General Papers ARKIVOC 2008 (xi) 227-237
group VIII transition metals supported on charcoal were tested for the conjugated reduction and, quite ironically, they were all found superior to Pd/C. Indeed, the treatment of tert-butyl acrylate with 5 mol.% of Pd/C in the presence of 1.2 equiv. of triethylamine at 70 °C for 4 h gave 25% conversion and yield, while Pt/C, Ru/C and Rh/C gave, respectively, 37%, 50% and 60% conversion and yield under similar conditions. This points out that rhodium supported on charcoal (Rh/C) might be an efficient catalyst for the hydrogen transfer from triethylamine to conjugated electro-deficient alkenes under relatively mild conditions, yet to be studied. The good conversion obtained with Rh/C is in striking contrast to the poor result obtained with Rh/Al2O3, indicating the crucial importance of the metal support.
Conclusions
We have discovered that the suspected hydrido-palladium iminium complex generated from Pd/C and triethylamine catalyses the isomerization of allylic alcohols into carbonyl compounds. The process is accompanied by Pd/C-catalyzed disproportionation of the substrate to give the saturated alcohol and the a,ß-unsaturated ketone. Also, Pd/C catalyses the conjugate reduction of activated double bonds, using triethylamine as the source of the two newly incorporated hydrogen atoms, probably via the same complex. During the conjugate reduction reaction, the triethylamine undergoes self-alkylation to produce elongated aliphatic tertiary amines and ethylamine which, in the reaction with methyl cinnamate and coumarin, resulted in the formation of the reduced ethyl amide following in situ domino transamidation. Although the Pd/C–NEt3 system is clearly not the reagent of choice for both the isomerization and reduction reactions presented above, it might be a useful and cheap satisfactory alternative in some cases. In light of these results, unexpected isomerizations and/or reductions occurring in Pd-catalyzed reactions involving alkyl amines can now be rationalized.
Experimental Section
General Procedures. All reactions were performed in oven-dried glassware under an argon atmosphere. All reagents were obtained from commercial sources and used as supplied. Toluene was dried by refluxing with CaH2 and then distilled under argon. Triethylamine was dried over solid KOH and then distilled under argon. The reactions were monitored either by NMR, HPLC coupled with a Brüker Esquire 6000 mass spectrometer equipped with an electrospray ionization source and a ion-trap detector, or GC. NMR data were recorded on a Brüker Avance 300 spectrometer in CDCl3 or d-8-toluene. HPLC was performed on a Hitachi LaChrom Elite system. GC was performed on a Varian 3900 system equipped with a capillary column (CP-S11-5-CB low bleed) and a flame ionization detector set at 240 °C. 1-Cyclohexyl-2-propen-1-ol was prepared by addition of cyclohexyl carboxaldehyde to a THF solution of vinylmagnesium
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