10.1002/anie.201708800
Angewandte Chemie International Edition
COMMUNICATION
conditions shown in Equation 1, using unlocked 31P NMR. After
2 h reaction time, we observed 2c and a new species with δ =
88.4 ppm in approximately a 1:1 ratio.[21] This new species
resonates significantly upfield of the known hydride complexes
derived from 2c.[22] It falls much closer to the reported values of
(PNN)RuCl2(CO), δ = 91.4 and [(PNN)RuCl2]2(µ-N2), δ = 87.8.[23]
Thus it seems likely that both complexes of the type
(PNN)RuHX(CO) and (PNN)RuX2(CO) are the catalyst resting
state.
In summary, we have developed a catalyst system for the
efficient transfer reduction of a range of unactivated and
functionalized alkyl and aryl halides, which requires only the
relatively inexpensive and safe stoichiometric reagents NaOt-Bu
and i-PrOH. Reaction setup and workup is simple. While many
iridium and ruthenium pincer complexes show catalytic activity,
Milstein’s complex 2c was key to obtaining high yields. The
reaction appears to proceed via a radical mechanism. Our
conditions offer a greener alternative for several types of
stoichiometric LiAlH4 or Bu3SnH-mediated reductions. Future
studies will be directed the reaction mechanism and catalyst
design.
Acknowledgements
As noted earlier, several reactivity trends point to a radical
mechanism for the C–X bond reduction step. To probe this
hypothesis, we subjected 5-bromohex-1-ene to our optimized
conditions (Equation 2). The reaction proceeded efficiently to
generate methylcyclopentane with >99:1 selectivity, implying the
intermediacy of the 5-hexenyl radical.[24] Alternatively, the
reaction could proceed via insertion of the olefin into a Ru–C
bond, however this insertion would be expected to be slow.[25]
Our proposed radical mechanism[26-27] is shown in Scheme 2.
Initiation likely occurs via homolysis of the benzylic C–H bonds
in (PNN)RuH2(CO), ultimately generating H2 and a putative 15e-
species, (PNN)RuH(CO). This reactive species could abstract a
bromine atom from the organic substrate, generating the
We acknowledge funding from King Fahd University of
Petroleum and Minerals. M.C.H. acknowledges funding from the
Resnick Sustainablility Instute in the form of a postdoctral
fellowship.
Keywords: halogens • hydrogen transfer • green chemistry •
hydrides • hydrocarbons
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(PNN)RuHBr(CO) can undergo facile hydrodehalogenation by i-
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atom donor towards the organic radical. Thus the observed
reduced organic product and the active intermediate
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the main cycle via the reaction with i-PrO-.
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when applied to a neopentyl bromide. b) B. Sahoo, A.-E. Surkus, M.-M.
Pohl, J. Radnik, M. Schneider, S. Bachmann, M. Scalone, K. Junge, M.
Beller, Angew. Chem., Int. Ed., in press. c) Dai and Li recently reported
Scheme 2. Proposed mechanism.
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