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A Nuclear Magnetic Resonance Spectroscopic Technique for the Characterization of Lithium Ion Pair Structures in THF and THF / HMPA Solution 1
| Content Provider | Semantic Scholar |
|---|---|
| Author | Reich, J. Joseph Borst, Piet Dykstra, Robert Richard Patrick, D. |
| Copyright Year | 2001 |
| Abstract | Lithium cations coordinated by HMPA undergo sufficiently slow dynamic exchange on the NMR time scale at low temperature that distinct cation-HMPA complexes can usually be observed. This observation is the basis for a new technique to quickly and unambiguously determine the ion pair structure of lithium reagents in THF and THF/HMPA solution using low-temperature 7Li and 31P NMR. In this paper we describe the procedures for assigning structures to HMPA-complexed separated and contact lithium cations. The application of this technique to ion pair structure determination of several lithium reagents is also presented. The HMPA titrations of fluorenyllithium, trityllithium, and lithium triphenylmercurate (PhjHgLi) in THF solution give similar results. All are solvent-separated ion pairs in THF. The progressive coordination of four HMPA molecules to the lithium cation can be observed in the 'Li and 31P NMR through characteristic chemical shifts and ' J ~ i p scalar coupling. Under the same conditions LiI, LiBr, and MeSeLi are contact ion pairs and become almost completely solvent-separated upon addition of 3 equiv of HMPA. Our results show that MeSLi and LiCl are aggregated in THF, and the addition of HMPA not only breaks down the dimers to monomers but also causes ion separation on addition of 6 equiv of HMPA. Phenyllithium can be deaggregated from dimer to monomer, but even a high concentration of HMPA fails to cause significant ion pair separation.. The tetrameric methyllithium, on the other hand, undergoes no visible dissociation to dimers and monomers on addition of HMPA; only coordination of the four corners of the methyllithium tetrahedron with HMPA is observed. Understanding the solution structure of lithium reagents is important for effectively rationalizing and predicting their reactivity and/or selectivity. In nonpolar and weakly polar solvents (hydrocarbons, diethyl ether) aggregation phenomena play an important role in determining reactivity. However, in the more polar media commonly used (THF and mixtures of it with more strongly coordinating solvents), many organolithium reagents, particularly those with one or two carbanion stabilizing groups, have become largely deaggregateda2 Under these conditions reactivity is dominated by details of ion pair structure. By this we mean the coordination of lithium by the solvent or solvent additives and the contact or solvent-separated ion pair dichotomy (CIP/SIP). In particular, strongly coordinating ligands will weaken coordination between lithium and the counterion and increase the anionic reactivity of the co~nter ion .~ Eventually a solvent-separated ion pair forms with greatly increased4a or substantially modified4b reactivity. (1 ) (a) Reich, H. J.; Green, D. P.; Phillips, N. H. J. Am. Chem. Soc. 1989, 111.3444. (b) Reich, H. J.; Green, D. P. J. Am. Chem. Soc. 1989,111,8729. (c) Reich, H. J.; Green, D. P.; Phillips, N. H. J. Am. Chem. SOC. 1991 , 113, 1414. (d) Green, D. P. Ph.D. Dissertation, University of Wisconsin-Madison, 1989. (e) Reich, H. J.; Borst, J. P. J. Am. Chem.Soc. 1991,113,1835. Reich, H. J.; Dykstra, R. R. J. Am. Chem. SOC. 1993,115, 7041. ( f ) Dykstra, R. R.; Reich, H. J. Unpublished