Kinetics and mechanisms of the reactions of the beta-dicarbonyl-substituted iodonium ylides 1(a-d) with several pi-conjugated carbenium and iminium ions have been investigated. All reactions proceed with rate-determining attack of the electrophile at the nucleophilic carbon center of the glides to give iodonium ions, which rapidly expel iodobenzene and undergo different subsequent reactions. The second-order rate constants k(2) for the reactions of the iodonium ylides with benzhydrylium ions correlate linearly with the electrophilicity parameters E of the benzhydrylium ions and thus follow the linear free energy relationship log k(20 degrees C) = s(N)(N + E) (eq 1), where electrophiles are characterized by one parameter (E), while nucleophiles are characterized by two parameters: the nucleophilicity N and the susceptibility S-N. The nucleophilicity parameters 4 < N < 8 for iodonium ylides 1(a-d) derived from these correlations show that substituting hydrogen for Ph-I+ at the carbanionic center of Meldrum's acid or dimedone, respectively, reduces the nucleophilicity by approximately 10 orders of magnitude. The iodonium ylides 1(a-d) thus have nucleophilicities similar to those of pyrroles, indoles, and silylated enol ethers and, therefore, should be suitable substrates in iminium-activated reactions. Good agreement of the measured rate constant for the cydopropanation of the imidazolidinone-derived iminium ion Ha with the iodonium ylide 1a with the rate constant calculated by eq 1 suggests a stepwise mechanism in which the initial nucleophilic attack of the iodonium ylide at the iminium ion is rate-determining. The reaction of cinnamaldehyde with iodonium ylide 1a catalyzed by (5S)'-5-benzyl-2,2,3-trimethyl-imidazolidin-4-one (11a, MacMillan's first-generation catalyst) gives the corresponding cyclopropane with an enantiomeric ratio of 70/30 and, thus, provides proof of principle that iodonium ylides are suitable substrates for iminium-activated cyclopropanations.