Lewis Acidic Properties of Alkali Metal Salt

1 Lewis Acidic Properties of Alkali Metal Salt 

Continuous effort by Righetti et al. was devoted to investigation of the effect of different metal salts on the rate of the Diels-Alder and hetero-Diels-Alder reaction in acetone solution (Sch. 3) [22]. LiClOq, NaC104, Ba(C104)2, and Mg(C104)2 were compared to evaluate the distinctive Lewis acidic nature of each species. When the effect of different cations is compared at the same concentration, Na is 2-3 times less effective than Li, which is 2-3 times less effective than Ba. The outstanding result, however, is that Mg is ca 

10-20 times more efficient than Li. The rate increases with increasing charge density of each cation, because the order of the charge/radius values (Mg Ba w Lr Na ) parallels the order of reactivity. 

An enormous rate acceleration in LPDE was also reported for the ionization of p-methoxyneophyl p-toluenesulfonate [24]. In the absence of LiC104, the rate of the ionization in AcOH k = 11.9 x 10 s at 50 °C) exceeds that in Et20 (0.5 x 10 s at 75 °C) by a factor of 2 x 10 , and is also much faster in other solvents such as DMSO (18.2 X 10 s at 75 C), acetone (0.857 x 10 s at 75 °C), and EtOAc (0.113 X 10 s at 75 °C). Et20 becomes a better ionization medium than AcOFI, however, when concentrations of LiC104 exceed 0.036 m (at 50 °C). Finally, the rate in Et20 is increased by a factor of 10 by 0.1 m LPDE. Similarly, the rate of ionization of spirodienyl p-nitrobenzoate was further examined to account for the remarkable effects of LPDE [25]. 

2 Lewis Acidic Functions in Lithium Amide and Organolithium 

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