Alkenyl, Allyl, Vinyl, Alkynyl and Related Derivatives

The asymmetric cyclisation of achiral olefinic organohthium reagents by a stereogenic alkah metal centre can be modulated by ( )-sparteine, which confers enantiofacial selectivity on the reaction such that the anionic cyclisation process discriminates between the enantiotopic faces of an unactivated C=C bond. Recently, modifications have been made to the well known hthium-ene cyclisation reaction whereby the subsequent expulsion of a thiophenoxide group yields a fused vinylcyclopropane. Moreover, allylic lithium oxyanion-induced reactivity and stereoselectivity in this intramolecular carbometallation has been demonstrated in the highly stereoselective synthesis of a natural bicyclo[3.1.0] hexane.  

Cryoscopy and NMR spectroscopy have been used to probe the structures of sulfur-stabilised allyllithium species in solution. In THE endo-monomers predominate, but it is also suggested that the structures are dependent on the oxidation state of sulfur. Hence, 1-thiophenylallyllithiumexhibits an equilibrium between two q -bonded complexes with the Ca-Li contact ion pair (CIP) being preferred, whereas the sulfoxide analogue reveals both static, THF-bonded q Ca-Li and q Cy-Li CIPs and for the lithiosulfone several solvated conformations are in rapid equilibrium.  

The mechanism of addition of allylzinc reagents to vinylmetallics (including vinyllithium) has been probed using DFT calculations with both the metallo-ene and metallo-Claisen processes being studied. Finally, an ab initio analysis of the structures of lithium and sodium pentadienyl points to the stability of a U-shaped structure which maximises metal centre coordination.  

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