Organic Syntheses Procedure

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Org. Synth. 2005, 82, 134

DOI: 10.15227/orgsyn.082.0134

ASYMMETRIC REARRANGEMENT OF ALLYLIC TRICHLOROACETIMIDATES: PREPARATION OF (S)-2,2,2-TRICHLORO-N-(1-PROPYLALLYL)ACETAMIDE

[Acetamide, 2,2,2-trichloro-N-[(1S)-1-ethenylbutyl]-]

Submitted by Carolyn E. Anderson, Larry E. Overman*, and Mary P. Watson.

Checked by Matthew L. Maddess and Mark Lautens.

1. Procedure

Caution! Part A should be carried out in a well-ventilated hood to avoid exposure to trichloroacetonitrile vapors.

A. Preparation of (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester. A 500 mL round-bottomed flask equipped with a stirring bar is flame dried under a stream of nitrogen and allowed to cool to room temperature. The flask is then charged with trans-2-hexen-1-ol (Note 1) (3.3 mL, 27.8 mmol), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (Note 2) (0.84 mL, 5.6 mmol) and 170 mL of methylene chloride (Note 3). The solution is cooled to 4 °C using an ice/water bath before trichloroacetonitrile (Note 2) (4.2 mL, 42 mmol) is added over five minutes by syringe. With time, the reaction solution is observed to change from clear to orange in color, and within 1 hour the starting materials are consumed (Note 4). The stir bar is removed with a magnetic rod and the solution is then concentrated under reduced pressure using a rotary evaporator to give a brown oil (Note 5). This oil is purified by flash chromatography through a plug of silica gel (Silicycle® 230-400 mesh, 6 cm tall, 5 cm diameter) using 2% ethyl acetate in hexanes (600 mL, 60 mL fractions) to yield 6.48 g (95%) of nearly pure (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester as a colorless oil (Notes 6, 7).

B. (S)-COP-Cl catalyzed rearrangement of (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester to (S)-2,2,2-trichloro-N-(1-propylallyl)acetamide. A 150-mL, round-bottomed flask is fitted with a stirring bar and then charged with (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester (6.81 g, 27.8 mmol), Di-μ-chlorobis[η⁵-(S)-(pR)-2-(2'-(4'-isopropyl)oxazolinylcycloentadienyl, 1-C, 3'-N))-(η⁴-tetraphenylcyclo-butadiene)cobalt]dipalladium [(S)-COP-Cl] (Note 8) (816 mg, 0.56 mmol) and 9.3 mL of methylene chloride (Note 3). The flask is sealed with a polyethylene cap, the cap is secured to the flask with Parafilm, and the flask is placed in an oil bath preheated to 38 °C +/− 2 °C. After 24 h, the solution is cooled to room temperature, the stir bar is removed using a magnetic rod, and the solution is concentrated using a rotary evaporator to yield a brown oil. This oil is purified by flash chromatography through a column of silica gel (Silicycle® 230-400 mesh, 20 cm tall, 5 cm diameter) using 0.5% to 2% ethyl acetate: hexanes as eluent (3 L 0.5% ethyl acetate:hexanes, 1 L 1% ethyl acetate:hexanes, 1 L 2% ethyl acetate: hexanes). Evaporation of solvent provides 6.61 g (97% yield) of (S)-2,2,2-trichloro-N-(1-propylallyl)acetamide, 94% ee, as a pale yellow oil (Notes 9, 10, 11, 12).

2. Notes

1. The checkers used trans-2-hexen-1-ol purchased from Aldrich Chemical Company, Inc. Although of sufficient purity for this series of transformations, the commercial reagent is contaminated with 3% of 1-hexanol.¹ The submitters used (E)-2-Hexen-1-ol (>99% E) prepared from butanal by Horner-Wadsworth-Emmons reaction with trimethyl phosphonoacetate to form hex-2-enoic acid methyl ester, followed by reduction of this product with diisobutylaluminum hydride (−78 °C to room temperature in THF).

2. 1,8-Diazabicyclo[5.4.0]undec-7-ene and trichloroacetonitrile were purchased from Aldrich Chemical Company, Inc. These chemicals were used as received.

