J . A m . Chem. SOC.1985, 107, 5693-5699
n
crown-6)' derivatives perturbation of the carboxylate ligands by the N a + was noted by spectral measurements (Table VI). Concomitantly, more facile loss of the carboxylate ligand was observed during CO, insertion reactions involving these counterions. It is worthy of note here that spectroscopic studies have demonstrated that although the rate of cationic exchange in the NaBPh4/18crown-6/THF system is slow (two 23Naresonances observed in excess Na' a t -7.9 and -17.1 ppm), the exchange rate is highly dependent on the accompanying anion.23e
L o OJ \.
Figure 5. Structure of the cation [Na.l8-crown-6]+. the carbon-carbon bond-forming process was investigated a t high carbon dioxide pressure (500 psi) both as a matter of convenience and in order to minimize subsequent carboxylate displacement reactions which are prevalent in the presence of interacting alkali metal ions (eq 3). As can be readily seen in Table VI1 and M(CO),O,CCH
