Peter O. Otieno

Synthesis and Characterization of 3-Cyano- and 3-Nitroformazans, Nitrogen-Rich Analogues of β-Diketimine Ligands

The synthesis and characterization of several formazans containing strong electron-withdrawing substituents (cyano and nitro) in the 3 position of the ligand backbone are described. Reactions of aryldiazonium cations with the conjugated bases of either cyanoacetic acid or nitromethane lead to 1,5-diaryl-3-cyano- or 3-nitroformazans, respectively. When these reactions are carried out in aqueous conditions, the range of aromatic groups is limited by the stability of the diazonium salt. However, 3-nitroformazans containing bulky substituents on the nitrogen atoms (2,6-dimethylphenyl, 2,4,6-trimetyhlphenyl, 2,6-diisopropylphenyl, and 3,5-ditert-butylphenyl) could be made by performing the reactions under nonaqueous and anhydrous conditions. NMR and electronic spectroscopic studies indicate that the 3-nitroformazans exist exclusively as closed ( trans-syn, s-cis) isomers whereas the 3-cyanoformazans exist as mixtures of isomers which are substrate-dependent. The crystal structures of five of the formazans are presented: two 3-nitroformazans, both of which are closed, and three 3-cyanoformazans, two of which are closed and one of which adopts an open ( trans-syn, s-trans) structure. Solid state (diffuse reflectance) spectroscopy has been employed to ascertain the isomeric preferences of the other formazans which could not be crystallographically characterized.

ErratumErratum to “Carbon–carbon bond formation via oxidative-addition processes of titanium(II) reagents with π-bonded organic substrates. Reactivity modifications by Lewis acids and Lewis bases. Part 22. Organic chemistry of subvalent transition metal complexes: [Journal of Organometallic Chemistry 624 (2001) 229–238]

Abstract A series of titanium(II) derivatives, TiE 2 , was prepared by alkylative reduction of TiE 4 by two equivalents of n -butyllithium in THF at −78 to 25°C (E=Cl, F, OBu n , OPr i and 0.5 NPh–CH 2 –CH 2 –NPh). The LiE by-product could usually be removed by THF evaporation and dissolution of the TiE 2 into toluene. All such TiE 2 derivatives were shown to effect the epimetallation and oligomerization of olefins, acetylenes and carbonyl derivatives in varying degrees. Particularly pertinent were the isolation and chemical reactions of titanium(II) isoproxide, the postulated intermediate in the Kulinkovich synthesis of cyclopropanols from ethyl Grignard reagents and organic esters, as well as an intermediate in many allied reactions developed by the Sato group. The findings of the present study corroborate completely the foregoing hypothesis that titanium(II) isopropoxide is the key intermediate in such novel reactions in organic synthesis. Furthermore, Ti(OPr i ) 2 can be prepared readily in a relatively pure state and has been found to react with 1-alkenes, alkynes and ketones by epimetallation at 25°C to form three-membered titanacycles, which can be utilized in organic synthesis. Finally, the ease with which such TiE 2 derivatives epimetallate unsaturated organic substrates has been shown to be decreased by the steric demands of E and by the coordination of Lewis bases or donor solvent to the titanium(II) center. Lewis acids, on the other hand, greatly increase the rate of epimetallation by TiE 2 . A dramatic illustration of this effect is in the action of TiCl 2 ·Me 2 AlCl on unsaturated hydrocarbons, wherein the polymerization of ethylene and of 1-alkene and the cyclotrimerization of alkynes are found to occur at room temperature.