Lithium pentaalkyl/aryl stannate complexes as intermediates in the lithium—tin exchange reaction

The lithium-tin exchange, a metathesis reaction used for the preparation of organolithium reagents, proceeds by way of intermediate pentorgano "ate" complexes. Under suitable conditions these complexes can be observed spectroscopically. The formation constants of the complexes are dramatically increased by polar coordinating solvents such as THF and HMPA. The "ate" complexes have trigonal bipyramidal structures, and undergo rapid pseudorotation even at very low temperatures. INTRODUCTION The lithium-tin exchange reaction is one of the premier methods for the preparation of organolithium reagents in situations where other less expensive methods such as metallation or the reduction of halides are not selective or mild enough to be effective (ref. 1,2,3). Aryl, vinyl (j, ref. lc,2a), and several kinds of functionalized alkyllithium reagents such as a-amino (2, ref. 2b), a-alkoxy (3, , ref. 2c) and those bearing electrophilic substituents (3, ref. 2d) are frequently made this way. The bistrimethylstannyl butadiene can be converted to either the monolithium reagent 2 or a dilithium reagent (ref. lc). Derivatization of 2 gives a new trimethylstannylbutadiene, which can be exchanged a second time to provide efficient syntheses of a series of 2,3-disubstituted butadienes. Bu3Sn. Me_2_L Li-CH2-OLi Et0Li PhN,iL,) NeLl, -78 L,_nMe3 MgBr2: PhSO2C, Me3Sn SnMe3 Me3S1 SnMe3 Me3Si SPh MeLs. MeSsCl MeLs; Ph2S2 91% 55% /\ C12H2 SeMe Se Se C6H1 CO2H Me2Si SiMe2 H H Examples of Lithium Reagents Prepared Using Li/Sn Exchange Although some kinetic work has been done (ref. 3), the mechanism of this interesting transformation has not been established securely, but has generally been assumed to proceed through a four-centered transition state. An alternative mechanism involving intermediate "ate" complexes has been proposed (ref. la, lb, 2c, 4). R R Sn Li R R: R' R—Sn" L1 R