Ring-opening mechanism

The monomer, norbomene (or bicyclo[2.2.l]hept-2-ene), is produced by the Diels-Alder addition of ethylene to cyclopentadiene. The monomer is polymerised by a ring-opening mechanism to give a linear polymer with a repeat unit containing both an in-chain five-membered ring and a double bond. Both cis-and trans- structures are obtainable according to the choice of catalyst used ... 

Starting from l.l-dichloro-7b-ethoxy-2-methyl-1,1 a,2,7-tetrahydrobenzo[/)]cyclopropa[prepared from the corresponding benzothiopyran by addition of dichlorocarbene, the three 1-benzothiepins 6a-c are formed upon treatment with strong bases, i.e. sodium methoxide or ethoxide in dimethyl sulfoxide.73 The optimal yield of each 1-benzo-thiepin compound depends on the molar equivalents of base, as follows from different ring-opening mechanisms. 

Organometallic catalysis insertion and ring opening mechanism of methylene-cyclopropane over Cp2LaH and Cp2LuH catalysts... 

The ring-opening mechanism was well supported by the snapshots and the overlap bond population obtained from TB-QCMD simulations, where the formation of new C-H and La-C bonds and the dissociation of La-H and proximal C-C bonds could be tracked. The obtained dynamic ring opening mechanism was similar to the static mechanism, however, a novel transition state was also proposed for insertion reaction of alkenes, with tetrahedral h4-coordination. This example perfectly illustrates the importance of mutual interplay between high-level first principle methodologies and simplified methodologies derived from ab initio quantum chemistry, massively applicable for real systems. 

The behavior of B-triamino-N-tris(trimethylsilyl)borazine was in good agreement with the ring opening mechanism. This compound was much more reactive than its phenyl analogues thus, the pure monomer could not be Isolated. The product obtained from the... 

These findings can be interpreted in terms of a normal ring-opening mechanism of intermediate 325 with proton transfer favored by protic solvent, whilst in aprotic solvent cycloreversion of the unstable aziridinium grouping in 325 followed by ring expansion prevails. Likewise, 2,3-disubstituted aziridines follow this reaction pattern, while N-substituted aziridines do not225. ... 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Additional studies of the enzyme-substrate complementarity in other complexes along the reaction path are under way. Since the initial report of an alternative pathway for lysozyme hydrolysis (8,9.28) work on the solution hydrolysis of glucosides has demonstrated the existence of a ring opening mechanism (29,30). ... 

Complex mixtures of oxidized products are produced in the ozonolysis of BaP on solid substrates. For example, BaP deposited on a glass fiber filter and exposed to 1 ppm 03 in air primarily formed products characteristic of the ring-opening mechanism, e.g., dialdehydes and dicarboxylic and ketocarboxylic acids. However, some quinones and phenols, were also formed. For details on the mechanisms and O-PAH products of these BaP-Ov gas-solid substrate reactions, see Van Cauwenberghe et al. (1979) and Van Cauwenberghe and Van Vaeck (1983 and references therein). 

Chromene CASPT2/CASSCF Structure, photochemical ring-opening mechanism 2005PCA8684... 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

Cycloolefins can be polymerized by a ring-opening mechanism with complex tungsten-aluminum catalyst, according to K. W. Scott and coworkers. The polymer formed is unsaturated and is either elastomeric or rigid, depending on the monomer and degree of polymerization. 

The IPNs prepared were composed of a rubbery polyurethane and a glassy epoxy component. For the polyurethane portion, a carbodiimide-modified diphenyl-methane diisocyanate (Isonate 143L) was used with a polycaprolactone glycol (TONE polyol 0230) and a dibutyltin dilaurate catalyst (T-12). For the epoxy, a bisphenol-A epichlorohydrin (DER 330) was used with a Lewis acid catalyst system (BF -etherate). The catalysts crosslink via a ring-opening mechanism and were intentionally selected to provide minimum grafting with any of the polyurethane components. The urethane/epoxy ratio was maintained constant at 50/50. A number of fillers were included in the IPN formulations. The materials used are shown in Table I. 

Pyridinium salts are converted to 4-phenylpyridines by treatment with methylammonium sulfite and 4-methylpyridinium compounds through a ring-opening mechanism <03IZV 1522>. 

Patiiway (c) is more speculative, but it has been previously suggested by Horikoshi et al., 2001, who investigated the OH oxidation of 2P in the aqueous phase, in the presence of Ti02. This pathway may occur for NMP under real tropospheric conditions. It proceeds via a ring-opening mechanism, leading to the formation of N-methyl-4-aminobutanoic acid. 

Tn strict logic, inversion must arise from an odd number and retention from an even number of displacements. In biological practice, however, it is easier for natural selection to develop a protein which stabilises one or two transition states, than one which stabilises 3, 5, 7,. .. or 4, 6, 8,. .. transition states, as the case may be. The same argument makes it unlikely that simple glycoside hydrolases which work by a ring-opening mechanism will ever be found. 

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