Two-step ionic mechanisms

The rr electrons of unsaturated carbons are susceptible to electrophilic attack, generally, by a two-step ionic mechanism as in (20). Alkenes react faster than alkynes since the p electrons in the latter are more... 

RX - [Ni(I), RX ], is superseded by more examples of the four types of processes concerted, radical, radical-chain, and two-step ionic. Under the mantle of reductive elimination, perhaps not quite correctly, /3 elimination of hydrogen is included since this process seems able to take place under very similar conditions to those under which a concerted reaction occurs. It is interesting to see the use of Hoffmann-type calculations in rationalizing elimination reaction pathways. Mention should be made also of the mechanism of processes involving two metal centers. 

The mechanism for reaction 13 is not well established yet, but it is likely to proceed through ionic intermediates (11) the Cl atom in chlorine nitrate is slightly electropositive, so that it readily combines with negative chloride ions to produce CI2 the HQ on ice is expected to be at least partially ionized. We have found that HCl has a very high mobility on the ice surface, so that even small amounts of HCl will enable reaction 13 to occur. It is also possible for this reaction to proceed in two steps the initial step is the reaction of chlorine nitrate with ice it is followed by the reaction of the product HOCl with HCl on the ice substrate ... 

The kinetics of the general enzyme-catalyzed reaction (equation 10.1-1) may be simple or complex, depending upon the enzyme and substrate concentrations, the presence/absence of inhibitors and/or cofactors, and upon temperature, shear, ionic strength, and pH. The simplest form of the rate law for enzyme reactions was proposed by Henri (1902), and a mechanism was proposed by Michaelis and Menten (1913), which was later extended by Briggs and Haldane (1925). The mechanism is usually referred to as the Michaelis-Menten mechanism or model. It is a two-step mechanism, the first step being a rapid, reversible formation of an enzyme-substrate complex, ES, followed by a slow, rate-determining decomposition step to form the product and reproduce the enzyme ... 

The previous section described a two-step mechanism. The first step is the coordination of the substrate into the metal coordination sphere. The second is the most characteristic step within the inner-sphere mechanism the insertion of the substrate into the M-H bond. Nevertheless there are other mechanistic options that include neither substrate coordination nor M—H insertion. They are outer-sphere mechanisms and in turn can be classified as bifunctional and ionic mechanisms. 

Note that this mechanism is closely related to that of the bifunctional catalysts. In fact, the latter is also said to work by means of an ionic mechanism since one proton and one hydride coming from the catalyst are added to the substrate. Note, however, that there is an important difference between both mechanisms ionic mechanism takes place in two separated steps for the proton and hydride transfers, respectively, whereas in those involving bifunctional catalysts the H+ and H transfers takes place conceitedly. 

The oxidation of guanine (G) and adenine (A) follows a two-step mechanism involving the total loss of four electrons and four protons showing current peaks at approximately 0.9 and 1.2 V, respectively. However, the redox properties are dependent on the pH, the ionic strength of the electrolyte, and the electrode material.2 The reader is referred to a recent review by Palecek and coworkers for a more comprehensive discussion regarding the electrochemical mechanism of the oxidation and reduction of DNA bases on carbon and mercury electrodes.3 4 Guanine oxidation is irreversible and occurs in two consecutive steps (Fig 10.1).5... 

Relatively few authenticated photoinduced concerted [w2 + w2] cycloaddition reactions involving pyrroles or indoles have been reported. The formation of many cycloadducts can be rationalized in terms of two-step thermally induced ionic mechanisms. 

Ionic additions which transform cyclopropenes into cyclopropanes are well known. The electrophilic addition of sulphenyl halides proceeds by a two-step mechanism to give trans-disubstituted products but with little regioselectivity. Cyclic sulphonium ion intermediates are probably involved as illustrated for 1-methylcyclopropene. Electrophilic... 

Usually 5-endo cyclizations cannot be formed via an intramolecular ene reaction because of a disfavored (strained) transition state. Cyclization occurs in a two-step mechanism via an ionic intermediate to give cyclopentanones or cyclopentanols. 

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