Valence electron sextet

Suppose a valence electron sextet occurs at a carbon atom and the possibility exists for a... 

Rearrangements in Species with a Valence Electron Sextet... 

The reader has already encountered semipinacol rearrangements in which the elimination of the leaving group was not followed but instead was accompanied by the [1,2]-rearrangement (Figures 14.20 and 14.22-14.24). In this way, the temporary formation of an energetically unfavorable valence electron sextet could be avoided. These rearrangements are summarized in the first two rows of Table 14.1. 

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. 

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