Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides

Frustrated Lewis Pair (FLP)-Like (Cyclo)Addition of Small Organic Molecules to a Stable Azomethine Ylide

Azomethine ylides are 1,3-dipolar zwitterions typically used for constructing N-heterocycles by 3+2-cycloaddition reactions. We report here a pyridyl-tethered isolable azomethine ylide (AY) that activates a series of H-E bonds (E═B, Si, Al, O) in FLP-like fashion. The reactions are probed mechanistically by DFT calculations and each case appears to be distinct from others. While the HBpin activation is stepwise, the same of PhSiH3 is concerted. The AlH3 activation is also stepwise but takes place across the 1,5-(C+/N-) dipole. The hydrolysis of AY fits better with a "relay" mechanism with two H2O molecules working in tandem. The B-B bond of B2pin2 is also cleaved but in an intriguingly different way, by delivering both the Bpin moieties at the carbenic site. Though the challenging H2 activation fails, a transfer hydrogenation of AY by NH3•BH3 is readily achieved. AY also undergoes cycloadditions with various dipolarophiles including unactivated alkene and alkynes. In this regard, AY makes an interesting distinction between CO2 and CS2 by staying inert to the former but easily cycloadding the latter. DFT analysis justifies this dichotomy by showing the cycloaddition of CO2 as thermodynamically disfavored.

An Aziridine and a 2-Pyrrolidone with Pyridyl Sidearms as Ligands for Cationic Rh(I)-Catalyzed Hydrosilylation and Hydrogenation of C=C and C≡C Bonds

Phosphine- or carbene-based soft ligands are customarily used in Rh and other late transition metal catalyzed alkene and alkyne hydrosilylation and hydrogenation. We report here an aziridine and a 2-pyrrolidone with pyridyl sidearms, whose cationic Rh(I) complexes prove as excellent catalysts for hydrosilylating terminal olefins by Et3SiH giving anti-Markovnikov products selectively. To the best of our knowledge, the [(2-pyrrolidone)-Rh]+ seems to be the most active Rh catalyst recording a highest TOF of 24000 h-1. It works remarkably (TOF: 714 h-1) even at 10 ppm concentration! Terminal alkynes are hydrosilylated too to give β-(Z)-vinylsilanes selectively. Both catalysts also hydrogenate alkenes and doubly-hydrogenate alkynes, both terminal and internal, under ambient and benchtop conditions. But in hydrogenation, the [(aziridine)-Rh]+ catalyst works better. Both ligands and the Rh catalysts are air-stable.

Cyclic (Alkenyl)(Amino)Carbene (SMeCAenAC): Introducing a Member to the Cyclic (Alkyl)(Amino)Carbenes Family Featuring a Narrow Energy Gap

Herein, we report the carbene-like activity of a nonisolable, highly ambiphilic cyclic (alkenyl)(amino)carbene (SMeCAenAC, 3), which is stabilized as [(SMeCAenAC)(H)N(SiMe3)2] (4). This protected form (4) is stable in air and moisture. Compound 4 can be used as a carbene source through deamination upon heating to 130-140 °C. Moreover, density functional theory (DFT) calculations indicate that SMeCAenAC has the smallest singlet-triplet gap (37.05 kcal/mol) and a narrow highest occupied molecule orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap (3.92 eV) among the cyclic (alkyl)(amino)carbenes (CAACs). The precursor of carbene (3) can be synthesized on a multigram scale with a good yield. Moreover, the SMeCAenAC-coordinated copper complex showed excellent efficiency in the catalytic addition of phenols to electron-deficient olefins. This study also highlights that [SMeCAenAC-H]OTf can be used for metal-free catalysis, a property uniquely characteristic of an ambiphilic carbene, even though the formation of free SMeCAenAC (3) was not achieved.

Cyclic (amino)(barrelene)carbene Ru-complexes: synthesis and reactivity in olefin metathesis

The synthesis of ruthenium-complexes with cyclic (amino)(barrelene)carbenes (namely CABCs) as ligands is reported. Isolated in moderate to good yields, these new complexes showed impressive thermal stability at 110 °C over several days. Good catalytic performances were demonstrated in various ring-closing metathesis (RCM), macrocyclic-RCM, ring-closing enyne metathesis (RCEYM), cross-metathesis (CM), and ring-opening cross metathesis (ROCM) reactions.