fig. 1) and congeners are readily available in enantiomeric form by
the classical Sommer procedure involving separation of diastereomeric methyl(1-naphthyl)phenyl[(−)-
menthoxy]silanes4,5 or by a catalytic method developed by Corriu and coworkers.6 For both methods
∗
Corresponding authors. E-mail: ernst.schaumann@tu-clausthal.de, jwicha@ichf.edu.pl and azar@ichf.edu.pl
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00022-1

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P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
Figure 1.
it is crucial that methyl(1-naphthyl)phenylsilane and many of its derivatives retaining the 1-naphthyl
substituent at the silicon atom occur as crystalline solids. Some reactions of nonracemic 1 and its
derivatives that have been examined with respect to asymmetric induction provided only moderate en-
couragement for further investigation7–12 [an exception is the methylation of optically active tert-butyl 2-
(methylnaphthylphenylsilyl)acetate which occurs with 9:1 diastereoselectivity13 ]. However, the relatively
low level of asymmetric induction in reactions involving 1 may by attributed to the lack of differentiation
of the two bulky aromatic substituents, the 1-naphthyl and phenyl groups. In this connection, it was
considered to be of interest to develop a practical synthesis of a silane with more pronounced alteration
in the steric bulk of the substituents at the silicon atom and to scrutinise its application as a tool in
asymmetric synthesis. Transformations of silanes to other silyl derivatives, such as silyl halides and
silyl alkoxides are well known. Enantiomeric tert-butyl(methyl)phenylsilanes 2 were chosen as targets
in the initial studies. Racemic tert-butyl(methyl)phenylsilane may be easily prepared14,15 from commer-
cially available dichloro(methyl)phenylsilane. Nonracemic tert-butyl(methyl)phenylsilane ([α]20 −0.65
D
or +0.65) has been prepared with the use of optically active borneol as a chiral agent ([α]20 +1.20) or by
a method involving hydrosilylation of benzophenone in the presence of a chiral catalyst.1D6 However, the
specific rotation of the described material was low, the enantiomeric excess was not determined and the
assignment of the absolute configuration was speculative.
2. Results and discussion
Our preliminary attempts to separate tert-butyl(methyl)phenylmenthoxysilanes by crystallisation or
chromatography failed. We then turned our attention to amino alcohols as resolving agents, reasoning that
even if their O-silyl derivatives will not crystallise instantly, the presence of the amine function will pro-
vide a handle for chemical modifications and consequently allow the generation of crystalline derivatives
[(+)-ephedrine had been utilised6 in the synthesis of the optically active methyl(1-naphthyl)phenylsilane
but in a quite different way]. 2-Amino-1-butanol, which is commercially available in both enantiomeric
forms, was chosen as potentially one of the most advantageous resolving agents. Reaction of (R)-(−)-
2-amino-1-butanol 4 (97% ee) (Scheme 1) with racemic tert-butyl(methyl)phenylsilyl chloride 3 in dry
toluene yielded a mixture of diastereomeric amino silyloxy derivatives 5 as hydrochlorides.
These hydrochlorides were partly crystalline, however, all attempts to isolate pure crystalline material
proved futile. It appeared likely that only one of the diastereomeric hydrochlorides had crystallised. The
mixture of hydrochlorides was thus transformed into the respective free amine 6 which was purified by
distillation (89% yield). Treatment of 6 in hexane–2-propanol with 0.5 mol equivalents of HCl (generated
in situ from Me3SiCl) indeed gave crystalline hydrochloride (R)Si-5 that could be easily separated by
filtration and purified further. The hydrochloride thus obtained (25% yield) was transformed into the free
base (R)Si-6 (an oil) and then into the N-acetyl derivative 8 (Fig. 2).
1H NMR analysis of the latter showed that it is diastereomerically pure (see experimental section).
Some other crystalline amides were also prepared, of which p-nitrobenzyl derivative (R)Si-9 provided
crystals suitable for X-ray analysis (long needles from 1-chlorobutane). The absolute configuration of
the silicon atom in the derivative (R)Si-9 was revealed by a single crystal X-ray analysis. The structure of

P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
521
Scheme 1.
Figure 2.
(R)Si-9 is represented in Fig. 3. It shows that with use of (R)-(−)-2-amino-1-butanol and crystallisation
of the respective hydrochloride, the diastereomer 5 with (R)-configuration of the Si atom is obtained.
