Structure and Photochemistry of 18-Diazo-1,4,7,10,13,16-hexaoxacyclononadeca-17,19-dione and Its Sodium and Potassium Complexes. Control of the Ground-State Conformation of 2-Diazo-1,3-dicarbonyl Fragment via Host−Guest Complexation

The macrocyclic 18-diazo-1,4,7,10,13,16-hexaoxacyclononadeca-17,19-dione (3-diazo-2,4-dioxo-19-crown-6, 1) readily forms complexes with potassium (2, stability constant in methanol is
KK+ = 229 ± 25 M-1) and sodium ions (3, KNa+ = 84.2 ± 7.9 M-1 in methanol). According toB3LYP/6-31G+(d,p) calculations and temperature-dependent 1H NMR spectroscopy, the predominant conformation of 1 has a Z,Z arrangement of the diazo and carbonyl groups. The X-ray crystal
structure analysis showed that the potassium complex (2) has the same Z,Z arrangement, while
the sodium analogue (2) exists in conformation with Z,E geometry of the diazodicarbonyl moiety.
Direct 254 nm photolysis of diazo compounds 1−3 in methanol results in the formation of 3-methoxy-2,4-dioxo-19-crown-6 (5), the product of the insertion of corresponding α,α‘-dicarbonylcarbene into
the O−H bond of the solvent. The triplet-sensitized photolysis of diazomalonates 1−3 produces
2,4-dioxo-19-crown-6 (6), which is apparently formed via the triplet state of the intervening carbene.