Lithium chelates, structure

A bis(chelate) structure was found for the closely related germylene [MeC(NPr )2]2Ge, which was also made from GeCl2(dioxane) and 2 equivalents of the lithium amidinate (colorless crystals, 81%). The same synthetic approach was used to make bis(amidinato) metal dichlorides of silicon and germanium in high yields (83-95%). Rapid oxidative addition of chalcogen atom sources (styrene sulfide and elemental Se) to the germylene derivatives resulted in a series... 

The reactants are usually /V-acyl derivatives. The lithium enolates form chelate structures with Z-stereochemistry at the double bond. The ring substituents then govern the preferred direction of approach. 

Selective reduction to aldehydes can also be achieved using /V-methoxy-iV-methyla-mides.49 Lithium aluminum hydride and diisobutylaluminum hydride have both been used as the hydride donor. The partial reduction is believed to be the result of the stability of the initial reduction product. The /V-mcthoxy substituent permits a chelated structure which is... 

Tomioka et al. reported the asymmetric Michael addition of lithium thiolates catalyzed by chiral aminoether 31 (Scheme 8D. 18) [39]. Thus, in the presence of catalytic amounts of 31 (10 mol %) and lithium 2-(trimethylsilyl)thiophenolate 32-Li (8 mol %), thiol 32 (3 equiv.) reacted with a,p-unsaturated esters at -78°C in toluene-hexane solvent to give the Michael adduct with up to 97% ee. In the ahsence of 31, the reaction of thiophenol proceeded in only 0.5% yield at room temperature. A monomeric complex consisting of 31 and lithium is proposed as the key reactive species in this asymmetric reaction. The trimethylsilyl group at the ortho-po-sition of the thiol moiety in 32 contributes to the formation of the stereochemically defined monomeric chelated structure, wherein the lithium cation is coordinated with the three heteroatoms of the tridentate ligand 31. The reactions of acyclic /nmv-a,P-unsaturated esters (R1 = Me, Et, Pr, Bu, Bu, PhCH9 R2 = H) proceeds with high enantioselectivity in... 

The finding that the stereochemistry of such reactions can be drastically affected as shown in Scheme 93, by (i) a solvent change,(ii) addition of macrocyclic polyethers (especially if the reaction is carried out in TMEDA), or (iii) addition of lithium salts has supported the idea that the stereochemistry may be largely governed by cation-carbanion interactions, and more particularly by the ability of Li to form a chelate structure involving the carbanionic carbon and the sulfoxide oxygen (Scheme 89 Scheme 91, entry h, structures 3 and 4). ... 

The lithium enolate is expected to have the (enolate) configuration 147. Chelation is impossible and it adopts the Houk conformation 147a with the H atom on the inside eclipsing the ir-bond. The enormous protected amine forces the allyl bromide to the opposite face. The potassium enolate prefers the chelated structure 148 and the same group directs the allyl iodide to the bottom face. 

Chelated structures analogous to (19) and (20) were first proposed by House and coworkers to explain the increased anti selectivity observed for lithium ketone enolates following addition of ZnCh (equation 30). Heathcock and coworkers determined the rate of equilibration as well as the equilibrium composition for a number of aldolates derived from benzaldehyde and zinc ketone enolates (equation 31). Again, the preference for anti aldolates is in accord with zinc-chelated structures. 

The rates of alkaline hydrolysis of the half-esters, potassium ethyl oxalate, malonate, adipate, and sebacate were studied in the presence of potassium, sodium, lithium. thallium(I), calcium(II), barium(II), and hexamminecobalt(III) ions (106). On the basis of the results obtained, chelate formation between the metal ions and the transition state of the substrate was postulated. In these chelate structures (structures XXXVIII), formally similar to those postulated in the hydrolysis of a-amino esters (26), the metal ion facilitates the attack by the hydroxide ion by positioning it in a suitable manner. The rate of hydrolysis of the oxalate half-ester is greater than that of the malonate, which in turn is greater than that of the adipate. This is in the expected order of the stability of the metal chelates. The order for the rate of hydrolysis of the ethyl oxalate and ethyl malonate is Ca2+ Ba2+ > [Co(NH3)6]3+ > T1+. The hexamminecobalt(III) ion seems to be less effective than expected, since it is too large to satisfy the steric requirements of the chelate structures. The alkali metals were found to have marked negative specific salt effects on the rates of reaction of the adipate and sebacate, but only a small negative salt effect on the hydrolysis of potassium ethyl malonate. 

Apart from the lithium carbanions, derived from secondary 2-alkenyl carbamates, no further types of configurationally stable a-oxyallyllithium derivatives have been reported in the last 15 years. One must conclude that the five-membered lithium chelate ring plays an important role for the stereochemical integrity. This structure has been nicely demonstrated by the X-ray structure analysis of a sparteine complex [158]. 

These outstanding results are connected to the nature of the acetonide, because the lithium counter ion attached to the enolate oxygen can chelate to one oxygen of the acetonide, as shown in Scheme 7.7. The chelated structure 21 was calculated with force field (MM+) and quantum mechanics (PM3) methods, both methods showed minima in favor of the chelated enolate 21 [19]. 

Optically active a,a-disubstituted acetic acids have also been obtained using oxazoline intermediates. The oxazoline can be alkylated via the lithium derivative. Subsequent deprotonation gives a new enolate which is considered to have the chelated structure shown. 

The monolithiated cyclopropene dimer was computed to have an unusual structure with two planar tetracoordinate carbon fragments the planar structure 6 is found to be about 11 kcal mol more stable than the perpendicular form 7 with tetrahedral tetracoordinate carbon. Calculations on model dimers indicated that the planar geometry is even stabilized further by heteroatom substituents at the vinylic Cfi, because of the formation of favorable chelate rings. The solid-state structure of a solvated cyciopropenyllithium dimer reveals the perpendicular geometry with tetrahedral tetracoordinate carbons, but lithium chelation by C/0 and C/N dianions results in dimeric cyclopropenyl structures with a high degree of planarization of the tetracoordinate carbon environments. ... 

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