Journal of the American Chemical Society, Vol.135, No.10, 3826-3833, 2013
Utilizing Redox-Mediated Bergman Cyclization toward the Development of Dual-Action Metalloenediyne Therapeutics
Reaction of 2 equiv of 1,2-bis((diphenylphosphino)ethynyl)benzene (dppeb, 1) with Pt(cod)Cl-2 followed by treatment with N2H4 yields the reduced Pt(0) metalloenediyne, Pt(dppeb)(2), 2. This complex is stable to both air oxidation and metal-mediated Bergman cyclization under ambient conditions due to the nearly idealized tetrahedral geometry. Reaction of 2 with 1 equiv of I-2 in the presence of excess 1,4-cyclohexadiene (1,4-CHD) radical trap rapidly and near-quantitatively generates the cis-Bergman-cyclized, diiodo product 3 (P-31: delta = 41 ppm, J(Pt-P) = 3346 Hz) with concomitant loss of 1 equiv of uncyclized phosphine chelate (P-31: delta = -33 ppm). In contrast, addition of 2 equiv of I-2 in the absence of additional radical trap instantaneously forms a metastable Pt(dppeb)(2)(2+) intermediate species, 4, that is characterized by delta = 51 ppm in the P-31 NMR (J(Pt-P) = 3171 Hz) and v(C C) = 2169 cm(-1) in the Raman profile, indicating that it is an uncyclized, bis-ligated complex. Over 24 h, 4 undergoes ligand exchange to form a neutral, square planar complex that spontaneously Bergman cyclizes at ambient temperature to give the crystalline product Pqdppnap-I-2)I-2 (dppnap-I-2 = (1,4-diiodonaphthalene-2,3-diyl)bis(diphenylphosphine)), 5, in 52% isolated yield. Computational analysis of the oxidation reaction proposes two plausible flattened tetrahedral structures for intermediate 4: one where the phosphine core has migrated to a trans-spanning chelate geometry, and a second, higher energy structure (3.3 kcal/mol) with two cis-chelating phosphine ligands (41 degrees dihedral angle) via a restricted alkyne-terminal starting point. While the energies are disparate, the common theme in both structures is the elongated Pt-P bond lengths (>2.4 angstrom), indicating that nucleophilic ligand substitution by I- is on the reaction trajectory to the cyclized product 5. The efficiency of the redox-mediated Bergman cyclization reaction of this stable Pt(0) metalloenediyne prodrug and resulting cisplatin-like byproduct represents an intriguing new strategy for potential dual-threat metalloenediyne therapeutics.