Page 376 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 3.29. Relative Lewis Acidity a 357
Acid H kcal/mol b eV c NMR d Relative acidity SECTION 3.7
∗
−6 6 −2 52 1 35 1.00 Catalysis
BCl 3
−25 6 −2 31 1 23 0.91
AlCl 3
−20 0 −2 03 1 15 0.80
C 2 H 5 AlCl 2
+4 1 −1 93 1 17 0.76
BF 3
C 2 H 5 2 AlCl −5 6 −1 82 0 91 0.71
C 2 H 5 3 Al −10 1 −1 68 0 63 0.63
+10 0 −1 58 0 87 0.61
SnCl 4
a. P. Laszlo and M. Teston, J. Am. Chem. Soc., 112, 8750 (1990).
b. As found by MNDO calculation
c. LUMO energy in eV.
d. Change of chemical shift of H(3) in butenal.

less than for covalent bonds between similar elements (see also Section 3.4.4). Some
Lewis acid-base complexes have weak bonds that are primarily electrostatic in nature
+
(e.g., CH CN –BF , 9.1 kcal/mol). 145
−
3
3
There have been several efforts to develop measures of Lewis acid strength.
One indication of Lewis acid strength of a number of compounds commonly used in
synthesis is shown in Table 3.29. The relative acidity values given are derived from
∗
the LUMO level of the orbital of the compound, with the BCl complex defined
3
as 1.00 and the uncomplexed but-2-enal LUMO energy taken as 0. 146 These values
1
correlate with H-NMR chemical shift of the H(3) proton of butenal. 147 In contrast,
the calculated (MNDO) bond strengths H do not correlate with the acid strength.
In another study, the 13 C chemical shift for acetone in complexes with various
Lewis acids was determined (Table 3.30). 148 One general trend that can be noted is
the increasing shift with the formal oxidation state of the metal.
A quite broad range of Lewis acid strengths was evaluated by comparing their
effect on the shift of the fluorescence of complexes with N-methylacridone. 149 The
shifts also correlated with the capacity of the Lewis acid to facilitate the reduction of

Table 3.30. 13 C Chemical Shifts in
Lewis Acid Complexes of Acetone

Lewis Acid 13 Chemical Shift
– 210
221
MgCl 2
227
ZnCl 2
239
Sc O 3 SCF 3 3
245
AlCl 3
250
SbF 5
145 V. Jonas, G. Frenking, and M. T. Reetz, J. Am. Chem. Soc., 116, 8741 (1994).
146 P. Laszlo and M. Teston, J. Am. Chem. Soc., 112, 8750 (1990).
147
R. F. Childs, D. L. Mulholland, and A. Nixon, Can. J. Chem., 60, 801 (1982).
148 D. H. Barich, J. B. Nicholas, T. Xu, and J. F. Haw, J. Am. Chem. Soc., 120, 12342 (1998).
149
S. Fukuzumi and K. Ohkubo, J. Am. Chem. Soc., 124, 10270 (2002).