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Extreme oxatriquinanes and a record C-O bond length

Günbaş, Emrullah Görkem
Sheppard, William L.
Olmstead, Marilyn M.
Stoyanova, Irini V.
Tham, Fook S.
Meyer, Matthew P.
Oxatriquinanes are fused, tricyclic oxonium ions that are known to have exceptional stability compared to simple alkyl oxonium salts. C-O bonds in ethers are generally similar to 1.43 angstrom in length, but oxatriquinane has been found to have C-O bond lengths of 1.54 angstrom. A search of the Cambridge Structural Database turned up no bona fide C-O bond length exceeding this value. Computational modelling of oxatriquinane alongside other alkyl oxonium ions indicated that the electronic consequences of molecular strain were primarily responsible for the observed bond elongation. We also show that substitution of the oxatriquinane ring system with alkyl groups of increasing steric demand pushes the C-O bond to unheard of distances, culminating in a tert-butyl derivative at a predicted 1.60 angstrom. Chemical synthesis and an X-ray crystallographic study of these compounds validated the results of the modelling work and, finally, an extraordinary 1.622 angstrom C-O bond was observed in 1,4,7-tri-tert-butyloxatriquinane.