Intra-aggregate exchange

Variable temperature measurements of the enriched samples show that exchange processes are operating and different states of aggregation are observed. It is not clear at this point if inter- or intra-aggregate exchange is dominating. 

A systematic study of the structures of thirteen lithiated hydrocarbons and of eighteen a-halolithium carbenoids in donor solvent (R20, R3N) mixtures down to — 150 °C has been performed by Seebach and coworkers 149,150,151). At room temperature no 13C—6Li coupling was observed in the 13C nmr spectrum because of fast inter- and intra-aggregate exchange processes. At lower temperatures (<—70 °Cj these reactions freeze out thus allowing a correlation of the multiplicities of the signals with the... 

Below 200 K the 13Ci resonances broaden differentially with decreasing temperature, indicating a slowing down of the intra-aggregate C—Li exchange at different rates for different species. 

From temperature-dependent NMR studies it has been shown that uncomplexed diamine 6 exhibits ligand exchange with complexed 6 in (Li-6)2 6 by a dissociative mechanism with AG 228 = 7.8 kcalmol-1. The (Li-6)2 6 chelate undergoes a fast intra-aggregate diamine-amide interconversion via degenerate proton transfer between diamine and amide with AG 268 = 10.9 kcalmol-1 (Scheme 8)39. 

Thus the chiral diamine 6 appears to catalyze intra-aggregate lithium-lithium exchange when coordinated in the complex (Li-6)2 6. 

The processes for the exchange of lithiums in Li-6/(n-BuLi)3 are likely similar to those responsible for the exchange of lithiums in n-BuLi tetramers. The intra-aggregate lithium exchange does not involve dissociation into free dimers (Scheme 10). 

As shown by C, Li coupling constants and multiplicities in the NMR spectra, vinyl-lithium exists in a dimer-tetramer equilibrium in THF. The tetramer undergoes an intra-aggregate C-Li bond exchange process. A comprehensive analysis of the NMR spin-spin coupling constants was made. The Jc c values in vinyl-lithium were determined to be very small,viz 35.0 Hz for the dimer and 36.3 Hz for the tetramer. 

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