Asymmetric Catalysis. Mechanism of Asymmetric Catalytic Intramolecular Hydrosilation

This work was supported by grants from the National Institutes of Health.Peer reviewe

Asymmetric Catalysis. Asymmetric Catalytic Intramolecular Hydroacylation of 4-Pentenals Using Chiral Rhodium Diphosphine Catalysts

Catalysts of the type [Rhfchiral diphosphine)]+ convert 4-substituted 4-pentenals into the corresponding 3-substituted cyclopentanones with generally high turnover numbers and frequencies at 25 °C. The enantioselectivities of various substituted 4-pentenals with two chiral diphosphines have been explored. It was found that with the binap catalyst, almost complete enantioselectivity is observed for 4-pentenal substrates bearing 4-substituted tertiary substituents and for ester groups. Ketonic substituents give very high enantioselectivities. The mechanism of intramolecular hydroacylation has been explored, and it is suggested that an important consideration for obtaining high turnover frequencies is related to the acyl-alkyl reductive elimination mechanism which is inferred to occur by a process similar to ester hydrolysis. The origin of the enantioselection is discussed in terms of the interactions between the phenyl groups of the phosphine and the substituent of the pentenalThis work was supported by grants from the National Institutes of Health.Peer reviewe

Asymmetric Catalysis. Production of Chiral Diols by Enantioselective Catalytic Intramolecular Hydrosilation of Olefins

Rhodium(1) chiral diphosphine complexes efficiently and rapidly catalyze the intramolecular hydrosilation of silyl ethers derived from allylic alcohols. The efficiency and rates of intramolecular hydrosilations were determined for a variety of silyl and olefin substituents. The catalysts were found to tolerate a wide variety of silyl substituents, although terminal alkyl olefin substituents were found to retard catalysis. Terminal aryl olefin substituents were found to be hydrosilated efficiently and at reasonable rates. One of the chiral catalysts is highly enantioselective for terminal aryl olefin substituents. Almost quantitative eels are obtained. Moreover, the ee’s are only slightly sensitive to aryl and olefin substituents, suggesting that this enantioselective catalysis can provide a wide range of chiral species. Oxidative cleavage of the hydrosilation products gives chiral diols.This work was supported by grants from the National Institutes of Health.Peer reviewe

Second Order Dependence on the Surface Fraction of Pt in PtRu_adatom of the Oxidation of 2‑PrOH in Base

The electrooxidation of 2-PrOH in alkaline electrolyte is studied over a series of Pt-Ru adatom catalysts with controlled surface compositions as a function of potential and temperature. Multivariable analysis is used to determine both kinetic and thermodynamic parameters of interest. PtRu surfaces are found to be more active at low potentials than either Pt or Ru alone. At moderate overpotentials, Pt is the most active catalyst due to a surface blocking effect by Ru. A clear second order dependence on the surface fraction of Pt is determined, and a kinetic isotope effect suggest that both CH­(D) and OH­(D) bonds are broken before or during the rate-determining step for the electro-dehydrogenation of 2-propanol to acetone. Possible mechanisms that account for these observations are discussed