Org. Synth. 1983, 61, 112
DOI: 10.15227/orgsyn.061.0112
PHOTOPROTONATION OF CYCLOALKENES: LIMONENE TO p-MENTH-8-EN-1-YL METHYL ETHER
[Cyclohexane, 1-methoxy-1-methyl-4-(1-ethenyl-1-methyl-)]
Submitted by F. P. Tise and P. J. Kropp
1.
Checked by R. L. Amey and R. E. Benson.
1. Procedure
A
250-mL photochemical reactor (see
Figure 1) is fitted with a
cylindrical Vycor filter sleeve, a
450-W Hanovia mercury lamp, and a
watercooled condenser which is connected to a
mineral oil bubbler. Tubing attachments are made so that water is circulated through the condenser and then through the Vycor filter sleeve. The tube leading from the bottom of the reaction vessel and containing the
glass frit is connected in series to a
trap fitted with a
fritted filter stick and then to a trap that is connected to a
nitrogen source. The system is flushed with
nitrogen, and sufficient anhydrous
methanol is placed in the trap containing the fritted stick to provide for a methanol-saturated gas stream during the course of the reaction
(Note 1).
Figure 1
The nitrogen-flushed reactor is charged with a solution of 20.0 g (147 mmol) of ( + )-limonene (Note 2), 5.0 g (53 mmol) (Note 3) of phenol, and 5 drops of concentrated sulfuric acid in 210 mL (167 g, 5.2 mol) of anhydrous methanol (Note 4). Water flow through the condenser is started (Note 5), and the nitrogen flow is adjusted to provide good agitation of the contents of the vessel. After 15 min, irradiation is started and the reaction followed by GLC (Note 6), with 48 hr being the approximate time needed for essentially complete conversion (Note 7).
The solution is poured into 900 mL of 5% aqueous sodium hydroxide solution containing 125 g of sodium chloride, and the mixture is extracted with two 100-mL portions of ether. The ether layers are combined, washed with 50 mL of saturated sodium chloride solution, and dried over anhydrous sodium sulfate. The drying agent is removed by filtration and the filtrate is concentrated with a Büchi rotary evaporator. After a preliminary distillation to separate the product from a small amount of nonvolatile material, the liquid is distilled at reduced pressure through a Teflon spinning band column (47 cm × 7 mm). The material that distills at 90–95°C (10 mm) is collected to give 12.8–13.2 g (52–53%) of a mixture of cis- and trans-p-menth-8-en-1-yl methyl ether (Note 8),(Note 9),(Note 10).
2. Notes
1.
The submitters used a reactor with a joint that was capped with a
rubber septum fitted with
two syringe needles, which were attached by means of a
Y-tube to a single nitrogen line. To one of these needles is attached a piece of 1.70-mm-o.d. polyethylene tubing of sufficient length to reach to the bottom of the reaction vessel. By use of pinchcocks,
nitrogen can be passed through either of the two needles. The solution was stirred with a
magnetic stirring bar.
2.
( + )-Limonene was obtained from Aldrich Chemical Company, Inc. and distilled before use.
3.
The checkers used reagent available from Fisher Scientific Company.
4.
The checkers used
fresh, acetone-free, absolute methanol available from Fisher Scientific Company.
5.
For best results the cooling water should pass through the condenser first and then through the immersion well. This arrangement lessens evaporation of
methanol.
6.
The submitters used a
3-m × 3.2-mm stainless steel column packed with 20% SF-96 on Chromasorb W (60–80 mesh) and a He flow rate of 60 mL/min. With a temperature program of 4 min at 50°C followed by an increase of 10°C/min to a maximum of 200°C, the retention times were 17.9 and 18.7 min.
7.
The checkers found that the reaction was impeded by the formation of a yellow film on the immersion well with very little further conversion occurring after 30 hr of irradiation.
8.
The checkers used a
2.4-m × 3.2-mm column packed with 7% SE-30 and 3% Silar on Chromasorb W (60–80 mesh) at 160°C. The retention time was 1.56 min for the trans isomer and 1.81 min for the cis isomer at a He flow rate of 55 mL/min.
9.
The spectral properties of the product (approximately 60% cis:40% trans isomers) are as follows: IR (neat) cm
−1: 3080 (=C-H); 2964, 2939, 2860, 2825, (C-H); 1645 (C=C); 1464, 1453, and 1442 (overlapping peaks); 1370, 1124, and 1082 (C-OC); 885 (=CH).
1H NMR (CDCl
3) δ: 1.10 [s, 3 H, C
H3 (trans)], 1.19 [s, 3 H, C
H3 (cis)], 1.30–2.00 (8 H, -C
H2−), 1.71 [s, 3 H, C
H3 (cis/trans)], 3.14 [s, 3 H, OC
H3 (trans)], 3.21 [s, 3 H, OC
H3 (cis)], 4.69 [s, 2 H, =C
H2 (cis/trans)].
10.
The submitters state that similar irradiation of
20.0 g of cyclohexene,
5.0 g of phenol, and
1.5 mL of concentrated sulfuric acid for 24 hr afforded
cyclohexyl methyl ether in 70% yield.
3. Discussion
Acid-catalyzed, ground-state additions to
limonene generally afford a mixture of products resulting from competing protonation of both double bonds.
2 In one case in which selective reaction was observed, attack occurred at the acyclic C
8-C
9 double bond.
3The photoprotonation of cycloalkenes, described in this procedure, is believed to proceed via initial light-induced cis → trans isomerization of the alkene.
4 The resulting highly strained trans isomer undergoes facile protonation. This procedure permits the protonation of cyclohexenes and cycloheptenes under neutral or mildly acidic conditions.
5 Since the process is irreversible, high levels of conversion to addition products can be achieved.
Photoprotonation is generally specific for cyclohexenes and cycloheptenes. Smaller-ring cycloalkenes are incapable of undergoing cis → trans isomerization, and the trans isomers of larger-ring or acyclic analogues have insufficient strain to undergo ready protonation. Thus, in addition to facilitating protonation of cycloalkenes, the procedure affords a means of selectively protonating a double bond contained in a six- or seven-membered ring in the presence of another double bond contained in an acyclic, exocyclic, or larger-ring cyclic environment.
6 When conducted in non-nucleophilic media, the photoprotonation procedure is also useful for effecting the isomerization of 1-alkylcyclohexenes and -heptenes to their exocyclic isomers.
4
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
sulfuric acid (7664-93-9)
methanol (67-56-1)
ether (60-29-7)
sodium hydroxide (1310-73-2)
phenol (108-95-2)
Cyclohexene (110-83-8)
sodium chloride (7647-14-5)
sodium sulfate (7757-82-6)
nitrogen (7727-37-9)
CYCLOHEXYL METHYL ETHER (931-56-6)
LIMONENE,
( + )-limonene
Cyclohexane, 1-methoxy-1-methyl-4-(1-ethenyl-1-methyl-)
p-Menth-8-en-1-yl methyl ether,
cis- and trans-p-menth-8-en-1-yl methyl ether (24655-71-8)
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