[766-04-1]  · C7H7Li  · Benzyllithium  · (MW 98.07)

(benzylating agent in substitutions and additions at carbon; used to attach benzyl groups to other metals and nonmetals)

Physical Data: by X-ray, the DABCO complex forms contact ion pairs with Li over planar benzyl anions2 and the THF-TMEDA complex has pyramidal benzylic carbon atoms.3 UV4 and 1H,5 13C,6 and 7Li7 NMR studies have been reported.

Solubility: sol ethers; DABCO complex crystallizes from hexane as it forms.8

Form Supplied in: prepared in situ and used directly.

Preparative Methods: preparation from benzyl ethyl ether and Lithium in THF/ether is recommended on a large scale; toluene metalation for pure samples.8 Metalation of toluene with n-Butyllithium/TMEDA gives 90-91% yield, but contaminated with 5-6% m-tolyllithium and 2% each of o- and p-tolyllithium;9 metalation of toluene with butyllithium/Potassium t-Butoxide, then reaction of the red solid benzylpotassium with LiBr in ether, gives purer product.10 Preparations from organotin compounds by exchange may be contaminated with organotin compounds.10 A method involving treatment of benzyl selenides with BuLi at -78 °C should be especially useful for benzyllithiums containing halogens or methoxyls.11

Handling, Storage, and Precautions: must be prepared and transferred under an inert gas to exclude oxygen and moisture.

Nucleophilic Substitutions at Carbon.

Resonance-stabilized Li reagents such as benzyllithium are stronger nucleophiles but weaker bases than alkyllithium reagents, permitting displacement of Br- and I- from cyclohexyl and cyclopentyl systems in Wurtz-type couplings (eq 1).12 Optically active secondary halides react with clean inversion.13

Nucleophilic Substitutions at Other Nonmetals.

Cinchonine (an optically active alcohol) can be displaced from phosphinites by benzyllithiums to give optically active phosphines with inversion of configuration (eq 2).14 The stereoselectivity was much greater with sterically hindered benzyl derivatives.

Vinyl azide acts as a NH2+ equivalent in reaction with benzyllithium (eq 3), providing a method for the amination of such compounds.15

Additions to Double Bonds.

Besides the usual additions to aldehydes and ketones, benzyllithium adds to ketenes as shown in eq 4 to give regiospecific enolates which can be used in aldol condensations.16 It also adds to a double bond of cyclooctatetraene and then acts as a base, leading to benzylcyclooctatetraene dianion (eq 5).17

The direction of addition of benzyllithium to vinyl groups depends on the substituents. It adds to the more-substituted carbon of the double bond of allyl alcohol (eq 6),18 but (in the presence of CuI) to the less-substituted carbon of the double bond in the epoxyalkene in eq 7.19

Metal Exchange.

Benzyllithium used in the presence of copper salts may react as lithium dibenzylcuprate.19 Benzyllithium has been used to make benzylzinc20 and rare earth benzyl derivatives.21

Related Reagents.

Table 1 lists some useful benzyllithium derivatives.22-30 See also Benzylmagnesium Chloride.

1. Wakefield, B. J. The Chemistry of Organolithium Compounds; Pergamon: Oxford, 1974.
2. Patterman, S. P.; Karle, I. L.; Stucky, G. D. JACS 1970, 92, 1150.
3. Zarges, W.; Marsch, M.; Harms, K.; Boche, G. CB 1989, 122, 2303.
4. Waack, R.; Doran, M. A. JPC 1964, 68, 1148.
5. Sandel, V. R.; Freedman, H. H. JACS 1963, 85, 2328.
6. (a) Breslow, R.; Schwarz, J. JACS 1983, 105, 6795. (b) Seebach, D.; Hässig, R.; Gabriel, J. HCA 1983, 66, 308.
7. Scherr, P. A.; Hogan, R. J.; Oliver, J. P. JACS 1974, 96, 6055.
8. Eisch, J. J. Organomet. Synth. 1981, 2, 94.
9. Chalk, A. J.; Hoogeboom, T. J. JOM 1968, 11, 615.
10. Schlosser, M.; Hartmann, J. AG(E) 1973, 12, 508.
11. Clarembeau, M.; Krief, A. TL 1985, 26, 1093.
12. Korte, W. D.; Cripe, K.; Cooke, R. JOC 1974, 39, 1168.
13. Sommer, L. H.; Korte, W. D. JOC 1970, 35, 22.
14. Chodkiewicz, W.; Jore, D.; Wodzki, W. TL 1979, 1069.
15. Hassner, A.; Munger, P.; Belinka, B. A., Jr. TL 1982, 23, 699.
16. Häner, R.; Laube, T.; Seebach, D. JACS 1985, 107, 5396.
17. Miller, J. T.; DeKock, C. W.; Brault, M. A. JOC 1979, 44, 3508.
18. Crandall, J. K.; Clark, A. C. JOC 1972, 37, 4236.
19. Araki, S.; Butsugan, Y. CL 1980, 2, 185.
20. Negishi, E. I.; Bagheri, V.; Chatterjee, S.; Luo, F. T.; Miller, J. A.; Stoll, A. T. TL 1983, 24, 5181.
21. Dolgoplosk, E. I.; Tinyakova, E. I.; Guzman, I. S.; Vollerstein, E. L.; Chigir, N. N.; Bondarenko, G. N.; Sharaev, O. K.; Yakovlev, V. A. JOM 1980, 201, 249.
22. (a) Paquette, L. A.; Henzel, R. P.; Wilson, S. E. JACS 1972, 94, 7780. (b) Screttas, C. G.; Micha-Screttas, M. JOC 1979, 44, 713.
23. Clarembeau, M.; Krief, A. TL 1985, 26, 1093.
24. Ludt, R. E.; Crowther, G. P.; Hauser, C. R. JOC 1970, 35, 1288.
25. Kaiser, E. M.; Petty, J. D.; Solter, L. E.; Thomas, W. R. S 1974, 805.
26. Ito, Y.; Kobayashi, K.; Saegusa, T. JACS 1977, 99, 3532.
27. Braun, M.; Ringer, E. TL 1983, 24, 1233.
28. Smith, A. B., III; Visnick, M. TL 1985, 26, 3757.
29. Fitt, J. J.; Gschwend, H. W. JOC 1976, 41, 4029.
30. Chong, J. M.; MacDonald, G. K.; Park, S. B.; Wilkinson, S. H. JOC 1993, 58, 1266.

Robert B. Bates & Sriyani Caldera

University of Arizona, Tucson, AZ, USA

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