Organometallics 1995, 14, 1213-1220
1213
Preparation of Ethereal Lithium Dimethylcuprates (Me2CuLi)2 and Me2CuLi.LiI Displaying Narrow Line Width 13C NMR Resonanceda Steven H. Bertz*Jb AT & T Bell Laboratories, Murray Hill, New Jersey 07974
A. Samuel Vellekoop and Robin A. J. SmithlC Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
James P. Snyderld Istituto di Ricerche di Biologia Molecolare P. Angeletti, 00040 Pomezia, Italy Received August 10, 1994@ Halide-free lithium dimethylcuprate (MezCuLi)~ (1)and the Gilman reagent MezCuLi-LiI (2)have been generated from an ether-soluble form of solid MeLi obtained by transmetalation between BuLi and MeI. The 13C NMR spectra of these cuprates show single narrow resonances at -9.25 and -9.28 ppm (Wll2 < 4 Hz), respectively. The preparative procedure can be adapted to allow for 13C enrichment and is thus suitable for mechanistic studies using 13CNMR. Ab initio geometry optimization and subsequent calculation of the cuprate 13Cchemical shifts indicates t h a t the structural environment around the methyl carbons in the two compounds is remarkably similar. Modeling of the LiI-mediated equilibrium between MezCuLi*LiIand (Me2CuLi)z suggests strongly t h a t the Gilman reagent consists primarily of dimer (Me2CuLi)2 and free LiI. The virtually identical I3C NMR shifts observed for 1and 2 are thereby explained, along with the effect of lithium halide on line width.
Introduction Organocuprates have found wide application in many areas of organic synthesis.2a Despite the great variety of carbon-carbon bond forming reactions which can be achieved with these reagents, in many cases the mechanistic details remain undetermined.2b Two reasons for this apparent paucity of mechanistic information are the complex nature of the reagents themselves2c and the difficulty in monitoring the reagents during reactions. A promising approach to solving these problems is the use of NMR spectroscopy to investigate the nature of the reagents in solution, preferably under conditions similar to those employed for maximum synthetic efficiency, and also to directly observe some of the reaction intermediates.2d The preferential use of I3C rather than lH NMR for studies of these species in solution3 is based upon its relatively large natural chemical shift range and the fact Abstract published in Advance ACS Abstracts, January 15, 1995. ( l ) ( a ) New Copper Chemistry, 23. For part 22, see: He, X.; Ruhlandt-Senge, K; Power, P. P.; Bertz, S. H. J . Am. Chem. SOC.1994, 116, 6963. (b) Present address: Lonza Inc., 79 Route 22 East, Annandale, NJ 08801. ( c ) Inquiries concerning MeLi preparation should be addressed to this author. (d) Inquiries concerning computational aspects should be addressed to this author. Present address: Department of Chemistry, Emerson Center for ScientificComputation, Emory University, Atlanta, GA 30322. (2)(a) Reviews: Bertz, S. H.; Fairchild, E. H. Encyclopedia of Reagents for Organic Synthesis; Wiley: New York, in press. Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135. Posner, G. H. An Introduction to Synthesis Using Orgamcopper Reagents; John Wiley & Sons: New York, 1980. Rossiter, B. E.; Swingle, N. M. Chem. Rev. 1992,92,771. (b) Review: Smith, R. A. J.; Vellekoop, A. S. Advances in Detailed Reaction Mechanisms Vol. 3, Coxon, J. M., Ed.; JAI Press: Greenwich, CT, 1994; p 79. (c) For a recent review of organocuprate structures, see: Power, P. P. Prog. Inorg. Chem. 1991,39, 75. (d) For an outstanding recent example, see: Krause, N.; Wagner, R.; Gerold, A. J . Am. Chem. SOC.1994,116, 381. @
that carbon nuclei, being directly attached to the metal site, are more sensitive to electronic perturbations than hydrogen nuclei two or more bonds removed. The problem with this potentially useful approach is related to the low NMR sensitivity of the 13C nucleus, particularly if used at natural abundance. Early work involving the direct NMR investigation of conjugate addition reactions with methyl cuprates demonstrated the formation of cuprate-enoate complexes and also reported a significant broadening of the 13C resonance of the methyl carbon attached to the metal4 A 13C-enriched substrate was necessary in order to obtain publicationquality spectra.4b In order to effectively investigate fundamental cuprate structures and also to accurately monitor the organocopper species during reactions, a methyl cuprate with a narrow line width 13C NMR methyl resonance produced by a route readily amenable to 13C enrichment is highly desirable. Methylcuprates are normally prepared by reaction of 2 equiv of MeLi with a suitable copper(1) salt, typically CUI or CuBr, which affords MeaCuLi-LiI or Me2CuLi-LiBr,respectively. Low-halide lithium dimethylcuprate (Me2CuLi)a (1) is used less often in normal synthetic procedures and has usually been prepared by the reaction of MeLi with methylcopper, (MeCu),, which is an insoluble yellow solid in the usual hydrocarbon and ethereal solvent^.^ We represent pure lithium (3) (a) Bertz, S. H.; Dabbagh, G.; He, X.; Power, P. P. J . Am. Chem. SOC.1993,115, 11640. (b) Bertz, S. H. J.Am. Chem. SOC.1991,113, 5470. (c) Bertz, S. H. J . Am. Chem. SOC.1990,112,4031. (4) (a) Hallnemo, G.; Olsson, T.; Ullenius, C. J. Organomet. Chem. 1985,282, 133. (b) Christenson, B.; Olsson, T.; Ullenius, C. Tetrahedron 1989,45,523. (c) Ullenius, C.; Christenson, B. Pure Appl. Chem. 1988, 60, 57. (5) Gilman, H.; Jones, R. G.; Woods, L. A. J . Org. Chem. 1952,17, 1630.