3. Methylene chloride was purified by passage through a solvent purification system. The submitters used a GlassContour alumina solvent purification columns and the checkers used a MBRAUN® solvent purification system.²

4. The reaction progress can be analyzed by thin layer chromatography (Silicycle®, plastic backed, 250 µm thickness). Using 10% ethyl acetate:hexanes as eluent, the product (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester has an R_f of 0.45, whereas the starting alcohol has an R_f of 0.12. Both the imidate and starting alcohol can be visualized by potassium permanganate stain.

5. Upon concentrating the solution, black semi-solids sometimes form. These unwanted byproducts are insoluble in 2% ethyl acetate:hexanes.

6. The product, (E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester, exhibits a spectrum that matches that which is reported in the literature.³ ¹H NMR pdf (300 MHz, CDCl₃) δ 8.27 (broad s, 1H, NH), 5.86 (dt, J = 15.6, 6.3 Hz, 1H, CH), 5.68 (dt, J = 15.6 Hz, 6.9 Hz, 1H, CH), 4.74 (d, J = 6.3 Hz, 2H, CH₂), 2.06 (tq, J = 6.9, 6.9 Hz, 2H, CH₂), 1.43 (tq, J = 6.9, 7.2 Hz, 2H, CH₂), 0.91 (t, J = 7.2 Hz, 3H, CH₃). ¹³C NMR (75 MHz, CDCl₃) δ 162.6, 136.9, 123.3, 91.7, 70.0, 34.5, 22.1, 13.7.

7. The product is contaminated with 2,2,2-trichloroacetimidic acid hexyl ester (3%) arising from the reaction of hexyl alcohol with trichloroacetonitrile.

8. (S)-COP-Cl was purchased from Aldrich Chemical Company, Inc. and was used as received. The specific rotation of this catalyst was measured to be: [α]_D23.8 = +1169 (c 0.25, CHCl₃). A detailed procedure for the synthesis of (S)-COP-Cl has been published: Anderson, C. E.; Kirsch, S. F.; Overman, L. E.; Richards, C. J.; Watson, M. P. Org. Syn. 2007, 84, 148-155.

9. The enantiomeric ratio was determined by HPLC analysis by comparison to a racemic sample:^3a Hewlett-Packard HP Series 1100, Chiracel OD with guard column, 99:1 hexanes:isopropyl alcohol, 0.4 mL/min, 30 °C, R_t (major) = 15.3 min, R_t (minor) = 16.4 min.

10. ¹H NMR pdf (400 MHz, CDCl₃) δ 6.60 (broad s, 1H, NH), 5.81 (ddd, J = 16.4, 10.4, 5.6 Hz, 1H, CH), 5.17–5.27 (m, 2H, CH₂), 4.39–4.47 (m, 1H, CH), 1.55–1.71 (m, 2H, CH₂), 1.35–1.46 (m, 2H, CH₂), 0.96 (t, J = 7.2 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) d 161.2, 136.7, 115.9, 92.8, 53.4, 36.5, 18.9, 13.7; IR (neat) cm⁻¹: 3424, 3322, 2961, 2935, 1714, 1520, 1249, 926, 821; HRMS (EI) m/z 244.0063 [244.0063 calcd for C₈H₁₃Cl₃NO (M + H)]; R_f = 0.36 (10% ethyl acetate:hexanes).

11. The submitters reported, that when commercially available starting material is used, the product is contaminated with observable quantities of 2,2,2-trichloroacetimidic acid hexyl ester. The checkers, however, report that 2,2,2-trichloroacetimidic acid hexyl ester is not observed to the detection limits of ¹H or ¹³C NMR.

12. On half-scale to that described, with prolonged drying under high vacuum the product was observed to solidify to a white crystalline solid (mp = 28 - 29 °C). On the scale described above, the product remained an oil but could be rapidly induced to crystallize by addition of a seed crystal.

Handling and Disposal of Hazardous Chemicals

The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.

These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.

