Metalated ethers

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

In all three diglycolate complexes the M—0(eq) i.e. metal-ether oxygen, distances are longer than the M—0 (carboxylate) distances 200, 201). This is... 

More than 50 macrocyclic crown ethers were synthesised by Pedersen, and many were found to solublise alkali metal salts in non-polar solvents. He isolated 1 1 complexes with metal salts (87) and also showed that if the cation was too large to fit in the central hole, complexes with ratios of 1 2 or 2 3 (metal ether) could be obtained (88). Some of the larger ethers have been shown to complex two metal atoms simultaneously (89). Stability constants in solution are affected by the nature of the anion and the solvent. Both are also important in obtaining crystalline products. 

Both 1 and 2 are stereochemically stable and shelf stable -metalated ethers. Subsequent tin-lithium exchange reaction with bulyllithium occurs as expected with retention of configuration in both cases51. 

Only in exceptional circumstances are aliphatic radicals sufficiently stable to be prepared in solution. Notable amongst those that have been studied is the anion of tetracyanoethylene (Phillips et al., 1960) and the ketyls (Hirota and Weissman, 1960) [(Me3C)2CO] and [Me3C.CO.CHMe2]. The anion of tetracyanoethylene is readily formed in aqueous solution by the action of such mild reducing agents as iodide ion. The ketyls were prepared at low temperature in metal-ether solutions, and are relatively unstable. 

Figure 3.24 Optimal amine and ether donor group orientations. Note that metal-ether interactions are optimal with a planar C2-0-M unit.

Convenient alternatives to direct deprotonation of ethers are tin-lithium exchange [199, 258-261], halogen-magnesium exchange [262], or reductive cleavage of 0,Se-acetals [263, 264], Another synthetic equivalent of a-metalated ethers are (alkoxymethyl)phosphonium salts [265]. 

The separation (A) of the hyperfine lines in the ESR spectra of metal-amine, and metal-ether solutions represents a direct measure of the average s-electron (spin) density of the unpaired electron at the particular metal nucleus (12,156). When this splitting is compared to that of the free (gas-phase) atom, we obtain a measure of the "percent atomic character of the paramagnetic species. The percent atomic character in all these fluid systems increases markedly with temperature, and under certain circumstances the paramagnetic species almost takes on "atomic characteristics (43, 53, 160). Figure 9 shows the experimental data for fluid solutions of K, Rb, and Cs in various amines and ethers, and also for frozen solutions (solid data points) of these metals in HMPA (17). The fluid solution spectra have coupling con-... 

Two conceptually different models have been proposed to explain the temperature dependence of the (metal) hyperfine coupling constant and electronic ge factor in metal-amine and metal-ether solutions. 

Fig. 13. A schematic representation of the multistate and continuum models for metal-amine and metal-ether solutions at (a) high and (b) low temperatures. The multistate picture shown here assumes, for simplicity, two species of atomic character X, and X2. The continuum model suggests a single species with an atomic character ofX(n and X(U> at the extremes of high and low temperatures, respectively. P(X) is a measure of the number of states with a given percent atomic character.

Metal etherates were used to obtain complexes by the ligand-exchange method in a series of transformations (3.102) ... 

The metalated ethers with EWG = acyl are simply etiolates and azaenolates. These allow for intramolecular bridging via the Li+ cation (Scheme 11). Agreement with the experiments would be attained if such bridging was favored on the endo face of the five-membered transition state (70). Indeed, the atom H-2 in the enolate is less sterically hindered in e/ido-(70) vj. exo-(71). In (70), the H-2 is directed towards a small hydrogen atom, while it would be confronted with the larger alkyl group R in (71). 

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