0276-733319512314-1213$09.00/0 0 1995 American Chemical Society
Bertz et al.
1214 Organometallics, Vol. 14, No. 3, 1995 dimethylcuprate in ether as a dimer (MezCuLi)~, based on the studies of Pearson6and Ashby.’ X-ray structures of 1 and MezCuLi-LiI (2) have not yet been determined. Solvent molecules are undoubtedly coordinated by the Li atoms, based upon analogy with known organocopper structureszc and the observations of Ullenius and Christen~on.~‘ Initial NMR investigations of methylcuprates in diethyl ether at low temperature reported a single lH NMR resonance at -1.16 ppm8*and a single 13C NMR peak ranging from -8.8 t o -9.6 ppm.8b During the course of our studies on cuprate-enone complexes, we found that the 13CNMR spectra of various preparations of ethereal lithium dimethylcuprate generally showed a single methyl resonance at ca. -9.2 ppm in diethyl ether,gbut that the line widths varied dramatically. We now report details of the preparation and characterization of especially pure solutions of (MezCuLi):! and MezCuLi-LiI with extremely sharp 13C NMR spectra. We also describe an improved preparation of ether-soluble MeLi, which is easily adapted to the preparation of (13CH&C~Li*LiI.In addition, we present theoretical calculations which shed considerable light on the experimental data. Results and Discussion
The 13C NMR resonance of MezCuLi.Li1 prepared from commercial “low-halide”MeLi in diethyl ether-dlo is a single broad peak (-9.2 ppm, W1/2 = 15-20 Hz). MezCuLi-LiI prepared initially from commercial MeLi in diethyl ether followed by evaporative solvent removal and replacement with diethyl ether-dlo gave a similar broad resonance (WVZ= 22 Hz). In addition, the lH NMR spectrum of the latter solution displayed several new high-field resonances, which indicated chemical change had taken place during the solvent manipulation.9 Methods for the preparation of low-halide (MezCuLi)~ are beset by several difficulties related t o the preparation of pure methylcopper. Methylcopper was originally obtained by the reaction of a suspension of CUIwith an equimolar amount of MeLi in diethyl ether followed by filtration or decantation of the ethereal LiI ~ o l u t i o n . ~ Fresh diethyl ether was added, and after stirring or shaking, the ether was again removed along with the dissolved salts. The first problem is the thermal instability of methylcopper, which makes the production of byproducts during the precipitation and washing process a significant consideration. Consequently, one cannot wash the organometallic an arbitrary number of times with fresh solvent. Another difficulty arises directly from the methylcopper washing procedure, as there is no convenient method for rapidly determining the amount of LiI removed or the amount of methylcopper lost during manipulation. Subsequent reaction of the methylcopper prepared in this manner with 1 equiv of ethereal lowhalide MeLi gives a solution characterized by a 13C N M R (6)Pearson, R. G.; Gregory, C. D. J.Am. Chem. SOC.1976,98,4098. (7)Ashby, E.C.; Watkins, J. J. J . Am. Chem. SOC.1877,99,5312. (8) (a) House, H. 0.; Respess, W. L.; Whitesides, G. M. J.Org. Chem. 1966,31,3128. (b) House, H. 0.; Chu, C.-Y. J . O g . Chem. 1976,41, 3083. (9)(a) Bertz, S. H.; Smith, R. A. J. J . Am. Chem. SOC. 1989,111, 8276. (b) Vellekoop, A. S.; Smith, R. A. J. J . Am. Chem. SOC. 1994, 116,2902.
methyl carbon resonance at -9.2 ppm with line width Wllz = 10 Hz,only marginally better than the material made without removal of LiI (vide supra). Even with extensive washing at low temperature, significant amounts of iodide (&lo%) are still present, as determined by atomic absorption spectroscopy. We speculate that CUI units are trapped in the 3-dimensional MeCu lattice, since both the starting material (CUI), and the product (MeCu), are very insoluble in ether. These problems with the purification of MeCu ultimately raise questions concerning the exact composition of the low-halide lithium dimethylcuprate subsequently prepared from it. MeCu from a Homogeneous Solution. We therefore desired a preparation of MeCu from homogeneous solution in order to avoid contamination by entrainment. The key to developing such a procedure was House’s observation that MezCuLi*LiIreacts with a-enones to afford MeCu and the corresponding Li enolate.la We have found that the MeCu prepared in this way is pure enough that only two or three washes are required to obtain (Me2CuLi)zwith a very low halide content (
