Pseudo-bases mechanism

There remains, of course, the question why apparently isobutene (and perhaps other aliphatic olefins) do not polymerise by the pseudo-cationic mechanism - or do so much less readily than, say, styrene. Probably the short answer lies in the relative stabilities of the esters in the polymerisation conditions, (e.g., perchlorate stabilised by co-ordination of styrene). The long answer will have to be based on a detailed understanding of all the factors which determine this stability and thus govern the equilibrium between ester and ions. 

Robinson, Robert, and Gertrude M. Robinson. "Researches on Pseudo-Bases. Pt. II. Note on some Berberine Derivatives and Remarks on the Mechanism of the Condensation Reactions of Pseudo-Bases," JCS 111 (1917) ... 

Very little is known about the mechanism of the chemliminescent reaction. Even the identity of the excited species is unknown at pre-ent. It is known that the deuterium isotope effect (i.e., 4a-FlEt—O—O —CH(OH)R/4a-FlEt—O—O—CD(OH)R) on the quantum yield is approximately 2 the breaking of the C—H(D) bond is at least partially rate controlling. The proposal (50) that the pseudo base (i.e., 4a-FlEtOH) is formed as an excited species seems unlikely. 

Figure 6. The restricted Berry pseudo-rotation mechanism proposed for the intramolecular metal core rearrangements observed in solution for the clusters [M2Ru4H2(CO)i2L2] (M = Cu, Ag or Au L = a variety of monodentate phosphine ligands or L2 = a variety of bidentate diphosphine ligands). The mechanism exchanges the two coinage metals in sites M(l) and M(2) of the trigonal bipyramidal M2RU3 unit in the metal skeletons of the clusters via a square-based pyramidal intermediate (reprinted by permission of the Royal Society of Chemistry from ref. 36).

FIGURE 12.7 Base catalyzed hydrolysis of this class of pentaaminecobalt(lll) complex is believed to proceed via a pseudo-aminate mechanism via deprotonation of the blue hydrogen atoms. 

Although the concept of pseudo-cationic mechanism may be accepted by some, it is not accepted by many. An alternative mechanisms based on ion pairs were offered instead in the past [111-113]. A publication by Scwarc claims that all detailed re-examinations of the evidence for pseudo-cationic polymerizations shows that all the features of styrene polymerization initiated by acids are accounted for satisfactorily and convincingly by the orthodox ionic mechanism. To support his claim, he reported that optical absorption and electric conductance of the styrene solutions undergoing polymerization induced by perchloric acid in stopped-flow studies demonstrated protonations of the monomer by the acid and formation of ionic species [114]. 

In Volume 6 of this series, evidence was summarized for a relatively new mechanism involving covalent hydrate and pseudo-base formation in hydrolysis reactions of complexes of bipy, phen, and terpy. Attack by HjO or OH ion usually occurs at the carbon atoms adjacent to the co-ordinated V-donors (e.g. the 2,9-positions of phen), or at the 4,7-positions of phen, and at similar positions for bipy and terpy. However, for 5-nitro-l,10-phenanthroline, n.m.r. evidence has been found for attack by MeO ion at the 6-position of free and co-ordinated... 

The structure, and the mechanism of the chemical transformations, of the pseudo ammonium bases which can be obtained from heterocyclic quaternary ammonium salts has been one of the most discussed phenomena in theoretical organic chemistry since the first publication in this field by Claus and Himmelmann. These questions have still not been finally decided. 

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