Dihydropyran, polymerization

Protocols have been developed which utilize an insoluble solid catalyst in combination with dihydropyran to effect the protection of alcohols as their corresponding THP ethers. These procedures are advantageous in that the catalyst may be recovered by simple filtration and the products isolated by evaporation of volatiles. In many cases the catalyst can be reused without regeneration. Reaction of alcohols with dihydropyran in the presence of Amberlyst H-15 (25 °C, 1 h, 90-98%) yields THP derivatives. Alternatively, a solution of dihydropyran and the alcohol may be passed slowly through a column of silica overlaid with Amberlyst H-15 to yield the THP ethers directly (73-97%). The acidic clay Montmorillonite KIO (25 °C, 15-30 min, 63-95%) is similarly applicable, Reillex 425 resin (86 °C, 1,5 h, 84-98%) is applicable with the advantage that it does not promote the sometimes troublesome polymerization of dihydropyran. Polymeric derivatives of pyridinium p-toluenesulfonate are also effective. Poly(4-vinylpyridinium p-toluenesulfonate) and poly(2-vinylpyridinium p-toluenesulfonate) catalysts yield tetrahydropyranyl derivatives of primary, secondary, and tertiary alcohols (24 °C, 3-8 h, 72-95%). ... 

Tamura etal.2 described only briefly in their paper on dihydropyran derivatives that 6,8-dioxabicyclo[3.2.1]octan-7-one 53 and its methyl derivative underwent polymerization in the presence of a large amount of boron trifluoride etherate to give polymers with molecular weights of several hundreds. 

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

Unsaturated tetrahydropyran derivatives have received only cursory attention in the literature as heterocyclic monomers. 2,3-Dihydropyran and several of its substituted derivatives apparently undergo cationic polymerization in a manner typical of vinyl ethers (72MI11103), while tetrahydropyranyl esters of methacrylic acid (123) are fairly typical free radically polymerizable monomers (Scheme 35) (74MI11105). The THP group was used in this study as a protecting group for the acid functionality, and it was found that deprotection of polymers (124) could be accomplished under extremely mild conditions. 

The polymerization of 2,3-dihydropyran has been reported. However, this compound is a vinyl ether. The polymerization occurs mainly through the double bond and not by ring opening [3]. Kamio et al. [117] found that the polymer obtained by polymerizing 2,3-dihydropyran with... 

Dihydropyrane appears to be the reagent of choice for blocking the 2 -hydroxy groups of 5 -nucleotide monomers that are to be polymerized. The THP-ethers are usually very sensitive to acids and should be prepared just prior to use or else stored in the dry state at the lowest possible temperature. 

Where both donor and acceptor molecules are vinyl monomers then the generation of radical ions might be expected to polymerize both species. This appears to be so [see reaction (9)] when unsaturated ethers, such as / -dioxene, dihydropyran, ethyl vinyl ether, isopropyl vinyl ether, and ketene diethylacetal, are each mixed with vinylidene cyanide. Cycloadducts are also important... 

The copolymerizations of dihydrofuran or dihydropyran monomers with maleic anhydride or vinylene carbonate were carried out with different concentrations of monomers in the presence of radical initiators (AIBN) in DMF or in bulk at 70 - 100 °C and the polymerization results are given in Table I. The polymer yields were increased with the monomer concentrations (runs I and 2, 3 and 4, 5 and 6), and with the initiator concentrations (runs 5 and 6, 9 and 10) as well as with the polymerization time (runs 14 and 15). The bulk copolymerization (run 13) resulted in higher yield than the solution polymerization (run 14). 

Ziegler-Natta Polymerization. NVK and other monomers, such as vinyl ethers, 1,5-hexadiene, dihydrofuran, and dihydropyrans can be polymerized using a titanium, hafnium or zirconium pentamethylcyclopentadienyl complex as an initiator in the presence of a borane co-initiator, e.g., ... 

The known dimerization reaction that takes place when glycals are treated with Lewis acids proceeded more efficiently with acetyl perchlorate (a known polymerization initiator of dihydropyran) in dichloromethane at —78 °C. For example tri-O-acetyl-D-galactal gave a 77 % yield of dimer 22. The report that tri-O-benzyl-D-glucal gives a cyclopentane derivative under these conditions has been corrected. ... 

A soln. of startg. tert. alcohol, dihydropyran, and 0.05 eq. triphenylphosphine hydro-bromide in methylene chloride stirred at room temp, for 24 h under inert gas - product. Y 88%. Dehydration and/or polymerization of highly sensitive tert. alcohols is avoided under these conditions. F.e. inch benzylic, allylic and propargylic derivs. s. V. Bolitt et al.. Tetrahedron Letters 29, 4583-6 (1988). 

When similar initiators were employed in the polymerization of 6,8-dioxa-bicyclo[3.2.1]octan-7-one (2) macrocyclic oligomers containing alternating tetrahydropyran and ester units resulted at low temperatures. At higher temperatures the lactone homopolymer was formed. The mechanism of polymerization of 2,3-dihydropyran derivatives (3) depended on the nature of the substituents, with low molecular weight polymers being formed in all cases studied. ... 

Fujimori showed that the above concept also holds for ethyl vinyl ether (EVE), 2,3-dihydropyran (DHP), and divinyl ether (DVE). The e values of DHP, EVE, and DVE (Table 10.14), respectively, become negatively larger as the equilibrium constants of their CTC with MA increase in the same order ii (DHP-MA) - 0.11 jK (EVE-MA) = 0.15, and i (DVE-MA) = 0.18 liter mole Under the same conditions, the conversion for the copolymerization of DHP with MA after 10 h was comparable to that of the copolymerization of EVE with MA after only 100 min. It is hard to conceive how such a small difference between the K values of DHP and EVE with MA could bring about such a great difference between the copolymerization rates of the two pairs. Zeegers and Butler claimed that the polymerization kinetics of DVE with MA could be explained without the intervention of a CTC, but charge-transfer complexes may participate in competing reactions. 

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