Abstract
Borrowing hydrogen (or hydrogen autotransfer) reactions represent straightforward and sustainable C–N bond-forming processes. In general, precious metal-based catalysts are employed for this effective transformation. In recent years, the use of earth abundant and cheap non-noble metal catalysts for this process attracted considerable attention in the scientific community. Here we show that the selective N-alkylation of amines with alcohols can be catalysed by defined PNP manganese pincer complexes. A variety of substituted anilines are monoalkylated with different (hetero)aromatic and aliphatic alcohols even in the presence of other sensitive reducible functional groups. As a special highlight, we report the chemoselective monomethylation of primary amines using methanol under mild conditions.
Highlights
Borrowing hydrogen reactions represent straightforward and sustainable C–N bond-forming processes
The atom-efficient creation of carbon–nitrogen bonds represents a key step for the synthesis of a plethora of compounds, which are widely applied in life sciences and the chemical industry[1,2]
We were attracted by the potential of PNP pincer complexes, which are known for their cooperative catalysis under mild conditions[54,55]
Summary
Borrowing hydrogen (or hydrogen autotransfer) reactions represent straightforward and sustainable C–N bond-forming processes. Due to the importance of these products, there is a number of powerful methods available that address the synthesis of amines, for example, classic nucleophilic substitutions, as well as Buchwald–Hartwig[3], Ullmann reactions[4] and hydroaminations[5] Most of these more advanced procedures require catalysts which are based on specific (noble metal) complexes. Heterogeneous catalysts are known for this latter transformation[38,39,40] and are applied in industry for the preparation of methylamines from methanol on bulk scale Most of these materials need drastic reaction conditions (250–500 °C) making them not suitable for the synthesis of more advanced amines, especially life science products such as pharmaceuticals. Complementary to that work, we describe the first well-defined manganese complex for N-alkylation of amines with a variety of alcohols including methanol through hydrogen autotransfer methodology
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