Communication
Catalysis Science & Technology
give IJZ)-2-pentylnon-2-enal and subsequent hydrogenation of
the CO bond of the α,β-unsaturated aldehyde. Whereas
traces of IJZ)-2-pentylnon-2-enal were found in the GC-MS
spectrum of the reaction mixture, no evidence for a partial
hydrogenation of the CC bond was observed by NMR and
GC-MS spectra. In order to avoid aldehyde condensation
catalyzed by the strong base KOtBu, the borohydride complex 2
was used as catalyst in absence of this base. Indeed, under
these conditions the selective hydrogenation of primary
aldehydes to primary alcohols was possible. Thus hydrogena-
tion of heptanal in the presence of 0.10 mol% catalyst 2
using 30 bar of H2 pressure at 50 °C gave 1-heptanol in 93%
yield after 16 h (Scheme 2).
In conclusion, we have developed a general method for
the iron-catalyzed hydrogenation of aldehydes to alcohols. The
well-defined iron pincer complex ij(iPr-PNP)FeIJH)IJCO)IJBr)] (1)
is an efficient precatalyst for the hydrogenation and of
secondary and tertiary aliphatic aldehydes and aryl alde-
hydes. These reactions proceed smoothly under mild condi-
tions (30 bar, 40 °C) and low catalyst loadings (0.025 mol%)
to give the products in good to quantitative yields. This proto-
col is not suitable for primary aldehydes, as aldol condensa-
tion proceeds faster than the hydrogenation of the primary
aldehydes so that the corresponding enols are obtained selec-
tively. However, selective hydrogenation of primary aldehydes
to primary alcohols is possible by using the iron pincer complex
ij(iPr-PNP)FeIJH)IJCO)IJBH4)] (2) as catalyst under KOtBu-free
conditions. Overall this constitutes the most active system for
iron-catalyzed hydrogenation of aldehydes, achieving unprece-
dented turnover numbers.
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Acknowledgements
This research was supported by the European Research
Council under the FP7 framework (ERC no. 246837) and by
the MINERVA foundation. T. Z. received a postdoctoral fellow-
ship from the MINERVA foundation and D. M. holds the Israel
Matz Professorial Chair and thanks the Humboldt Foundation
for the Meitner-Humboldt Research Award.
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