Consequences of correlated solvation on the structures and reactivities of RLi-diamine complexes: 1,2-addition and alpha-lithiation reactions of imines by TMEDA-solvated n-butyllithium and phenyllithium

6Li and (13)C NMR spectroscopic studies were carried out on [(6)Li]n-BuLi and [(6)Li]PhLi (RLi) in toluene-d(8) containing the following diamines: N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetraethylethylenediamine, 1,2-dipyrrolidinoethane, 1,2-dipiperidinoethane, N,N,N',N'-tetramethylpropanediamine, trans-(R,R)-N,N,N',N'-tetramethylcyclohexanediamine, and (-)-sparteine. Dimers of general structure (RLi)(2)S(2) (S = chelating diamine) are formed in each case. Treatment of RLi with two different diamines (S and S') affords homosolvates (RLi)(2)S(2) and (RLi)(2)S'(2) along with a heterosolvate (RLi)(2)SS'. Relative binding constants and associated free energies for the sequential solvent substitutions are obtained by competing pairs of diamines. The high relative stabilities of certain heterosolvates indicate that solvent binding to the RLi dimer can be highly correlated. Rate studies of both the 1,2-addition of RLi/TMEDA to the N-isopropylimine of cyclohexane carboxaldehyde and the RLi/TMEDA-mediated alpha-lithiation of the N-isopropylimine of cyclohexanone reveal monomer-based transition structures, [(RLi)(TMEDA)(imine)], in all cases. The complex relationships of solvent binding constants and relative reactivities toward 1,2-additions and alpha-lithiations are discussed.

Solution structures of the mixed aggregates derived from lithium acetylides and a camphor-derived amino alkoxide

Low-temperature (6)Li, (13)C, and (15)N NMR spectroscopies reveal that mixtures of lithium cyclopropylacetylide or lithium phenylacetylide (RCCLi) and a vicinal amino alkoxide derived from camphor (R*OLi) in THF/pentane afford an asymmetric (RCCLi)(3)(R*OLi) mixed tetramer and a C(2)-symmetric (RCCLi)(2)(R*OLi)(2) mixed tetramer depending on the stoichiometries. The corresponding (RCCLi)(R*OLi)(3) mixed tetramer is not observed. R*OLi-mediated additions of PhCCLi to benzaldehyde proceed with up to an 8:1 enantiomeric ratio that depend on both the choice of R*OLi and the PhCCLi/R*OLi stoichiometries. The results are considered in light of a previously proposed mechanism for the 1,2-addition to a trifluoromethyl ketone.

NMR spectroscopic investigations of mixed aggregates underlying highly enantioselective 1,2-additions of lithium cyclopropylacetylide to quinazolinones

The solution structures of mixed aggregates derived from lithium alkoxides and lithium acetylides were investigated as part of a program to develop practical syntheses of quinazolinone-based nonnucleoside reverse transcriptase inhibitors. Low-temperature (6)Li, (13)C, and (15)N NMR spectroscopies reveal that mixtures of lithium cyclopropylacetylide (RCCLi), a (+)-carene-derived amino alkoxide (ROLi), and lithium hexamethyldisilazide (LiHMDS) in THF/pentane afford a (RCCLi)(3)(ROLi) mixed tetramer, a C(2)-symmetric and asymmetric (RCCLi)(2)(ROLi)(2) mixed tetramer, and a C(3)-symmetric (RCCLi)(ROLi)(3) mixed tetramer. Analogous mixtures of RCCLi/ROLi in Et(2)O and Me(2)NEt also provide 3:1, 2:2, and 1:3 mixed tetramers. The stereochemistry of aggregation is highly sensitive to the medium. The C(2)-symmetric (RCCLi)(2)(ROLi)(2) mixed tetramer is formed in Et(2)O, whereas the asymmetric isomer is formed in Me(2)NEt. LiHMDS in THF is shown to be an efficient proton scavenger without forming LiHMDS-RCCLi or LiHMDS-ROLi mixed aggregates. LiHMDS-RCCLi mixtures form mixed aggregates in Me(2)NEt.

Solution structures and reactivities of the mixed aggregates derived from n-butyllithium and vicinal amino alkoxides

Low-temperature (6)Li, (13)C, and (15)N NMR spectroscopies reveal that mixtures of n-BuLi and (1R,2S)-R'(2)NCH(R)CH(Ph)OLi (ROLi; R = Ph or Me; R'(2)N = pyrrolidino or Me(2)N) in THF/pentane afford a (n-BuLi)(3)(ROLi) (3:1) mixed tetramer and a C(2)-symmetric (n-BuLi)(2)(ROLi)(2) (2:2) mixed tetramer depending on the proportions of the reactants. The corresponding (n-BuLi)(ROLi)(3) (1:3) mixed tetramer is not observed. ROLi-mediated additions of n-BuLi to benzaldehyde proceed with up to 21:1 enantiomeric ratios that depend on the n-BuLi/ROLi stoichiometries. The enantioselectivities are considered in light of a previously posited mechanism involving reaction via the C(2)-symmetric 2:2 mixed tetramer.

Lithium diisopropylamide: oligomer structures at low ligand concentrations

One- and two-dimensional (6)Li and (15)N NMR spectroscopic studies of lithium diisopropylamide (LDA) solvated by substoichiometric concentrations of oxetane, THF, Et(2)O, and diisopropylamine are described. Partially solvated dimers and trimers are identified. Possible benefits of carrying out organolithium chemistry at low ligand concentrations are discussed.