Proton scavengers

Chloroaluminate laboratory preparations proved to be easily extrapolated to large scale. These chloroaluminate salts are corrosive liquids in the presence of protons. When exposed to moisture, they produce hydrochloric acid, similarly to aluminium chloride. However, this can be avoided by the addition of some proton scavenger such as alkylaluminium derivatives. In Difasol technology, for example, carbon-steel reactors can be used with no corrosion problem. 

Figure 3. Effects of temperature and proton scavenger additives in TD / C2 CHa/CDa. Dose rate = 4.72 X 1012 e.v./cc.sec.

Furthermore, intercepting the furylpalladium(II) species 130 with an electrophile would result in a carbodepalladation in place of protodepalladation. Therefore, a tandem intramolecular alkoxylation of p.y-acetylenic ketone 127 was realized to afford trisubstituted furan 131 when allyl chloride was added to the original recipe [103]. 2,2-Dimethyloxirane was used as a proton scavenger, ensuring exclusive formation of 3-allylated 2,5-disubstituted furan 131 without contamination by protonated furans. 

FIGURE 3.16 V-Acylation at neutral pH employing zinc dust as a proton scavenger.39 The zinc destroys the acid that is present or produced by reducing it to hydrogen. 

At the time of writing, neither ionic liquids nor fluorous solvents have been used as solvents for commercial processes, although BASF use an ionic liquid as a proton scavenger in phosphine manufacture [36], These are relatively new technologies and time will tell whether or not they will fulfil current high expectations. 

In addition, the Diels-Alder reaction was found to be catalyzed by salt 18 giving the e d(( -product in 97% yield in 1 h. Apphcation of Me3SiOTf resulted in no product formation, while MejSiNTfj and Et3SiNTf2 gave a yield of 6 and 13%, respectively (Scheme 12). Both reactions proceeded in the same way in the presence of the proton scavenger 2,6-di-fe/t-butylpyridine with salt 18, which should rule out a proton promoted reaction. 

The electrochemical cyclization of enol ethers in methanol uses an undivided cell and 2,6-lutidine is added as a proton scavenger. Acid catalysed hydrolysis of the enol function is thus avoided. An advantage is gained by diluting the methanol witli a non-nucleophilic co-solvent. This lowers the extent of dimethoxylation of... 

A library of piperazine containing fused azepine-tetrazoles 183 was built by Nixey et al. via Ugi reaction in the solution phase [55]. This library comprises an example of a building block introduced piperazine (Scheme 32). The reaction of A-Boc-a-amino aldehyde 184, methyl isocyanoacetate 185, substituted piperazines 186, and trimethylsilylazide 181 in methanol, followed by acid treatment, proton scavenging, and reflux affords bicyciic azepine-tetrazoles 183. This efficient protocol with three diversity points can be used to generate arrays of biologically... 

Ene and 2 + 2 cycloaddition reactions of acetylenic esters (7, 7 8 8, 13). In the early studies on the reaction of acetylenic esters with alkenes, Snider used A1C1( as the Lewis acid catalyst. The presently preferred catalyst is C2H5A1C12, which is a Lewis acid and also serves as a proton scavenger by reaction with HC1 to give ethane and AIC13. It is generally used in amounts close to 1 equivalent for a neutral Hlkene. 

Ene reaction of aldehydes. Aliphatic and aromatic aldehydes are not reactive enophiles however, in the presence of dimethylaluminum chloride, which serves as u mild Lewis acid catalyst and proton scavenger, ene reactions occur in reasonable to high yield. Use of C2HSA1C1 results in complex mixtures of products. This ene reaction is a useful route to homoallylic alcohols.2... 

Notes Because of the severe steric constraints of this amine, it serves well as a proton scavenger and is quite resistant to quatemization. 

The cationic polymerization was further evidenced by studying the effect of added proton scavengers, such as ammonia and trimethylamine, and the copolymerization with a-methylstyrene and isobutylvinylether. It is now believed that both radical and cationic polymerization of styrene are able to proceed by ionizing radiations even in bulk at room temperature, and the latter polymerization is much more predominant in the absence of cation scavengers, though it is effectively suppressed in their presence. 

The protection of the amine as a hydrochloride, allows the selective oxidation of the alcohol with 62% yield. However, the protection of the amine is not complete by protonation, because the DMSO present in the medium is basic enough to compete as proton scavenger. A better protection of the amine by the addition of ca. 0.5 eq. of concentrated sulfuric acid, as an extra proton source, allows to increase the yield to 78%. 

Allylic suffoxides. Alkenes can be converted directly into allylic sulfoxides by an ene reaction withp-toluenesulfinyl chloride catalyzed by ethylaluminum dichloride in ether at 25°. In this reaction C2H5A1C12 serves as a Lewis acid and as a proton scavenger by reaction with the HC1 liberated to form A1C13 and ethane.4... 

Combination of amines and dendrimers functionalized proton scavenger resins efficiently removes excess reagents from amide-formation reactions. Marsh et al.41 used a supported branched polyamine and... 

The UDC concept can be further extended by application of ethyl glyoxylate (a convertible aldehyde ). Simple reaction of 48 in the Ugi MCR with /V-Boc anthranilic acids, /V-Boc-rt-amino acids, mono-A-Boc diamines, and niono-A-Boc phenylenediamines, followed by acid treatment and in some cases proton scavenging, affords 1,4-benzodiazepines 49, diketopiperazines 50, ketopiperazines 51, and dihydroquinoxalinones 52, respectively.25 Note that products differ from those obtained from convertible isocyanides in that they contain an additional exocyclic amide... 

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