C-13 NUCLEAR MAGNETIC-RESONANCE STUDY OF NUCLEOPHILIC ADDITIONS TO BENZO[A]PYRENE 4,5-OXIDE AND OF ITS ACID-CATALYZED REARRANGEMENT

Nucleophilic additions to benzo[a]pyrene 4,5-oxide labeled in position 4 or 5 with carbon-13 were studied by 13C NMR spectroscopy. Under neutral or basic conditions, only soft nucleophiles such as tert-butylmercaptan, glutathione, N-acetylcysteine, and azide are able to add to the oxirane ring to give the trans adducts. No preferential attack at position 4 or 5 is observed. With chiral nucleophiles, such as glutathione and N-acetylcysteine, all four diastereomers are observed by 13C NMR spectroscopy. Under acidic conditions with methanesulfonic acid in aqueous dioxane, benzo[a]pyrene 4,5-oxide preferentially undergoes the NIH shift to form the 4-phenol and 5-phenol in a 60:40 ratio. In methanol, the two phenols are obtained in a 60:40 ratio along with the four possible hydroxy ethers. However, if HC1 in dioxane is used, the trans dihydro diol and two chlorohydrins are formed in addition to the two phenols. Likewise, these same products form if methanesulfonic acid is used as the acid in the presence of LiCl. Chloride appears to be especially effective as a nucleophile in trapping the carbocation from acid-catalyzed ring opening of benzo[a]pyrene 4,5-oxide. The acid-catalyzed dehydration of trans-4,5-dihydrobenzo[a]pyrene-4,5-diol also gives a 60:40 ratio of the 4-phenol and the 5-phenol. The lack of significant selectivity observed in nucleophilic addition reactions and acid-catalyzed rearrangement of benzo[a]pyrene 4,5-oxide as well as in the acid-catalyzed dehydration of the trans diol are consistent with the identical Dewar reactivity numbers for the 4- and 5-positions. © 1979, American Chemical Society. All rights reserved.