Abstract
The coupling of two equivalents of ethynylferrocene (2) with one equivalent of 1,2-diiodocyclohexene (1) and 1,2-diiodobenzene (4) using Sonogashira cross-coupling conditions led to 1,2-bis(ferrocenylethynyl)cyclohexene (3) and 1,2-bis(ferrocenylethy-nyl)benzene (5), respectively. At high temperatures enediynes 3 and 5 showed exothermic signals in differential scanning calorimetry (DSC) measurements, suggestive of intramolecular diradicaloid ring formation (Bergman (C1−C6) or Schreiner-Pascal (C1−C5) cyclizations). The oxidation of 3 and 5 to the mono-oxidized enediynes 3+ and 5+ decreased the onset temperatures drastically. Equally, 1-ferrocenylethynyl-2-(p-nitro-phenyl)ethynylbenzene (8) displayed a significant decrease in the onset temperature after oxidation to 8+. Because the insoluble nature of the polymeric material formed in the thermolysis of the oxidized enediynes prevented characterization, the origin of this drastic effect was studied by DFT. Contrary to expectations, one-electron oxidation does not lower the barrier for intramolecular cyclization. Rather, the computations suggest that the polymerization is initiated by a bimolecular process.
Highlights
The discovery of the antibacterial and antitumor properties of the naturally occurring enediynes [1], such as calicheamicin, dynemicin and kedarcidin, has attracted the interest of many scientists over the last three decades [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
Because the insoluble nature of the polymeric material formed in the thermolysis of the oxidized enediynes prevented characterization, the origin of this drastic effect was studied by DFT
Reactive benzenoid σ,σ-diradicals [17,18,19,20] that induce single and double strand DNA cleavage, a significant amount of interest has focused on the design of simple acyclic enediynes with similar properties
Summary
The discovery of the antibacterial and antitumor properties of the naturally occurring enediynes [1], such as calicheamicin, dynemicin and kedarcidin, has attracted the interest of many scientists over the last three decades [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. Because of their ability to afford (under physiological conditions!). Since the activation of neutral enediynes has been accomplished by photoinduced electron transfer (PET) [38], we evaluate the possibility to activate ferrocenyl enediynes by thermal one-electron oxidation, a methodology that has been successful in our hands over many years [39,40]
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