Triflic acid initiator

There are relatively few entries in the non-fused dioxepin area, and most of these focus on reactions of these systems. For example the triflic acid-initiated polymerisation of 1,3-dioxepane in the presence of acetic acid and hexanedicarboxylic acid has been studied and mechanistic aspects discussed <00JPS(A)1232>. Biodegradable microspheres for the controlled delivery of drugs have been made from copolymers and homopolymer blends of L-lactide and l,5-dioxepan-2-one <00PP1628>. Ring contraction of 5-methylene-l,3-dioxepanes (eg. Ill) on reaction with trimethylsilyl trifluoromethanesulfonate in the presence of base afforded the exo tetrahydropyrans, in good yields <00TL2171>. 

For most of the systems reported in the literature, C/K is not known—very often, neither K nor C is known. For two-component initiator-coinitiator systems, C is usually taken to he the initiator concentration [YZ] when the coinitiator is in excess or the coinitiator concentration [I] when the initiator is in excess. C may be lower than [YZ] or [I] due to association that is, only a fraction of [YZ] or [I] may be active in polymerization. This may also he the case for one-component initiators such as triflic acid. It would be prudent to determine the actual value of C in any polymerization system—usually a difficult task and seldom achieved. Experimental difficulties have also limited our knowledge of K values, which are obtained most directly from conductivity measurements or, indirectly, from kinetic data. A comparison of polymerization in the absence and presence of a common ion salt (e.g., tetra-n-butylammonium triflate for the triflic acid initiated polymerization) is useful for ascertaining whether significant amounts of free ions are present in a reaction system. 

Strong protonic acids such as trifluoroacetic, fluorosulfonic, and trifluoromethanesulfonic (triflic) acids initiate polymerization via the initial formation of a secondary oxonium ion... 

When protonic acids are used as the initiators, nucleophiles do not complex oxyanions and therefore only form onium ions. The kinetic scheme of triflic acid initiated polymerizations of isobutyl vinyl ether in the presence of sulfides is presented in Eq. (80) [58,133]. The rate of polymerization described by Eq. (81) takes into account propagation by both carbenium kp ) and sulfonium ions ( /). 

The exo double bond is formed first in polymerizations of a-methylstyrene, but is later isomerized by protonic acid to the more stable endo isomer [14]. Carbocationic polymerizations initiated by protonic acids with extremely basic counteranions, as in triflic acid-initiated polymerizations of isobutene, produce predominantly the unsaturated dimer [285]. The exo dimer forms first and then isomerizes to the more stable endo isomer [Eq. (90)]. 

A variety of initiator systems of the types used in the cationic polymerization of alkenes (Chapter 8) can be used to generate the tertiary oxonium ion prpoagating species. Strong protonic acids such as sulfuric, trifiuoroacetic, fluorosulfonic, and trifluoromethanesulfonic (triflic) acids initiate polymerization via the initial formation of a secondary oxonium ion ... 

The reason for the exceptional ability of triflate counterion to mediate the copolymerization in favor of m-DMB enchainment is difficult to ascertain. A recent study of Hasegawa and Higashimura on the cationic polymerization of divinylbenzene showed reduced propagation tendency relative to chain transfer when triflic acid initiation was used. This implies that increased m-DMB incorporation from triflic acid initiation is a result of reduced propagation rather than enhanced alkylation. 

Studies of cationic polymerization, carried out in parallel, solved an initial controversy concerning the nature of artive species, that is, acylium versus tertiary oxonium cations, in favor of the latter ones. Attempts of several laboratories to prepare high-molar-mass aliphatic polyesters in the controlled cationic process eventually failed. The first example of a controlled cationic process involving cyclic ester is the activated monomer (AM) polymerization of CL, conducted in the presence of an alcohol. More recendy, Basko and Kubisa " published a series of papers that confirm applicability of the AM mechanism to the controlled synthesis of aliphatic polyesters. Interestingly, it has been also shown for the first time that triflic acid initiation of L,L-lactide (l,l-1A) polymerization, without a purposely introduced alcohol, leads to the living process proceeding in agreement with the AM mechanism. ... 

Triflic acid is strong enough to protonate polycyclic saturated hydrocarbons [77, 78, 79], and even -butane [80, 81], and to initiate skeletal rearrangements Acidic treatment of homoadamantane [77] (equation 32), 2-homoprotoadamantane [78] (equation 33), or as 2,3-trimethylenebicyclo[3 3 Ojoctane [79] (equation 34) causes their rearrangement to isomenc hydrocarbons... 

