Stepwise direct route

Conceptually, two routes are available to acyclic carbonates from C02, the so-called indirect and direct routes. The first strategy is to operate in stepwise fashion,... 

These stepwise reactions raise yield and selectivity issues in the final acyclic carbonates, due to thermodynamics constraints. The second strategy, the so-called direct route, consists of converting C02 in one-step to create an end-product (Equation 7.8). This reaction is also thermodynamically equilibrated (see below). 

The kinetic isotope method was also applied to study whether the stepwise or direct route is valid for the dehydrogenation of the cyclohexane ring.[bi4,67] mentioned before, plotting specific radioactivities as a function of the contact time confirmed the predominance of direct pathway on Re and the stepwise dehydrogenation on chromia. 

With increasing reaction severity, the concentrations of the individual isomers approach their equilibrium values. The monomolecular route is the most effective for achieving high yields of PX, which is typically the most desirable for petrochemical applications. The schematic above shows the stepwise interconversion of OX to MX and MX to PX, which is consistent with a 1,2-methyl shift route. However, the results of kinetics studies provide some indications in favor of a reaction step that directly converts OX to PX [62]. It is not clear what the form of the reaction intermediate for this transformation is. Some in situ time-resolved spectroscopic methods have been used to look at how modification of zeoMtes like MFl affects the monomolecular mechanism by constraining the diffusion of MX [63]. 

In the various reactions involving 4,6-dimethoxyindolin-2- one 73 and phosphoryl chloride, there is the possibility that the likely intermediate, 2-chloro-4,6-dimethoxyindole 102, could react with itself, as it is both electrophilic at C-2 and nucleophilic at C-3 and C-7. Even though no products of such a self-condensation process were observed in any of the reactions with indoles, an investigation of this combination of reactants was carried out. The reaction was indeed quite slow, but eventually very small yields (2 and 1%) of the rather interesting heterocyclic system 103 and its further substituted product 104 were isolated44 (Scheme 29). A possible mechanism is indicated in Scheme 29. A similar 1-methylindole trimer has been synthesized from 2-iodo-1-methylindole by both direct and stepwise routes.39,40 So far attempts to generalize the phosphoryl chloride reaction and improve the product yields have not met with success. 

In reality, the situation is more subde because very often the synthesis of direct analogues is justified by a desire to improve the existing drug. Thus, for penicillins the chemical structure that surrounds the beta-lactam ring is still being modified. Current antibiotics that have been derived from this research (e.g., the cephalosporins) are more selective, more active on resistant strains, and can be administered by the oral route. They are as different from the parent molecule as a recent car compared to a 40-year-old model In other words, innovation can result from the sum of a great number of stepwise improvements, as well as from a major breakthrough. 

The first reported synthesis of this compound involved the stepwise procedure described in Scheme 5.8.5 One can note that there are two cyclization reactions which were accomplished in 59% and 60% yields. The synthesis required ten steps and the overall yield was 7%. In order to gain more direct access to the macrobicyclic compound a shorter route involving a tripod-tripod coupling sequence was developed (Scheme 5.9). 

Equation (1.36) conforms to the relationship deduced by Horiuti and Boreskov in the 1940s for a particular case of stationary single route catalytic reactions [1—4]. This relationship states that the overall rate of such a stepwise catalytic process can be expressed as the difference between the rates of the direct (v ) and reverse (v ) routes of this stepwise process ... 

The synthetic importance of reaction (a) ° comes from the fact that it reduces to one step the pathway for conversion of an acid chloride into a nitrile (instead of the classical and rather inconvenient two-step route via an acid amide). Reaction is an example of a new transformation for aliphatic amines. Previously, there were no methods available for the direct transformation of an amino into a nitro group and the stepwise procedures were too cumbersome to be of practical use. Transformation of a nitro group into a carbonyl is a well-known reaction. Its modification, shown in reaction represents a welcome opportunity to obtain a protected carbonyl group as the immediate result of such a transformation. The viability of the sequential reactions (b) plus (c) enables the employment of a > CHNH2 moiety as a synthetic equivalent to a protected carbonyl group. A one-pot sequence of imine formation and its reduction with sodium triacetoxyborohydride represents a convenient... 

In order to make a more clear distinction between the two suggested routes for the interaction of the complex with oxygen, namely, the direct oxidation to Fe(lll), and the stepwise replacement of pyridine with oxygen, further investigations are in progress with studying separately the behaviour of Fe(Pc)-Y catalyst centres without addition of pyridine. 

The photochemical production of vinylcyclopropane derivatives from compounds having two 7t-moieties bonded to an sp3-hybridized carbon648 is termed the din-methane rearrangement, also known as the Zimmerman reaction.649 A very broad spectrum of di-7t-systems can lead to photoproducts that are usually not obtainable by alternative routes.632,633 The reaction may be classified formally as a [l,2]-shift but, according to the proposed stepwise biradical mechanism,650 651 1,3- and 1,4-biradical (BR) intermediates and also the second 7t-bond may be involved652 (Scheme 6.29). A concerted (pericyclic) pathway for the di-7t-methane reaction from the excited singlet state is, however, not excluded. Typically, the singlet state reaction occurs upon direct... 

There are three possible routes for the formation of a Schiff base chelate (LXX) (a) There can be an equilibrium between the heterocycle and the Schiff base. In this event, a metal chelate could be formed with the Schiff base. (b) There can be an equilibrium between the heterocycle and the two starting materials. The latter can react with the metal ion in a stepwise manner to give the Schiff base chelate, (c) Alternatively, the metal ion can react directly with the heterocycle inducing a rearrangement reaction which results in the Schiff base chelate. From a kinetic study of the reactions of 2,2 -bisbenzothiazoline and its 2,2 -dimethyl derivative with Cd(II) and Zn(II), it was determined that the pathway (b) was inconsistent with the experimental results in all cases. The most probable course of the reaction involves a metal ion-induced rearrangement reaction, although it is possible in some instances that pathway (c) might simultaneously contribute to the formation of these Schiff base chelates (109). 

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