Ring-opening reactions reactants

Lactam polymerization represented by reaction 5 in Table 5.4 is another example of a ring-opening reaction, the reverse of which is a possible competitor with polymer for reactants. We shall discuss this situation in Sec. 5.10. 

Scheme 3. Schematic representation of the ring-opening reactions ofESBO with various reactants (23).

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

Scheme 5 Effective charge distribution (e) in the transition structure of the sultone ring opening reaction deducedfrom substituent effects on both forming and breaking bonds. Values of Ae are written relative to respective atoms in the reactant structure (cf...

Entropy is a sophisticated concept, and this is not the place to give a rigorous definition of disorder suffice it to say that a disordered system is more probable than an ordered one, and the entropy ofa system is proportional to the logarithm ofits probability [101]. Intuitively, we see that A5 > 0 for a process in which the product orthe transition state is less symmetrical or has more freedom of motion than do the reactants. For example, ring-opening reactions, since they relieve constraints on intramolecular motion, should be accompanied by an increase in entropy. Note that an increase in entropy favors a process it increases a rate constant (activation entropy) or an equilibrium constant (reaction entropy), while an increase in enthalpy disfavors a process. 

Over alkaline earth oxides and Na/Na0H/Al203, the double bond isomerization selectively takes place. The reactants contain three membered and four membered rings in the molecules. If solid acid catalysts were used, the ring opening reactions would occur, and the selectivity for doubie bond isomerization should markedly decrease. One characteristic feature of solid base catalyst is a lack of C-C bond cleavage ability. Therefore, the double bond isomerization selectively occurs without C-C bond cleavages. 

Direct reaction of epoxy functionalities with amine-terminated reactants in a nucleophilic ring-opening reaction. 

As we ve just seen nucleophilic ring opening of ethylene oxide yields 2 substituted derivatives of ethanol Those reactions involved nucleophilic attack on the carbon of the ring under neutral or basic conditions Other nucleophilic ring openings of epoxides like wise give 2 substituted derivatives of ethanol but either involve an acid as a reactant or occur under conditions of acid catalysis... 

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