Studies on the mechanism of metal-catalyzed hydrogen transfer from alcohols to ketones

The mechanism of metal-catalyzed hydrogen transfer from alcohols to ketones has been studied. Hydrogen transfer (H-transfer) from (S)-alpha -deuteroa- phenylethanol ((S)-1) to acetophenone was used as a probe to distinguish be tween selective carbon-to-carbon H-transfer and nonselective transfer invol ving both oxygen-to-carbon and carbon-to-carbon H-transfer. The progress of the reaction was monitored by the decreasing enantiomeric excess of (S)-1. After complete racernization, the alcohol was analyzed for its deuterium c ontent in the alpha -position, which is a measure of the degree of selectiv ity in the H-transfer. A number of different rhodium, iridium, and rutheniu m complexes (in total 21. complexes) were investigated by using this probe. For all rhodium complexes a high degree of retention of deuterium at alpha -carbon (95-98%) was observed. Also most iridium complexes showed a high d egree of retention of deuterium. However, the results for the ruthenium com plexes show that there are two types of catalysts: one that gives a high de gree of deuterium retention at alpha -carbon and another that gives about h alf of the deuterium content at alpha -carbon (37-40%). Two different mecha nisms are proposed for transition-metal-catalyzed hydrogen transfer, one vi a a monohydride (giving a high D content) and another via a dihydride (givi ng about half of D content). As comparison non-transition-metal-catalyzed h ydrogen transfer was studied with the same probe. Aluminum- and samarium-ca talyzed racernization of (S)-1 gave 75-80% retention of deuterium in the al pha -position of the alcohol, and involvement of an electron transfer pathw ay was suggested to account for the loss of deuterium.