Monomeric Titanium Complexes Research Articles

Dual Lewis Acid/Lewis Base Catalyzed Acylcyanation of Aldehydes: A Mechanistic Study.

A mechanistic investigation, which included a Hammett correlation analysis, evaluation of the effect of variation of catalyst composition, and low-temperature NMR spectroscopy studies, of the Lewis acid-Lewis base catalyzed addition of acetyl cyanide to prochiral aldehydes provides support for a reaction route that involves Lewis base activation of the acyl cyanide with formation of a potent acylating agent and cyanide ion. The cyanide ion adds to the carbonyl group of the Lewis acid activated aldehyde. O-Acylation by the acylated Lewis base to form the final cyanohydrin ester occurs prior to decomplexation from titanium. For less reactive aldehydes, the addition of cyanide is the rate-determining step, whereas, for more reactive, electron-deficient aldehydes, cyanide addition is rapid and reversible and is followed by rate-limiting acylation. The resting state of the catalyst lies outside the catalytic cycle and is believed to be a monomeric titanium complex with two alcoholate ligands, which only slowly converts into the product.

Thermally or Photochemically Induced Reductive Cleavage of Metal−Metal Bonds of Metal Carbonyl Dimers by a Titanocene(III) tert-Butoxide: Novel Reversible Access to Heterobimetallic Complexes

Novel synthetic methods for Ti−Mo, Ti−W, and Ti−Ru heterobimetallic complexes are established by the reaction of a certain titanium(III) alkoxide and the corresponding metal carbonyl dimer, [CpM(CO)n]2 [M = Mo, W (n = 3); M = Ru (n = 2)]. Thus, a novel monomeric titanium complex, Cp2Ti(OtBu) (1), which is synthesized from Cp2TiCl and KOtBu and characterized by spectroscopy and crystallography, smoothly reacts with [CpM(CO)3]2 (M = Mo (2a), W (2b)) to give heterobimetallic complexes, Cp2Ti(OtBu)(μ-OC)M(CO)2Cp (M = Mo (3a) and W (3b)), of which two metallic moieties are linked by the isocarbonyl bridge. Similar reaction of 1 with [CpRu(CO)2]2 (4) does not occur thermally, but is accomplished under photoirradiation to afford Cp2(OtBu)Ti−Ru(CO)2Cp (5), of which two metallic moieties are linked by a direct metal−metal bond. Use of 1 is particularly important for these reactions; other titanium alkoxides such as [Cp2Ti(OMe)]2 and Cp2Ti[O(2,6-tBu2-4-Me)C6H2] do not react with the metal carbonyl dimers. An interesting feature of these new heterobimetallic complexes, 3a, 3b, and 5, is the existence of the thermal fragmentation process to regenerate the starting materials: 3a or 3b is actually in equilibrium with a mixture of 1 and 2a or 1 and 2b, respectively [ΔG0298K= −4.1 ± 0.2 kcal mol-1 (3a), −4.3 ± 0.2 kcal mol-1 (3b)]. The Ti−Ru compound 5 thermally undergoes fragmentation to regenerate 1 and 4. The formation of these heterobimetallic complexes is formally considered as the metal−metal bond cleavage of metal carbonyl dimers by a Ti(III) reducing reagent; possible mechanisms are discussed.