The Role of Lewis Acidity

Most working zeolite catalysts are likely to have both Bronsted and Lewis acidity, the latter resulting from aluminium, either as extra-framework cations 

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The final class of reactions to be considered will be the [4 + 2]-cycloaddition reaction of nitroalkenes with alkenes which in principle can be considered as an inverse electron-demand hetero-Diels-Alder reaction. Domingo et al. have studied the influence of reactant polarity on the reaction course of this type of reactions using DFT calculation in order to understand the regio- and stereoselectivity for the reaction, and the role of Lewis acid catalysis [29]. The reaction of e.g. ni-troethene 15 with an electron-rich alkene 16 can take place in four different ways and the four different transition-state structures are depicted in Fig. 8.16. 

The role of Lewis acids in the formation of oxazoles from diazocarbonyl compounds and nitriles has primarily been studied independently by two groups. Doyle et al. first reported the use of aluminium(III) chloride as a catalyst for the decomposition of diazoketones.<78TL2247> In a more detailed study, a range of Lewis acids was screened for catalytic activity, using diazoacetophenone la and acetonitrile as the test reaction.<80JOC3657> Of the catalysts employed, boron trifluoride etherate was found to be the catalyst of choice, due to the low yield of the 1-halogenated side-product 17 (X = Cl or F) compared to 2-methyI-5-phenyloxazole 18. Unfortunately, it was found that in the case of boron trifluoride etherate, the nitrile had to be used in a ten-fold excess, however the use of antimony(V) fluoride allowed the use of the nitrile in only a three fold excess (Table 1). 

Collman JP, Hendricks NH, Kim K, Bencosme CS. 1987. The role of Lewis acids in promoting the electtocatal3dic four-electron reduction of dioxygen. J Chem Soc Chem Commun 1537. 

H-bond energy is weaker in (5.70b) than in (5.70a), apparently due to weaker steric repulsions in the geometry of Fig. 5.38(a). Overall, PtH2 again serves surprisingly well as a mimic for H20 in the role of Lewis acid. 

The role of Lewis acid is to activate the substrate alkenes and initiate an acid-catalyzed hydride transfer from an allylic position. 

The low-temperature reaction of allyl cyanide with the Ni hydrido dimer [NiH(dippe)]2 gives the Ni(0) complex Ni(77 -CH2=CHCH2CN)(dippe), which rearranges to the nickel(ii) allyl(cyano) complex 71 (via C-CN bond activation) and the olefin-isomerization product Ni(77 -CH3CH=GHGN)(dippe) (via C-H bond activation).The kinetics of these bond-activation processes has been studied and the role of Lewis acids in these competitive reactions has been clarified. 

The role of Lewis acidity is particularly clear in the reaction of H-[A1]-MCM-41 with propene (Bonelh B, Garrone E, unpublished results). This reaction was studied to assess whether Bronsted acidity was strong enough to promote polymerization, a reaction clearly visible in the IR. Basically, the result was that, imlike protonic zeolites, H-[A1]-MTS systems do not polymerize propene. Traces of polymerization products observed increased with temperature of pretreatment, thus suggesting that the reaction occurs on Lewis and not on Bronsted sites. 

Hu Z, Metiu H Halogen adsorption on Ce02 the role of Lewis acid—base pairing, J Phys Chem C 116(11) 6664-6671, 2012. 

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