Initiators triflic esters

To initiate efficiently, rapidly and quantitatively the polymerization of these heterocycles it was searched for cationic species which add to the monomer and are associated to stable counterions. Suitable initiators are oxocarbenium salts (I), stabilized car-benium salts (II) and trifluorosulfonic derivatives (III) (triflic esters or anhydrides)42"44 ... 

Unfortunately, there are no data for triflic anhydride (CF3802)20 available which, according to the existing reports, gives rise to much faster initiation than esters. 

Initially, Gandini and Plesch proposed that the perchloric acid-initiated low temperature polymerization of styrene is based on monomer insertion on the nonionic perchlorate chain ends, which was based on the observation that the polymerization mixture was not conductive [68, 69]. These nonionic polymerizations were referred to as pseudo-cationic polymerizations. However, more detailed investigations by stopped-flow UV-vis spectroscopy revealed the presence of short-lived carbocations indicating that these are the propagating species in the cationic polymerization of styrene [70, 71]. This was also confirmed for the polymerization of styrene with trifiic acid for which Matyjaszewski and Sigwalt showed that the covalent triflic ester adduct was unstable even at -78 °C leading to carbocationic propagating species [72]. 

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)). 

Treatment of the alcohol ( ) with trifluoromethylsulfonic anhydride (triflic anhydride) at -78 C afforded the ester (1 ) which could be isolated and characterized. We knew from previous experience (2J that sulfonyl esters vicinal to an isopropylidene acetal are relatively stable. The triflate T,) reacted cleanly with potassium azide and 18-crown-6 in dichloromethane at room temperature. The crystalline product [68% overall from (1 )] was not the azide ( ) but the isomeric A -triazoline ( )- Clearly the initially formed azide (18) had undergone intramolecular 1,3-cyclo-addition to the double bond of the unsaturated ester (21- ). The stereochemistry of the triazoline (1 ), determined by proton nmr spectroscopy, showed that the reaction was stereospecific. There are several known examples of this reaction ( ), including one in the carbohydrate series ( ). When the triazoline was treated with sodium ethoxide ( ) the diazoester ( ) was rapidly formed by ring-opening and was isolated in 85% yield, Hydrogenolysis of the diazo group of (M) gave the required pyrrolidine ester ( ) (90%). 

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. 

Even very small amounts of anhydrous perchloric and triflic acids (<10-3 M) polymerize styrene rapidly and quantitatively [123,126-130], Carbenium ions were initially not detected in these systems, and they were therefore proposed to proceed by a pseudocationic mechanism in which covalent esters react directly with styrene in a concerted muticenter rearrangement [128], However, short-lived carbenium ions have since been detected directly by stopped-flow UV [17-19,131]. The mechanism of the propagation step in these systems is discussed in more detail in Section lV.D.2.a. 

Esters of very strong protonic acids (trifluoromethanesulfonic, fluoro-sulfonic, perchloric), however, are sufficiently strong alkylating agents to initiate the polymerization of even weakly nucleophilic monomers (cyclic acetals, ethers) [2-6], Also their anhydrides (e.g., triflic anhydride) are efficient initiators. This last compound is especially interesting, because in the polymerization of cyclic ethers it leads to macromolecules with two identical growing chain ends (difunctional initiator) [30] ... 

Anhydrides of strong i x)tonic acids provide a group of initiators able to give dicationically terminated macromolecules °° The anhydride of trifluoromethane-sulfonic acid (triflic anhydride) initiates the polymerization of THF in this way both reactions, with rate constants ki and kai, are faster than the formation of the alkyltetrahydrofuranium cation with the corresponding triflic acid ester ... 

Neither the first nor the second step in Scheme (42) has been directly observed, but it has conclusively been shown by studies of the number of the end groups in poly-THF initiated with triflic anhydride that one molecule of the anhydride gives two growing ends of the cationic and/or ester structure in one macxomolecufe. These structures are interconvertible. Recently, Smith separated and characterized two intmnediate products tetramethylene-bis-triflate, resulting from the attack of the anion on the o-methylene carbon atom in salt 42a, and the corresponding dicationic trimer... 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

Sen and coworkers have examined systems based on Pd(OAc)2 in triflic acid (TfOH) at 80 °C (equation 5). Na2Cr207/Pd(0Ac)2/Tf0H aromatizes cyclohexanes for example, decalin gives naphthalene (9%) and a-tetralone (4%). Normally the functionalization product of an alkane is more reactive than the alkane itself and so only the low conversion prevents the initial product from being oxidized further. Shilov and Sen s use of triflic acid in this context means that the initial functionalization product at the alcohol oxidation level is protected as the triflate ester, which is relatively insensitive to oxidation. 1,4-Dimethylbenzene is oxidized to triflates with a 50 1 preference for oxidation at the ring rather than of the side chain, unlike the selectivity expected in a radical process. A kinetic isotope effect of 5 was measured. In certain cases the reaction can be... 

Most commonly initiation proceeds as direct addition of initiator to monomer molecule (route 1). Cationic polymerization of cyclic ethers may be initiated by both Bronsted and Lewis acids. Most commonly used initiators include strong protic acids such as trifluoromethanesulfonic (triflic) acid (also its anhydride or esters), fiuorosulfonic acid, perchloric acid, or heteropolyacids, oxonium salts such as triethyloxonium (e.g., EtsC A ), carbenium (e.g., Ph3C A ), or carboxonium (e.g., CeHsCO A ) salts where A should be stable, weakly nucleophilic counterion (e.g., BF4, PFg, and SbFg) or Lewis acids (most commonly used is BF3 -Et20). Several other initiation systems have been used (e.g., rare earth triflates) but the advantages over typically used simple and easily available initiators have not always been shown. 

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. ... 

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