Protonic initiation

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Fig. 9. Initiation of epoxy cure. Irradiation of a triaryl sulfonium salt produces a radical cation that reacts with an organic substrate RH to produce a cation capable of releasing a proton. The proton initiates ring-opening polymerization. X = BF , PFg, AsFg, and SgFg. ...

When a protic coinitiator HB is present, the proton initiation dominates. When a coinitiator is absent, the selfionization of the Lewis acid according to... 

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride, the net result being the addition of hydrogen and the nucleophile to the alkene. The regio-selectivity is excellent and is in the same sense as is observed for proton-initiated additions.17... 

Whitmore proposed a mechanism of proton initiated polymeri-sation in 1932. It can be represented as under ... 

As far as the polymerisations of DCA other than DXL by non-protonic initiators are concerned, Kops and Spanggaard [11] favour the ring expansion mechanism for the dimerisation and polymerisation of the cis- and tnms-7,9-dioxabicyclo[4.3.0] nonanes by phosphorus pentafluoride or triethyloxonium hexachloroantimonate, although they have not obtained any definitive evidence for it. 

Nitrobenzene radical-anion is more stable in aprotic solvents than its aliphatic counterparts. Nitrobenzene shows two one-electron polarographic waves in acetonitrile with By, -1,15 and -1.93 V vj. see, Tire first wave is due to tlie formation of the radical-anion and this species has been characterised by esr spectroscopy [6]. Nitrobenzene radical-anion can also be generated in steady-state concentration by electrochemical reduction in aqueous solutions at pH 13 [7] and in dimethyl-formamide [8]. It is yellow-brown in. solution with A., ax 435 nm. Protonation initiates a series of reactions in which niti osobenzene is formed as an intermediate and... 

Thietanes all have, to varying degrees, the tendency to polymerize. The rate of polymerization is mainly dependent on the conditions. In sunlight the process takes place at a relatively slow rate. The strained polycyclic thietanes, such as 6-thiobicyclo[3.3.1]heptane polymerize spontaneously when photon or proton initiated. ... 

The following mechanistic possibilities are to be considered (i) in acidic solvents, a proton-initiated addition-elimination process which may lead to products of formal substitution (equation 22) ... 

Esterification in the gas phase was studied by the use of ICR in the 1970s [ 130, 211]. These studies revealed an unexpected richness in chemical reactivity. There are two different ways to promote the gas phase analogue of acid catalysed esterification depending on whether the carboxylic acid or the alcohol is protonated initially ... 

By dissociation, strong mineral acids liberate a proton-initiating polymerization... 

Slow exchange may lead to extremely broad polydispersities and often to polymodal MWDs. It is recommended to study the evolution of molecular weights with conversion and especially the proportion and position of various peaks in the MWD by size-exclusion chromatography (SEC). Use of scavengers is helpful in the identification of the origin of peaks in SEC (MWD) traces. For example, salts with common ions suppress free ions and reduce the intensity of peaks formed by free ions. Hindered pyridines trap protons and reduce peaks resulting from protonic initiation, especially in systems with adventitious moisture. Apparently, stability of complex anions MtX +, and MtX OH can be different and slow exchange may lead to polymodal MWD. 

In the case of /J-pinene (Fig. 13), it is proposed that the protonic initiation creates a cyclic tertiary carbenium ion which rearranges into a more stable pendant species, and the successive addition-isomerization processes lead to a chain structure with alternating isobutyl and cyclohex-enyl subunits. The presence of one unsaturation per repeating unit has been confirmed by polymer analysis. 

This same research group also reported [318] the cationic polymerization of p-methoxystyrene in miniemulsion. DBSA was used as both a protonic initiator and surfactant. A monomer conversion of 100% was achieved in eight hours at 60 °C. Molecular weights were low (approximately 1,000) and solids of up to 40% could be achieved with good colloidal stability. Polymerization takes place at the interface, initiated by the proton, and terminated by water. Molecular weight increased with conversion, suggesting either reversible termination or decreasing termination. 

The use of alkenes as sources of electrophiles in Friedel-Crafts alkylations has also been studied. The intramolecular alkylation of l-(2-tolyl)-( )-pent-3-ene gives 1,5-dimethyl-l,2,3,4-tetrahydronaphthalene in 95 % yield [55]. BF3 has been shown to form a complex with nitromethane which is particularly effective in catalyzing proton-initiated cascade cyclization like that shown in Eq. (29) [56]. 

They also applied this method to the intermolecular ene reactions of aliphatic and aromatic aldehydes with alkenes containing a disubstituted vinylic carbon, a potentially valuable route to homoallylic alcohols [50]. Proton-initiated rearrangements do not take place, because the alcohol-Lewis acid complex formed in the ene reaction reacts readily to give methane and a non-acidic aluminum alkoxide. Formaldehyde and excess Me2AlCl gave good yield of ene adducts with all types of alkene, as exemplified in Sch. 26. 

These studies firmly establish the role of 2,3-epoxysqualene (32) as progenitor of the polycyclic triterpenoids and steroids. However, a number of triterpenoids lack an oxygen function at C(3). In these cases, it has been suggested that a plausible biogenesis mechanism would involve the proton-initiated cyclization of squalene rather than of the terminal epoxide. An alternative hypothesis... 

Small concentrations of H2O, added before and after irradiation, induce a postcross Iinking process that proceeds to completion within a short time and appears to be proton-initiated. 

In conventional hydration of alkenes by water in the presence of a strong acid, there is little direct stereochemical consequence, but we mention it as a useful contrast to the content of the next section. A proton initially adds to the alkene, and in the case of 2-methylpropene (44) gives the intermediate carbocation 45 (Scheme 4.9), which is then captured by the nucleophile, water, and yields the product 2-methylpropan-2-ol (46). 

The explanation for the differing nature of the products is that protonation initially yields a terminal alkylidene intermediate that will rearrange, if... 

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