Mechanisms of Cleavage

Tin-carbon bonds can be broken by reaction with electrophiles (e.g. protic acids, Lewis acids, halogens), nucleophiles (e.g. RLi), or free radicals (e.g. succimidyl, t-butoxyl), or with certain transition metal (particularly palladium) compounds. Fragmentation can also be induced through the radical cations which are formed by electron transfer. 

The substitution reactions of organometallic compounds, in which heterolytic attack at the carbon is usually by electrophiles, are not susceptible to the simple mechanistic classification (SnI, Sn2, and S i) which is so useful in discussing nucleophilic reactions of organo-nonmetallic compounds. The situation is complicated by the propensity of the metal to increase its coordination number, and a spectrum of mechanisms has to be considered in which electrophilic attack at carbon is accompanied by or preceded by nucleophilic attack at the metal. 

Further, these basic models must be modified to include the effect of catalysts. Other mechanisms may be involved when the organic group has a special structure (e.g. aryl, vinyl, benzyl, and allyl). 

Evidence for these mechanisms comes mainly from kinetic measurements including steric, solvent, and salt effects, and from some studies of the stereochemistry at the carbon centre which is difficult to obtain because the products R3S11X are configuration-ally unstable when X is an electronegative group. Pope and Peachy described the optical resolution of methylethylpropyltin iodide in 1903 by crystallisation of the (+)-camphorsulfonate from water in the presence of KI,26 and reported the iodide to be optically stable, but this has since been discounted.27 

These mechanisms are discussed, together with the reactions, below. By most of these mechanisms, the alkyl-tin bond is less reactive than the aryl-, alkenyl-, alkynyl-, allyl-, or benzyl-tin bond, and when two types of group are bonded to tin, reactions are selective for this second class of group. 

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Rossi and Bunnett64 studied the chemical reductive cleavage of diphenyl sulfoxide, diphenyl sulfone and methyl phenyl sulfone under the action of potassium metal in liquid ammonia in the presence of acetone. The enolate ion is used to trap phenyl radicals formed eventually during the process, in order to determine whether one or two electrons are required for the mechanism of cleavage (Scheme 7). In all the runs, phenyl anion is... 

The 4-pentenoyl group is easily removed from amides by I2 and can be used as a protecting group. The mechanism of cleavage involves iodocyclization and hydrolysis of the resulting iminolactone (see Section 4.2.1).239... 

Figure 14-7. Snapshots of the active site structures near the transition state of (top) the nucleophilic attack and (bottom) the exocyclic cleavage for the in-line monoanionic O2p mechanism of cleavage transesterification in the hairpin ribozyme. The yellow and red colored cartoon is for the substrate and ribozyme strands, respectively, and water molecules interacting with non-bridging oxygens and O5/ are shown...

Figure 18 The mechanism of cleavage of amide bonds by cathodic current. (From Ref. 40.)...

Not even traces of strong acids should be added to high strength solutions of the hydroperoxide [1], The thermal stability and mechanism of cleavage of mixtures with 4-toluenesulfonic acid have been studied under adiabatic conditions, and there is potential for development of a thermal runaway [2]. 

Before discussmg the mechanism of cleavage of carboxylic acid esters and amides by hydrolases, some chemical principles are worth recalling. The chemical hydrolysis of carboxylic acid derivatives can be catalyzed by acid or base, and, in both cases, the mechanisms involve addition-elimination via a tetrahedral intermediate. A general scheme of ester and amide hydrolysis is presented in Fig. 3. / the chemical mechanisms of ester hydrolysis will be... 

An explanation for the smaller-than-expected effect of strain energy upon the rate of hydrolysis of /3-lactam rings can be found upon examination of the mechanism of cleavage of the C-N bond amides and lactams are hydro-... 

Fig. 8.21. Mechanisms of cleavage ofN-[(acyloxy)methyl] derivatives of primary amides (8.170, R = H) and secondary amides (8.170, R = methyl) via an acid-catalyzed pathway (.Reaction a), a pH-independent pathway (.Reaction b), and base-catalyzed pathways (Reactions c and d) [218].

Fig. 4A The mechanism of cleavage by ribonuclease A. Two imidazole residues function as general acid-base catalysts. B The single-metal-ion mechanism proposed for cleavage by the hammerhead ribozyme. One metal ion binds directly to the pro-Rp oxygen and functions as a general base catalyst. C The double-metal-ion mechanism proposed for cleavage by the hammerhead ribozyme. Two metal ions bind directly to the 2 -oxygen and the 5 -oxygen...

In the catalysis of the lyase from C. perfringens, the participation of lysine residues forming intennediary Schiff bases between enzyme and substrate molecules, and of histidine residues, has been demonstrated with the aid of photooxidation, reagents for histidine modification, and borohydride reduction in the presence of substrate.408-418 Thus, according to Frazi and coworkers,414 the lyase belongs to the class I lyases (aldolases). The catalytic mechanism proposed is outlined in Scheme 3. Evidence has been educed for the existence of a similar mechanism of cleavage of sialic acid by the lyase enriched from pig kidney.411... 

Not only oxidation but also thermolysis of X -phosphorins can lead to cleavage of both 1.1-substituents. According to Markl l.l-dibenzyl-2.4.6-triphenyl-X -phosphorin splits off 1.2-diphenylethane at temperatures higher than 220 °C to form 2.4.6-triphenyl-X -phosphorins22 (seep. 24). Other l.l-carbo-2.4.6-tri-phenyl-X -phosphorins also split off C groups at high temperatures to form 2.4.6-triphenyl-X -phosphorin. The mechanism of cleavage may involve radicals. 

The most unclear part of this cycle is the mechanism of cleavage of the 0—0 bond. Transfer of the second electron leads formally to the presence of a peroxo group bound to Fe111, but the actual electronic distribution is not known. It seems unlikely that the FeIn(022 ) group would be... 

The hydrolysis of adenosine 3. 5 -cyclic monophosphate (cAMP) by the cobalt complexes (215) was considered here earlier,187 as was the Ce(IV)-catalysed hydrolysis of phospho monoesters in nucleotides.189 A review (ca 100 references) on current data on the mechanism of cleavage-transesterification of RNA has appeared.258 In this review special attention was focused on the two crucial steps in the hydrolysis of RNA, i.e. cleavage-transesterification and hydrolysis of the cyclic phosphodiester (Scheme 14). The catalysis of various amines for the hydrolysis of RNA has been looked at and ethylenediamine and propane-1,3-diamine are highly active under physiological conditions because they exist as the catalytically active monocation forms.259... 

Scheme 2.5 Mechanism of cleavage of />-methoxybenzyl ethers with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Note SET=single-electron transfer.

The mechanism of cleavage of the a-(l—A) linkage by starch lyase involves removal of the proton on C-2, with subsequent formation of a double bond between C-2 and C-l, and simultaneous displacement of the protonated glycosidic oxygen atom. This results in the release of the starch chain with one less glucosyl unit and the formation of the enol of 1,5-anhydro-D-fructose (Figure 7.20). 

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