[37974-18-8]  · C3H5Cu  · Allylcopper  · (MW 104.62)

(reagent for allyl transfer to enones,2 acid chlorides,3 aldehydes,3 and imines4)

Alternate Name: 2-propenylcopper.

Preparative Methods: prepared by reaction of Allyllithium or allylmagnesium halides and Copper(I) Iodide, or by reaction of allyl halides and activated copper metal.

Handling, Storage, and Precautions: sensitive to air and moisture. Use in a fume hood.

Allylcopper and Lithium Diallylcuprate were initially prepared by House and Fischer in 1969.5 Addition of allylcopper to cyclohexenone occurred in excellent yield, but b-substituted enones resulted only in 1,2-addition products (eq 1).

In a more recent study, Bertz and Dabbagh disclosed that the extent of 1,2-addition in the reaction of allylcopper with cyclohexenone was dependent on the reaction solvent.6 There have been reports on the difficulties associated with regiocontrol in the reactions of allylcopper and lithium diallylcuprate.2,3 Hutchinson and Fuchs speculated that lithium diallylcuprate existed as an equilibrium mixture of bis-h3-allyl and h1-allyl-h3-allyl species.7 Only the h1-allyl species was able to undergo conjugate addition due to the fact that this complex was formally a 16-electron complex, whereas the 18-electron bis-h3-allyl complex could not undergo conjugate addition. Lipshutz and co-workers2 developed a method for the preparation of allylcopper which provided an apparent solution to the problems associated with the reagent generated under the conditions reported by House.5 Preparation of the neutral allylcopper reagent from Allyllithium (obtained from transmetalation of allyltributyltin with butyllithium) and Copper(I) Iodide solubilized by Lithium Chloride resulted in a reagent that undergoes 1,4-addition with sterically hindered enones (eq 2). The conjugate addition reaction required the presence of Chlorotrimethylsilane for reproducible results. The allylcopper reagent could also be obtained from Allylmagnesium Bromide and soluble copper iodide-Lithium Bromide in THF. The Grignard-derived allylcopper reacted with a hindered cyclopentenone to give the 1,4-adduct in very good yield (eq 3).

Lipshutz noted that the allylcopper reagent prepared from soluble copper(I) complexes exhibited atypical reactivity when compared to other less reactive neutral organocopper reagents. 1,2-Addition becomes competitive with 1,4-addition reactions of allylcopper in cases where dialkylcuprate reagents predominately give the 1,4-addition product. Yamamoto and co-workers reported that allylcopper prepared by the method of Lipshutz undergoes conjugate addition to ethyl 4-benzyloxypent-2-enoate, while the same reagent prepared by the method of House did not react (eq 4).8

However, Shultz and Harrington noted that allylcopper prepared by the Lipshutz method did not provide a conjugate addition product in reaction with a 2-amido substituted cyclohexenone.9 Only copper bromide-catalyzed addition of allylmagnesium bromide provided the 1,4-addition product in modest yield. More recently, Rieke and co-workers have reported the direct preparation of allylcopper from Allyl Chloride or allyl acetate using activated copper metal generated from the reaction of Lithium Naphthalenide with soluble Copper(I) Cyanide-lithium bromide.3 Allylcopper formed in this fashion reacted cleanly with acid chlorides to provide ketones in good yields (eq 5).

1,2-Addition products could be obtained with aldehydes and ketones, and 1,4-addition reactions with enones were also realized in the presence of trimethylsilyl chloride. Substituted allyl halides readily produce the corresponding allylcopper derivative. Conjugate addition of the substituted allylcopper reagents occurs exclusively by a-attack of the allyl group (eq 6). The activated copper metal method also allows for the generation of substituted allylcopper reagents bearing remote functional groups such as enones, ketones, esters, nitriles, alkyl chlorides, or carbamates.

Conjugate addition reactions of allylcopper obtained by reaction of allylmagnesium halides with copper(I) salts is also common. Corey and co-workers reported that allylcopper prepared from allylmagnesium chloride and copper iodide added to alkynic esters in a 1,4-fashion to provide the dienyl ester products in good yield.10 Paquette and co-workers demonstrated that the enolate generated by allylcopper addition to an enone could be trapped by addition of trimethysilyl chloride to the reaction (eq 7).11 Crimmins and co-workers have been able to trap the intermediate formed by allylcopper addition to an alkynic ester by an intramolecular acylation reaction (eq 8).12 In this example, the initial allyl addition product isomerizes to the trans-propenyl substituent upon work-up. Displacement reactions of allylcopper obtained from allylmagnesium halides have also been used in an improved synthesis of allyl sulfones.13 Allylcopper has also undergone 1,2-addition to imines to provide homoallylic amines with excellent enantioselectivity (eq 9).4 The high enantioselectivity of the reaction was attributed to the formation of a rigid chelated complex.

Related Reagents.

Lithium Diallylcuprate; Allylmagnesium Bromide-Copper(I) Iodide.

1. (a) Posner, G. H. OR 1972, 19, 1. (b) Posner, G. H. OR 1975, 22, 253. (c) Lipshutz, B. H.; Sengupta, S. OR 1992, 41, 135. (d) Normant, J. F. S 1972, 63. (e) Yamamoto, Y. AG(E) 1986, 25, 947. (f) Posner, G. H. An Introduction to Synthesis Using Organocopper Reagents; Wiley: New York, 1980.
2. Lipshutz, B. H.; Ellsworth, E. L.; Dimock, S. H.; Smith, R. A. J. JACS 1990, 112, 4404.
3. (a) Stack, D. E.; Dawson, B. T.; Rieke, R. D. JACS 1991, 113, 4672. (b) Stack, D. E.; Dawson, B. T.; Rieke, R. D. JACS 1992, 114, 5110.
4. Bocoum, A.; Boga, C.; Savoia, D.; Umani-Ronchi, A. TL 1991, 32, 1367.
5. House, H. O.; Fischer, W. F., Jr. JOC 1969, 34, 3615.
6. Bertz, S. H.; Dabbagh, G. T 1989, 45, 425.
7. Hutchinson, D. K.; Fuchs, P. L. TL 1986, 27, 1429.
8. Yamamoto, Y.; Chounan, Y.; Nishi, S.; Ibuka, I.; Kitahara, H. JACS 1992, 114, 7652.
9. Shultz, A. G.; Harrington, R. E. JACS 1991, 113, 4926.
10. Corey, E. J.; Kim, C. U.; Chen, R. H. K.; Takeda, M. JACS 1972, 94, 4395.
11. Paquette, L. A.; Annis, G. D.; Schostarez, H. JACS 1982, 104, 6646.
12. Crimmins, M. T.; Mascarella, S. W.; DeLoach, J. A. JOC 1984, 49, 3033.
13. Frye, L. L.; Sullivan, E. L.; Cusack, K. P.; Funaro, J. M. JOC 1992, 57, 697.

Russell J. Linderman

North Carolina State University, Raleigh, NC, USA

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