Molecular lattice fragment of LiI. Crystal structure and ab initio calculations of [LiI(NEt3)]4

Lithium hexamethyldisilazide-mediated enolizations: influence of triethylamine on E/Z selectivities and enolate reactivities

Lithium hexamethyldisilazide (LiHMDS) in triethylamine (Et 3N)/toluene is shown to enolize acyclic ketones and esters rapidly and with high E/ Z selectivity. Mechanistic studies reveal a dimer-based mechanism consistent with previous studies of LiHMDS/Et 3N. E/ Z equilibration occurs when <2.0 equiv of LiHMDS are used. Studies of the aldol condensation and Ireland-Claisen rearrangement of the resulting Et 3N-solvated enolates show higher and often complementary diastereoselectivities when compared with analogous reactions in THF. The Et 3N-solvated enolates also display a marked (20-fold) acceleration of the Ireland-Claisen rearrangement with evidence of autocatalysis. A possible importance of amine-solvated enolates is discussed.

Synthesis and Characterization of Low-Coordinate Divalent Aryl Transition-Metal Halide Analogues of Grignard Reagents: Precursors for Reduction to Metal−Metal-Bonded Complexes

The synthesis and characterization of the series of divalent first-row aryl transition-metal(II) halide compounds [Cr(mu-Cl)Ar']2 (1) and (Li(OEt2)Ar'MI2]2 (M=Mn (2), Fe (3), and Co (4); Ar'=C6H3-2,6-(C6H3-2,6-iPr2)2) are described. 1-4 were prepared by the addition of one equiv of Ar'Li to the respective transition-metal dihalides. They were characterized by UV-vis spectroscopy, magnetic measurements, and by X-ray crystallography. In dimeric 1, each chromium center has quasi-four-coordinate, square-planar geometry, in which the metal is terminally bound to a terphenyl ligand through the ipso carbon of the central ring and to two bridging chloride ligands. There is a further interaction between chromium and an ipso carbon from one of the flanking -C6H3-2,6iPr2 rings. In contrast, for the iodo derivatives 2-4, LiI is not eliminated upon addition of LiAr' to MI2. Instead, the diethyl ether solvated adducts, [Li(OEt2)Ar'MI2]2 (M=Mn (2), Fe (3), or Co (4)) were isolated. These possess a distorted cubane Li2M2I4 core, in which the lithiums are bound to an ether and the transition metals are bound to a terphenyl group. Magnetic measurements between 2 and 300 K reveal the expected weak antiferromagnetic exchange coupling in each of the complexes.