Transition metals hydride shuttles

As shown in Scheme 2.1, the catalytic cycle of the metal(O) catalyst 94 is understood by combination of the aforementioned unit reactions. The reductive elimination of 97 regenerates M(0) 94, which undergoes oxidative addition to afford 96 and starts the new catalytic cycle, then subsequent insertion gives 98 or transmetallation affords 97. The catalytic species M(0) 94 can be reproduced from X—M—H 95, which is a -elimination product of 98. The metal hydride 95 itself can also serve as a catalytic species through the insertion of alkenes. The ability of transition metals to undergo facile shuttling between two or more oxidation states contributes to making these reactions catalytic. 

Boranes as hydride shuttles hydride shift from boron to transition metals... 

The mechanism of this unique H-D exchange is of considerable importance. The usual mechanism proposed for H-D exchange in d-block transition metals involves a series of reductive-elimination, oxidative-addition cycles. The prerequisite in this type of process is the ability of the metal atom to shuttle between two readily available oxidation states. This type of mechanism could be invoked to explain the exchange reaction in the uranium hydride since uranium (VI) is a well-known oxidation state. The observation that the thorium hydride also undergoes exchange at a comparable rate shows that such a process is not viable as thorium (VI) is unknown. Scheme III outlines an... 

The direct synthesis of HCO2H from CO2 and H2 catalyzed by transition metal systems has been much investigated in the past 20 years, and catalysts have been developed that have a high TON (turnover number) and TOP (turnover frequency) close to industrial utilization. The drawback in this field is the fact that the intermediate hydrido-formate species L MH(OCHO) are kmetically stable and require a shuttle that helps to eUminate the hydride (amines that are converted into ammonium compounds) or the formate anion (group 1 metal... 

---