Polar Organometallic Reagents: an Evergreen Tool for Tuning the Reactivity of Unsaturated Systems

The advice of professor Dieter Seebach “Thinking of polar organometallics compounds as carbanions is an impoverishment rather that a simplification” (ISCC, Durham 1984), perfectly fits the statement of professor Manfred Schlosser, who coined the term polar organometallic”, according which no difference in reactivity patterns of organometallic reagents can be rationalized without taking into account the metal counterion and its specific interactions with the accompanying carbon backbone. Both statements stress the need of considering the metal as a key component of these reagents, that can initiate and govern the subsequent reactions. The class of the socalled polar organometallics comprises the reagents of the most common alkali and alkaline earth metals and extend into the area of organozinc derivatives. All these reagents are characterized by a polar carbon-metal bond and the pattern of polarity vary significantly with the metal. Metalation is one of the widely used synthetic routes in order to obtain a labile and reactive carbon-metal bond from the relatively inert carbon-hydrogen bond. Until recently, alkyllithium reagents and bulky lithium amides have been commonly employed for this purpose. More recently, some groups have pioneered alternative metalating agents made up of two or more distinct compound types. Several examples include Schlosser's superbases, Kondo and Uchiyama's 2,2,6,6-tetramethylpiperidide (TMP)-zincate complexes, and Knochel's turbo-Grignard reagents. In this review, we wish to report the most recent contributions appeared in the literature from 2005 to date, concerning the use of polar organometallics in synthesis. The topic has been divided into three sections, the first of which deals with metalated 1,2-dienes and the second one with metalated 1,3-dienes (the metal is in turn lithium, potassium or magnesium). The third section is dedicated to miscellaneous applications that cannot be comprised into the previous classification.