SnIcB mechanism

This is an example of an SnIcB mechanism (p. 448). The other two compounds react by the normal mechanisms. Carbon tetrachloride cannot give up a proton and dichloromethane is not acidic enough. 

Reaction with chloroform under basic conditions is a common test for primary amines, both aliphatic and aromatic, since isocyanides have very strong bad odors. The reaction probably proceeds by an SnIcB mechanism with dichlorocarbene as an intermediate ... 

Table IV. Calculated Stereochemical Results in Base Hydrolysis of ci5-Co(en)2LX Assuming SnICB Mechanism...

Studies of the base hydrolysis of cis-[Co(en)2(GlyOR)X] ions led to interesting developments. Base hydrolysis of halopentamine complexes of cobalt(III) occurs by an SnICB mechanism leading to a five-coordinate intermediate. In the N-bonded ester complexes of the [Co(en)2(GlyOR)Cl] type, the ester carbonyl group and solvent water can compete for the vacant site in the five-coordinate intermediate (Scheme 5) giving rise to the chelated glycine ester species and the hydroxypentamine respectively. 

The kinetics of aquation and base hydrolysis of the cis-[Co(en)2(NH2Et)02CR] ions (R = H or Me) have been studied in detail. Aquation is strongly acid-catalyzed and rate and activation parameters for this process are reported. Similar rates are observed for both complexes in spite of the differences in basicity of coordinated formate and acetate. Aquation rates (k q) are also very similar, but base hydrolysis of the formato complex is some five times faster than that of the acetato complex, consistent with a dissociative SNiCB mechanism and cleavage of the Co—O bond. 

The observed rate expression is of the form expected for an (A) or (I) type reaction, but the mechanism is in fact dissociative since the slow step is the dissociation of the conjugate base Co(NH3)4(NH2) t This process is normally called the SnICB mechanism, but it would be preferable to call it Dcb process. This reaction is considered in detail in Chapter 3. 

It is now generally accepted that the base hydrolysis pathway occurs by a l cB (SnICB) mechanism. During the period 1955-65 there was a lively controversy on the topic, an account of which has been written by Basolo [5] and by Pearson [18]. The currently accepted Dcb mechanism was first proposed in 1937 by Garrick [6] on the basis of the similarity of the kinetics of these reactions to those of proton exchange of amine complexes. The mechanism was revived and developed by Basolo and Pearson in the 1950 s [7]. The Dqb mechanism is outlined by the set of equations (1-3). Hydroxide ion acts as a base rather than as a nucleophile and removes a proton from a... 

Fig. 5.2 The SnICB mechanism for rapid base hydrolysis of cobalt ammine chloride complexes...

If this SnICB mechanism is responsible for the unique effectiveness of OH- then it would not be such a good reagent for analogous cobalt(III) complexes containing no acid N—H bonds. Experiments do in fact show that complexes of the type frans-[Co(py)4Cl23 (py = pyridine) and... 

The most direct evidence in support of an SnICB mechanism would be to establish the formation of a five-coordinated intermediate. This intermediate would be extremely reactive and therefore very difficult to detect. Experiments in water have not been successful because water is such a good coordinating solvent that it immediately enters the five-coordinated system. However, experiments in the solvent dimethyl sulfoxide do rule out an Sn2 mechanism in this solvent and support the SnICB process. This was demonstrated by experiments showing that the reaction of []Co(NH3)5Cl]p+ with NOa is catalyzed by hydroxide ion, Eqs. (31) and (32). 

Since this is an SnICB mechanism, it follows that the rate of the catalyzed reaction is determined by the rate of dissociation of the conjugate base, Eq. (29). Experiments do show that the rate of reaction at the same OH concentration does not depend on the concentration of NO2- and that it is the same for the entry of other nucleophilic reagents such as NCS- and Nj". Furthermore, other bases such as piperidine also act as catalysts. 

The CHsO, analogous to OH in water as the solvent, is believed to react by an SnICB mechanism. 

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