Organic Syntheses, CV 7, 223
OSMIUM-CATALYZED VICINAL OXYAMINATION OF OLEFINS BY N-CHLORO-N-ARGENTOCARBAMATES: ETHYL threo-[1-(2-HYDROXY-1,2-DIPHENYLETHYL)]CARBAMATE
[Carbamic acid (2-hydroxy-1,2-diphenylethyl),-ethyl ester, (R*R*)-]
Submitted by Eugenio Herranz and K. Barry Sharpless
1.
Checked by Steven D. Young and Clayton H. Heathcock.
1. Procedure
A
1-L, one-necked, round-bottomed flask is equipped with a
magnetic stirring bar and a
100-mL addition funnel. The flask is placed in an
ice bath and charged with
13.36 g (0.15 mol) of ethyl carbamate (Note
1) and
100 mL of reagent-grade methanol. Vigorous stirring is begun and to the ice-cold solution is carefully added
16.9 mL (16.2 g, 0.15 mol) of tert-butyl hypochlorite (Note
2). Fifteen minutes after the addition of the
tert-butyl hypochlorite is complete, a
methanolic solution (75 mL) of sodium hydroxide (6.43 g, 0.158 mol) is added dropwise over a period of several minutes (Note
3). After addition of the
sodium hydroxide is complete, the ice bath is removed and stirring is continued for a further 10 min. The solvent is removed using a
rotary evaporator (bath <60°C) to give the crude
ethyl N-chloro-N-sodiocarbamate as a white solid (Note
4). Addition of
400 mL of reagent-grade acetonitrile and
26.33 g (0.1 mol) of silver nitrate (Note
5) results in the gradual appearance of a brown suspension. The solution is stirred for 5 min at room temperature;
18.23 g (0.1 mol) of (E)-stilbene (Note
6),
10 mL (
1.0 mmol) of a solution of OsO4 in tert-butyl alcohol (Note
7), and 8.1 mL (0.45 mol) of water are then added. The milky brown suspension that results is stirred for 18 hr at room temperature. Filtration of the reaction mixture through a
Celite mat on a sintered-glass funnel gives a yellow–brown solution (Note
8). The filtrate is refluxed for 3 hr with
200 mL of 5% aqueous sodium sulfite (Note
9). The resulting mixture is concentrated at aspirator pressure using a rotary evaporator until
acetonitrile no longer distills. The residue, which is primarily aqueous, is extracted with two
60-mL portions of methylene chloride (Note
10). The organic phase is dried (
MgSO4) and concentrated to give
24.6 g of crude product as a pale-yellow solid. Crystallization from
50 mL of hot toluene affords
18.6–19.8 g (
66–69%) of almost pure
ethyl threo-1-(2-hydroxy-1,2-diphenylethyl)carbamate, mp
120–122°C (Note
11). Concentration of the mother liquors yields an additional 0.6–0.8 g of the hydroxy carbamate (Note
12).
2. Notes
6.
(E)-Stilbene was used as obtained from Aldrich Chemical Company. The olefin should be added in small portions to avoid overheating of the reaction mixture.
7.
Osmium tetroxide was supplied by Matthey-Bishop, Inc. in 1-g amounts in sealed glass ampuls. The procedure that we describe below should be followed to prepare the
osmium tetroxide catalyst solution. Work in a
well-ventilated hood. One ampul is scored in the middle, broken open, and the two halves are dropped into a clean,
brown bottle containing
39.8 mL of reagent grade tert-butyl alcohol and
0.20 mL of 70 or 90% tert-butyl hydroperoxide (Aldrich). The bottle is capped (use caps with Teflon liners) and then swirled to ensure dissolution of the OsO
4. These solutions are stored in the hood at room temperature and seem to be very stable.
8. In this way the silver salts (AgCl) are removed from the reaction mixture. The precipitate is washed twice with
20-mL portions of acetonitrile.
9. The purpose of this sulfite treatment is to reduce and thereby remove the small amount of
osmium that is bound to the organic products.
10. If an emulsion forms, addition of Celite and subsequent filtration through a sintered-glass funnel gives a clear separation of the two phases. The checkers found that extraction with three
100-mL portions of methylene chloride avoids emulsion formation.
11. Crystallization occurs at room temperature over a period of ca. 12 hr. The crystals are washed once with
15 mL of toluene or
50 mL of petroleum ether (bp
40–60°C). The product is quite pure. A product of higher purity, however, mp
122–123.5°C, can be obtained by a second crystallization from
toluene.
12. After 24 hr at high vacuum (0.1 mm), some crystals appear. Addition of
15 mL of ether, filtration, and washing with
10 mL of ether gives more product, mp
110–121°C. The checkers found that a higher overall yield was obtained if the mother liquors from the first recrystallization were dissolved in
50 mL of boiling diethyl ether. The solution is then brought to cloudiness by addition of
petroleum ether (bp
40–60°C). When this mixture is stored at 0°C overnight, brown crystals are deposited. Recrystallization of this material from
10 mL of hot toluene provides an additional
2.25–3.51 g of hydroxy carbamate, mp
114–117°C.
3. Discussion
This new procedure
2 for vicinal, cis addition of an
oxygen and a
nitrogen to an olefinic bond constitutes a major improvement over earlier methods,
3,
4 since the
nitrogen is introduced bearing an easily removed protecting group. Although the procedure described here employs
ethyl carbamate, both
tert-butyl carbamate and
benzyl carbamate can also be used. In fact, in most cases, higher yields are realized in oxyaminations using the latter carbamates.
N-Chloro-
N-argentocarbamates are generated
in situ by reaction of the corresponding
N-chlorosodiocarbamates with
silver nitrate in
acetonitrile. The
N-chlorosodiocarbamates are prepared from the carbamates according to the method of Campbell and Johnson.
5 There are conflicting statements in the literature about the stability of these
N-chlorosodiocarbamates.
6 On one occasion, when EtOCONNaCl was prepared by the submitters on a 250-mmol scale, it decomposed rapidly (but not explosively), turning dark and releasing heat and gases. However, this same chloramine salt has been prepared on a 100-mmol scale without incident. The submitters have found that acidic conditions (which lead to contamination by the
N-chlorocarbamate) are responsible for the spontaneous decomposition of these salts at room temperature. A simple modification of Campbell's procedure for preparing
N-chloro-
N-sodiocarbamates avoids this problem. By adding
5% more sodium hydroxide than the calculated amount, it is assured that all the
N-chlorocarbamate in the reaction mixture is neutralized. No spontaneous decomposition has occurred in the batches of
N-chloro-
N-sodiocarbamates prepared in this way.
The regioselectivity of this new procedure toward terminal olefins is considerably better than that realized with the earlier catalytic oxyamination procedures based on
chloramine-T3. However, the catalytic procedure cannot compete with the regiospecificity exhibited by the stoichiometric
tert-alkyl imido osmium reagents.
3
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved