On the mechanism of nitrosoarene-alkyne cycloaddition

The thermal reaction between nitrosoarenes and alkynes produces N-hydroxyindoles as the major products. The mechanism of these novel reactions has been probed using a combination of experimental and computational methods. The reaction of nitrosobenzene (NB) with an excess of phenyl acetylene (PA) is determined to be first order in each reactant in benzene at 75 degrees C. The reaction rates have been determined for reactions between phenyl acetylene with a set of p-substituted nitrosoarenes, 4-X-C(6)H(4)NO, and of 4-O(2)N-C(6)H(4)NO with a set of p-substituted arylalkynes, 4-Y-C(6)H(4)C[triple bond]CH. The former reactions are accelerated by electron-withdrawing X groups (rho = +0.4), while the latter are faster with electron-donating Y groups (rho = -0.9). The kinetic isotope effect for the reaction of C(6)H(5)NO/C(6)D(5)NO with PhC[triple bond]CH is found to be 1.1 (+/-0.1) while that between PhC[triple bond]CH/PhC[triple bond]CD with PhNO is also 1.1 (+/-0.1). The reaction between nitrosobenzene and the radical clock probe cyclopropylacetylene affords 3-cyclopropyl indole in low yield. In addition to 3-carbomethoxy-N-hydroxyindole, the reaction between PA and o-carbomethoxy-nitrosobenzene also affords a tricyclic indole derivative, 3, likely derived from trapping of an intermediate indoline nitrone with PA and subsequent rearrangement. Computational studies of the reaction mechanism were carried out with density functional theory at the (U)B3LYP/6-31+G(d) level. The lowest energy pathway of the reaction of PhNO with alkynes was found to be stepwise; the N-C bond between nitrosoarene and acetylene is formed first, the resulting vinyl diradical undergoes cis-trans isomerization, and then the C-C bond forms. Conjugating substituents Z on the alkyne, Z-C[triple bond]CH, lower the calculated (and observed) activation barrier, Z = -H (19 kcal/mol), -Ph (15.8 kcal/mol), and -C(O)H (13 kcal/mol). The regioselectivity of the reaction, with formation of the 3-substituted indole, was reproduced by the calculations of PhNO + PhC[triple bond]CH; the rate-limiting step for formation of the 2-substituted indole is higher in energy by 11.6 kcal/mol. The effects of -NO(2), -CN, -Cl, -Br, -Me, and -OMe substituents were computed for the reactions of p-X-C(6)H(4)NO with PhC[triple bond]CH and of PhNO and/or p-NO(2)-C(6)H(4)NO with p-Y-C(6)H(4)C[triple bond]CH. The activation energies for the set of p-X-C(6)H(4)NO vary by 4.3 kcal/mol and follow the trend found experimentally, with electron-withdrawing X groups accelerating the reactions. The range of barriers for the p-Y-C(6)H(4)C[triple bond]CH reactions is smaller, about 1.5 and 1.8 kcal/mol in the cases of PhNO and p-NO(2)-PhNO, respectively. In agreement with the experiments, electron-donating Y groups on the alkyne accelerate the reactions with p-NO(2)-C(6)H(4)NO, while both ED and EW groups are predicted to facilitate the reaction. The calculated kinetic isotope effect for the reaction of C(6)H(5)NO/C(6)D(5)NO with PhC[triple bond]CH is negligible (as found experimentally) while that for PhC[triple bond]CH/PhC[triple bond]CD with PhNO (0.7) differs somewhat from the experiment (1.1). Taken together the experimental and computational results point to the operation of a stepwise diradical cycloaddition, with rate-limiting N-C bond formation and rapid C-C connection to form a bicyclic cyclohexadienyl-N-oxyl diradical, followed by fast tautomerization to the N-hydroxyindole product.