Three-membered rings TMEDA

The crystal structure of 2-lithiobenzofuran (TMEDA) dimer 3 shows that, despite its expected symmetry, only one Li-C-O group forms a three-membered ring <19890M1688>. This arises because one of the benzofuryl groups twists substantially with respect to the other. In the cyclic moiety t/(Lii-Oi) = 2.09(1) A, while in the acyclic moiety (Li2-02) = 3.00(1) A. As in 2, carbon Ci lies asymmetrically between the Li ions, with [Pg.757]

Two structural types can be distinguished for tetrameric alkali metal amides. [ Li(pyrrolidyl) 2(tmeda)]2, 119, associates in the form of the familiar ladder or step-like structure containing three fused four-membered rings. The same central core is present in the related hexanuclear adduct [ Li(pyrrolidyl) 3(pmdta)]2, 125... 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

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