Paper
Green Chemistry
allows for hydrogenations to be run on many types of olefins
without recourse to alternative catalysts, whether derived from
base or precious metals, that must first be made and then
used under more vigorous and far less environmentally
responsible conditions. Switching to olefin hydrogenations in
water, following Nature’s lead, is long overdue.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support provided by the NSF (CHE 18-56406),
Novartis, and PHT International (postdoctoral fellowship to
B. S. T.) is warmly acknowledged.
Scheme 3 ICP-MS analysis of residual Pd in products 2, 19, 45, and 46.
Conditions A = H2 balloon (1 atm); Conditions B = 1.2–1.5 equiv. Et3SiH.
hours to give 21. Hydrogenation of α-methyl p-chlorostyrene
proceeded smoothly under nano-to-nano conditions without
dehalogenation to give the desired product 57 in 99% yield. By
contrast, to achieve similar results, 100 times higher loading
of an iron catalyst was used in organic solvent in a system
designed to handle hydrogen under high pressure.12 Lastly, a
triglyceride underwent triple hydrogenation to arrive at stearin
60 (90% isolated yield), also run on a gram scale using
1000 ppm Pd/C, or only 333 ppm Pd (0.0333 mol%) per alkene.
The same cobalt catalyst (2.9% loading) required 150 °C at 40
bar of hydrogen pressure to obtain 93% conversion.16
Heterogeneous catalysis oftentimes offers the advantage of
low levels of catalyst leaching into a reaction mixture, leading
to limited product contamination by residual metal(s). Given
the low loadings of Pd involved in these hydrogenations, and
as expected from prior studies,20–22 ICP-MS analyses of pro-
ducts 2, 19, 45, and 46 (Scheme 3) led, in all cases, to residual
levels below 1 ppm Pd;30 the FDA limit for residual Pd per
dose is ≤10 ppm.9
Notes and references
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C. J. Elsevier, Wiley-VCH, 2008, ch. 37.
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innovation/endangered-elements.html, accessed April 13,
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acs.org/articles/91/i33/Trace-Metals-Debate.html, accessed
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3. Conclusions
In conclusion, the time-honored, fundamental process of
alkene hydrogenation has been newly defined as a modern 10 P. J. Chirik, Acc. Chem. Res., 2015, 48, 1687–1695.
protocol that (1) avoids waste-generating organic solvents, 11 R. Xu, S. Chakraborty, S. M. Bellows, H. Yuan,
using recyclable water; (2) eliminates the investment of energy
in the form of heating past ambient temperatures; and (3) is
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suffices. Overall, this unprecedented technology in water
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Green Chem.
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