Ring-opening process, conditions

The alkylcyclopentane (AGP) to aromatics process (ACP ACH Ar) is less efficient than ACH dehydrogenation, owing to the slowness of the first step and to ACP ring opening. Under conditions where cyclohexane is converted to benzene with close to 100% efficiency, only 50—75% of methylcyclopentane may be converted to benzene. 

Cyclosilazanes are found to be reluctant to polymerize by the ring-opening process, probably for thermodynamic reasons. On the other hand, six- and eight-membered silazoxane rings are able to undergo anionic polymerization under similar conditions to those which have been widely used for cyclosiloxane polymerization provided there is no more than two silazane units in the cyclic monomer. They can also copolymerize with cyclosiloxanes however, the chain length of the linear polymer formed is substantially decreased with increasing proportion of silazane units. 

Extensive optimization studies identified highly electron-deficient 2,4-dinitrobenzyl-substituted aziridines as the most reactive substrates, chromium as the metal of choice, and indanol-derived Schiff bases as the most effective ligands. In this ring-opening process, catalyst 61 provided the highest selectivities. Using these optimized conditions, a variety of aziridines were selectively opened in a very efficient manner (Scheme 17.21).51 This reaction can provide an easy access to C2-symmetric 1,2-diamines, a valuable class of chiral auxiliaries, and even to less accessible non-C2-symmetric 1,2-diamines because of the differentially protected amines of the ring-opened products. 

Pyrimidine 1-oxides 72, which are less electrophilic than triazines, do not react with cyanamide in the presence of a base. However, treatment of 72 with cyanamide under acidic conditions (dry HCl) results in the formation of 4-aryl-2-ureidopyrimidines 75 (Scheme 49) [107]. It is evident that in situ protonation of these N-oxides significantly enhances their ability to give ring opening process (73 74 75), thus resulting in the formation of 75. Hydrolysis of the latter on reflux in formic acid provides the corresponding 2-aminoperimidines. 

The first question is if no cyclization to the (Cy5) compounds is observed when the 5-hexenyl radical is chosen, is it possible to rule out the formation of an alkyl radical R on the reaction pathway The answer is no, not if fast competitive intermolecular reactions are expected. In this case, it is necessary to work at low concentrations in order to favor the intramolecular process but even under these conditions, very low yields of cyclized products are sometimes obtained. The use of the faster ring-opening processes (see Section XII. 1.D) will be then a useful complementary probe. But there is a case where even these faster reactions cannot afford a positive answer, when the radical intermediate reacts, by dimerization or disproportionation, with another radical in the solvent cage, since these processes are faster than the rearrangement processes. 

The ring-opening process of Equation 8.45 is, of course, simply the reverse of the process by which oxiranes (oxacyclopropanes, epoxides) are formed from halohy-drins (e.g., see item 3,Table 7.6). Further, as written, the processes shown in Schemes 8.90-8.92 are reversible and thus, at least in principle, carbonyl compounds can be converted to enol ethers, acetals (and ketals), and orthoesters. However, while acetals and ketals readily form from alcohols and acids under dehydrating conditions (Chapter 9) and esters undergo exchange reactions with alcohols in the... 

Aumann et al. have observed an unusual formal [6S+2C] cycloaddition reaction when they performed the reaction between an alkynylcarbene complex and 1-aminobenzocyclohexenes. The solvent used in this reaction exerts a crucial influence on the reaction course and products of different nature are obtained depending on the solvent chosen. However, in pentane this process leads to cyclooctadienylcarbene complexes in a reaction which can be formally seen as a [6S+2C] cycloaddition [117] (Scheme 71). The formation of these compounds is explained by an initial [2+2] cycloaddition reaction which leads to a cy-clobutenylcarbene derivative which, under the reaction conditions, undergoes a cyclobutene ring opening to furnish the final products. 

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