Carbenium ions, /3-cleavage

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

The leaving group doesn t have to be a water molecule any group or substituent which upon cleavage from the carbon skeleton under appropriate reaction conditions leaves behind a carbenium ion—e.g. a halogen substituent—will suffice. The other substituents can be hydrogen, alkyl or aryl. ... 

A mechanism for its formation was also proposed. Essentially, this involved protonation of 2-methylfuran followed by dimerization and trimerization to a 2,4-difuryl tetrahydrofuran derivative which suffered an acid catalysed cleavage of the saturated ring to produce a carbenium ion possessing an alcoholic function at the other end of... 

FIGURE 3.23 Protectors that are more sensitive or more stable to acidolysis than the parent protector. Electron-donating groups favor protonation and hence aid cleavage electron-withdrawing groups disfavor protonation. The numbers indicate the relative ease of cleavage, which also depends on the stability of the carbenium ion that is released. Bpoc = 2-(biphenyl-4-yl)-prop-2-yloxycarbonyl. 

Example The El mass spectmm of -decane is typical for this class of hydrocarbons (Fig. 6.18a). Branching of the aliphatic chain supports cleavage of the bonds adjacent to the branching point, because then secondary or tertiary carbenium ions and/or alkyl radicals are obtained (Fig. 6.18b,c). This allows for the identification of isomers to a certain degree. Unfortunately, hydrocarbon molecular ions may undergo skeletal rearrangements prior to dissociations, thereby obscuring structural information. 

Fig. 6.28. El mass spectrum of (3-methylpentyl)-benzene. McLafferty rearrangement and benzylic cleavage are clearly dominating. In the low-mass range carbenium ions and the aromatic fragments are present. Spectmm used by permission of NIST. NIST 2002.

On the one hand, this particular feature makes it more difficult to distinguish between reactions involving radical cations, free radicals or carbenium ions, but on the other hand the chemist acquires an additional tool to control the course of the intended reaction. Some illustrative examples of cyclization reactions that utilize cleavage of the radical cations, primarily generated by single-electron oxidation, will be given in the following sections. 

Finally, we point to the possibility of P = 0 bond formation from 1-alkoxy-X -phosphorin derivatives 124 or 125 by cleavage of alkyl cations. Also the reverse process, /. e. alkylation of the P = O moiety to form P—O—R groups is possible. The synthesis of X -phosphorins having functional groups at the C-atoms of the phosphorin ring was first made possible by the preparation of new stable X -phosphorin carbenium ions 140. Here again, the fundamental difference between phosphorin and pyridine systems comes to light Whereas carbanionic structures 139 b are stabilized in the pyridine series, in the X -phosphorin series carbenium ions as 140 b are stabilized. 

Fig. 25. Two possible pathways proposed by Sommer and co-workers explaining the observed H/D exchange of the alkanes. Pathway 1 the carbenium ion Rh (nondeuterated) is formed by protolytic cleavage of a C-H bond (via a carbonium ion intermediate). Pathway 2 the olefin R-is formed by acid-base bifunctional dehydrogenation. (Reprinted with permission from Sommer et al. (130a). Copyright 1995 American Chemical Society.)...

The first report of a distinct effect of silicon on a neighbouring carbenium ion was published as early as 1946. Sommer, Whitmore and coworkers34 35 reported that under conditions of basic hydrolysis, (l-chloroethyl)trichlorosilane 58 and (1-chloropropyl)trichlorosilane 59 failed to give cleavage of the a-C—Cl bond, while under the same conditions the analogous 2-chloroethyl 60 and 2-chloropropyl compounds 61 hydrolyse rapidly. 

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