results. (9) Reich, H. J.; Green, D. P.; Phillips, N. H.; Borst, J. P. Phosphorus Sulfur 1992, 67, 63. (h) Reich, H. J.; Gudmundsson, B. 0.; Dykstra, R. R. J. Am. Chem. SOC. 1992,114,7937. (i) Reich, H. J.; Phillips, N. H. J. Am. Chem. SOC. 1986, 108, 2102. (2) The following lithium reagents are partially or completely monomeric in THF: phenyllithium,'8,C,28 sec-butyllithium,& tert-butyllithium,2P (phenyldimethylsilyl)lithium,2b (diphenylphosphinomethyl)lithium,2c and 7-norbornadienyllithium:x (a) Bauer, W.; Winchester, W. R.; Schleyer, P. v. R. Organometallics 1987,6, 2371. (b) Edlund, U.; Lejon, T.; Venkatachalam, T. K.; Buncel, E. J. Am. Chem. SOC. 1985, 107, 6408. (c) Fraenkel, G.; Winchester, W. R.; Williard, P. G. Organometallics 1989, 8, 2308. (d) Goldstein, M. J.; Wenzel, T. T. Helu. Chim. Acta 1984, 67, 2029. ( 3 ) Bartlett, P. D.; Goebel, C. V.; Weber, W. P. J. Am. Chem. SOC. 1969, 91, 7425. ( 4 ) (a) Hogen-Esch, T. E. Adu. Phys. Org. Chem. 1977, 15, 1 5 3 . (b) Cohen, T.; Abraham, W. D.; Myers, M. J. Am. Chem. SOC. 1987,109,7923. Brown, C. A.; Yamaichi, A. J. Chem. SOC., Chem. Commun. 1979, 100. 0002-7863/93/1515-8728$04.00/0 Reich, Green, and Phillips,la~b in connection with NMR studies of the metal-halogen exchange,Icvd and independently Snaith and co-worker~,~ as part of their extensive series of crystal structures of HMPA complexes, reported the first lithium-phosphorus ('J~i-p) coupling for an HMPA-complexed lithium cation in THF. This observation provides valuable direct evidence for solution structures of organolithium species since it enables determination of the number of HMPA molecules attached to lithium. Recently we have reported the application of this NMR spectroscopic technique to carbanions which are solvent-separated and contact ion pairs in THF and have identified the ion separation process for 2-lithio-2-(phenyldimethylsilyl)1 ,3-dithiane.le The technique has also been used to study solvation and reactivity of lithium amides6 and lithium phenoxide^.^ This paper expands on the procedures we use for making assignments of structure to HMPAcomplexed contact and separated lithium cations and applies the technique to several lithium reagents. The observation of slow exchange on the NMR time scale between HMPA and its complexes with metal ions,s as well as J-coupling between the phosphorus of HMPA and a coordinated metal ion, is precedented9 and has been used to provide structural and mechanistic information about metals more Lewis acidic than lithium, such as aluminum and zinc. The present study explores the coordination chemistry of HMPA and lithium cations in a variety of compounds. ( 5 ) (a) Barr, D.; Doyle, M. J.; Mulvey, R. E.; Raithby, P. R.; Berd, D.; Snaith, R.; Wright, D. S. J. Chem. Soc., Chem. Commun. 1989, 318. (b) Raithby, P. R.; R d , D.; Snaith, R.; Wright, D. S. Angew. Chem., Int. Ed. Engl. 1991,30, 101 1 . (c) Barr, D.; Clegg, W.; Mulvey, R. E.; Snaith, R. J. Chem. Soc., Chem. Commun. 1984, 79. (6 ) Romesberg, F. E.; Gilchrist, J. H.; Harrison, A. T.; Fuller, D. J.;Collum, D. B. J. Am. Chem. SOC. 1991,113,5751. Romesburg, F. E.; Collum, D. B. J. Am. Chem.Soc. 1992, 114, 2112. (7 ) Jackman, L. M.; Chen, X . J. Am. Chem. SOC. 1992, 124, 403. ( 8 ) Tkaczuk, M. N.; Lincoln, S. F. Aust. J. Chem. 1980, 33, 2621. (9 ) Delpuech, J.-J.; Khaddar, M. R.; Peguy, A. A.; Rubini, P. R. J. Am. Chem. Soc. 1975,97,3373. Wharf, I.; Onyszchuk, M. J. Organomet. Chem. 1980, 190, 417. |
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| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