Diastereomerically pure aminosilane (R)Si-6 was treated with lithium aluminium hydride in diethyl
ether. The apolar product was collected and distilled to give silane 2 (75% yield) as a colourless oil ([α]20
D
+4.24 (hexane)). As it has been demonstrated that lithium aluminium hydride reduction of alkoxysilanes
occurs with retention of configuration at the silicon atom,4,17 the (S) configuration at the silicon atom has
been attributed to the product, i.e. 2(S). As regards the relationship between the absolute configuration
of tert-butyl(methyl)phenylsilane and the direction of its specific rotation, the suggestion of the previous
workers16 has been confirmed.
At this stage, a relatively simple approach to optically active silane (S)-2 was achieved. The overall
efficiency of the method suffered, however, from processing of only one enantiomer of the starting
racemic tert-butyl(methyl)phenylsilyl chloride 3. A further improvement would be a method that would
allow purification of the major diastereomer of aminosilane 6 remaining after separation of crystalline
diastereomeric hydrochloride (R)Si-5. It was eventually found that the mother liquors from precipitation
of the (R)Si-5, after drying and treatment with an excess of anhydrous oxalic acid in 2-propanol, afford
crystalline oxalate (S)Si-7 that can be isolated by filtration (33% yield from 6). Recrystallisation from
chloroform and decomposition of the salt with ammonium hydroxide gives the free base (S)Si-6 ([α]20
D
−53.44).
For larger scale preparations, a restriction of the developed procedure may come from the use of 2-

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P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
Figure 3.
amino-1-butanols of 97% ee, which, although available in both enantiomeric forms (Aldrich), is relatively
expensive. (R)-(−)-2-Amino-1-butanol of only 80% ee (Fluka) is approximately 100 times cheaper.
Fortunately, application of the latter reagent to the above described procedure gave the respective silanes
with no decrease in the enantiomeric excess, but with a somewhat lower overall yield.
With enantiomeric silanes 2 showing a high specific rotation in hand, we faced the problem of
determining the enantiomeric excess. Some separation of enantiomers was observed on a Chiralcel®
OJ column (Daicel Chemical Industries, Tokyo, 25×0.46 cm, hexane, tR ca. 12 min, 0.5 ml/min; no
separation occurred on a Chiralcel OD-H column). However, the separation was not sufficient for
any quantitative conclusions. Screening of NMR spectra of 2 with chiral shift reagents was equally
unfruitful. After this failure of direct methods, we turned our attention to chemical reactions. Our previous
experience18 has shown that enantiomeric vinylsilanes 11 are very well suited to HPLC analysis on a
chiral column. Hydrosilylation of protected propargyl alcohol 10 with (S)-2 ([α]20 +4.24) in the presence
D
of H2PtCl6 followed by hydrolysis of the protecting group in the reaction product gave a mixture of
regioisomeric vinylsilanes 11 and 12 in a ratio of 2:3, which were separated on a silica gel column
(Scheme 2). The minor isomer was identical to 11 prepared via another route.18 Its HPLC analysis
using a Chiralcel OD-H column indicated the presence of only one enantiomer. We conclude that the
starting material consisted of one enantiomer within the limit of error of the HPLC analysis which
we estimate at 2%. It is noteworthy that hydrosilylation of ethylene derivatives has been shown19 to
occur with retention of configuration at the silicon atom. In conclusion, the synthesis of epimeric (R)-
and (S)-tert-butyl(methyl)phenylsilanes 2 starting from racemic tert-butyl(methyl)phenylsilyl chloride
and involving separation of the corresponding diastereomeric 2-amino-butan-1-ol derivatives, has been
achieved. It has been shown that hydrosilylation of a propargyl alcohol derivative 10 with enantiomeric
tert-butyl(methyl)phenylsilane affords product 11 of 98% ee.

P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
523
Scheme 2.
3. Experimental
3.1. General
1
Melting points were determined on a Kofler hot-stage melting point apparatus. H and 13C NMR
spectra were performed in CDCl3 on a Varian Gemini 200 MHz spectrometer and multiplicities were
assigned using DEPT techniques. All chemical shifts are quoted on the δ scale relative to internal
tetramethylsilane. Optical rotations were measured on a Perkin–Elmer model 141 polarimeter at the
concentration (g/100 ml) indicated. Chiral HPLC analyses were performed using a Shimadzu LC-8A
chromatograph provided with an SPD-6A variable λ detector and fitted with a Chiralcel OD-H column
(25×0.46 cm). Conditions A: hexane as eluent, flow 0.5 ml/min, λ=254 nm. Conditions B: hexane:2-
propanol (9:1) as eluent, flow 0.5 ml/min, λ=254 nm. Elemental analyses were carried out by the
Microanalysis Service of the Institute of Organic Chemistry, Warsaw.
