1,1′-Methylene-bis(1,1′,2,2′,3,3′,4,4′-octahydroisoquinoline): synthesis, reaction, resolution, and application in catalytic enantioselective transformations

Stereodivergent Palladium- and Rhodium-Catalyzed Intramolecular Addition of Tosylureas to Allenes: Diastereoselective Synthesis of Tetrahydropyrimidinones

The intramolecular addition of tosylureas to allenes is highly syn-/anti-diastereoselective when employing a palladium or rhodium-based catalytic system and affords 1,3-cyclic ureas. Under palladium catalysis a range of thermodynamic anti-tetrahydropyrimidinones are accessible, while rhodium catalysis allows synthesis of the kinetic syn-tetrahydropyrimidinones. For a representative scope of substrates both cyclic ureas were obtained in excellent yields and diastereoselectivities. The obtained tetrahydropyrimidinones were shown to be easily deprotected and modified to demonstrate the synthetic value.

Redox induced oxidative C-C coupling of non-innocent bis(heterocyclo)methanides

Redox driven C-C bond formation has gained recent attention over the traditional sequence of oxidative addition, insertion and reductive elimination reactions. In this regard, the transient radical mediated diverse reactivity profile of bis(heterocyclo)methanes (H-BHM: HL1-HL4) has been demonstrated as a function of varying metal ions and ligand backbones. It highlighted the following events: (a) redox induced homocoupling of deprotonated HL1 and HL4 on coordination to M(OAc)₂ precursors (M = Cu^II, Zn^II, Pd^II, Ag^I), including the effective role of molecular oxygen in the transformation process; (b) steric inhibition of C-C coupling of HL1 or HL4 on inserting the substituent at the bridged methylene centre (Ph in HL2 or CH₃ in HL3); (c) competitive C-C coupling versus oxygenation of free HL1 with varying concentrations of Pd^II(OAc)₂ as the ease of oxygenation over dimerisation of the deprotonated HL1 was corroborated by the DFT calculated lower activation barrier and greater thermodynamic stability of the former; and (d) redox non-innocence of BHMs on a coordinatively inert ruthenium platform, which in turn favored the involvement of a radical pathway for the aforestated coupling or oxygenation process. A combined structural, spectroscopic and DFT calculated transition state analysis demonstrated the mechanistic outline for the metal assisted oxidative coupling of BHMs.

Henry Reaction Revisited. Crucial Role of Water in an Asymmetric Henry Reaction Catalyzed by Chiral NNO-Type Copper(II) Complexes

Chiral copper(II) and cobalt(III) complexes (1-5 and 6, respectively) derived from Schiff bases of (S)-2-(aminomethyl)pyrrolidine and salicylaldehyde derivatives were employed in a mechanistic study of the Henry reaction-type condensation of nitromethane and o-nitrobenzaldehyde in CH₂Cl₂ (CD₂Cl₂), containing different amounts of water. The reaction kinetics was monitored by ¹H and ¹³C NMR. The addition of water had a different influence on the activity of the two types of complexes, ranging from a crucial positive effect in the case of the copper(II) complex 2 to insignificant in the case of the stereochemically inert cobalt(III) complex 6. No experimental support was found by ¹H NMR studies for the classical Lewis acid complexation of the carbonyl group of the aldehyde by the central copper(II) ion, and, moreover, density functional theory (DFT) calculations support the absence of such coordination. On the other hand, a very significant complexation was found for water, and it was supported by DFT calculations. In fact, we suggest that it is the Brønsted acidity of the water molecule coordinated to the metal ion that triggers the aldehyde activation. The rate-limiting step of the reaction was the removal of an α-proton from the nitromethane molecule, as supported by the observed kinetic isotope effect equaling 6.3 in the case of the copper complex 2. It was found by high-resolution mass spectrometry with electrospray ionization that the copper(II) complex 2 existed in CH₂Cl₂ in a dimeric form. The reaction had a second-order dependence on the catalyst concentration, which implicated two dimeric forms of the copper(II) complex 2 in the rate-limiting step. Furthermore, DFT calculations help to generate a plausible structure of the stereodetermining transition step of the condensation.