C-F bond substitution via aziridinium ion intermediates.

Aliphatic 1,2-aminofluorides undergo extremely fast substitution reactions under the influence of lanthanum tris(hexamethyldisilazide). The substitution proceeds via an in situ generated aziridinium ion intermediate, which subsequently undergoes ring opening by addition of a nucleophile, yielding various ß-substituted amines.

Mixed dimer and mixed trimer complexes of nBuLi and a chiral lithium amide.

Multinuclear and multidimensional NMR spectroscopy have shown that lithium (S)-N-isopropyl-O-methyl-valinol (1-[6Li]) exists in a mixed 2:1 complex with nBu[6Li], (1-[6Li])2/nBu[6Li], in non-coordinating solvents such as hexane or toluene. A 6Li,1H-HOESY NMR spectrum indicates that the complex is a cyclic trimer with a large distance between the di-coordinated lithium and the carbanion of nBu[6Li]. Such arrangements are present in the solid state as previously reported by Williard and Sun. The exchange of lithium atoms within the trimer is slow at -33 degrees C. The exchange barrier (deltaG++) was determined to be 14.7 kcal x mol(-1) from quantitative 6Li,6Li-EXSY spectra. Addition of diethyl ether results in the formation of mixed dimers of (1-[6Li])/nBu[6Li], tetramers of nBu[6Li], and homodimers (1-[6Li])2. The apparent equilibrium constant of the mixed dimer was determined from the 6Li NMR integrals as K = 7.

An NMR and computational study of a lithium amide and ethereal ligand exchange

The activation parameters for the exchange of an ethereal ligand in a chiral lithium amide was determined from full bandshape analysis of the dynamic NMR spectra. For diethyl ether, the activation parameters were deltaH++=11.0 kcalmol(-1) and deltaS++=12.0 cal K(-1)mol(-1). The exchange of the tetrahydrofuran ligand proceeds with a similar activation enthalpy deltaH++=11.2 kcalmol(-1); however, the entropy is close to zero. deltaS++=1.6 cal K(-1)mol(-1). The dissociation of the ethereal ligand was also modeled by means of semiempirical (PM3) and density functional (DFT) methods. The experimental 13C NMR chemical shifts for the carbons of uncoordinated and coordinated ethers were calculated with the GIAO-DFT (B3PW91/6-31G(d)) computational method.

Guest exchange in an encapsulation complex: a supramolecular substitution reaction.

Encapsulation complexes are reversibly formed assemblies in which small molecule guests are completely surrounded by large molecule hosts. The assemblies are held together by weak intermolecular forces and are dynamic: they form and dissipate on time scales ranging from milliseconds to days-long enough for many interactions, even reactions, to take place within them. Little information is available on the exchange process, how guests get in and out of these complexes. Here we report that these events can be slow enough for conventional kinetic studies, and reactive intermediates can be detected. Guest exchange has much in common with familiar chemical substitution reactions, but differs in some respects: no covalent bonds are made or broken, the substrate is an assembly rather than a single molecule, and at least four molecules are involved in multiple rate-determining steps.

Isolation and Spontaneous Resolution of Eight-Coordinate Stereoisomers.

The right- and left-handed propeller-shaped enantiomers of the eight-coordinate SmI(2) complexes shown can be resolved by crystallization from dimethoxyethane (dme) at ambient temperature. Apart from representing a new type of chiral metal complex, such enantiomers are potential reagents for enantioselective reductions.