LINEARLY FUSED VS BRIDGED REGIOSELECTION IN THE INTRAMOLECULAR 1,3-DIYL TRAPPING REACTION

The intramolecular diyl trapping reaction can now be used to obtain synthetically useful quantities of either bridged or linearly fused cycloadducts in a selective manner and by design. Bridged cycloadducts arise by intercepting the triplet diyl, while linearly fused products can be produced from either the singlet or the triplet. When an electron-withdrawing group is attached to the diylophile, the singlet diyl leads selectively to fused cycloadducts. On the other hand, the presence of a large alkyl group attached to the internal carbon of the diylophile affords bridged cycloadducts selectively from cycloaddition with the triplet. Four diazenes, 4–7, differing only in the electronic and steric properties of the substituent located on the internal carbon of the diylophile, were studied. The diyl trapping reactions were conducted using ca. 1 mM solutions of diazene in THF at reflux for periods of 3–4 h; cycloadduct yields ranged from 68% (beginning with the dimethyl ketal 7) to 98% (from keto diazene 4). To determine the origin of the bridged cycloadducts, the effect of oxygen upon the product distribution was examined. The results show that the rate of the intramolecular triplet diyl cycloaddition is slower than the rate of the intermolecular reaction of the triplet with oxygen. The rate of triplet intramolecular cycloaddition can be estimated to be less than 4 ˟106 to 4 ˟ 107 s-1. © 1990, American Chemical Society. All rights reserved.