Lewis acids, proton sponge reactions

The Lewis acid of choice for most of the cyclization reactions is ethylaluminum dichloride, because of its exceptional properties it is a mild Lewis acid, and, as an organometallic compound, can serve as a proton sponge , and thereby inhibit competitive protodesilyla-tion37. The desired precursors reacted smoothly with 1.1 equivalents of ethylaluminum dichloride in toluene or dichloromethane at low temperature to generate diastereoselectively the desired spiro[4.5]decanones38. 

An interesting bifunctional system with a combination of In(OTf)3 and benzoyl-quinine 65 was developed in p-lactam formation reaction from ketenes and an imino ester by Lectka [Eq. (13.40)]. High diastrereo- and enantioselectivity as well as high chemical yield were produced with the bifunctional catalysis. In the absence of the Lewis acid, polymerization of the acid chloride and imino ester occurred, and product yield was moderate. It was proposed that quinine activates ketenes (generated from acyl chloride in the presence of proton sponge) as a nucleophile to generate an enolate, while indium activates the imino ester, which favors the desired addition reaction (66) ... 

In rare cases the proton sponge can react with Lewis acids as a monodentate ligand. Thus, reaction of 1 with highly electrophilic borane B(C6F5)3 in CD2C12 led to the... 

The 1/BQ system alone or with Lewis acid additives was also employed for asymmetric catalysis in the synthesis of /3-amino acids235. During an elaboration of the tandem catalytic asymmetric chlorination/esterification process, Lectka and coworkers found that proton sponge 1 competes with ketenes in the reaction with halogenating agents, such... 

Some applications of Proton Sponge (1) for pericyclic reactions have been reported. Intramolecular Diels-Alder (IMDA) reaction of triene proceeded under thermal conditions to give bicyclic compound. In this reaction, the yield of the IMDA reaction for the Lewis acid labile substrate was significantly improved to 57% in the presence of a catalytic amount of Proton Sponge (1) (0.3 equiv.) [28] (Scheme 8.9). A similar effect was observed in the IMDA reaction of ester derivative. Whitney et al. conducted the IMDA reaction of acrylate... 

Hossain et al have already found that the cyclopentadienyl dicarbonyl iron Lewis acid, [CpFe(CO)2(THF)] BF4 (108), catalyzes a variety of reactions including cy-clopropanation [34a], aziridination [34b], and Diels-Alder reactions [34c]. Recently, they have reported that the iron Lewis acid [CpFe(CO)2(THF)]+BF4 (108) catalyzed synthesis of 3-hydroxy-2- arylacrylic acid ethyl ester (106) [34d]. The product distribution of (108) is not affected in the presence of proton sponge, but is dependent on temperature and the nature of the substrate aldehyde. 

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