[134067-47-3] · Ni · Nickel-Graphite
Form Supplied in: not commercially available.
Preparative Methods: Ni-Gr1 catalyst is prepared in situ by reduction of bis(dimethoxyethane)dibromonickel (NiBr2.2DME) by Potassium-Graphite in THF/HMPA at room temperature under argon.1 Exposure of Ni-Gr1 to air produces Ni-Gr2, a less active catalyst.2
Handling, Storage, and Precautions: due to the air sensitivity of Ni-Gr1, it is advisable to prepare and use it in situ for each application. Exposure of Ni-Gr1 to air produces a less active catalyst, Ni-Gr2, which can be stored in polyethylene bottles for at least four months before catalytic activity is decreased. Neither form of the catalyst is pyrophoric.
Catalytic hydrogenation of terminal alkynes with Ni-Gr1 affords the alkene as the major product (eq 1).1 Prolonged exposure to hydrogen will result in complete conversion to the alkane regardless of the presence of ethylenediamine. Disubstituted unconjugated alkynes are stereospecifically converted to the corresponding (Z)-alkenes (eq 2). The specificity is comparable to that obtainable with other catalysts such as Lindlar's catalyst and P2 nickel. Ni-Gr2 also reduces disubstituted alkynes, but with slightly lower stereoselectivity than Ni-Gr1.2 Attempts to improve the selectivity by addition of amines such as ethylenediamine, quinoline, or piperidine failed, due to complete loss of activity.
The reactivity of Ni-Gr2 is in agreement with most common catalysts. Reactivity towards catalytic hydrogenation decreases in the following order: alkyne > alkene &AApprox; nitro > carbonyl > nitrile. Stereoselective reduction of disubstituted alkynes affords (Z)-alkenes, although further reduction to the alkane also takes place (eqs 3 and 4).2 Alkenes are quantitatively reduced to alkanes (eq 5).2 Conjugation of the double bond with an aromatic ring increases reactivity toward alkenes and ketones.
Ni-Gr2 is a poor catalyst for reduction of nitro groups,2 presumably due to poisoning of the catalyst. In order to achieve some reaction, a large excess of catalyst relative to substrate is needed.2 Additionally, nitriles can be reduced to amines even in the presence of ketones.2
As a consequence of its reduced activity, Ni-Gr2 selectively reduces the double bond of an a,b-unsaturated ketone in the presence of other alkenes (eq 6),2 although the reactivity appears to be temperature dependent. In all cases, no reaction takes place at room temperature and elevated temperatures are needed to reduce carbonyl groups as well as hindered double bonds. Similarly, in unsymmetrical b-diketones the less hindered carbonyl group is reduced most easily (eq 7).2
Ellen M. Leahy
Affymax Research Institute, Palo Alto, CA, USA