Open sequence reactions

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

Bond energy considerations indicate that the initiation reaction (4.2.2) should be quite slow because its activation energy must be quite high (at least equal to the bond dissociation energy). If one were dealing with an open sequence reaction mechanism, such a step would imply that the overall reaction rate would also be low because in these cases the overall reaction becomes approximately equal to that of the rate limiting step. In the case of a chain reaction, on the other hand, the overall reaction rate is usually much faster because the propagation steps occur many times for each time that an initiation step occurs. 

Non-chain reactions are also called open sequence reactions, this adjective possibly better indicates their fundamental difference with closed sequence reactions, to which category so-called chain reactions and catalytic reactions belong, whether they are homogeneous or heterogeneous. 

First of all, it is obvious that this is really an open sequence reaction mechanism, as the two free radicals involved in this mechanism are not regenerated in the step following the one in which they first appear. 

This is an open sequence reaction, as the O atom produced by reaction (39) disappears in reaction (40) without being regenerated later on. 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

A reaction mechanism may involve one of two types of sequence, open or closed (Wilkinson, 1980, pp. 40,176). In an open sequence, each reactive intermediate is produced in only one step and disappears in another. In a closed sequence, in addition to steps in which a reactive intermediate is initially produced and ultimately consumed, there are steps in which it is consumed and reproduced in a cyclic sequence which gives rise to a chain reaction. We give examples to illustrate these in the next sections. Catalytic reactions are a special type of closed mechanism in which the catalyst species forms reaction intermediates. The catalyst is regenerated after product formation to participate in repeated (catalytic) cycles. Catalysts can be involved in both homogeneous and heterogeneous systems (Chapter 8). 

The derivation of a rate law from a postulated mechanism is a useful application of reaction mechanisms. It shows how the kinetics of the elementary reaction steps are reflected in the kinetics of the overall reaction. The following example illustrates this for a simple, gas-phase reaction involving an open sequence. The derivations typically employ the stationary-state hypothesis (SSH) to eliminate unknown concentrations of reactive intermediates. 

Open chain hydrocarbons, skeletal rearrangements, 29 298-302 aromatic selectivities, 29 302 atomization, 29 298-302 cyclization, 29 298-302 Open sequence in reaction mechanisms, 32 ... 

A strategy to accelerate p-BL formation was therefore developed, consisting of the synthesis of unsymmetrical tridentate Ci Schiff base) complexes (Fig. 44) that might open a reaction channel for this particular sequence after chloride abstraction with Na[Co(CO)4]. 

For reasons evident from the figure we call the two types of sequences respectively open and closed sequences. Since 1921 the latter type of sequences has been called chain reactions (11,12), a name which should be reserved for sequences of the closed type. Sometimes it has been used also for open sequences, but as the characteristics of the two types are very different this use may cause confusion and should be abandoned. 

From the equations of the open sequence (p. 319), it is evident that the first intermediate X2 (in the open sequence) must be formed from a reactant (Ai) in the overall reaction. If now we introduced this intermediate from the outside it would disappear either by the reaction (— 1) X2 + B2 —> A2 or by formation of resultants of the overall reaction. (The time taken for the disappearance of X2 would be the time it takes for the system to reestablish the steady state which is supposed to be very short.)... 

It is well known that the mechanism found by experiment is often incomplete, this expression meaning that the equations for the partial reactions which have been derived from the kinetics do not add together to give the equation for the overall reaction. Let us consider then an open sequence of four steps with the orientated diagram... 

Reaction of 4-oxo l/7-chromene-3-sulfonyl chloride with an excess of 1,3-diaminopropane believed to proceed through acylation/ring-opening sequence. Open-chain intermediate 175 undergoes intramolecular cyclization accompanied by elimination of amine and formic acid to afford 1,2,6-thiadiazocine ring system 176 (Scheme 46 <1994AP819>). 

The metabohc cycle of living organisms is an open sequence of reactions where an ahmentary substrate (which is the initial reactant for the conjugat ing process) is transformed (assimilated) in such a way that the initial state of... 

Homogeneous catalysis is observed in a large number of organic solution reactions. A homogeneous catalyst reacts with the substrate to form an intermediate in the first step of a sequence of several reaction steps leading to the product. In such a way, catalysis opens a reaction pathway with a sufficiently low energy barrier which is not otherwise available. 

An approach to 2,5-disubstituted tetrahydrofurans is outlined in Scheme 29. Solid-phase-bound 2-nitroethanol 108 was synthesized in two steps from Merrifield resin. TMS protection and reaction with phenyl isocyanate in the presence of 1,5-hexadiene gave the dihydroisoxazole 109 in a 1,3-dipolar cycloaddition of the nitrile oxide formed by dehydration of the nitroalkane. Treatment with ICl at -78 °C initiated a ring-closing-ring-opening sequence (electrophilic cyclization), which resulted in cleavage from the resin with the formation of tetrahydrofuran 110 in 40% yield as a mixture of diastereomers [44, 45]. 

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