3.2. tert-Butylchloro(methyl)phenylsilane 3
The procedure of Tacke20 was simplified: in a two-necked flask provided with a reflux condenser was
placed dichloro(methyl)phenylsilane (2 ml, 2.319 g, 12.1 mmol), and a 1.7 M solution of tert-butyllithium
in pentane (Aldrich, 8 ml, 13.6 mmol) was added through a septum, all at once. The flask was immersed in
a cold water bath and the reaction mixture was stirred for 24 h. The resulting slurry was concentrated on a
rotary evaporator and then distilled in a Kugelrohr apparatus at 8 torr. A fraction collected at 120–150°C
consisted of chloride 3 (2.291 g, 89% yield from MePhSiCl2); δH 0.66 (3H, s, CH3Si), 0.98 (9H, s,
(CH3)3C), 7.41 (3H, m), 7.61 (2H, m).
3.3. 2-(R)-Amino-1-(tert-butylmethylphenylsilyloxy)butane 6
A mixture of silyl chloride 3 (3.459 g, 16.7 mmol), (R)-(−)-2-amino-1-butanol 4 (Aldrich, 97% ee,
1.570 g, 17.6 mmol), and dry toluene (5 ml) was stirred at 95–100°C (oil bath) for 24 h. The slurry was
diluted with toluene (5 ml), cooled to rt and treated with 3 ml of 25% aqueous ammonia and 3 ml of
water. After stirring for 2 h, the organic phase was separated, dried with anhydrous sodium sulfate and
concentrated in vacuo. Kugelrohr distillation (175°C at 22 torr) afforded 6 as a colourless liquid (3.511
g, 82% based on silyl chloride 3); δH 0.37 (3H, s, SiCH3), 0.91 (9H, s, SiC(CH3)3), 0.93 (3H, m, CH3),
1.1–1.6 (2H, bm, CH2), 2.8 (1H, m, CHN), 3.3–3.5 (1H, m, CHO), 3.55–3.67 (1H, m, CHO), 7.38 (3H,
m, Ph), 7.55 (2H, m, Ph).
3.4. 2-(R)-Amino-1-(tert-butylmethylphenyl-Si(R)-silyloxy)butane hydrochloride (R)Si-5
Trimethylchlorosilane (923 mg, 8.50 mmol) was added in one portion to a solution of aminosiloxane 6
(4.371 g, 16.47 mmol) in 2-propanol (4.5 ml). At first the mixture remained liquid then turned gelatinous.

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P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
The flask was placed in an oil bath maintained at 70°C. When a clear solution resulted it was diluted with
hexane (18 ml). The heating was turned off and the mixture was stirred magnetically while the bath
temperature gradually dropped. After crystals appeared, the flask was placed in a refrigerator for several
hours. The crystals were collected and washed three times with a cold mixture of hexane:2-propanol (4:1,
10 ml in total), and air-dried for 24 h. Product (R)S-5i was obtained (1.768 g, 36% from aminosiloxane
6), m.p. 186–188°C, [α]20 +31.7 (c 11.16, CHCl3).
D
A sample of this material (11 mg) was acetylated (Ac2O, Et3N, CH2Cl2, 2 h at rt) and its diastereomeric
1
purity was determined by H NMR; the integration ratio of CH3CON signals at δ 1.92 and δ 2.00
was 95:5, respectively. The remaining part of the product (1.757 g) was recrystallised twice from
hexane:chloroform (7:3, 35 ml) yielding 1.148 g (65%) of crystals, m.p. 193–194°C, [α]20 +34.48 (c
D
10.92, CHCl3); δH 0.41 (3H, s, CH3Si), 0.93 [9H, s, (CH3)3C], 0.98 (3H, t, J=7 Hz, CH3), 1.7–2.1
(2H, bm, CH2), 3.2 (1H, m, CHN), 3.80 (2H, dq, J1=11 Hz, J2=4.3 Hz, CH2O), 7.37 (3H, m, Ph), 7.55
(2H, m, Ph), 8.55 (3H, m, H3N+); δC −7.49 (CH3Si), 10.06 (CH3), 18.33 [(CH)3C], 22.60 (CH2), 25.89
[(CH3)3C], 54.91 (CHN), 82.27 (CH2O), 96.11, 127.72, 129.58, 134.48. Anal. calcd for C15H28ClNOSi:
C, 59.67%; H, 9.35%; Cl, 11.74%; N, 4.64%. Found: C, 59.60%; H, 9.57%; Cl, 11.89%; N, 4.61%.
