An aminated series: a well-defined iron-catalyzed reductive amination reaction of aldehydes and ketones with aliphatic amines using molecular hydrogen is presented. Under mild conditions, good yields for a broad range of alkyl ketones as well as aldehydes were achieved.
Alkylation of ketones usually involves halides or pseudohalides such as tosylate and triflate derivatives in the presence of a stoichiometric amount of as trong base. [1] Such methodology generates wastes, as all these electrophiles are prepared from alcohols, and requires the use of hazardous chemical materials. Synthetic chemists seek nowadays for more environmentally friendly ways to construct carbon-carbon bonds. In recent years, several efficient strategies were proposed for their creation:i )directly from two simple carbon-hydrogen bonds (catalytic dehydrogenative cross-coupling reaction), [2] andi i) from ketonesa nd alcohols (hydrogen autotransfero rb orrowing hydrogen strategy). [3] Carbon-carbon bond formation via the borrowing hydrogen strategy is ap owerful strategy for the alkylation of ketones (Scheme1). [3] Advantages of this approach are the use of easily-to-handle alcohols, asasourceo fa lkylating reagents, andt he formationo fw ater as the sole byproduct. Indeed, following as implified mechanism,t he alcohol is initially oxidized (dehydrogenation step), and then an aldolizationdehydration step liberates an enone intermediate, whichc an be reduced into ak etone (Scheme 1).Many efficient catalysts are based on expensive noble metals such as iridium, [4] ruthenium [5] or rhodium. [6] Owing to economic constraint and sustainability concerns,t he replacement of platinum metals by first-row-based metals could be an attractive alternative. Recent reports described the use of iron, [7] cobalt [8] and manganese [9] as non preciousm etals in hydrogen autotransfer processes. However,a ll these complexes requiredat emperature threshold of 140 8Ca nd/ore xpensive phosphine ligands. The scope of substrates could also be rather limited. As example, Darcel and co-workers showed recently that only aromatic ketones could be engaged in alkylation reactions in the presence of an in situ iron catalystg enerated from Knçlker's complex and triphenylphosphine. [10] Moreover,y ields were moderate andn om echanism was proposed. [11] Then, even if these works pavet he way to new opportunities in sustainable chemistry,s omel imitationsw ere still present,amechanistic understanding of this iron-catalyzed alkylation reaction and someimprovementwere needed.In our ongoing work on iron-catalyzed reduction, [12] we have recently brought to light that cyclopentadienone iron tricar-Cyclopentadienone iron dicarbonyl complexes were applied in the alkylation of ketones with variousa liphatic and aromatic ketones and alcohols via the borrowing hydrogen strategy in mild reactionc onditions. DFT calculations and experimental works highlight the role of the transition metal Lewis pairs and the base. These iron complexes demonstrated ab road applicability in mild conditions and extended the scope of substrates.Scheme1.Simplified acceptedm echanism of alkylation of ketones.
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