Carbanions coordination geometry

When a methylene group spacer is inserted between the ortho heteroatom and the carbanionic center, the coordination geometry of the anionic center is no longer restricted to be planar for intramolecular chelation to occur. Hence, o-(dimethylaminomethyl)phenyllithium (126) crystallizes from an ether/hex-ane solution as the internal-chelated tetramer (127). This structure is analogous to tetrameric phenylli-thium (119). When an additional dimethylaminomethyl group is substituted at the ortho position as in 2,3,5,6-tetrakis(dimethylaminomethyl)phenyllithium, the aggregate crystallizes as the dimer (128). Hie lithium atoms in both (127) and (128) are coordinated to four other nonlithium atoms this coordination can only be achieved by dimerization and tetramerization respectively. 

One final structure in this series is the bis-TMEDA-solvated bis(phenylethynyl)magnesium species characterized as a monomer (111). ° Note the octahedral geometry of the central magnesium with two axial ethynyl ligands. This series of alkynic structures, (106)-(lil). serves to underscore the unpredictability of carbanion crystal structures. The alkynic carbanions have coordination numbers of one, two or three in these complexes. 

Structural features of the ir carbanion organometallic complexes are discussed here in three sections (1) 7r-carbanion geometry, (2) metal-base coordination, and (3) metal—7r-carbanion geometry. The primary tool used in these studies was single-crystal x-ray crystallography. Se-... 

Metal—Base Coordination and Metal—w-Carbanion Geometry... 

The reactions we are studying are also concerted. A concerted reaction is one that occurs in a single step without any intermediate, while a stepwise process has one or more intermediates. Most of the reactions discussed in Chapters f 0 and 11 are stepwise with intermediates (carbocations, radicals, carbenes, or carbanions), and are thus distinct from pericyclic reactions. Pericyclic and concerted, however, are not inextricably linked. Not all concerted reactions are pericyclic. For example, the Sn2 reaction is concerted, but it is not pericyclic. Pericyclic and concerted really pertain to two very different aspects of a reaction. Pericyclic refers to the geometry of the transition state a cyclic array is required. Concerted refers to a particular kind of reaction coordinate diagram—one without intermediates. The reactions of this chapter are indeed both concerted and pericyclic, but keep clear in your mind the differing implications of these two terms. 

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