Achiral and chiral, C2-symmetric bicyclic guanidinates based on 1,5,7-triazabicyclo[4.4.0.]dec-5-ene as ligands in high- and mid-valent early transition metal chemistry

The syntheses of achiral and chiral, C2-symmetric bicyclic guanidine derivatives of 1,5,7-triazabicylo[4.4.0]dec-5-ene (hppH) are described.

Quantum mechanical MO calculations based on Spartan were performed in order to study the effects (electronic and steric) of substituents on the basicity of hppH derivatives. These calculations showed that substituting only one methyl group on each carbon next to the bridgehead nitrogen of hppH gives high basicity. Based on these calculations, syntheses of achiral (hpp*H, with four methyl groups) and chiral C2-symmetric (hpp'H, with two methyl groups) derivatives of hppH were developed based on cyclization of 1,5,9-triaminononanes with C1 reagents.

The synthesis of hpp*H started from commercially available ethyl cyanoacetate and was completed in six steps with 2.5% overall yield. The synthesis of the chiral, C2-symmetric hpp'H was carried out using L-alanine as the chiral precursor. The key step was the diastereoselective coupling of L-alaninol (derived from L-alanine) with commercially available hydroxyacetone via a reductive amination approach. An important step was successful diastereoselective resolution of N-benzyl-protected meso and C2-symmetric aminodinitriles. Chiral hpp'H was synthesized in six steps starting from L-alaninol in 9% overall yield and was isolated as hpp'H2+I-. Both hpp*H and hpp'H2+I were characterized by 1H and 13C NMR spectroscopy and high resolution mass spectrometry. hpp'H2+I was also characterized by single-crystal X-ray diffractometry.

The coordination chemistry of non-methylated hpp- with tantalum is described. The new tantalum complexes TaV(hpp)Cl4, TaV(hpp)2Cl3, Cp*TaV(hpp)Cl3 (Cp* = C5Me5) and Cp''TaV(hpp)Cl3 (Cp'' = C5Me4Et) were synthesized. The first mid-valent hpp- complexes of tantalum, Cp*TaIV(hpp)Cl2 and Cp"TaIV(hpp)Cl2, were obtained by reduction of (C5Me4R)Ta(hpp)Cl3 (R =Me, Et) or reaction of (C5Me4R)2Ta2(µ-Cl)4 with (hpp)SiMe3. 1H NMR spectroscopy and comparative single-crystal X-ray diffractometry of these complexes is discussed.