Dimethyl- and Bis[(trimethylsilyl)methyl]cuprates Show
Aggregates Higher than Dimers in Diethyl Ether: Molecular
Diffusion Studies by PFG NMR and Aggregation−Reactivity
Correlations
posted on 2003-01-18, 00:00authored byXiulan Xie, Carsten Auel, Wolfram Henze, Ruth M. Gschwind
The molecular sizes of higher aggregates of dimethylcuprates (Me2CuLi (1), 1·LiI, and 1·LiCN)
and bis[(trimethylsilyl)methyl]cuprates ((Me3SiCH2)2CuLi (2), 2·LiI, and 2·LiCN) in diethyl ether (Et2O) were
determined by pulsed field gradient (PFG) NMR diffusion measurements. The obtained diffusion coefficients
show molecular sizes larger than those of dimers for all systems. In these higher aggregates, steric hindrance
and dilution reduce aggregation, whereas LiCN increases it. The molecular sizes were first determined by
a spherical model-free approach and then refined by structure models of higher aggregates. These models
were built by a combination of diffusion results, known NMR studies, and crystal structures. Thus, polymeric
chains with homodimeric cores connected by solvent (salt-free case) or solvent and salt (salt-containing
case) were proposed. These models were confirmed by a solvation analysis, whereby the number of solvent
molecules attached to the aggregates was determined by a weighted average study. On the basis of these
structure models, the number of repetition units (length index) was determined to be between 1.3 and 5.2,
with the general trends in aggregation independent of the structure model used. A combined analysis of
the determined length indices and known relative reactivities led for the first time to a correlation between
higher aggregation and reactivity of dimethylcuprates in the addition reaction with enones: aggregates
higher than dimers reduce the reactivity. Consequently, despite their consistent homodimeric core structures,
for the first time the remaining reactivity differences between iodo- and cyanodimethylcuprates in Et2O are
explained by the difference in their aggregation.