Polymeric transfer reagents

Polymeric Transfer Reagents for Organic Synthesis with Self-Control... 

In such a reaction, which may be termed a "transfer reaction", an active species A, is transferred to the acceptor B. In practice, an excess amount of one of the components in such a reaction often is used, in order to effect complete conversion of reagent B to the required product BA, and to shorten the reaction time. At the aid of the reaction, the resulting product BA, must be separated from the excess reagent By using polymeric transfer reagents the separation is facilitated by a simple filtration step ... 

In a recent development, a new process of preparing borane-terminated isotactic polypropylene (z -PPs) via an in situ chain-transfer reaction was achieved by a styrene/hydrogen consecutive chain-transfer reagent, which avoids the use of a B—H containing chain-transfer agent.74 This has resulted in the utilization of milder polymerization conditions due to the use of the alkylaluminoxane cocatalyst (MAO) (50) (Fig. 33), which cannot normally be used in the presence of a B—H chain-transferring... 

Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

PO proceeded in a living manner to yield highly regioregular polyethers with narrow MWDs. These authors also developed the immortal polymerization of epoxides where polymers with narrow MWDs were obtained with the number of polymer chains exceeding the number of initial aluminum-porphyrin complexes (Scheme I). The key in the immortal polymerization is a reversible chain transfer, which is much more rapid than the chain propagation. In the presence of an alcohol (R OH) as a chain-transfer reagent, an aluminum-porphyrin complex with a growing species reacts with R OH reversibly, so that the polymerization takes place from all the molecules of aluminum-porphyrin complex and R OH. 

Immortal polymerization of epoxides with la and an alcohol is also accelerated by co-use of bulky Lewis acid 2a. The polymerization of PO with la/2-propanol system ([PO]/[la]/[2-propanol] = 1000/1/49) in the presence of 2a ([PO]/[2a] = 1000/1) proceeds rapidly to achieve 86% conversion in 1.5 h, while the polymerization in the absence of 2a requires 380 h to reach 84% conversion (Table 1). The polyether produced in the presence of 2a has an of 900 gmoP and an MJM of 1.10, which indicates that almost all of la and 2-propanol participate in the initiation of the polymerization. Other protic chain-transfer reagents, such as methanol, benzyl alcohol, and 4-/ r/-butylphenol, are also applicable to the high-speed immortal polymerization to give similar results as 2-propanol. As a substrate, ECH is also employable. Polymerization of ECH ([EGH]/[la]/[2-propanol]/[2a] = 1000/1/49/1) gives a polymer with and/n of 1100gmol close to the value estimated from the conversion and [PO]/([la] + [2-propanol]) ratio, and a narrow M IM of 1.10, while the conversion is lower than the case of PO. 

An advantage of polymer-based reagents is that both the excess and the spent reagent are easily separated from the product. Bruno Linclau of the University of Southampton has reported J. Org. Client. 2004, 69,5897) the preparation of a polymer-bound carbodiimide. Exposure of the polymer to alcohol gives a family of O-alkylisoureas that smoothly convert carboxylic acids to the corresponding esters. Methyl, benzyl, ally and p-nitrobenzyl transfer smoothly. The polymeric (-butyl reagent could not be prepared. 

Kumari, K. A. Sreekumar, K. Polymeric Acyl Transfer Reagents Synthesis of Amides Using Polystyrene Supported Oximino Esters, Polymer 1996,37, 171. 

Oxygen transfer reactions.1 Iodosobenzene can function as an oxygen transfer reagent. Thus it converts tetracyanoethylene into the oxide in 74% yield. It also effects epoxidation of ketenes to yield, initially, a-lactones that polymerize to polyesters (equation I). 

Thermal cycloreversion of the adducts can be accomplished at a convenient rate when heated in toluene under reflux. If a new diene is present in the reaction mixture, the thioaldehyde thus generated in the retro-Diels-Alder reaction may give a new adduct. Therefore, adducts 81 and 82 act as thioaldehyde or thioketone transfer reagents. These adducts dissociate reversibly on heating, thus ensuring that the concentration of the labile species remains very low. For this reason, polymerization is not a serious problem especially in the case of thioaldehydes224. The transient thiocarbonyl compounds can be trapped not only by dienes but also by 1,3-dipolar cycloadditions332 (equation 85). 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. We recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylene-diamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that one-electron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical (VII). 

Polyfunctional dithiocarbamate derivatives, (VI) and (VII), were prepared by charmot [4] and used as chain transfer reagents in free radical polymerization reactions. 

Organotungsten reversible addition-fragmentation chain transfer reagents, (IV), prepared by Lo [5] were used with AIBN to polymerize isobutyl acrylate. 

Divalent organolanthanide complexes can also initiate MMA polymerization. A divalent lanthanide complex, as a single-electron transfer reagent, can readily react with the monomer to generate a radical anion species, which subsequently couples into a bimetallic trivalent lanthanide enolate intermediate, which is the active center. Therefore, divalent organolanthanide complexes serve as bisinitiators for MMA polymerization [160]. 

By the addition of hydrogen to PMS, the number of chains increased without any broadening of the molecular weight distribution. Hydrogen did not act as a chain transfer reagent but the number of polymer chains showed that almost all Ti compounds turned to active sites. After the polymerization with hydrogen, ethylmethylbenzene was observed as a by-product. The amount of ethylmethyl-benzene is almost equimolar to the hydrogen added. These results showed that almost all of the titanium compounds turned to active sites, but about half of the active sites become dormant by some type of coordination with... 

The commercially available 3-amino-1,2,4-dithiazole-5-one, (3), has been attached to a hydroxyl resin via a succinic acid linker, and has been used as an efficient sulfur-transfer reagent for the solution-phase synthesis of phos-phorothioates. DNA synthesis scale-up to lOOg of a 20-mer phosphorothioate has been examined " key to this was the purification of the product, which was carried out using a high efficiency polymeric anion exchange chromatographic media. 

Two-phase systems, usually consisting of a strongly alkaline aqueous rfiase and methylene chloride, are also effective and convenient. Trialkylsulfonium salts with methylsulfate counterions can be used alone under these conditions, whereas those with halide counterions generally benefit from added phase transfer catalysts (equation 2). Dodecyldimethylsulfonium chloride was found to be an effective phase transfer reagent. A sulfonium salt covalently attached to a polymeric resin was found most effective when a phase transfer agent was also employed. ... 

Radical polymerization of acrylates can be used to make low-molecular-weight oligomers under high dilution conditions with a relatively large concentration of chain transfer reagent. These conditions were extended to the intramolecular cycli-zation of two acrylate entities tethered by a chiral diol in the formation of a remote stereocenter and a medium-sized ring (Eq. (13.38)) f49J. 

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