METAL-CATALYZED CYCLOPROPENE REARRANGEMENTS FOR BENZANNULATION - EVALUATION OF AN ANTHRAQUINONE SYNTHESIS PATHWAY AND REEVALUATION OF THE PARALLEL APPROACH VIA CARBENE-CHROMIUM COMPLEXES

The reaction of 3-arylcyclopropenes with Cr(CO)(6) and Mo(CO)(6) produ ces naphthols, in an example of metal-promoted benzannulation. Substit uents at C-3 (in addition to aryl) have a strong effect on the success of the process: 3-H derivatives are generally effective, but the yiel ds decrease for 3-alkyl derivatives as the size of the alkyl group inc reases. The 3,3-diphenyl and 3-cyano derivatives are unreactive. The m echanism is postulated to involve metal-complexed vinyl carbene units, parallel with the benzannulation reaction involving arylcarbene compl exes with alkynes. The regioselectivity has been probed with various u nsymmetrically 1,2-disubstituted 3-arylcyclopropenes. The results sugg est a simple correlation with steric size, consistent with initial cle avage of the cyclopropene sigma bond bearing the smaller substituent. The result of this regioselectivity is a product structure showing a s ubstituent arrangement opposite to that from the carbene-chromium appr oach; the smaller substituent of the cyclopropyl double bond ends up a djacent to the phenol OH in the product. Catalytic activity at low eff iciency was observed, using a Mo(CO)a catalyst. However, the use of Mo (CO)(6) also promotes formation of indenes as significant byproducts a t the expense of naphthalenes. Attempts to use the arylcyclopropene re arrangement to convert a 3-(1,4-dimethoxy-2-naphthyl)cyclopropene to a n anthraquinone skeleton produced instead a phenanthrene via an unusua l substitution for a methoxy substituent. A related example previously reported to produce anthraquinones via the naphthylcarbene-chromium/ alkyne reaction was shown to be in error; the correct structure is aga in a phenanthrone, and the product is exactly parallel with that obser ved in the cyclopropene rearrangement. A naphthoquinone substituted wi th a cyclopropene at C-2 does provide the first example of metal-promo ted benzannulation to give an anthraquinone.