The procedures in Sections 3.3 and 3.4 were then repeated using (R)-(−)-2-amino-1-butanol of 80%
ee (Fluka). Since this reagent contains some water which cannot be removed by a standard distillation,
it was diluted with four parts of benzene, stirred overnight with calcium hydride and then distilled under
reduced pressure. Reaction of purified aminobutanol (1.665 g, 18.7 mmol) with silyl chloride 3 (3.796 g,
17.8 mmol) yielded aminosiloxane 6 (4.152 g, 88%), which in turn gave hydrochloride (R)Si-5, (1.768 g,
36%). Two recrystallisations of the product from toluene (9 ml per gram) provided pure product (1.497
g, total yield 32% based on 6).
3.5. 2-(R)-Amino-1-[tert-butyl(methyl)phenyl-Si(R)-silyloxy]butane (R)Si-6
The purified hydrochloride (R)Si-5 (1.124 g) was suspended in dichloromethane (10 ml), treated with
25% aqueous ammonia solution (0.5 ml) and diluted with water (0.5 ml). The mixture was stirred at rt for
2 h, then the organic phase was separated, washed with water, dried with sodium sulfate and concentrated
in vacuo. Kugelrohr distillation of the residue (150°C at 2 torr) afforded (R)Si-6 as a colourless liquid,
941 mg (95%), [α]20 +38.89 (c 13.2, hexane). Anal. calcd for C15H27NOSi: C, 67.87%; H, 10.25%; N,
D
5.28%. Found: C, 67.50%; H, 10.51%; N, 5.42%.
3.6. tert-Butyl(methyl)phenylsilane (S)-2
To a suspension of lithium aluminium hydride (150 mg) in ether (10 ml), a solution of the siloxyamine
(+)-6 (1.047 g) in ether (10 ml) was added dropwise. After stirring for 24 h at rt, methanol (1 ml)
was added carefully, followed by 10% hydrochloric acid (20 ml). The organic phase was washed
successively with water and saturated sodium hydrogen carbonate solution, dried with sodium sulfate,
and concentrated on the rotary evaporator (rt water bath). The residue was filtered through a short silica
gel column in hexane solution; Kugelrohr distillation of the concentrated eluent (115°C at 22 torr)
afforded silane (S)-2 as a colourless liquid, 100% pure by GLC, 528 mg (yield 75%), [α]20 +4.24 (c
D
8.04, hexane); δ 0.33 (3H, d, J=3.9 Hz, CH3Si), 0.93 [9H, s, (CH3)3C], 4.13 (1H, q, J=3.9 Hz, SiH),
7.2–7.7 (5H, m, aromatic).

P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
525
3.7. 2-(R)-Amino-1-[tert-butyl(methyl)phenyl-Si(S)-silyloxy]butaneoxalate (S)Si-7
The filtrate and washings from the first precipitation of the hydrochloride (R)Si-5 were concentrated
in vacuo. The semisolid residue was stirred with dichloromethane (3 ml), conc. ammonia solution (1 ml)
and water (1 ml) until two liquid phases formed. Hexane (10 ml) was added and stirring was continued
for about 5 min. The organic phase was separated, washed with brine, dried with sodium sulfate and
concentrated in vacuo. The residue (2.295 g) was dissolved in 2-propanol (7 ml). The solution was
warmed to 80°C and treated with anhydrous oxalic acid (387 mg, 4.28 mmol) in 2-propanol (4 ml). The
mixture was cooled with stirring and then set aside in the refrigerator. Crystals were collected, washed
with cold 2-propanol (total 10 ml) and air-dried. The product (S)Si-7 was obtained (1.676 g, 33% from
6), m.p. 167–169°C. To determine its diastereomeric purity, a sample (14 mg) was converted to the N-
1
acetyl derivative (see above). H NMR showed a mixture of 12% of the (R)Si diastereomer (CH3CON
signal at δ 1.92). The bulk of the product (1.662 g) was then recrystallised twice from 2-propanol to give
pure (S)Si-7 (577 mg), m.p. 170–173°C, [α]20 −31.3 (c 9.1, MeOH). Anal. calcd for C32H56N2O6Si2:
D
C, 61.89%; H, 9.09%; N, 4.51%. Found: C. 61.72%; H, 9.05%; N, 4.43%.
