Neighboring group participation in carbene chemistry. Effect of neighboring carboxylate group on carbene reactivities

Article abstract of DOI:10.1021/ja00177a033

Reactivities of (alkoxycarbonyl)carbenes are shown to be dramatically changed as one substitutes the ester group with carboxylate group. Thus, (methoxycarbonyl)- or carboxyl(4-nitrophenyl)carbene (2a and 2b, respectively), generated by photolysis of the corresponding diazo compounds in a binary mixture of 2-methyl-2-butene and methanol, gives both cycloaddition products to the butene and OH inserts products into methanol, the relative reactivities (k_OH/k_add) being 0.3-0.5. In marked contrast, the "carboxylate" carbene (2c) generated from the sodium salt of the diazoacetate under the same conditions produces mostly the OH insertion product at the expense of the cyclopropanes, k_OH/k_add being > 100. The marked effect of the carboxylate group is nicely explained in terms of the participation by the neighboring carboxylate group, which interacts strongly with the vacant p orbital of the singlet carbene, resulting in the reduction of the electrophilicity. The competition experiments using two sets of alkenes with different electron density also support the above idea Thus, 2c is > 2000 times more reactive to α-chloroacrylonitrile relative to l-hexene while 2a is only 3-4 times more reactive. More intriguingly, a Hammett treatment of the addition of 2 to a series of substituted styrenes demonstrates that philicity of the carbene is converted from electrophilic to nucleophilic in going from 2a to 2c. The geometries of the singlet state of the parent carboxylate carbene optimized by the ab initio molecular orbital using the STO-3G basis set is very much like that of α-lactone anion, where strong interaction between carboxylate oxygen anion and the vacant p orbital is possible. ESR studies show, however, that both 2a and 2c have a triplet ground state with comparable thermal stability and that the geometry of the triplet state is not affected by the neighboring carboxylate group.