Monocationic catalysts

Synthesis of the Pt(ll) monocationic catalysts la-h bearing a perfluorophenyl residue. 

The DFT calculated energetics for the main hydroformylation reaction steps based on 15r starting with the 15r-alkene complex are shown in Fig. 14. The two largest activation barriers are for the initial alkene-hydride migratory insertion step (16.8 kcal/mol) and for the final reductive elimination of the acyl and hydride (21.6 kcal/mol). The computational prediction, therefore, is that the final aldehyde reductive elimination is the rate determining step for the monocationic catalyst 15r. The largest activation barrier for the dicationic dirhodium catalyst (Fig. 8) is only 13 kcal/mol, indicating that the monocationic dirhodium catalyst should be less active on a per molecule basis, which is completely consistent with the impact of... 

The other significant difference between the two bimetallic catalysts is that the monocationic monohydride dirhodium catalyst needs to oxidatively add Hj in order to gain the hydride(s) to allow the reductive elimination of aldehyde. The dicationic dihydride system has the second hydride already present and ready to go for the acyl reductive elimination step. H2 then oxidatively adds to the dicationic catalyst to regenerate the dihydride llr/llr. But since the monocationic catalyst system has a low activation barrier for H2 oxidative addition (8.7 kcal), this is not a bottleneck in the catalysis cycle. 

Addition of 2 equivalents of NEts to either the dicationic catalyst in acetone or the monocationic catalyst in water/acetone dramatically slows the hydroformylation. In acetone the initial TOF is reduced by 34%, while in water/acetone... 

Fitting the current response for each individual H-bond donor revealed the stoichiometry of binding to be 2 1 for the monocationic and neutral catalysts, and 1 1 for the bisamidinium catalyst. To further support this proposed mechanism, additional kinetic smdies revealed a second-order dependence on the rate of quinone reduction with monocationic catalysts, and a first-order dependence on the rate of reduction using the biscationic amidinium salt. 

In transition metal chemistry, ligand variation has proven to be the key to obtaining highly active polymerization catalysts. In particular, sterically hindered monocationic alkyl complexes with an empty site seem to be well suited for polymerization. The steric bulk prevents (associative) -hydrogen transfer, while the positive charge destabilizes the free hydride and thus opposes (dissociative) /(-elimination. 

With a greater understanding of the chemistry and mechanism of stoichiometric organometallic reactivity inside of 1, catalytic systems were investigated. The prevalence of monocationic rhodium catalysts in the literature, and the water solubility of... 

In recent years, a large number of mono- and dicationic lanthanide alkyl complexes have been found to be efficient catalysts for ethylene polymerization, and in some cases, the dicationic lanthanide derivatives show higher activity and selectivity than their monocationic counterparts. Ionic radii of lanthanide metals also affect the catalytic behavior, and polymerization activity often increases with ionic radius [5, 76],... 

Hydroformylation in Water/Acetone Formation of a Monocationic Dirhodium Catalyst... 

Although we initially proposed that the water was inhibiting the phosphine ligand dissociation and bimetallic fragmentatiOTi from generating inactive 12r and 13rr [44], the actual situation is quite different. The dicationic dihydride catalyst llr/llr can easily deprotonate to form a new monocationic monohydride dirhodium catalyst. This is supported by in situ FT-IR, NMR, the acidity of the catalyst solution, and DPT computational studies. A 1 mM catalyst solution in 30% water/acetone after exposure to H2/CO has a pH of 3.1, while a 10 mM solution has a pH of 2.2 - consistent with a strong monoprotic acidic species. 

Comparing this spectrum with that of the fragmentation-prone dicatirmic system in acetone (Fig. 4) reveals the dramatically improved stability of the monocationic bimetallic catalyst toward deactivation. 

Another key point is that for most of the complexes shown in Fig. 13, one of the rhodium centers is formally cationic, which helps labilize the CO ligands to keep the bimetallic catalyst from becoming saturated. Since 15r is monocationic and more electron-rich, we believe that on a per molecule basis it is less active compared to the dicationic catalyst llr/llr. But it is far more resistant to fragmentation reactions, which increases the concentration of the active catalyst in solution producing higher overall activity. 

We believe that the presence of free H in the acetone/water solvent system plays a role in the monocationic system. The rate determining step, once again, is the reductive elimination of aldehyde with a calculated barrier of 21.6 kcal (Fig. 14). Protonation of the monocationic dirhodium acyl is an alternate and likely pathway for eliminating aldehyde and forming the dicationic dirhodium catalyst Hr. Due to the very low activation barrier for the monocationic aUcyl-CO migratory insertion step, protonation of Rh-alkyl species to produce alkane is far less likely and consistent with the much lower alkane side reactions for 15r. 

While effective bimetallic catalyst design has the potential to lead to an enhancement of the reaction rate, the use of chiral bimetallic catalysts has also been explored to enhance the enantioselectivity of a reaction. Such bimetallic chiral induction is excellently demonstrated by the use of digold catalysts for the hydroamination of prochiral substrates such as allenes and alkenes [59]. The bimetallic Au catalyst 66, for example, was shown to be an effective catalyst for the hydroamination of amino-allenes in the presence of a silver salt activator (Scheme 24) [106]. The highest enantioselective induction for this reaction was achieved with a 1 1 ratio of AgBp4 to 66 (51 % ee) suggesting that the monocationic... 

---