Organic Syntheses, Vol. 75, 78
Checked by Pradeep Madan and Steven Wolff.
1. Procedure
2. Notes
1.
3-Butyn-2-ol, 99%, is purchased from Aldrich Chemical Company, Inc.
3.
Dimethylphenylsilane is purchased from United Chemical Technologies, Inc (formerly Hüls Petrarch Inc.).
5. The
platinum catalyst is prepared according to the procedure of Chandra and Lo.
2 Chloroplatinic acid, H2PtCl6, (5.0 g, 12.22 mmol) and 0.6 mL of water are added to
61.71 mL (268.84 mmol) of 1,3-divinyltetramethyldisiloxane. The heterogeneous solution is heated to 55°C in an
oil bath for 4 hr. The now homogeneous solution is cooled to room temperature and
5.0 g of sodium bicarbonate is added. The solid is filtered, and the resulting yellow solution is treated with
3.33 mL (13.44 mmol) of tri-tert-butylphoshine[(tert-Bu)3P]. The catalyst precipitates as
4.8 g (
67%) of a white solid that is filtered and washed with
1,3-divinyltetramethyldisiloxane (25 mL), and dried under vacuum. The checkers found that the mixture remained heterogeneous after 4 hr. Heating was continued for 18 hr.
6. Flash chromatography is performed on E. Merck silica gel 230-400 mesh:
250 g of silica gel is loaded on a
16- × 2-in size column using a minimum amount of
hexanes as loading solvent.
7. The hydrosilylation product
2a is sufficiently pure for use as starting material in procedure B. The spectral properties are as follows:
1H NMR (400 MHz, CDCl
3) δ: 0.38 (s, 6 H), 1.30 (d, 3 H, J = 6.5), 4.34 (m, 1 H), 5.99 (dd, 1 H, J = 1.3, 18.7), 6.22 (dd, 1 H, J = 4.9, 18.7), 7.28-7.56 (m, 5 H);
13C NMR (67.5 MHz) δ: −2.7(2C), 22.8, 70.4, 125.9, 126.6, 127.6, 128.9, 133.8, 151.3; IR (neat) cm
−1: 3380, 3080, 2960, 1730, 1620, 1430, 1250, 1110, 860; CIHRMS M + NH
4+ (calcd for C
12H
22NOSi): 224.1470, (found): 224.1416.
8. The amount of enzyme extract is roughly calculated to be 0.5 wt equiv. The crude enzyme preparation may be purchased from Amano Enzyme U.S.A. Co., Ltd., Rt. 2, Box 1475, Troy, VA 22974.
9. A marked increase in reaction rate is noted when
vinyl acetate is freshly distilled.
10. Reaction progress is monitored by
1H NMR (CDCl
3); 0.5-mL of sample is withdrawn periodically (at 30-min intervals after 3 hr), filtered through a cotton plug, and the solvent is removed under reduced pressure. The intergration ratio of proton 4.34 (m, 1 H) (S)-
3a, and 5.38 (m, 1 H) (R)-
3b is measured and the reaction is terminated when the integration ratio is 1:1.
11. The enantiomeric purity of vinylsilane (S)-
3a and (R)-
3c are determined to be >95% ee by
1H-NMR (400 MHz) on the derived mandelate ester, obtained by a DCC-promoted coupling to
(R)-O-acetylmandelic acid, and absolute stereochemical assignment is accomplished by
1H NMR analysis of the derived (R)-O-acetylmandelate esters. For details of this procedure see the published method of Trost.
3 4 5
12. The spectral properties are as follows: (S)-
3a:
1H NMR (400 MHz, CDCl
3) δ: 0.38 (s, 6 H), 1.30 (d, 3 H, J = 6.5), 4.34 (m, 1 H), 5.99 (dd, 1 H, J = 1.3, 18.7), 6.22 (dd, 1 H, J = 4.9, 18.7), 7.28-7.56 (m, 5 H);
13C NMR (67.5 MHz) δ: −2.7 (2C), 22.8, 70.4, 125.9, 126.6, 127.6, 128.9, 133.8, 151.3; IR (neat) cm
−1: 3380, 3080, 2960, 1730, 1620, 1430, 1250, 1110, 860; CIMS (NH
3), m/e (relative intensity): 224 (M+NH
4+, 100), 206 (M, 44); CIHRMS (NH
3), m/e M+NH
4+ (calcd for C
12H
22NOSi): 224.1470, (found): 224.1416;
[α]D23 +4.3° (CHCl
3,
c 0.8); (R)-
3b:
1H NMR (400 MHz, CDCl
3) δ: 0.37 (s, 6 H), 1.32 (d, 3 H, J = 6.5), 2.08 (s, 3 H), 5.38 (m, 1 H), 5.9 (dd, 1 H, J = 1.33, 18.8), 6.1 (dd, 1 H, J = 4.64, 18.8), 7.3-7.5 (m, 5 H);
13C NMR δ: −2.7 (2C), 19.8, 21.3, 72.1, 127.7, 127.8, 128.0, 129.0, 133.8, 146.6, 170.2; IR (neat) cm
−1: 3040, 2950, 1740, 1430, 1570, 1240, 1110, 1040, 830, 740; CIMS (NH
3), m/e (relative intensity) 209 (33), 179 (24), 171 (68), 135 (100), 117 (97); CIHRMS (NH
3), m/e M
+(calcd for C
14H
20O
2Si): 248.1233, (found): 248.1299;
[α]D23 +58.3° (CHCl
3,
c 0.5).
14. The specific rotation of alcohol (R)-
3c obtained from
LiAlH4 reduction of acetate (R)-
3b is
[α]D23 −3.8° (CHCl
3,
c 0.8).
16. The determination of enantiomeric excess (96% ee) of the Claisen rearrangement products
4a and
4b is accomplished by a Mosher
1H NMR analysis of the (R)-O-acetylmandelate esters that are derived from the primary alcohols. For example,
4a is reduced with LiAlH
4 (1.0 equiv/THF/0°C) followed by esterification of the resulting primary alcohol with
(R)-O-acetylmandelic acid (DCC, 1.5 equiv/cat. DMAP/CH
2Cl
2) to afford the mandelate ester in
91% yield (two steps).
3,4,5
17. The spectral properties are as follows:
1H NMR (400 MHz, CDCl
3) δ: 0.28 (s, 6 H), 1.64 (d, 3 H, J = 4.7), 2.20 (m, 1 H), 2.27 (dd, 1 H, J = <1.0, 14.5), 2.33 (dd, 1 H, J = 6.3, 14.5), 3.57 (s, 3 H), 5.28 (m, 2 H), 7.35-7.49 (m, 5 H);
13C NMR (67.5 MHz) δ: −5.5, −4.5, 18.1, 28.7, 34.3, 51.4, 123.8, 127.7, 129.1, 129.8, 133.9, 136.8, 173.9; CIMS (NH
3), m/e (relative intensity) 280 (M+NH
4+, 48), 263 (M
+, 39), 184 (100), 151 (40); CIHRMS (NH
3), m/e M+NH
4+ (calcd for C
15H
26NO
2Si): 280.1733, (found): 280.1734;
[α]D23 −11.4° (CHCl
3,
c 0.5) and
[α]D23 −15.4° (CH
2Cl
2,
c 1.4) for [(R)-
4b derived from Claisen rearrangement of (R)-
3c. For (S)-
4a [α]D23 +11.9° (CHCl
3,
c 0.5) and
[α]D23 +15.9° (CH
2Cl
2,
c 1.6).
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
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