Computational study of solvation and stereoselectivity in deprotonation of cyclohexene oxide by a chiral lithium amide

Configurational stability of chiral lithiated cyclopropylnitriles: a density functional study

Chiral, configurationally stable lithiated nitriles would be valuable intermediates for asymmetric carbon-carbon bond-forming reactions. To gain insight into the design of such species, Walborsky's attempted enantioselective deprotonation/trapping reactions of a chiral cyclopropylnitrile were studied computationally up to the MP2(fc)/6-31+G* and B3LYP/6-31+G* levels. Investigation of cyclopropylnitrile/LiNH(2) deprotonation transition structures demonstrated a significant (20-23 kcal/mol) kinetic preference for N-lithiation, and a facile (4-6 kcal/mol barrier) "conducted tour" racemization pathway for the N-lithiated nitrile product. Addition of a model directing group (formyl) to the beta-carbon of the cyclopropyl ring is predicted to significantly favor C-lithiation over N-lithiation, both kinetically and thermodynamically. Thus, chiral beta-Lewis base substituted cyclopropylnitriles may serve as precursors to chiral, configurationally stable organolithium reagents.

Effective computational modeling of constitutional isomerism and aggregation states of explicit solvates of lithiated phenylacetonitrile

We present the first calculations which accurately account for the position of metalation and aggregation state of lithiated nitriles. Solvation is found to be a key determinant of structure. Five known solvates of lithiated phenylacetonitrile were examined computationally to determine the minimum level of theory required to reproduce the observed X-ray and multinuclear NMR structures. In all cases Hartree-Fock 3-21G energies of explicit solvates calculated at PM3 geometries correctly predict the observed N-lithiated constitutional isomer. Selected density functional theory (B3LYP/6-31+G*//PM3) energy calculations reproduce this trend. We also show that 3-21G//PM3 calculations which do not include explicit solvent molecules, or which include water as a model for diethyl ether, may lead to incorrect predictions of the preferred constitutional isomer. 3-21G//PM3 energies also adequately account for observed aggregation states of the TMEDA, diethyl ether, and THF solvates. Finally, calculations of THF-solvated monomers up to the B3LYP/6-31+G*//B3LYP/6-31+G level indicate a significant (6.8 kcal/mol) preference for N-lithiation.

New catalysts for the base-promoted isomerization of epoxides to allylic alcohols. Broadened scope and near-perfect asymmetric induction

Optically active (1S,3R,4R)-3-[N-(trans-2,5-dialkyl)pyrrolidinyl]methyl-2-azabicyclo-[2.2.1]heptanes were evaluated as catalysts for the enantioselective beta-elimination of meso-epoxides. The (2R,5R)-dimethylpyrrolidinyl-substituted catalyst 4 exhibited exceptionally high enantioselectivity and reactivity, and several substrates were rearranged with enantioselectivities of 98-99% ee. In addition, the use of 4 allowed the first successful, true catalytic rearrangement of the difficult substrates cyclopentene oxide (81%, 96% ee) and (Z)-4-octene oxide (80%, 91% ee).

A novel mixed dimer of a norephedrine-derived chiral lithium amide and 2-lithium-1-methylimidazole, and catalytic enantioselective deprotonation of cyclohexene oxide

Improved stereoselectivity has been obtained by using 2-lithium-1-methylimidazole, 2, as a replacement for lithium diisopropylamide (LDA) as a bulk base in catalytic deprotonations. The chiral lithium amide 6 of (1R,2S)-N-methyl-1-phenyl-2-pyrrolidinylpropanamine, 5, has been found to deprotonate cyclohexene oxide 3 in the presence of compound 2 to yield (S)-cyclohex-2-en-1-ol, 4, in 96% ee. Compound 2 is a carbenoid species conveniently generated from nBuLi and 1-methylimidazole, 1. The base 2 has also been found to play a more intimate role in the deprotonation. Investigations by 1H, 6Li and 13C NMR of the 6Li/15N isotopologue 8 of 6 have shown that 6 is homodimeric in THF and that, in the presence of 2, it forms a novel heterodimer 10. This heterodimer is found to be the dominant reagent in the initial state, rather than the homodimer of 6. Computational investigations with PM3 and B3LYP/6-311 + G(d,p) have shown possible structures of the heterodimers, as well as the role of THF and I in the solvation of the dimers. The results are in line with the NMR results. Favoured complexes in the equilibria between homo- and heterocomplexes are also reported.