The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species.

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(µ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

Cobalt-Catalyzed Enantioselective Hydrogenation of Trisubstituted Carbocyclic Olefins: An Access to Chiral Cyclic Amides.

The enantioselective hydrogenation of cyclic enamides has been achieved using an earth-abundant cobalt-bisphosphine catalyst. Using CoCl2 /(S,S)-Ph-BPE, several trisubstituted carbocyclic enamides were reduced with high activity and excellent enantioselectivity (up to 99 %) to the corresponding saturated amides. The methodology can be extended to the synthesis of chiral amines by base hydrolysis of the hydrogenation products. Preliminary mechanistic investigations reveal the presence of a high spin cobalt (II) species in the catalytic cycle. We propose that the hydrogenation of the carbon-carbon double bond proceeds via a sigma-bond-metathesis pathway.

Selective Reductions of N,N-Bis{chloro(aryl)-phosphino}-amines Yielding Three-, Five-, Six-, and Eight-Membered Cyclic Azaphosphanes.

To date, a plethora of λ3 -P nitrogen-containing compounds is known. A large number of them are used as ligands in coordination chemistry and homogeneous catalysis. PN-containing compounds tend to build up cyclic moieties, which have received less attention in regard to their application as ligands in transition metal chemistry. Hence, different dehalogenation reactions of N,N-bis{chloro(aryl)-phosphino}-amines have been developed to synthesize different P-N-P containing cyclic compounds. Their coordination behavior to group VI transition metal carbonyls was explored.