α-Diazo esters containing an amido group in the γ-position have been found to undergo a rhodium(II)-catalyzed transformation, producing five-membered ammonium or carbonyl ylides depending on the reaction
conditions used. In the absence of an external dipolarophile, ammonium ylides are the exclusive products
formed. In most cases these ylides cannot be isolated as they readily undergo sigmatropic rearrangement or
fragmentation reactions. In the presence of typical dipolarophiles such as DMAD or N-phenylmaleimide,
cycloaddition products derived from cyclic carbonyl ylide dipoles are formed as the major products. The rhodium
carbenoid intermediate generated in these reactions can either attack the lone pair of electrons on the amide
nitrogen (ammonium ylide formation) or the lone pair of electrons on the carbonyl oxygen (carbonyl ylide
formation). The experimental observations reflect a catalyst-promoted system of equilibria with a clear-cut
thermodynamic bias. To examine the underlying mechanism in detail, density functional theory (DFT)
calculations were performed on all plausible intermediates, including the full dirhodium tetracarboxylate
functionality. A semiquantitative energy manifold is developed that rationalizes the empirical observations
and provides a detailed picture of the role of the dirhodium(II) catalyst.
The Rh(II)-catalyzed reaction of 1-acetyl-1-(diazoacetyl)cyclopropane and ethyl 3-(1-acetyl-cyclopropyl)-2-diazo-3-oxopropiolate with various dipolarophiles afforded dipolar cycloadducts in good yield. The reaction involves the formation of a rhodium carbenoid and subsequent transannular cyclization of the electrophilic carbon onto the adjacent keto group to generate a five-membered cyclic carbonyl ylide which undergoes a subsequent dipolar cycloaddition reaction. The regiochemical results encountered can be rationalized on the basis of FMO considerations. For carbonyl ylides, the HOMO dipole is dominant for reactions with electron deficient dipolarophiles, while the LUMO becomes important for cycloaddition to more electron rich species. A short synthesis of several members of the pterosin family of sesquiterpenes is described in which the key step involves a dipolar cycloaddition using a carbonyl ylide. The Rh(II)-catalyzed reaction of 1-acetyl-1-(diazoacetyl)cyclopropane with cyclopentenone afforded a dipolar cycloadduct in good yield as a 4:1 mixture of diastereomers. Treatment of the major cycloadduct with triphenylphosphonium bromide in the presence of sodium hydride gave the expected Wittig product. The reaction of this compound with acid in the presence of various solvents gave rise to several members of the pterosin family. The overall sequence of reactions can best be described as proceeding by an initial oxy-bridge ring opening followed by dehydration and a subsequent acid-catalyzed cyclopropyl ring opening. The facility of the process is undoubtedly related to the aromaticity gained in the final step.
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