Carbenium ions cycloaddition reactions

Figure 15.20 shows the multistep mechanism of the [4+2]-cycloaddition between 1-(dimethylamino)-l,3-butadiene and cis-dicyanoethenedicarboxylic acid diester. The reaction proceeds via an intermediate, which must be zwitterion conformer B. The anionic moiety of this zwitterion is well stabilized because it represents the conjugate base of a carbon-acidic compound (Section 13.1.2). The cationic moiety of zwitterion B also is well stabilized. It is an iminium ion (i.e., a species with valence electron octet) rather than a carbenium ion (which is a species with valence electron sextet). Moreover, the iminium ion is stabilized by conjugation to a C=C double bond. 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

The cycloaddition using allylsilanes as 1,2-dipole equivalents is apphcable to fhe construction of six-membered rings. j5-Oxyaldehydes as well as u-oxyaldehydes undergo Lewis acid-promoted cycloaddition with allylsilanes to provide substituted THP derivatives in moderate yields (Scheme 10.190) [509]. When N-t-butoxycarbo-nyl-O,N-acetals are used as the substrates, silylated oxazinones are obtained as major products [510]. In this reaction the t-butoxycarbonyl group adds to a /7-silyl-carbenium ion intermediate for ring-closure. 

A number of alkene radical cations have been generated in matrices at low temperature and have also been studied by ESR, CIDNP, and electrochemical methods. However, until recently very little absolute kinetic data have been available for the reactions of these important reactive intermediates in solution under conditions comparable to those used in mechanistic or synthetic studies. In a few cases, competitive kinetic techniques have been used to estimate rates for nucleophilic additions or radical cation/alkene cycloaddition reactions. In addition, pulse radiolysis has been used to provide rate constants for some of the radical cation chemistry relevant to the pho-topolymerization of styrenes. More recently, wc and others have used laser flash photolysis to generate and characterize a variety of alkene radical cations. This method has been extensively applied to the study of other reactive intermediates such as radicals, carbenes, and carbenium ions and is particularly well-suited for kinetic measurements of species that have lifetimes in the tens of nanoseconds range and up and that have at least moderate extinction coeffleients in the UV-visible region. 

The kinetic data discussed above demonstrate the effects of varying the structure of both the styrene radical cation and the alkene on the initial step in the cycloaddition reaction. However, the transient experiments do not provide any evidence that would permit one to distinguish between a concerted or stepwise mechanism. The kinetic data obtained for additions to a range of alkenes do show considerable similarities to those reported for the addition of carbenium ions to the same substrates. For example, rate constants for the addition of the bis(4-methyl-phenyl)methyi cation to a series of ring-substituted styrenes also correlate with the Hammett a and a parameters with p and p values of-5.2 and -5.0, respectively." The latter reactions are thought to proceed via a partially bridged transition slate and might, therefore, be expected to show similarities to concerted... 

Miscellaneous Reactions. Isobutene, as the carbenium ion source for Ritter reactions, produces Al-r-butyl amides (eq 34). Isobutene undergoes highly regioselective cycloaddition with isocyanate to form azetidinones and with nitrones to yield isox-azolidines (eqs 35 and 36). Allyl thioethers are formed by the electrophilic addition of Benzenesulfenyl Chloride to isobutene (eq 37). Alkenylphosphonous dichlorides and alkenylthionophosphonic dichlorides are also formed by addition of PCI5 and P2S5 (eqs 38 and 39). ... 

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