[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.
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
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
Philipps Universität Marburg, Germany