Benzaldehyde, diprotonated

Benzaldehyde reacts readily with benzene under superacid conditions to give tri-phenylmethane in high yields.427,428 The reaction is slow in triflic acid, giving a yield of 90% in 24 h, whereas with the strong superacid CF3SO3H2 B(0S02CF3)4 the product is formed instantaneously in 83% yield.427 Experimental evidence supports the involvement of diprotonated benzaldehyde in the reaction [Eq. (5.76)]. Theoretical calculations show that the 44 0,C(aromatic)-diprotonated cation is the reactive intermediate and not the 45 0,0-diprotonated cation ... 

CF3S03H2+B(0S02CF3)4 ] to give triphenylmethane (Scheme 5.28). Experimental evidence supports the involvement diprotonated benzaldehyde in the reaction. Calculations showed (MP2/6-31G7/MP2/6-31G +ZPE level) that the reactive intermediate is the 0,C(aromatic)-diprotonated dication 62 and not the 0,0-diprotonated 63 dication, which is less stable by 20.6kcalmol 1. 

Later, they reported Friedel-Crafts type cyclodehydration of 1,3-diphenyl-1-propanone [35], Pictet-Spengler reaction [36], andphenylation ofbenzaldehyde [37], which was also reported by Olah et al. at the same time (Scheme 2.14) [38]. These reactions proceed through a dicationic intermediate. As dicationic intermediate of benzaldehyde, Shudo proposed 0,0-diprotonated compound, while Olah proposed 0,meta-C-diprotonated benzaldehyde. 

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