Stepwise Reaction Mechanisms

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

While direct photolysis of (46) gave (48), the sensitized photolysis of (46) with acetophenone gave only the cyclopropane product (>93%). These authors favor a stepwise oxa-di-w-methane reaction mechanism (8.69) inasmuch as a concerted reaction should produce the product without a change in multiplicity. That is, the product would have to be formed in the triplet state and not enough energy is available for the product to be formed in an excited state ... 

According to the stepwise electrophilic reaction mechanism, the differences in the stereochemistries of the products from the reactions of alkenes with cyclic 49 and acyclic 51 disulfonium dications can be explained by the larger rates of the intramolecular reactions. In the case of a cyclic dication, the carbocationic center in intermediate 94, which is formed as the result of initial attack by a S-S dication on a double C=C bond reacts with nucleophile intramolecularly, thus conserving the configuration of the substituents at the double bond. On the other hand, an acyclic dication undergoes transformation to two separate particles (95 and dimethylsulfide) with a consequent loss of stereoselectivity. Additional experiments with deuteretad alkenes confirm that reaction is not stereoselective, lending further support to the stepwise mechanism (Scheme 36).106... 

The reaction mechanisms of these transition metal mediated oxidations have been the subject of several computational studies, especially in the case of osmium tetraoxide [7-10], where the controversy about the mechanism of the oxidation reaction with olefins could not be solved experimentally [11-20]. Based on the early proposal of Sharpless [12], that metallaoxetanes should be involved in alkene oxidation reactions of metal-oxo compounds like Cr02Cl2, 0s04 and Mn04" the question arose whether the reaction proceeds via a concerted [3+2] route as originally proposed by Criegee [11] or via a stepwise [2+2] process with a metallaoxetane intermediate [12] (Figure 2). 

All reaction mechanisms presented here should be understood as hypotheses to rationalize the outcome of the processes. Alternative explanations such as stepwise and dissociative mechanisms cannot be excluded. However, as long as the constitution and configuration of the product can be described as that of a 3,3-sigmatropic rearrangement, the present hypotheses seem acceptable. 

Oxidation of benzyl alcohol catalysed by chloroperoxidase exhibits a very high prochiral selectivity involving only the cleavage of the pro-S C-H bond. The reaction mechanism involved the transfer of a hydrogen atom to the ferryl oxygen of the iron-oxo complex. An a-hydroxy-carbon radical and the iron-hydroxy complex P-Fe -OH form. They may lead to the hydrated benzaldehyde or stepwise with the formation of the intermediate a-hydroxy cation. 

The mechanism of the thermal 5 rnI reaction, using 4-nitrocumyl chloride and 2-nitropropanate ion as a model has been investigated. The results provided unambiguous evidence that a decrease in driving force is able to change the mechanism of homogeneous reductive cleavage reactions from stepwise to concerted. ... 

A stepwise reaction mechanism which involves adsorption of nitrate at a bimetallic site, reduction to nitrite, desorption in to the aqueous phase and re-adsorption at a monometallic e.g. Pd) site has been proposed and is supported by theoretical prediction. A reaction scheme based on the use of a bimetallic catalyst is illustrated in Fig. 2. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

The benzylic substrates X-l-Y and X-2-Y have provided a useful platform for examining the changes in reaction mechanism for nucleophilic substitution that occur as the lifetime of the carbocation intermediate is decreased systematically by varying the meta- and para- aromatic ring substituents. When X is strongly resonance electron-donating, X-l-Y and X-2-Y react by a stepwise mechan-... 

Students of reaction mechanism will recognize intuitively that the difference between the narrow and broad borderline regions observed for nucleophilic substitution of azide ion at secondary and tertiary carbon (Fig. 2.2) is due to the greater steric hindrance to bimolecular nucleophilic substitution at the tertiary carbon. This leads to a large difference in the effects of an a-Me group on (s ) for the stepwise solvolysis and s ) for concerted bimolecular nucleophilic... 

The change from a stepwise preassociation mechanism through a triple ion intermediate to an uncoupled concerted reaction occurs as the triple ion becomes too unstable to exist in an energy well for the time of a bond vibration ( 10 s). The borderline between these two reaction mechanisms is poorly marked, and there are no clear experimental protocols for its detection. These two reaction mechanisms cannot be distinguished by experiments designed to characterize their transition states, which lie at essentially the same position in the inner upper right hand corner of Figure 2.3. Only low yields of the nucleophilic substitution product are obtained from both stepwise preassociation and uncoupled concerted reactions, because for formation of the preassociation complex in water is small... 

Before discussing the experimental results, let us make the connection to the topic of this section—PE prohles. The representation of concerted or stepwise mechanisms on a PE prohle seems clear cut. If the reaction is stepwise it involves an intermediate, appearing as a local minimum on the PE profile—such as that near the center of the prohle in Eigure 21.1. On the other hand, a concerted reachon can have no intermediate, and so its PE prohle should have just a reactant minimum and a product minimum connected by a single transition state. 

Intramolecular ionic Diels-Alder reactions were carried out in highly polar media to afford carbocyclic ring systems. The strategy, which obviates the need for high temperatures and pressures, features in situ generation of heteroatom-stabUized allyl cations that undergo subsequent (4 + 2) cycloaddition at ambient temperature. Typically, reactions were complete within 1 hour after addition of substrate. Some cycloadducts were the result of a concerted process, whereas others were formed via a stepwise reaction mechanism (Grieco, 1996). 

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