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1.
J Am Chem Soc ; 135(10): 4103-9, 2013 Mar 13.
Article in English | MEDLINE | ID: mdl-23413774

ABSTRACT

The lithium enolate of tert-amylacetate solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA) is shown to be a doubly chelated dimer. Adding the dimeric enolate to 4-fluorobenzaldehyde-N-phenylimine affords an N-lithiated ß-amino ester shown to be monomeric using (6)Li and (15)N NMR spectroscopies. Rate studies using (19)F NMR spectroscopy reveal reaction orders consistent with a transition structure of stoichiometry [(ROLi)2(TMEDA)2(imine)](‡). Density functional theory computations explore several possible dimer-based transition structures with monodentate and bidentate coordination of TMEDA. Supporting rate studies using trans-N,N,N',N'-1,2-tetramethylcyclohexanediamine showing analogous rates and rate law suggest that TMEDA is fully chelated.


Subject(s)
Aldehydes/chemical synthesis , Aza Compounds/chemical synthesis , Ethylenediamines/chemistry , Imines/chemistry , Lithium/chemistry , Organometallic Compounds/chemistry , Aldehydes/chemistry , Aza Compounds/chemistry , Dimerization , Molecular Structure , Organometallic Compounds/chemical synthesis , Solubility
2.
J Am Chem Soc ; 131(36): 13142-54, 2009 Sep 16.
Article in English | MEDLINE | ID: mdl-19702308

ABSTRACT

The method of continuous variation in conjunction with (6)Li NMR spectroscopy was used to characterize lithium phenolates solvated by tetrahydrofuran and 1,2-dimethoxyethane. The strategy relies on the formation of ensembles of homo- and heteroaggregated phenolates. The symmetries and concentration dependencies of the heteroaggregates attest to the aggregation numbers of the homoaggregates. The structurally diverse phenols afford substrate- and solvent-dependent combinations of lithium phenolate monomers, dimers, trimers, tetramers, and pentamers. We discuss the refinement of protocols for characterizing O-lithiated species. Computational studies examine further the substituent and solvent dependencies of aggregation.


Subject(s)
Lithium/chemistry , Phenols/chemistry , Ethyl Ethers/chemistry , Furans/chemistry , Magnetic Resonance Spectroscopy , Models, Molecular , Molecular Structure , Solvents/chemistry
3.
J Am Chem Soc ; 130(51): 17334-41, 2008 Dec 24.
Article in English | MEDLINE | ID: mdl-19090750

ABSTRACT

The solution structures of four enolates derived from beta-amino esters are investigated using (6)Li NMR spectroscopy in conjunction with the method of continuous variation (method of Job). Ensembles of homo- and heteroaggregated enolates are generated by mixing enantiomers of a single enolate (R/S mixtures), opposite antipodes of two different enolates (R/S' mixtures), and the same antipodes of two different enolates (R/R' mixtures). The numbers of observable aggregates and their dependence on the mole fraction of the two enolates confirm the hexamer assignments. Inherent symmetries observable in the (6)Li NMR spectra show the stereochemistry of chelation about the hexagonal drum.


Subject(s)
Chemistry, Organic/methods , Esters/chemistry , Drug Design , Lithium/chemistry , Magnetic Resonance Spectroscopy , Models, Chemical , Models, Molecular , Models, Statistical , Software , Stereoisomerism
4.
J Org Chem ; 73(19): 7743-7, 2008 Oct 03.
Article in English | MEDLINE | ID: mdl-18781812

ABSTRACT

The method of continuous variation was used to characterize lithium enolates, phenolates, carboxylates, and alkoxides solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA). The method relies on characterizing an ensemble of homo- and heteroaggregates using (6)Li NMR spectroscopy. A combination of aggregate counts and symmetries, nearly statistical distributions, and quantitative parametric fits revealed that cyclic dimers are the dominant forms. Nonstatistical distributions favoring heteroaggregated dimers were observed when hindered enolates and carboxylates were mixed with unhindered enolates. Hindered (tertiary) alkoxides form higher aggregates (possibly hexamers), whereas hindered lithium phenolates appear to form TMEDA-solvated monomers.


Subject(s)
Carboxylic Acids/chemistry , Ethylenediamines/chemistry , Hydroxybenzoates/chemistry , Lithium , Dimerization , Magnetic Resonance Spectroscopy , Molecular Structure , Solutions/chemistry
5.
J Am Chem Soc ; 130(14): 4859-68, 2008 Apr 09.
Article in English | MEDLINE | ID: mdl-18336025

ABSTRACT

The method of continuous variation in conjunction with 6Li NMR spectroscopy was used to characterize lithium enolates derived from 1-indanone, cyclohexanone, and cyclopentanone in solution. The strategy relies on forming ensembles of homo- and heteroaggregated enolates. The enolates form exclusively chelated dimers in N,N,N',N'-tetramethylethylenediamine and cubic tetramers in tetrahydrofuran and 1,2-dimethoxyethane.


Subject(s)
Ketones/chemistry , Lithium Compounds/chemistry , Organometallic Compounds/chemistry , Cyclohexanones/chemistry , Cyclopentanes/chemistry , Indans/chemistry , Magnetic Resonance Spectroscopy
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