Lithium Methyl(phenylseleno)cuprate1


[91365-06-9]  · C7H8CuLiSe  · Lithium Methyl(phenylseleno)cuprate  · (MW 241.60)

(heterocuprate designed for 1,4-addition-elimination reactions2 with aryl b-(phenylseleno)vinyl sulfones3)

Solubility: sol ether, THF.

Analysis of Reagent Purity: an assay to determine relative thermal stabilities4 can be used to estimate reagent quality and concentration: a sample of known volume and temperature is quenched with excess PhCOCl; the yield of acetophenone is measured by GC, precalibrated using authentic product and n-dodecane as internal standard.

Preparative Methods: Phenylselenocopper(I) is prepared by portionwise addition of 1.0 equiv Diphenyl Diselenide to a mechanically stirred, rt solution of 2.0 equiv Sodium Borohydride in 1:1 2 M aqueous NaOH:EtOH. The resultant yellow suspension is heated at reflux under N2 for 30 min; a clear solution results. Cooling to 0 °C and acidification to pH 5-6 using conc HCl is followed by addition of 1.0 equiv Cu2O in one portion. The mixture is heated at reflux under N2 for 18 h. Cooling is followed by filtration and washing of the brownish-yellow precipitate with ethanol and water; drying overnight in vacuo provides the selenide.3

MeCu(PhSe)Li is prepared by stirring a 0 °C suspension of 1.0 equiv PhSeCu in THF under N2 and adding 1.0 equiv of an ether solution of Methyllithium dropwise. After 15 min the resultant clear green-brown solution is cooled to -78 °C and is ready for use.

Handling, Storage, and Precautions: best results will be obtained with PhSeCu prepared from high purity Cu2O, dry, O2-free solvents, and alkyllithium solutions free of contaminating alkoxides or hydroxides.5 MeLi is pyrophoric;6 care must be exercised in its handling. The reagent appears to have thermal stability equal to that of Lithium Methyl(phenylthio)cuprate. Use in a fume hood.

Lithium methyl(phenylseleno)cuprate is a novel representative of the heterocuprate class of organocopper reagents.7 The methyl group acts as a nucleophile, while the SePh group does not. This latter nontransferable or dummy ligand8 provides enhanced thermal stability and solubility compared to the analogous homocuprate. Heterocuprates generally exhibit less nucleophilic reactivity than the corresponding homocuprates; however, increased thermal stabilities coupled with greater efficiency in use of the transferable ligand make them reasonable alternative reagents. This is particularly true when higher reaction temperatures are required, or when the organolithium used to form the cuprate is difficult or expensive to prepare.

Designed for 1,4-addition-elimination reactions with aryl b-(phenylseleno)vinyl sulfones, these cuprates offer specific advantages over the corresponding homocuprates and alkyl(phenylthio)cuprates (eq 1).3 Applications include stereospecific net anti addition of R-H to alkynes via a selenosulfonation/1,4-addition-elimination/reductive sulfurization9 sequence complementary to traditional syn carbocupration,10 and the synthesis of marine sterols.11

Related Reagents.

Lithium t-Butoxy(t-butyl)cuprate; Lithium Dimethylcuprate; Lithium Methyl(phenylthio)cuprate.

1. (a) Lipshutz, B. H.; Sengupta, S. OR 1992, 41, 135. (b) Posner, G. H. An Introduction to Synthesis Using Organocopper Reagents; Wiley: New York, 1980.
2. (a) Perlmutter, P. Conjugate Addition Reactions in Organic Synthesis; Pergamon: New York, 1992. (b) Kozlowski, J. A. COS 1991, 4, 169. (c) Hulce, M.; Chapdelaine, M. J. COS 1991, 4, 237. (d) Chapdelaine, M. J.; Hulce, M. OR 1990, 38, 225. (e) Posner, G. H. OR 1972, 19, 1.
3. Back, T. G.; Collins, S.; Krishna, M. V.; Law, K.-W. JOC 1987, 52, 4258.
4. Bertz, S. H.; Dabbagh, G. CC 1982, 1030.
5. Corey, E. J.; Naef, R.; Hannon, F. J. JACS 1986, 108, 7114.
6. Wakefield, B. J. Organolithium Methods; Academic: New York, 1988; pp 11-15.
7. Posner, G. H.; Whitten, C. E.; Sterling, J. J. JACS 1973, 95, 7788.
8. Lipshutz, B. H. SL 1990, 119.
9. Larock, R. C. Comprehensive Organic Transformations; VCH: New York, 1989; pp 33, 229.
10. Knochel, P. COS 1991, 4, 865.
11. Black, T. G.; Proudfoot, J. R.; Djerassi, C. TL 1986, 27, 2187.

Martin Hulce

Creighton University, Omaha, NE, USA

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