16725-79-4Relevant articles and documents
Enantioselective synthesis of α-phenyl- and α-(dimethylphenylsilyl)alkylboronic esters by ligand mediated stereoinductive reagent-controlled homologation using configurationally labile carbenoids
Barsamian, Adam L.,Wu, Zhenhua,Blakemore, Paul R.
, p. 3781 - 3786 (2015/03/30)
Chain extension of boronic esters by the action of configurationally labile racemic lithium carbenoids in the presence of scalemic bisoxazoline ligands was explored for the enantioselective synthesis of the two title product classes. Enantioenriched 2° carbinols generated by oxidative work-up (NaOOH) of initial α-phenylalkylboronate products were obtained in 35-83% yield and 70-96% ee by reaction of B-alkyl and B-aryl neopentyl glycol boronates with a combination of O-(α-lithiobenzyl)-N,N-diisopropylcarbamate and ligand 3,3-bis[(4S)-4,5-dihydro-4-isopropyloxazol-2-yl] pentane in toluene solvent (-78 °C to rt) with MgBr2·OEt2 additive. Enantioenriched α-(dimethylsilylphenylsilyl)alkylboronates were obtained in 35-69% yield and 9-57% ee by reaction of B-alkyl pinacol boronates with a combination of lithio(dimethylphenylsilyl)methyl 2,4,6-triisopropylbenzoate and ligand 2,2-bis[(4S)-4,5-dihydro-4-isopropyloxazol-2-yl]propane in cumene solvent (-45 °C to -95 °C to rt). The stereochemical outcome of the second type of reaction depended on the temperature history of the organolithium·ligand complex indicating that the stereoinduction mechanism in this case involves some aspect of dynamic thermodynamic resolution. This journal is
Chelation of 2-substituted-1-lithoxides: Structural and energetic factors of relevance to synthetic organic chemistry
Nichols, Michael A.,McPhail, Andrew T.,Arnett, Edward M.
, p. 6222 - 6233 (2007/10/02)
A number of lithium 2-(methylamino)-, 2-(dimethylamino)-, 2-methoxy-, and 2-(isopropylthio)-substituted-1-phenyl-1-propoxides were studied as models for asymmetric synthetic strategies for which lithium chelation between two electronegative atoms has frequently been invoked. The heats of formation of these alkoxides were determined by deprotonating the alcohols with lithium bis(trimethylsilyl)amide in a solution calorimeter. Aggregation numbers for the substituted alcohols and their corresponding lithium alkoxides were obtained with freezing point depression and vapor pressure osmometry in THF, benzene, and dioxane. In several cases, solution structures were obtained through 1H, 6Li, and 2D 6Li-1H NOE (HOESY) NMR spectroscopy. Solid-state structures of lithium (+)-N-methylpseudoephedrate and (-)-N-methylephedrate (+)-N-methylpseudoephedrate and (-)-N-methylephedrate (as the benzene solvate) were obtained by X-ray crystallography, and both were found to be present as tetramers in which the dimethylamino nitrogen atoms were coordinated to the lithium cations to form five-membered chelate rings. The lithium alkoxides were either tetramers or hexamers in nonpolar solvents; however, the alkoxides' solution structures were very complex in THF as evidenced by several 6Li resonances observed in the 6Li NMR spectra at low temperatures. Intramolecular lithium chelation was found to occur in each alkoxide in dioxane and benzene. The enthalpies of chelational stabilization were estimated by comparing their heats of deprotonation with those of nonchelatable 2-alkyl-substituted analogues. The stabilization enthalpies ranged from 5 to 11 kcal/mol per alkoxide molecule.
