catalyst loading to 7 mol% delivered an improved 84% yield
of the desired saturated product (Scheme 8).
Scheme 10 Selective hydrogenation of alkyne 22.
Scheme 8 Hydrogenation of 20 with complex 1c.
are compatible with functional groups which are sensitive to
alternative and more widely employed hydrogenation methods.
Complex 1a has emerged as the most generally effective of the
three complexes and leads to hydrogenation efficiencies that
are elevated over those displayed by Crabtree’s catalyst.
Additionally, with more demanding, tri- and tetrasubstituted,
olefin substrates the less encumbered complex (1c) shows
activity levels which are, again, at least competitive with those
shown with Crabtree’s catalyst. Furthermore, employing
benzamide as a poison, catalyst 1a has been shown to selectively
hydrogenate an alkyne to an alkene with high Z-selectivity.
Finally, since the reduction of carbon–carbon double bonds
was so successful, we decided to probe the applicability of our
complexes in alkyne hydrogenation and, indeed, the semi-
hydrogenation of the same class of substrate. Using the
internal alkyne (22) we initially employed complex 1a at
0.1 mol% loading, and, after a reaction time of only 1 h, the
alkyne was reduced completely, yielding the alkane (23)
quantitatively (Scheme 9).
We thank AstraZeneca, R&D Molndal for postgraduate
¨
studentship funding (S.I.), and the EPSRC Mass Spectrometry
Service, University of Wales, Swansea, for analyses.
Scheme 9 Complete hydrogenation of alkyne 22.
Notes and references
From this result, it was clear that complex 1a was extremely
active in the hydrogenation of alkynes and so, in order to be
able to achieve the selective reduction of the alkyne to the
alkene, the catalytic efficiency of the complex would have to be
lessened. In this regard, an effective way to tune the reactivity
of transition metal complexes is to use a suitable poison,
with this method often being employed within palladium
catalysis.10 With a view to finding a suitable poison for our
catalysts, we considered an additive compound which has
previously been found to be a challenging substrate within
associated hydrogen–deuterium exchange reactions, namely
benzamide.11 Although complexes 1a–c are capable of
labelling this substrate at low catalyst loadings,8 the levels of
deuteration are less elevated than those typically observed for
other substrates. This is thought to be due to strong coordina-
tion of this substrate to the iridium centre hindering the
progress of the required catalytic cycle. Thus, we thought that
benzamide could be an effective poison to allow the selective
hydrogenation of alkynes with the emerging class of catalysts
(1). Indeed, when 15 mol% of benzamide was employed with
0.1 mol% of complex 1a, we were pleased to obtain an 85%
conversion of alkyne 22 to alkene 24, with an excellent
selectivity for the Z-alkene (Scheme 10).12
1 (a) P. N. Rylander, Hydrogenation Methods, Academic Press,
London, 1985; (b) P. N. Rylander, Catalytic Hydrogenation in
Organic Syntheses, Academic Press, New York, 1979.
2 H. Brunner, in Applied Homogenous Catalysis in Organometallic
Compounds, ed. B. Cornils and W. A. Herrmann, VCH, Weinheim,
1996, vol. 1, ch. 2, p. 201.
3 (a) J. F. Young, J. A. Osborn, F. H. Jardine and G. Wilkinson,
Chem. Commun., 1965, 131; (b) J. A. Osborn, F. H. Jardine,
J. F. Young and G. Wilkinson, J. Chem. Soc. A, 1966, 1711.
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5 D. F. Chodosh, R. H. Crabtree, H. Felkin and G. E. Morris,
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6 H. M. Lee, T. Jiang, E. D. Stevens and S. P. Nolan, Organome-
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Commun., 2002, 2518; (b) L. D. Vazquez-Serrano, B. T. Owens and
J. M. Buriak, Inorg. Chim. Acta, 2006, 359, 2786.
8 (a) J. A. Brown, S. Irvine, A. R. Kennedy, W. J. Kerr,
S. Andersson and G. N. Nilsson, Chem. Commun., 2008, 1115;
(b) G. N. Nilsson and W. J. Kerr, J. Labelled Compd. Radiopharm.,
2010, 53, 662.
9 J. A. Brown, PhD Thesis, University of Strathclyde, 2007.
10 H. Lindlar, Helv. Chim. Acta, 1952, 35, 446.
11 (a) A. Y. L. Shu, W. Chen and J. R. Heys, J. Organomet. Chem.,
1996, 524, 87; (b) J. R. Heys, A. Y. L. Shu, S. G. Senderoff and
N. M. Phillips, J. Labelled Compd. Radiopharm., 1993, 33, 431;
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Tetrahedron, 2001, 57, 9487.
12 Following this process, the organic mass balance was made up of
alkyne starting material 22 (5%) and the fully saturated alkane 23
(10%).
In summary, new iridium(I) complexes (1) bearing a bulky
NHC/phosphine ligand combination have been established as
extremely efficient hydrogenation catalysts.w These species can
be used at low catalyst loadings over short reaction times and
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11653–11655 11655