The element tin has played an increasingly important role in organic chemistry as well as organometallic chemistry, serving as a source of new reagents for selective transformations. The main activity in these fields has been focused for a long time on tin(IV) compounds, and tin(II) compounds have been used primarily as reductants of aromatic nitro compounds to aromatic amines.
During the last decade, generation and reactions of various metal enolates have been extensively studied, and successful applications to the controlled formation of carbon–carbon bonds have been realized under mild conditions. The chemistry of tin(IV) enolates has also been studied and several interesting features of these enolates have been reported, whereas tin(II) analogs were relatively unknown in synthetic organic chemistry, probably because of the lack of general methods for generating them.
In 1979, tin(II) fluoride was employed as a reductant of several α-halocarbonyl compounds and allylic halides to generate tin(IV) species, which subsequently reacted with aldehydes to form new carbon–carbon bonds. When metallic tin was used instead of tin(II) fluoride, similar reactions proceeded smoothly. Because the reducing ability of metallic tin is superior to that of tin(II) fluoride, allyl bromide and α-bromoesters, which could not be reduced by tin(II) fluoride, were easily reduced by metallic tin to generate tin(II) enolates, which in turn reacted with aldehydes to yield homoallyl alcohols or β-hydroxyesters. The latter reaction was the first example of the reaction of tin(II) enolates. More conveniently, tin(II) enolates could be generated directly from ketones by using tin(II) triflate and a tertiary amine under neutral conditions.
While strongly basic conditions are required to prepare lithium enolates, tin(II) enolates can be generated under extremely mild conditions. Tin(II) enolates can behave as interesting chemical species in synthetic reactions that cannot be realized with other metal enolates. One of the characteristic features of tin(II) enolate mediated reactions is a highly enantioselective version employing chiral diamines.
Recent developments in the field of stereoselective aldol reactions have resulted in exploitation of the asymmetric version of this reaction, and several successful methods have been reported using chiral carbonyl compounds and/or chiral enolates. However, the efficiency of these reactions is greatly diminished by the tedious procedures for attachment and removal of the chiral auxiliaries. Thus development of a highly enantioselective aldol reaction between two achiral carbonyl compounds utilizing chiral chelating agents became desirable, though the use of chiral addends in the aldol reaction had not met with much success. Chiral diamines derived from (S)-proline, which are postulated to form rigid cis-fused 5-membered bicyclic structures by chelation to a metal center, were found to be effective ligands for several highly enantioselective reactions. Coordination of a chiral diamine to the metal center of the tin(II) enolate effected highly enantioselective cross aldol and Michael reactions between two prochiral reactants.
This review covers the literatures on tin(II) enolates to the middle of 1991.