Ylide formation carbon atom reactivity

From the discussion in the foregoing section, it should be clear that the structure of the product(s) from the cycloaddition reactions is dependent on the nature of the reactants as well as reaction conditions. In this section an attempt will be made to look at the general factors influencing the selectivity and reactivity of the nonclassical A,B-diheteropentalenes. In simple terms, cycloaddition or bond formation between terminal carbon atoms occurs when the topology and symmetry of the orbitals of the reacting ylide system and the dipolarophile allow parallel approach (Figure 2). 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

When diazoimide 19 (or 20) was deacetylated [36] and the resulting diazoamide 23 (or 24) was subjected to rhodium(II) acetate, the yield of the corresponding cycloadduct (i.e. 25 or 26) was significantly diminished. One explanation for this, different reactivity is the inherent decrease in electrophilic character conferred upon the intermediate rhodium carbenoid when the diazo carbon bears a hydrogen atom rather than an acetyl group. This decrease in electrophi-licity may alter the rate of carbenoid attack on the remote carbonyl group to the point where alternative reactions can occur. Another possible explanation to account for the diminished reactivity is that the preferred conformation of the intermediate rhodium carbenoid may not be the one that results in carbonyl ylide formation [35]. 

For the azomethyne ylide 86, the presence of the electron-releasing methyl group on the Cl position polarizes the HOMOdipo e through the C3 carbon atom. As a result, the unsubstituted C3 carbon atom presents a larger ff value compared to that at the Cl site. Therefore, along the cycloaddition reaction, the more favourable reactive channel takes place through the C3-C4 bond formation by the nucleophilic attack of the C3 carbon atom of 86 to the more electrophilic C4 site of 14, according to the experiment results. 

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