Steric effects on Lewis acidity and basicity

As discussed in Section 6.3.6, reactions of a series of substituted pyridines with hydrogen ions show this order of Brpnsted-Lowry base strengths  

This matches the expected order for electron donation by alkyl groups the t-butyl group has counterbalancing inductive and steric effects. However, reaction with larger Lewis acids, such as BF3 or BMes, shows the following order of Lewis basicity  

The larger fluorine atoms or methyl groups attached to the boron and the groups on the ortho position of the substituted pyridines interfere with each other when the molecules approach each other, so reaction with the substituted pyridines is less favorable. Interference is greater with the 2,6-substituted pyridine and greater still for the t-butyl substituted pyridine. This is an example of F strain. 

EXERCISE 6.4 Based on inductive arguments, would you expect boron trifluoride or trimethylboron to be the stronger acid in reaction with NH3 Which of these acids would you expect to be stronger towards 2,6-dimethylpyridine or 2-t-butylpyridine  

The proton affinities of quinuclidine and triethylamine are nearly identical, 983.3 and 981.8 kJ/mol (Table 6.6). When mixed with trimethylboron, whose methyl groups are large enough to interfere with the ethyl groups of triethylamine, the quinuclidine reaction is twice as exothermic as that of triethylamine (-84 vs. -42 kJ/mol). Whether the 

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