The reaction of 3-arylcyclopropenes with Cr(CO)6 and Mo(CO)6 produces naphthols, in an example of metal-promoted benzannulation. Substituents 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 yields decrease for 3-alkyl derivatives as the size of the alkyl group increases. The 3,3-diphenyl and 3-cyano derivatives are unreactive. The mechanism is postulated to involve metal-complexed vinyl carbene units, parallel with the benzannulation reaction involving arylcarbene complexes with alkynes. The regioselectivity has been probed with various unsymmetrically 1,2-disubstituted 3-arylcyclopropenes. The results suggest a simple correlation with steric size, consistent with initial cleavage of the cyclopropene σ bond bearing the smaller substituent. The result of this regioselectivity is a product structure showing a substituent arrangement opposite to that from the carbene-chromium approach; the smaller substituent of the cyclopropyl double bond ends up adjacent to the phenol OH in the product. Catalytic activity at low efficiency was observed, using a Mo(CO)6 catalyst. However, the use of Mo(CO)6 also promotes formation of indenes as significant byproducts at the expense of naphthalenes. Attempts to use the arylcyclopropene rearrangement to convert a 3-(1,4-dimethoxy-2-naphthyl)-cyclopropene to an anthraquinone skeleton produced instead a phenanthrene via an unusual 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 again a phenanthrone, and the product is exactly parallel with that observed in the cyclopropene rearrangement. A naphthoquinone substituted with a cyclopropene at C-2 does provide the first example of metal-promoted benzannulation to give an anthraquinone.
All Science Journal Classification (ASJC) codes
- General Chemistry
- Colloid and Surface Chemistry