Lithium Tri-t-butylzincate


[63676-98-2]  · C12H27LiZn  · Lithium Tri-t-butylzincate  · (MW 243.72)

(1,4-addition to enones;1 halogen-zinc exchange reactions)

Alternate Name: tri-t-butylzinclithium.

Solubility: sol ether, THF.

Preparative Methods: lithium triorganozincates (R3ZnLi) such as lithium tri-t-butylzincate are prepared by the reaction of RLi (3 equiv) with Zinc Chloride or by the treatment of R2Zn with RLi in ether or THF.1-4

Handling, Storage, and Precautions: like other polar organometallics, lithium tri-t-butylzincate is very sensitive to oxygen and moisture. It is best generated in situ and should be stored at low temperature.

Halogen-Zinc Exchange Reactions.

Lithium tri-t-butylzincate is a highly efficient reagent for performing bromine-zinc exchange reactions with various types of 1,1-dibromoalkanes,5 1,1-dibromoalkenes,6,7 and 1,1-dibromocyclopropanes.8,9 The initial halogen-zinc exchange reaction is followed by a 1,2-migration of the t-butyl group, leading to a zinc reagent which can be trapped with various electrophiles (eqs 1-3).5,6,9 Other primary and secondary lithium triorganozincates behave in a similar way.5-9 Interestingly, the halogen-zinc exchange proceeds with a high stereoselectivity and, in the case of b-disubstituted 1,1-dibromoalkenes, the Br/Zn exchange takes place preferentially at the sterically more hindered bromine atom (eq 4).7 This constitutes an excellent method for the stereospecific preparation of alkenyl halides (eq 5).7

Michael Addition Reactions.

Lithium tri-t-butylzincate undergoes a Michael addition to enones (eq 6).1 A similar reaction is observed with magnesium tri-t-butylzincate (eq 7).4 If mixed trizincates are used, the t-butyl group is transferred with moderate selectivity (eq 8).4 In strong contrast, an n-butyl group is transferred with a high chemoselectivity (eq 9).4,10-12 Related mixed heterozincates, R2(O-t-Bu)ZnMgBr.TMEDA, prepared by the reaction of ZnCl2.TMEDA, t-BuOK, and RMgX (2 equiv), add readily to enones to provide the 1,4-adducts in satisfactory yields. By using chiral TMEDA analog ligands, moderate optical yields (5-14% ee) are obtained.13-15 The addition of Bu3ZnLi to nitrostyrene in the solvent mixture pentane-(S,S)-1,4-dimethylamino-2,3-dimethoxybutane (DDB) affords the Michael adduct in optically active form (eq 10).3

1. Isobe, M.; Kondo, S.; Nagasawa, N.; Goto, T. CL 1977, 679.
2. Seebach, D.; Langer, W. HCA 1979, 62, 1701.
3. Seebach, D.; Langer, W. HCA 1979, 62, 1710.
4. Tückmantel, W.; Oshima, K.; Nozaki, H. CB 1986, 119, 1581.
5. Harada, T.; Kotani, Y.; Katsuhira, T.; Oku, A. TL 1991, 32, 1573.
6. Harada, T.; Hara, D.; Hattori, K.; Oku, A. TL 1988, 29, 3821.
7. Harada, T.; Katsuhira, T.; Oku, A. JOC 1992, 57, 5805.
8. Harada, T.; Hattori, K.; Katsuhira, T.; Oku, A. TL 1989, 30, 6035.
9. Harada, T.; Katsuhira, T.; Hattori, K.; Oku, A. TL 1989, 30, 6039.
10. Kjonaas, R. A.; Hoffer, R. K. JOC 1988, 53, 1433.
11. Watson, R. A.; Kjonaas, R. A. TL 1986, 27, 1437.
12. Kjonaas, R. A.; Vawter, E. J. JOC 1986, 51, 3993.
13. Jansen, J. F. G. A.; Feringa, B. L. TL 1988, 29, 3593.
14. Jansen, J. F. G. A.; Feringa, B. L. CC 1989, 741.
15. Jansen, J. F. G. A.; Feringa, B. L. JOC 1990, 55, 4168.

Paul Knochel

Philipps Universität Marburg, Germany

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