Cram selectivity in the reaction of 2-phenylpropanal with alkyllithium reagents: Myth and reality

Chiral aldehydes in hydrocarbons: diastereoselective nucleophilic addition, NMR, and CD spectroscopy reveal dynamic solvation effects

Temperature-dependent studies on the diastereoselective nucleophilic addition of n- BuLi to alpha-chiral aldehydes as (S)-O-(t-butyl-dimethylsilyl)lactal, (S)-O-(t-butyl-dimethylsilyl) mandelic aldehyde, and (R)-2-phenylpropanal in n-decane and n-dodecane reveal dynamic solvation phenomena with the presence of inversion temperatures (T(inv)) in the Eyring plots of ln (anti/syn) vs. 1/ T. These dynamic solvent effects were disclosed by temperature-dependent studies of the (13)C NMR, CD, and UV spectra of the starting aldehydes in solution of n-decane and n-dodecane. The concomitant presence of three peculiar temperatures T(CD), T(UV), and T(NMR), whose values are identical and match T(inv), clearly confirms our earlier interpretation of the solvent-dependent nature of T(inv). The inversion temperature, as well as T(CD), T(UV), and T(NMR) represents the interconversion temperature of two different solvation clusters which act as two different supramolecules with different stereoselectivities.

Use of Manganese(II)-Schiff Base Complexes for Carrying Polar Organometallics and Inorganic Ion Pairs

This report concerns the carrier properties of [Mn(acacen)]-derived compounds toward polar organometallics, inorganic ion pairs, and salts. Such properties are the consequence of Mn(II) behaving as a Lewis acid and the O&arcraise;O bite of the bidentate Schiff base ligand toward alkali cations. The starting compounds, which occur in a dimeric form, [Mn(acac-L-en)](2) [L' = CH(2)CH(2) (1); L" = C(6)H(10) (2); L"' = R,R-C(6)H(10) (3)] have been synthesized either via a metathesis reaction from MnCl(2) or using [Mn(3)Mes(6)]. The reaction of 1-3 with lithium organometallics allowed the isolation of [Mn(acac-L-en)(R)Li(DME)] [R = Me, L = L' (4); R = Ph, L = L' (5); R = Mes, L = L' (6); R = Me, L = L" (7); R = Me, L = L"' (8)] as metalated forms, where the alkyl or aryl group is sigma-bonded to Mn(II), while the lithium cation is anchored to the Schiff base ligand. The metalated forms 4-8 react with PhCHO to give the corresponding lithium alkoxide, which remains bound in its ion-pair form to the [Mn(acacen)] skeleton in [Mn(2)(acac-L'-en)(2)Li(2)(OCH(Ph)Me)(2)](n)() (9). The use of 8, which has a chiral bridge across two nitrogen atoms, did not lead to a significant asymmetric induction in the reaction with PhCHO, because of the long separation between the lithium cation and the stereogenic center. The metalated form 4 was able to transfer the methyl group to the nitrile function to give the corresponding lithium-imide which then remains bonded to [Mn(acacen)] as the ion pair in a dimeric structure, as revealed for [Mn(2)(acac-L'-en)(2)Li(2)(DME){N=C(Ph)Me}(2)](n)() (10). Their reaction with 1 appears to depend on the steric bulkiness of the alkyl group in NaOR, resulting in either monomeric adducts, i.e. in [Mn(acac-L'-en)(2,6-Bu(t)(2)C(6)H(3)O)Na(DME)(2)] (11.2DME), or polymeric structures, like in [Mn(acac-L'-en)Na(DME)(&mgr;-OEt)](n)() (13). All the dimeric units reported in this paper show a slight antiferromagnetic coupling between the two Mn(II) assisted by bridging alkoxo groups.