Oxophilic metal centers

The conversion of the dithiocarbonates into alkenedithiolates involves base hydrolysis, which is usually effected with sodium alkoxides in alcohol. With the dianion in hand, the synthesis of complexes follows the usual course, as described above. Obviously, oxophilic metal centers, for example, Ti(IV) and Nb(V) (62), are incompatible with the usual alcohol solutions of in situ generated alkenedithiolates. In such cases, the anhydrous salts Na2S2C2R2 are employed in nonhydroxylic solvents, although after complex formation protic solvents are typically employed for cation exchange. 

In particular, Schrock-type catalysts suffered from extreme moisture and air sensitivity because of the high oxidation state of the metal center, molybdenum. Due to the oxophilicity of the central atom, polar or protic functional groups coordinate to the metal center, poisoning the catalyst and rendering it inactive for metathesis. Since late transition metal complexes are typically more stable in the presence of a wide range of functionalities, research was focused on the creation of late transition metal carbene complexes for use as metathesis catalysts. 

However, once again, the main obstacle to further development was one of limited substrate scope resulting from the oxophilic titanium, molybdenum, and tungsten metal centers. The problem is illustrated in Figure 6.2, which summarizes the reactivity of early and late transition metal olefin metathesis catalysts with common... 

Since the keto component reacts neither with the catalyst 5 (with or without phosphine) nor with the diazoalkane (exceptions below), it is likely to enter the catalytic cycle at the metal-carbene stage 8. If this proposal withstands future mechanistic studies, then the regioselectivity - oxygen to the oxophilic rhenium center-is reasonable by considering a metallacyclic intermediate 10 (eq. (6)). This hypothetical rhena-oxetane is assumed to eliminate the olefin. As a matter of fact, catalyst 5 has been detected from such precursors (e. g., 7a, b -1- diazoalkane f aldehyde) by gas chromatography [9, 10]. 

The usual representation of Schrock-type nucleophilic carbenes as electron rich at carbon can be especially misleading in the case of the Tebbe reagent and related complexes. These high oxidation state complexes are electron-deficient and electrophilic at the metal center, and it is unlikely for polarization of the metal-carbon bond to remove even more electron density from the metal under these circumstances. Thus, the reactivity of the Tebbe reagent is more closely related to the electrophilicity and oxophilicity of the metal center than to the nucleophilicity of a polarized carbene carbon that is, the reactivity is due to carbonyl polarization upon complexafion, not attack of the alkylidene carbon on an unactivated, electrophilic carbonyl carbon. 

The crystal structure of pure Pt is face-centered cubic (fee), while that of Ru is hexagonal close packed (hep). For Ru atomic fractions up to about 0.7, Pt and Ru form a solid solution with Ru atoms replacing Pt atoms on the lattice points of the fee structure. The lattice constant decreases from 0.3923 (pure Pt) to 0.383 nm (0.675 atomic fraction of Ru). In contrast to bulk Pt-Ru alloys, it has to be remarked that in carbon supported catalysts the amount of Ru alloyed with Pt is lower than the nominal Ru content in the material the amount of Ru alloyed with Pt depends on the preparation method of the supported catalyst. In Pt-Ru-M catalysts, the third metal is an oxophilic element as W, Mo, Os, Ni, Ir, etc. Some of these elements can be fully alloyed, while several form alloys to a limited extent or not at all with Pl ... 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

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