Iodonium ylide 2, derived from dimethyl malonate, undergoes facile thermal cycloaddition with alkenes and dienes catalyzed with Rh 2 (OAc) 4 to form the corresponding cyclopropanedicarboxylates and vinylcyclopropanedicarboxylates, respectively, in excellent yields. Iodonium ylides, 3 a class of hypervalent iodine-compounds, 4 have been used quite often as rather reactive alternatives to diazo compounds without major drawbacks such as explosive and health hazards. Quite generally speaking, an iodonium ylide can be easily prepared 5 from the corresponding active methylene compound and a hypervalent iodine precursor, i.e. PhI(OAc) 2 . Its reactivity pattern 3 consists of its decomposition in various solvents, C-H insertion reactions, oxidation reactions, and/or cycloaddition reactions leading to a variety of products, usually new heterocyclic compounds. A major question arising from the literature 6 is whether or not a carbene (or a carbenoid) is involved in this rich reactivity pattern. A singlet or triplet carbene (or a carbenoid) could easily explain the observed results but this could not be always the case.We were more fascinated with the synthetic applications than the mechanistic aspects of the reactivity of iodonium ylides. Within our studies, we had seen rich reactivity of a b-dicarbonyl 7a,b iodonium ylide with various substrates, improvement 7 of the reaction yield by shifting from the cheap Cu-catalyst into the expensive Rh-catalyst, while we were observing the loss of stereochemistry. 7c,d This fact had been explained as the result of a two step polar reaction although there is no strong evidence that an initially-formed cyclopropane rearranged into dihydrofuran. On the other hand, recently it was reported 8 that a phenyliodonium ylide is a precursor for dicarboethoxycarbene. So, with the aim to develop and define the scope and limitation of this synthetic methodology, we extended our study to the reactivity of phenyliodonium bis(carbomethoxy)methylide towards alkenes and dienes.The reaction of methyl malonate (1) with diacetoxyiodobenzene in basic methanol at -10 °C leads to the isolation 6b of iodonium ylide 2 in moderate yields (Scheme 1). Heating or irradiating a suspension of iodonium ylide 2 in an appropriate solvent yields tetracarbomethoxyethylene (3) quantitatively. This alkene is a side product of all attempted cycloadditions of iodonium ylide 2, possibly produced by attack of a "carbene intermediate" on 2. Scheme 1Nevertheless, this formal carbene dimer was produced in minor amounts, when an excess of an alkene is present. The reaction 9 with styrene 4a and indene 4h were used as a probe to examine the overall yield of the desired reaction. Heating a mixture of 2 and styrene 4a in the presence of catalytic amounts of Rh 2 (OAc) 4 or Cu(acac) 2 led to the isolation of cyclopropanedicarboxylate 5a (Scheme 2) in 80% and 57% yield, respectively. On the other hand, irradiating a suspension of 2 and 4a in CH 3 CN led to the isolation of 5a in 51% yield. Although the photochemical reaction yields cy...

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