The Ramberg-Bäcklund Reaction

Taylor, Richard J. K.; Casy, Guy


The base-mediated conversion of alpha-halosulfones into region-defined alkenes was first described by Ludwig Ramberg and Birger Backlund. This transformation has proved to be of wide synthetic value and is known as the Ramberg-Backlund reaction (RBR). The facile nature of the RBR is surprising given the difficulties encountered when attempting to carry out nucleophilic substitution reactions on alpha-halosulfones. In contrast to halogens adjacent to carbonyls and related electron-withdrawing groups, polar, steric and field effects appear to combine leading to the marked deactivation of alpha-halosulfones. The stereochemical outcome of the reaction was also unexpected: a predominance of Z-alkenes is often observed with relatively weak bases, whereas stronger bases tend to favor E-alkenes. The mechanism of the RBR was therefore the subject of intense interest, particularly in the 1950s and 1960s.

From the synthetic viewpoint, the RBR is attractive for a number of reasons, for example, the accessibility of the precursor sulfide and sulfones; the conjunctive nature of the process; the unambiguous location of the resulting double bonds, among others. Organic sulfones are readily available, but the preparation of alpha-halosulfones can be problematic. In view of this, perhaps the most significant synthetic advance concerning RBR has been the development of Meyer’s modification, which involves in-situ halogenation-RBR sequence and enables sulfones to be converted directly into alkenes. This chapter discusses the mechanism of RBR, its scope and limitations, applications of the reaction in natural product synthesis, and comparison of the RBR with related procedures. Representative experimental procedures are also given.