Soft Catalysis with Transition Metal Compounds

2 Soft Catalysis with Transition Metal Compounds 

Typical catalysts for the isomerization, hydrogenation, ohgomerization, and carbonyla-tion of olefins are characterized by a low oxidation state of the central atom, which is sta-bihzed by a-n interactions with soft ligands such as H , CO, PR3, and X [7]. 

Numerous metal hydrides, such as [HCo(CO)4] and [HRh(CO)(PPh3)3], or combinations of a metal complex and a hydride source (e.g., [Co2(CO)8]/H2, [Ni P(0Et3) 4]/H2S04 catalyze the isomerization of 1-alkenes to 2-alkenes. In the industrial carbonylation of ot-olefins, this double-bond isomerization is undesirable since the linear end products are of greater industrial importance. 

Two mechanisms are discussed for the double-bond isomerization of olefins  

Therefore, the isomerization depends crucially on the hydride character of the hydrogen atom. The hydride hgands of soft complexes such as [HRh(CO)(PPh3)3] preferably undergo anti-Markownikow addition with the following polarization  

---

Scheme 2-4 Hard and soft catalysis with transition metal compounds...

Carbenoids present indeed a full range of reactivities. Those involved in catalysis are extremely reactive species (e.g. copper carbenoids have never been observed, even by spectroscopy). On the other hand, Fischer carbene XIX are stable compounds [24]. Such a dramatic stabilization occurs when some "soft" transition metal complexes (Cr, Mo, W, Re) are associated with a stabilized (soft) carbene (e.g. alkoxy or dialkoxycarbenes, heterocyclic carbenes, etc). 

---