Reaction electrophilic cleavage

Several mechanisms have been proposed for these electrophilic cleavage reactions. In general, these mechanisms range from pure SE1 to pine Se2 processes although these two extreme mechanisms are usually reserved for special cases. 

ELECTROPHILIC CLEAVAGE REACTIONS OF CARBON SILICON BONDS IN FLUORO-ARYLSILANES WITH OR WITHOUT FRIEDEL-CRAFTS CATALYSTS... 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

A surprising exception has been reported with evidence for a cleavage reaction in the case of divinyl sulphone. In non-aqueous and slightly acidic media, the behaviour of a., ji-unsaturated aromatic sulphones is also complex (see Table 7) since the cleavage and the saturation may compete. Strongly electrophilic double bonds undergo Michael additions in aprotic solvents by slowly protonated anions. Transfer of labile hydrogen may also lead to unactivated bases. It is noteworthy that in numerous cases (Table 6) the saturation is the preferred route. 

The compounds described herein were prepared by three methods. The first route involves deprotonation/substitution at the N-H sites of 1, the second consists of a cleavage reaction of an Si-N derivative of 1 with PhBCI2, and the third route is a transamination reaction between a bis(dimethylamino)boryl derivative of 1 and an aliphatic diamine. In the first approach, compound 1 is deprotonated by treatment with one equivalent of n-BuLi. Quenching of the resulting anion with various electrophiles produces the monosubstituted products 2-4 (eq 3). 

The cause for the high P-P bond reactivity in metathesis and addition reactions of P-phosphino-NHPs was further rationalized when it was established that attack of electrophiles at the PR -substituent facilitates bond cleavage reactions by further boosting the polarization and weakening of the P-P bond and may thus be considered to induce an autocatalytic activating effect [77],... 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

The intermediate formation of the ferrocenyl-substituted silylium ion 16 by protonation of the ansa-ferrocenyl silane 17 can be regarded as a special case of electrophilic cleavage of an activated C-Si bond (see Scheme 7). The driving force for this reaction is the release of a strain by formation of the silyl cation. In a... 

---