3. Discussion

This procedure illustrates a general method for the preparation of enantioenriched chiral allylic trichloroacetamides from readily available prochiral (E)-allylic alcohols by catalytic asymmetric rearrangement of trichloroacetimidate intermediates.⁴ The rearrangement tolerates a variety of alkyl and Lewis basic substituents at C3 of the starting allylic alcohol; however, substitution at C2 is not permitted (Table 1).⁴^,⁵ Although 5 mol % COP-Cl is convenient to use for small scale reactions, the catalyst loading

can be decreased to as low as 1 mol % if the concentration is simultaneously increased (Entry 3). The reaction conditions exemplified in this example (2 mol % COP-Cl, CH₂Cl₂ (3 M), 38 °C, 24 h) were chosen to insure complete conversion of the allylic imidate intermediate within 24 hours. (Z)-Allylic trichloroacetimidates do not undergo COP-Cl catalyzed rearrangement at a practical rate. However, allylic trichloroacetamides of opposite absolute configuration can be prepared using (R)-COP-Cl, synthesized from (R)-valinol.

The COP-Cl catalyzed transformation of prochiral allylic alcohols to chiral allylic trichloroacetamides is technically simple as allylic trichloroacetimidate intermediates require minimal purification. Additionally, no special precautions are required to protect the rearrangement reaction from light, air or traces of moisture. Moreover, the trichloroacetyl group of the product amide can be readily cleaved or this functional group can be converted to other functional arrays.⁶

amide

entry	R	yield (%)^b	% ee^c/conf

1	n-Pr	99	95/S
2	i-Bu	95	96/S
3^d	i-Bu	92	98/S
4	CH₂CH₂Ph	93	93/S
5	(CH₂)₃OAc	97	92/S
6	(CH₂)₂COMe	98	95^e/S
7	CH₂OTBDMS	98	96/S
8	(CH₂)₃NBn(Boc)	96	95/S

(a) Conditions: 5 mol % (S)-COP-Cl, CH₂Cl₂ (0.6 M), 38°C, 18 h. (b) Duplicate experiments (±3%). (c) Determined by HPLC analysis of duplicate experiments (±2%). (d) 1 mol % (S)-COP-Cl, CH₂Cl₂ (1.2 M). (e) Determined by chiral GC analysis of duplicate experiments (±2%).

References and Notes

Hill, J. G.; Sharpless, K. B.; Exon, C. M.; Regenye, R. Org. Syn., Coll. Vol. 7, 461.
(a) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518–1520. (b) http://www.glasscontour.com/ or http://www.mbraunusa.com/.
(a) Overman, L. E. J. Am. Chem. Soc. 1976, 98, 2901–2910. (b) Bongini, A.; Cardillo, G.; Orena, M.; Sandri, S.; Tomasini, C. J. Org. Chem. 1986, 51, 4905–4910.
Anderson, C. E.; Overman, L. E. J. Am. Chem. Soc. 2003, 125, 12412–12413.
(a) Overman, L. E. Acc. Chem. Res. 1980, 13, 218–224. (b) Overman, L. E. Angew. Chem., Int. Ed. Engl. 1984, 23, 579–586.
For examples, see: (a) Cardillo, G.; Orena, M.; Sandri, S. J. Chem. Soc., Chem. Commun. 1983, 1489–1490. (b) Nagashima, H.; Wakamatsu, H.; Itoh, K. J. Chem. Soc., Chem. Commun. 1984, 652–653. (c) Atanassova, I. A.; Petrov, J. S.; Mollov, N. M. Synthesis 1987, 734–736. (d) Yamamoto, N.; Isobe, M. Chem. Lett. 1994, 2299–2302. (e) Urabe, D.; Sugino, K.; Nishikawa, T.; Isobe, M. Tetrahedron Lett. 2004, 45, 9405–9407.

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

trans-2-Hexen-1-ol:
2-Hexen-1-ol, (2E)-; (928-95-0)

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU):
Pyrimido[1,2-a]azepine, 2,3,4,6,7,8,9,10-octahydro-; (6674-22-2)

Trichloroacetonitrile; (545-06-2)

(E)-2,2,2-trichloroacetimidic acid hex-2-enyl ester:
Ethanimidic acid, 2,2,2-trichloro-, (2E)-2-hexenyl ester; (51479-70-0)

(S)-COP-Cl:
Cobalt, bis[1,1',1",1'''-(η⁴-1,3-cyclobutadiene-1,2,3,4-tetrayl)tetrakis[benzene]](di-μ-chlorodipalladium)bis[μ-[(1-η:1,2,3,4,5-η)-2-[(4S)-4,5-dihydro-4-(1-methylethyl)-2-oxazolyl-κN3]-2,4-cyclopentadien-1-yl]]di-; (581093-92-7)

Notes

References/EndNotes

Article Compounds

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