Relay catalysis using a rhodium complex/chiral Brønsted acid binary system: enantioselective reduction of a carbonyl ylide as the reactive intermediate.

Carbonyl ylides are generally non-isolable reactive intermediates and have been extensively utilized as the dipole in 1,3-dipolar cycloaddition reactions with electron-deficient and electron-rich dipolarophiles to afford polycyclic compounds including five-membered oxacycles. The most efficient method for generating the carbonyl ylide is the interaction of a metal carbene complex with the oxygen atom of a carbonyl group. One practical way of generating metal carbene complexes as a precursor of carbonyl ylides is the decomposition of a-diazocarbonyl compounds by rhodium catalysts, and significant progress has been made in developing stepwise sequences for rhodium carbene/carbonyl ylide formation. However, this sequential process with carbonyl ylides has been applied only to 1,3-dipolar cycloaddition reactions and, to the best of our knowledge, further utilization of this attractive intermediate has never been demonstrated in any range of organic transformations. To broaden the scope of the synthetic applicability of the carbonyl ylide, we envisioned the combination of an organocatalytic method with a rhodium-catalyst-initiated reaction sequence in one pot. Combined use of a transition-metal catalyst and an organocatalyst has stimulated intensive interest in recent years, as it could potentially enable highly efficient and/or unprecedented transformations in a one-pot operation. Indeed, excellent transformations have been established by taking advantage of both of these catalytic approaches, where two types of catalyst combinations have been developed in the binary catalytic system. One is that each reactant is activated simultaneously by one type of catalyst; for instance, a metal catalyst is used to activate the nucleophile while an organocatalyst is used to activate the electrophile in a cooperative manner. The other is the consecutive transformation using a binary catalytic system, that is, relay catalysis for a multistep sequence in which each catalyst promotes one type of reaction in the sequence in one pot. Herein, we report an unprecedented relay catalysis for a carbonyl ylide formation/enantioselective reduction sequence using a binary catalytic system that consisting of the dirhodium(II) tetracarboxylate 1 and chiral phosphoric acid 2 as a chiral Brønsted acid catalyst (Scheme 1). The proposed relay catalysis involves a four-step transformation: a) decomposition of the a-diazocarbonyl compound 3 by 1 to generate