Multicomponent reactions (MCRs) are one-pot processes in which three or more starting materials form a product that incorporates the structural features of each reagent. These reactions date back to the mid-19th century, when Strecker first prepared α-aminonitriles through the condensation of aldehydes with ammonia and hydrogen cyanide. In addition to affording products with structural complexity and diversity, MCRs offer the advantages of simplicity, synthetic efficiency, synthetic convergence, and atom economy. Therefore, they have played an important role in modern synthetic organic chemistry and drug-discovery research. The irreversible trapping of an active intermediate generated from two components by a third one offers an effective way to discover novel MCRs. In cases where the intermediate from the first two components is reactive enough to generate a two-component byproduct, it becomes challenging to control of the chemoselectivity of these MCRs over the side reaction. For example, researchers had expected that ammonium/oxonium ylides, high energy intermediates that have acidic protons and basic carbanions attached to adjacent carbons, would be too reactive to be intercepted by external electrophiles. Instead, a very fast 1,2-proton transfer would neutralize the charge separation, resulting in a stable N-H/O-H insertion product. In this Account, we present our efforts toward the development of novel MCRs via trapping of the active ammonium/oxonium ylide intermediates with a number of electrophiles. In these reactions, a "delayed proton transfer" that occurs after the trapping process produces novel multicomponent coupling products. Thus, transition-metal-catalyzed MCRs of diazocarbonyl compounds, anilines/alcohols, and electrophiles efficiently afford polyfunctional molecules such as α-amino-β-hydroxy acids, α-hydroxy-β-amino acids, α,β-diamino acids, and α,β-dihydroxy acid derivatives. We have also applied a cooperative catalysis strategy to some of these MCRs leading to reactions with high chemo-, diastereo-, and enantioselectivity. These MCRs also provide solid experimental evidence for the existence of the active protic onium ylides.

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The diastereoselectively switchable enantioselective trapping of protic carbamate ammonium ylides with imines is reported. The intriguing Rh(2)(OAc)(4) and chiral Brønsted acid cocatalyzed three-component Mannich-type reaction of a diazo compound, a carbamate, and an imine provides rapid and efficient access to both syn- and anti-α-substituted α,β-diamino acid derivatives with a high level control of chemo-, diastereo-, and enantioselectivity.

Urban vitality is an important indicator of urban development capacity. Streets’ metrics can depict intro-urban fabrics and physiognomy in detail, and thus street vitality affected by street metrics is a concrete manifestation of urban vitality. However, few studies have evaluated dynamic vitality or explored how it is influenced by land use. To bridge this gap, we fully evaluated street dynamic vitality and explored how to enhance the street dynamic vitality by changing the distribution and combination of land use. Specifically, we examined the street dynamic vitality and land use diversity in the main urban zone of Xining city in China using mobile communication and point of interest data, adopted optimized K-means clustering to identify street dynamic vitality types, evaluated the classification result based on vitality intensity and vitality stability, and explored the link between land use and dynamic vitality. Since vitality intensity limitations were found in describing street dynamic vitality, it was necessary to introduce vitality stability. We also found a positive correlation between the vitality intensity and land use density, there were outstanding traffic facilities in high-intensity vitality streets, and improving the abundance and uniformity of land use was beneficial to increase vitality stability. Overall, describing street vitality from a dynamic perspective can improve resource utilization efficiency and rationally plan layouts.