The Oxacyclobutylidenecarbene-Oxacyclopentyne System: Experimental Access and Theoretical Studies

Photolysis of 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene)oxetane in benzene generates the putative 3-oxacyclobutylidenecarbene, which rearranges into 3-oxacyclopentyne, the smallest heterocyclic alkyne reported to date that is comprised entirely of second-row elements in the ring. This strained heterocyclic alkyne could be intercepted as a [4 + 2] adduct with a cyclopentadienone derivative that subsequently loses CO to form a phthalan core, a heterocyclic motif that is found in many biologically active natural products, pharmaceuticals, and advanced materials. Calculations at the FIC-NEVPT2/def2-TZVPP//CASSCF[8,8]/def2-TZVPP level of theory, incorporating a CPCM model for benzene, show that singlet 3-oxacyclobutylidenecarbene lies well below the triplet (ΔE = −42.8 kcal/mol) and has to surmount a free energy barrier of 7.80 kcal/mol to ring expand into 3-oxacyclopentyne. The ring expansion of singlet carbene to the cycloalkyne was calculated to be essentially thermoneutral with ΔGrxn° estimated to be +0.68 kcal/mol. The gas phase calculated geometry of 3-oxacyclopentyne reveals a molecule with C2v symmetry and extremely distorted bond angles of 112.2° at each of the two carbons involved in the triple bond. The triple bond itself is more typical with a calculated bond length of 1.24 Å. The gas phase electronic strain energy (ΔESE) of 3-oxacyclopentyne, computed via a homodesmotic scheme, was estimated to be 70.1 kcal/mol.