Organic Syntheses, CV 6, 215
Submitted by E. L. Eliel
1, T. W. Doyle, R. O. Hutchins
2, and E. C. Gilbert.
Checked by Mitchell Winnik and Ronald Breslow.
1. Procedure
To a solution of
4.0 g. (0.012 mole) of iridium tetrachloride (Note
1) in
4.5 ml. of concentrated hydrochloric acid is added 180 ml. of water followed by
52 g. (50 ml., 0.42 mole) of trimethyl phosphite (Note
2). This solution then is added to a solution of
30.8 g. (0.200 mole) of 4-tert-butylcyclohexanone (Note
3) in
635 ml. of 2-propanol contained in a
2-l. flask equipped with a
reflux condenser. The solution is heated at reflux for 48 hours (Note
4), at which time the
2-propanol is removed with a
rotary evaporator. The remaining solution is diluted with 250 ml. of water and extracted with four
150-ml. portions of diethyl ether. The combined
ether extracts are washed with two 100-ml. portions of water, which are combined with the aqueous residue (Note
5), dried over
magnesium sulfate or
potassium carbonate, and concentrated on a rotary evaporator, yielding
29–31 g. (
93–99%) of
cis-4-tert-butylcyclohexanol as a white solid. Analysis of the crude product by GC shows it contains
95.8–96.2% cis-alcohol and 4.2–3.8% of the
trans isomer with essentially no ketone remaining (Note
6). Recrystallization from
40% aqueous ethanol affords greater than
99% pure
cis-alcohol, m.p.
82–83.5° after sublimation (Note
7).
2. Notes
1.
Iridium tetrachloride was originally obtained from Platinum Chemicals, Inc., Box 565, Asbury Park, New Jersey 07712, or from Alfa Products, Thiokol/Ventron Division, P.O. Box 299, 152 Andover St., Danvers, Massachusetts 01923. More recently, the procedure has been repeated successfully with material obtained from Pfaltz and Bauer, Inc., a subsidiary of Aceto Chemical Co., Inc., 375 Fairfield Ave., Stamford, Connecticut 06902.
2. The order of mixing the catalyst components is required for good results, and the sequence described should be followed. Particular care should be taken
not to add the
trimethyl phosphite before the water, as the reaction between it and concentrated
hydrochloric acid is extremely violent.
4. The reaction solution is often dark-colored at the beginning but lightens as reflux continues. The reflux time may be varied with the amount of ketone to be reduced. The completeness of the reaction may be followed by removing small aliquots, working up these samples as described in the text, and analyzing the product mixture by GC (see (Note
6)).
5. The
iridium catalyst used in this preparation may be regenerated by reducing the volume of the aqueous residue to about 200 ml. at diminished pressure. This solution is then used instead of the
iridium tetrachloride and water called for in the procedure.
6. The product was analyzed by GC using a 9-ft. 20% Carbowax 20M on 45/60 Chromosorb W column at 150°. The order of increasing retention times is: ketone,
cis-alcohol,
trans-alcohol.
7. Recrystallization is best accomplished by dissolving the crude product in hot
ethanol (approx. 35 ml. per 10 g.) followed by adding water (approx. 25 ml. per 10g.) and allowing the solution to cool slowly to 0°. The fluffy white needles are filtered using a
sintered-glass funnel and dried over
P2O5 at atmospheric pressure. Recooling the filtrate affords a second crop of product, for an overall yield of
75–87%.
3. Discussion
The present
9 procedure employs a readily available starting material and produces essentially pure
cis isomer in good yield. In view of the fact that the catalyst may be reused several times with little loss in stereoselectivity, the expense of the
iridium tetrachloride is not a serious impediment.
Since this preparation was submitted, a number of reductions of
4-tert-butylcyclohexanone to the
cis alcohol, with 93–100% selectivity, using various bulky, complex metal hydrides have been described:
lithium tri-sec-butylborohydride (L-Selectride),
12,13 lithium trisiamylborohydride,
13,14,15 lithium tri-trans-2-methylcyclopentylborohydride,
14 lithium dimesitylborohydride,
16 lithium 2,6-di-tert-butylphenoxyneopentoxyaluminumhydride;
17 high (94–99%) selectivity is also attained by catalytic hydrogenation with various
rhodium catalysts.
18,19 Hydrogenation over
rhodium-on-carbon18 (94%
cis) followed by purification with liquid chromatography appears to be an attractive method, one which avoids the need for special reagents.
Appendix
Compounds Referenced (Chemical Abstracts Registry Number)
cis-rich 4-tert-butylcyclohexanol
lithium trisiamylborohydride
rhodium-on-carbon
ethanol (64-17-5)
potassium carbonate (584-08-7)
hydrogen chloride,
hydrochloric acid (7647-01-0)
acetic acid (64-19-7)
ether,
diethyl ether (60-29-7)
hydrogen (1333-74-0)
platinum oxide
carbon (7782-42-5)
2-propanol (67-63-0)
aluminum isopropoxide
magnesium sulfate (7487-88-9)
Cholestanone (566-88-1)
trimethyl phosphite (121-45-9)
iridium tetrachloride (10025-97-5)
iridium (7439-88-5)
rhodium (7440-16-6)
isobornyloxyaluminum dichloride
3,3,5-trimethylcyclohexanone (873-94-9)
cholestan-3α-ol
2,4,4-trimethylcyclohexanone
lithium dimesitylborohydride
4-tert-Butylcyclohexanone (98-53-3)
4-tert-Butylcyclohexanol (98-52-2)
p-tert-butylphenol (98-54-4)
cis-4-tert-Butylcyclohexanol,
Cyclohexanol, 4-(1,1-dimethylethyl)-, cis- (937-05-3)
3-tert-butylcyclohexanone (936-99-2)
lithium tri-sec-butylborohydride
lithium tri-trans-2-methylcyclopentylborohydride
lithium 2,6-di-tert-butylphenoxyneopentoxyaluminumhydride
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