General Papers ARKIVOC 2006 (ix) 265-291
Introduction
Numerous methods exist for the generation of carbanions in organic synthesis, including reactions of a metal or radical anion metal salt with an organic halide, hydrogen-metal exchange, metal-halogen exchange, and transmetallation. In the last category, tin-lithium exchange1-3 has proved to be a reliable and effective method for the generation of a variety of organolithium reagents. In this case, an organostannane is reacted with an organolithium reagent, resulting in the formation of the new organolithium reagent and concomitant formation of a tetraalkylstannane. In general, an equilibrium is established in which the more stable organolithium compound predominates; thus, reactions of alkyllithiums such as n-BuLi with vinyl- and arylstannanes result in effective tin-lithium exchange. The advantages of this method are the rapidity of the reaction even at low temperatures (-100ºC), the absence of potentially reactive byproducts (e.g. alkyl halides in the case of lithium-halogen exchange), and the variety of synthetic pathways that exist for the preparation of the required organostannane precursors.
This paper presents an overview of recent applications of tin-lithium exchange involving vinyl-, aryl- and hetarylstannanes. Due to an excellent review of this chemistry covering the literature up through 1984,4 the focus of this overview will be on reports published after this date.
1. Vinylstannanes In 1959, Seyferth reported the generation of vinyllithium from the reaction of phenyllithium and tetravinyltin in ethereal solvents (Equation 1),2 where tetravinyltin was prepared from the reaction of a vinylic Grignard reagent with tin tetrachloride:5,6
Et2O
(CH2=CH)4Sn + 4PhLi
4 CH2=CHLi + Ph4Sn (Eq 1)
The preparation of vinyllithium in this manner is well established and has been used numerous times in organic and organometallic synthesis. This synthetic method has also been successfully applied to the preparation of functionalized vinyllithiums, where at least one of the three remaining hydrogen atoms is replaced with a non-hydrogen atom. The syntheses of these substituted compounds are the focus of this section.
1.1 Carbon substituents Perhaps the simplest derivative of vinyllithium is the replacement of one hydrogen atom of the vinylic group with a methyl group. The first syntheses of cis-and trans-propenyllithium via tinlithium exchange were reported in 1964 by Seyferth and coworkers.7 More recently, the cis-
ISSN 1424-6376 Page 266 ©ARKAT
