Transition Metal Phosphine Oxygen Complexes

The primary focus of research using transition metal phosphine complexes for oxidations is in the complexation and activation of molecular oxygen. These oxygen complexes have been variously regarded as complexes of coordinated peroxide, superoxide, or singlet oxygen, and their reactivity with reduced substrate has been interpreted on such a basis. In this chapter, we will focus on the chemical reactivity of these compounds for oxygen atom transfer oxidation reactions, with a particular emphasis on the mechanistic features of these processes. 

Atom-transfer-type oxidation reactions can occur by electrophilic, nucleophilic, or radical pathways. Electrophilic mechanisms are prevalent with metal compounds in high-oxidation states. Common among these reagents are oxo complexes of Mn(VII), Cr(VI), V(V), as well as organic 

---

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Nitrogen nucleophiles, in a similar manner to oxygen- and sulfur-based functionality, undergo transition metal-catalyzed cross-coupling with halopyridines. The use of palladium(O) catalysts is most effective in combination with chelating bis-(phosphine) ligands such as BINAP that prevent the formation of pyridine-palladium complexes that... 

Similarly, a recent study141 of the homogeneous oxidation of cyclohexene by various low-valent phosphine complexes of Group VIII transition metals yielded no definite proof for initiation by oxygen activation. Results were consistent with reactions involving chain initiation via the usual redox reactions of the metal complexes with traces of hydroperoxides. Long induction periods were observed with peroxide-free hydrocarbons. 

Reactions between molecular oxygen and well-defined transition-metal complexes have been the subject of extensive study for decades [21]. Recent studies demonstrate the suitability of NHCs as ancillary ligands in this chemistry, and in several cases, the enhanced stability of NHCs over phosphines is noted. Certain limitations associated with NHC-ligand instability have also been identified, particulary in the study of first-row transition metals. 

Molecular oxygen has become a commonly used co-catalyst for inactive or weakly active transition metal complexes [1-5]. In addition, other oxidizing agents, mainly peroxides, have recently been used in active rhodium complexes in particular, but also in metal carbonyls, as catalysts for hydrosilylation. The catalytic activity of bis(triphenylphosphine)carbonylrhodium(I) in the hydrosilylation of C=C and C=0 bonds can be much increased by the addition of about a 50 % molar excess of tert-butyl hydroperoxide [100]. Chromium triad carbonyls M(CO)e, where M = Cr, Mo, W, have been tested to examine the effect of various organic peroxides on the hydrosilylation of 2,3-dimethyl-1,3-butadiene by triethyl-, triethoxy- and methyldiethoxysilanes [100]. The evidence for organic oxidant promotion of RhCl(cod)phosphine-catalyzed hydrosilylation of 1-hexene was demonstrated previously [101]. 

---