Free radical ring-opening catalysts

The Re(III) complex Re(PPh3)2(MeCN)Cl3 (2 mol%) catalyzes the ATRA of tetrachloromethane or bromotrichloromethane to terminal alkenes in 39-76% yield [303]. p-Pinene suffered a cyclobutylcarbinyl radical ring opening, thus supporting the free radical mechanism. With l, -dienes double addition was found, while 1,3-dienes gave the 1,4-addition product. Internal alkenes were almost inert under the reaction conditions. 1,6-Dienes 158 underwent a tandem radical addition/ cyclization reaction to cycles 159 in 64—87% yield with 3-6 1 c/s-diastereos-electivity (cf. Fig 43). This compares well to the results obtained with the most frequently used catalyst Ru(PPh3)3Cl2 (see Part 2, Sects. 3.3.1 and 3.3.2). 

There are very few homolytic reactions on triazolopyridines. A suggestion that the ring opening reactions of compound 1 involved free radical intermediates is not substantiated (98T9785). The involvement of radical intermediates in additions to ylides is discussed in Section IV.I. The reaction of radicals with compound 5 and its 1-substituted derivatives gives 4-substituted compounds such as 234 (96ZOK1085). A more detailed study of the reaction of the 1-methyl and 1-phenyl derivatives with r-butanol and ammonium persulfate produced 4-methyl substitution with a silver nitrate catalyst, and the side chain alcohol 235 without the catalyst (96ZOK1412). 

Monomers, (IV), which were crosslinkable using free radical polymerization with azoisobutyronitrile or with ring-opening metathesis polymerization catalyst, C12Ru(=CHC6H5)[P(C6H11)3] were prepared by Liaw et al. (4) and used in thermosets. 

Catalyst for Ionic Ring-Opening Addition Polyesterification Initiator for Free-Radical Styrene/Maleic Ester Copolymerization (Cross-linking Reaction)... 

On the other hand, Gavrilova and Gonikberg opened the furan ring with hydrogen, without the use of catalysts.213 The reaction is carried out at high temperature (350-375°) and at a pressure between 240 and 750 atmospheres, in the absence of a solvent. The following products are obtained from 2-methylfuran acetone, methyl ethyl ketone, methyl butyl ketone, methyl amyl ketone, acetophenone, methylpropylcarbinol, w-pentane, and water. The authors suggest that these products are formed by a free radical mechanism. 

Titanium framework-substituted aluminophosphate, TAPO-5, gave a 30.3% selectivity to AA at total cyclohexene conversion, in 72 h reaction time at 80 °C [35c]. However, when the filtered TAPO-5 catalyst was reused its activity was significantly diminished. The major by-product was 1,2-cyclohexanediol, which is formed as reaction intermediate. The trans isomer was more slowly transformed into 2-hydroxycyclohexanone than the cis isomer. A detailed investigation of the mechanism indeed showed that the trans-diol formed by ring opening of the cyclohexene epoxide, whereas the cis isomer formed via a free-radical mechanism. Scheme 7.14 shows the mechanism proposed. 

Catalysts are often needed to facilitate ring-opening polymerization reactions. Although the Sg-S equilibration is believed to be a free-radical process, most ringopening polymerizations follow an ionic pathway ... 

Carbenes in Free Radical and Ring-Opening Polymerization Catalysts (Type 3)... 

During ring-opening metathesis polymerization (ROMP) of norbornadiene (NBD) with the Grubbs catalyst a free radical is also observed as a 1 2 1 triplet, which is also formed, but to a much weaker extent, with norbornene (NBE), cyclopentene and 1,7-octadiene. The identity of this triplet, and that of a transient doublet observed together with the triplet in the case of benzonorbornadiene, are discussed as well as the possible role of radicals in initiation of ROMP, crosslinking of ROMP products, and polymerization of a-methylstyrene. 

---