Catalysis with molybdenum-palladium

The molybdenum-catalyzed asymmetric reaction differs from the palladium-catalyzed reaction in several ways, the most important of which is the different regios-electivity achieved. Molybdenum-catalyzed reactions favor the most sterically hindered position (Eq. 11.39), in contrast with palladium catalysis. The molybdenum-catalyzed allylations also suffer from significantly lower reactivity. 

In addition to the successful reductive carbonylation systems utilizing the rhodium or palladium catalysts described above, a nonnoble metal system has been developed (27). When methyl acetate or dimethyl ether was treated with carbon monoxide and hydrogen in the presence of an iodide compound, a trivalent phosphorous or nitrogen promoter, and a nickel-molybdenum or nickel-tungsten catalyst, EDA was formed. The catalytst is generated in the reaction mixture by addition of appropriate metallic complexes, such as 5 1 combination of bis(triphenylphosphine)-nickel dicarbonyl to molybdenum carbonyl. These same catalyst systems have proven effective as a rhodium replacement in methyl acetate carbonylations (28). Though the rates of EDA formation are slower than with the noble metals, the major advantage is the relative inexpense of catalytic materials. Chemistry virtually identical to noble-metal catalysis probably occurs since reaction profiles are very similar by products include acetic anhydride, acetaldehyde, and methane, with ethanol in trace quantities. 

These heterogeneous catalysis contain nickel, cobalt, molybdenum, tungsten, platinum, or palladium on acidic aluminum silicate or zeolite supports. As with reforming catalysts, the catalysts here are also believed to be... 

The synthesis of quaternary amino acids 86 have been shown using azlac-tones 85 as nucleophiles and the Trost ligand 39 under palladium [179] or molybdenum catalysis (Scheme 8) [180]. The allylic alkylation of glycine imino esters under biphasic conditions has also been achieved using a chiral phase-transfer catalyst in combination with an achiral Pd catalyst producing the unnatural amino acid derivatives [181]. 

Allyl complexes were amongst the earliest studied nomigid complexes, and there are a number of reviews on their rearrangements. Interest has been particularly high owing to their importance in catalysis and synthesis, with palladium and molybdenum complexes playing leading roles. 

Other examples of microwave-assisted catalysis include allylic alkylation, both palladium catalyzed and molybdenum catalyzed. In the latter case, air stable precursor complexes could be used under non-inert conditions. Microwave-enhanced Pauson-Khand reactions have also been reported, as have hydroamination of alkynes, and metathesis of functionalized alkynes. " Recently, microwave enhancement has been applied to C-H activation reactions, for example, for the formation of functionalized heterocycles, allowing the reaction to be performed with no solvent purification and minimal precautions to exclude air. A solvent-free chelation-assisted hydroacylation... 

On the other hand, molybdenum catalysis results in predominant attack at the more-hindered position when non-bulky nucleophiles are employed (Schemes 9.22 and 9.23). While the use of the palladium-phosphine system gives the product under steric control, with the molybdenum carbonyl system, attack is now under charge control. The carbon better able to support positive charge, i.e. the more substituted carbon, is the site of attack. More-hindered nucleophiles, however, again favour attack at the less hindered terminus, ... 

---