Double trap: A single product from the THF-initiated interception of a cyclopropylidene(oid) and its rearranged strained cyclic allene

Abstract

The reaction of 1,1-dibromo-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene (3) with butyllithium in THF at ∼ −70 °C, followed by quenching with water at room temperature, led to a complex mixture of products which included a trace amount of the unusual pyran derivative, 3-(5H-dibenzo[a,c][7]annulen-5-yl)-2-(phenanthren-9-yl)tetrahydro-2H-pyran (6). The X-ray crystal structure of 6 revealed a phenanthrene unit α to the oxygen and a dibenzocycloheptene moiety in the β position. The formation of 6 is mechanistically rationalized on the basis of the initially generated cyclopropylidenes(oid) (4/5) interacting with THF to form an oxonium ylid 7 which then undergoes a ring expansion to produce the pyran framework in 10. Subsequent reaction of 10 with the strained dibenzocycloheptatetraene 2, resulting from ring opening of 4/5, leads to the observed product 6. To the best of our knowledge, this work documents the first example of a cyclopropylidene(oid) and its ring-opened allene trapped into a single product.