3.8. 2-(R)-Amino-1-[tert-butyl(methyl)phenyl-(S)-silyloxy]butane (S)Si-6
The oxalate (S)Si-7 (242 mg, 0.39 mmol) was suspended in dichloromethane (3 ml) and stirred with
25% ammonia solution (0.2 ml) diluted with water (0.4 ml) until the solid had disappeared completely.
Usual work-up followed by a Kugelrohr distillation (155°C at 3 torr) afforded (S)Si-6 as a colourless
liquid, 172 mg (83%), [α]20 −53.44 (c 8.94, CHCl3).
D
3.9. tert-Butyl(methyl)phenylsilane (R)-2
Reduction of (S)Si-8 (170 mg, 0.64 mmol) with lithium aluminium hydride (60 mg, 1.58 mmol) in
ether (total volume 4 ml) and work-up were performed as above. Kugelrohr distillation (105°C at 19 torr)
afforded (R)-2 as a colourless liquid, 80 mg (69.9%), [α]20 −4.1 (c 6.15, hexane).
D
3.10. Hydrosilylation of propargylic derivative 10 with silane (S)-2
The hydrosilylation catalyst was prepared by dissolving H2PtCl6·H2O (Fluka, 50 mg) in 2-propanol
(1 ml).
A mixture of silane (S)-2 (186 mg, 1.04 mmol), protected propargylic alcohol 10 (179 mg, 1.28 mmol)
and the catalyst (two drops) was stirred at 50°C for 48 h under argon. The solution was diluted with
hexane (1 ml), the cloudy mixture was put on a short silica gel column and the THP-protected products
were eluted with hexane:ethyl acetate (9:1). The corresponding fractions were combined, concentrated,
dissolved in methanol (10 ml) and treated with a few crystals of p-toluenesulfonic acid. After 2 h at rt,
an excess of potassium carbonate was added, the mixture was evaporated to dryness and the residue was
chromatographed on silica with hexane–ethyl acetate. The fraction consisting of pure 11 (40 mg) was
analysed on a Chiralcel OD-H column; only one enantiomer 11 was detected (tR=16.7 min, hexane:2-
propanol 9:1, 0.5 ml/min), identical with an authentic sample.18

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P. Jankowski et al. / Tetrahedron: Asymmetry 10 (1999) 519–526
3.11. N-{2-(R)-1-[tert-Butyl(methyl)phenyl-Si(R)-silyloxy]}butyl-4-nitrobenzamide (R)Si-9
Compound (R)Si-6 (117 mg, 0.44 mmol), dissolved in benzene (2 ml) and pyridine (0.2 ml), was
allowed to react with 4-nitrobenzoyl chloride (105 mg, 0.57 mmol) for 1 h at rt. The excess reagent
was decomposed with water (1.5 ml). Work-up followed by chromatography on silica with hexane–ethyl
acetate afforded pure amide (R)Si-9, 157 mg, m.p. 123–124°C (long needles from 1-chlorobutane); δH
0.39 (3H, s, CH3Si), 0.92 [9H, s, (CH3)3C], 1.01 (3H, t, J=7.5 Hz, CH3), 1.75 (2H, m, CH2), 3.82 (2H,
m, CH2O), 4.15 (1H, m, CHN), 6.4 (1H, m, NH), 7.35 (3H, m), 7.5 (2H, m), 7.74 (2H, d, J=9 Hz), 8.23
(2H, d, J=9 Hz). Crystal data: colourless orthorhombic crystal (0.25×0.3×0.45 mm); empirical formula,
C22H30N2O4Si (fw 414.57); space group P212121; a=9.5140(10) Å, b=12.6970(10) Å, c=20.1500(10) Å,
V=2434.1(3) Å3, Dcalc=1.131 g/cm3; α=β=γ=90°; Cu-Kα radiation (1.54184 Å) at 293 K.
Acknowledgements
Financial support from the State Committee of Scientific Research, Poland, and the Volkswagen
Foundation, Germany, is gratefully acknowledged.
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