Cyclic Diacylcarbene Generated from Iodonium Ylides and Diazodiketones

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Abstract
The preparative method of iodonium ylides, Ph–\overset⊕I–\overset\ominusC(COR)(COR′), is improved by condensation of active methylene compounds, H2C(COR)(COR′), with iodosobenzene in the presence of acetic anhydride. Thermal and Cu-catalyzed decomposition of iodonium ylides can be differentiated and both are explained in terms of diacylcarbene intermediate with different multiplicity. For the sake of comparison diazodimedone and related compounds are also decomposed by Cu-catalysis as well as by photochemical reaction. Carbene is also found to be a highly probable intermediate. Throughout these investigations the spin multiplicity of the carbene is discussed on the basis of product distributions. Notably in the presence of homogeneous Cu-catalyst in ethanol, diazodimedone gives an insertion product to O–H bond, 2-ethoxy-3-hydroxy-5,5-dimethyl-2-cyclohexenone (XVII), whereas the iodonium ylides do not give such a compound. The formation of XVII is explained by assuming a ketocarbene-Cu-chelate complex.