Tetraene 4 (Scheme 1.3), when treated with 40 mol % of triflic acid in methylene chloride at -23 °C for 1 h, gives the adducts 5 and 6 in a 1 1 ratio as the main reaction products. The formation of these adducts has been justified [21] by a stepwise mechanism that requires an initial reversible protonation of 4 to produce the allyl cation 7, which then cyclizes to 8 and 9 in a non-reversible process. Deprotonation of 8 and 9 gives 5 and 6, respectively. 

The alkylation reaction is initiated by the activation of the alkene. With liquid acids, the alkene forms the corresponding ester. This reaction follows Markovnikov s rule, so that the acid is added to the most highly substituted carbon atom. With H2S04, mono- and di-alkyl sulfates are produced, and with HF alkyl fluorides are produced. Triflic acid (CF3S020H) behaves in the same way and forms alkyl triflates (24). These esters are stable at low temperatures and low acid/hydrocarbon ratios. With a large excess of acid, the esters may also be stabilized in the form of free carbenium ions and anions (Reaction (1)). 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

Table 5-1 shows the various kinetic parameters, including k+ and kp, in the polymerization of styrene initiated by triflic acid in 1,2-dichloroethane at 20°C. Data for the polymerization of isobutyl vinyl ether initiated by trityl hexachloroantimonate in methylene chloride at 0°C are shown in Table 5-2. Table 5-3 shows values for several polymerizations initiated... 

This type of initiation is limited hy the nucleophilicity of the anion A derived from the acid. For acids other than the very strong acids such as fluorosulfonic and triflic acids, the anion is sufficiently nucleophilic to compete with monomer for either the proton or secondary and tertiary oxonium ions. Only very-low-molecular-weight products are possible. The presence of water can also directly dismpt the polymerization since its nucleophilicity allows it to compete with monomer for the oxonium ions. 

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, tri aryl 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 triflic acid (158,163—166). Coordination polymerization of THF maybe in a different class (167). 

When cyclohexene is mixed with anhydrous triflic acid under a high pressure of carbon monoxide (120 atm) followed by the addition of benzene, cyclohexyl phenyl ketone and the isomeric cyclohexenyl cyclohexyl ketones are obtained with little isomerization of the initially formed cyclohexyl cation 22 to methylcyclopentyl cation 23 (Scheme 5.46).422... 

In a recent communication, a microsystem allowing controlled polymerization and block copolymerization of vinyl ethers with triflic acid as the initiator at 25°C has been described.977 The system allows a high level of control on molecular weight distribution. 

Triflic acid catalyzes the arylation of Af-methyl-l,2,3,6-tetrahydropyridine 227 to give 4-phenylpiperidine 229 (Scheme 59) <2001TL5821>. The reaction is believed to proceed by initial formation of a 1,4-dication 228, which forms in preference to the 1,3-dication 231. When yV-methyl-5-phenyl-l,2,3,6-tetrahydropyridine 230 is arylated under the same conditions, 3,3-diphenylpiperidine 232 is formed as the sole product showing the stabilization of the 1,3-dication intermediate 231 by the tertiary 5-position. Intramolecular arylation of 2- and 6-benzyl-substituted 1,2,3,6-tetrahydropyridines can also be catalyzed by triflic acid <20050L4309>. 

In their initial report on the use of the 5-V-acetyl Neu5 Ac donors, Demchenko and Boons observed that the addition of the second acetyl group to the standard donor 6, resulting in the imide 7, increased the reactivity of the donor considerably on activation with. V-iodosuccinimide (NIS) and triflic acid (TfOH) at -40°C in acetonitrile (Scheme 5.3). The precise origins of this increased activity were not identified,... 

Bis(iodosyl)benzene reacted with triflic anhydride to afford a bis iodine (III) derivative [21], DIB or iodosylbenzene, however, do not afford with triflic acid, or its anhydride, the expected analogues of HTI, although these are initially formed. The reaction of iodosylbenzene and triflic anhydride leads to two different products, depending on reaction time. When triflic acid was allowed to react with iodosylbenzene in dichloromethane for about 20 min the yellow p-compound 1 (m.p. 100-110°C) was obtained it was the same with the so-called Zefirov s reagent, which was originally prepared from DIB and triflic acid in chloroform. When the reaction time was extended to 12 h, then 1 isomerized to the slightly pale yellow compound 2 (m.p. 125-132°C). For preparative purposes the direct reaction of iodosylbenzene with triflic acid was preferable for 2, since it was isolated in 94% yield. 

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