Lithium Trimethylzincate


[52196-05-1]  · C3H9LiZn  · Lithium Trimethylzincate  · (MW 117.45)

(strong nucleophile; undergoes 1,4-additions to enones;1-6 can be used for performing halogen-zinc exchange reactions8,9)

Solubility: sol ether, THF; reacts with H2O, protic solvents, and oxygen.

Preparative Methods: THF or ether solutions can be prepared by the reaction of Methyllithium (3 equiv) with Zinc Chloride or by the treatment of Me2Zn with MeLi.1-4

Handling, Storage, and Precautions: should be handled under an inert atmosphere; like organolithiums, it may react with THF; best generated in situ and used rapidly.

Michael Additions.

Lithium trimethylzincate, like most lithium or magnesium triorganozincates, undergoes 1,4-additions to enones with high chemoselectivity (see Lithium Tri-t-butylzincate). The nature of the lithium salts present in the reaction mixture strongly influences the yield obtained (eq 1).1 It must also be noted that lithium trimethylzincate is one of the least reactive triorganozincates and that the transfer of a methyl group proceeds generally with moderate yield (eq 2).4 This has been exploited synthetically by preparing mixed alkyldimethylzincates which were shown to transfer selectively the alkyl group in the Michael addition (alkyl = n-Bu, i-Pr, t-Bu, i-Bu, Ph, vinyl, Me2PhSi; eqs 3 and 4).4,5 If the addition of lithium trimethylzincate to cyclohexenone is performed in a chiral solvent like (+)-1,4-dimethylamino-2,3-dimethoxybutane (DDB), the resulting 3-methylcyclohexane is obtained in low enantioselectivity.3 a,b-Unsaturated epoxides react rapidly affording a mixture of SN2 and SN2 substitution products (eq 5).6 Lithium trimethylzincate adds with moderate stereoselectivity to 4-t-butylcyclohexanone, producing a mixture of axial and equatorial alcohols (70:30).7

Halogen-Zinc Exchange Reactions.

Interestingly, lithium triorganozincates can be used for performing halogen-zinc exchange reactions.8,9 1,1-Dibromoalkanes such as (1) react readily with Me3ZnLi at -85 °C, providing the corresponding zinc carbenoid (2) which undergoes 1,2-migration10,11 upon warming to 0 °C to afford the organozinc reagent (3); after protonolysis, this gives the cyclopropane derivative (4) (eq 6).8 The reaction can be extended to acyclic 1,1-dibromoalkanes, thus offering unique homologation possibilities (eq 7).9

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. Kjonaas, R. A.; Hoffer, R. K. JOC 1988, 53, 4133.
6. Hardinger, S. A.; Fuchs, P. L. JOC 1987, 52, 2739.
7. Ashby, E. C.; Chao, L.-C.; Laemmle, J. JOC 1974, 39, 3258.
8. Harada, T.; Hattori, K.; Katsuhira, T.; Oku, A. TL 1989, 30, 6035.
9. Harada, T.; Kotani, Y.; Katsuhira, T.; Oku, A. TL 1991, 32, 1573.
10. Knochel, P.; Jeong, N.; Rozema, M. J.; Yeh, M. C. P. JACS 1989, 111, 6474.
11. Knochel, P.; AchyuthaRao, S. JACS 1990, 112, 6146.

Paul Knochel

Philipps Universität Marburg, Germany

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