Partial Reduction of Alkynes To Obtain Cis or Trans Alkenes
Last updated: February 27th, 2020 |
Alkynes to Alkenes via Partial Reduction With Lindlar’s Catalyst and Na/NH3
Alkynes bear many similarities to alkenes, but as we have already seen, their chemistry can differ in subtle and interesting ways. Today’s post is another case in point. One very important set of reactions of alkynes involves partial reduction to give either cis or trans alkenes depending on the reagents.
Table of Contents
- Hydrogenation of Alkynes With Pd-C and H2 Gives Alkanes
- Partial Hydrogenation of Alkynes Gives Alkenes
- Catalytic Hydrogenation Using the “Poisoned” Lindlar’s Catalyst Gives cis Alkenes
- Reduction of Alkynes Using Sodium and Ammonia (Na / NH3) Gives Trans Alkenes
- The Stereochemistry of Partial Reduction Has Important Consequences In Synthesis
- (Advanced) References and Further Reading
The reduction of alkenes by hydrogen in the presence of a metal catalyst (“catalytic hydrogenation”) is a time-honoured reaction recognized by Sabatier’s receipt of the Nobel Prize for Chemistry in 1904. Incidentally, the products of this reaction are a part of our daily lives – modern margarine is produced from hydrogenation of vegetable oils for example [Trans-fats are an unfortunate byproduct of catalytic hydrogenation].
Bearing two carbon-carbon π bonds, alkynes may likewise be hydrogenated. Under conditions used for the hydrogenation of alkenes, both bonds are reduced, producing alkanes.
[It’s reasonable to think that you could prevent over-reduction simply by only using one molar equivalent of hydrogen gas; in practice, this doesn’t work very well ]
If that’s all there was to the hydrogenation of alkynes, we’d quickly have to move on. The chemistry of alkanes, is – to put it bluntly – kind of dull*, and although reduction of alkynes to alkanes certainly has its place, you won’t find many reactions in organic chemistry which begin with alkanes. If you think of functional groups like airports, with all the reactions they can perform like flights that connect them to other hubs, a reaction that leads only to alkanes is a bit like taking a one-way flight to Saskatoon, Saskatchewan. Not nowhere, mind you, but a little far from the action.*
That’s not the whole story, of course. Imagine for a second that instead of hydrogenating both double bonds, we’d be able to stop at hydrogenating just one. This would allow us to convert alkynes into alkenes. Using the “functional groups as airports” analogy, a reaction that produces alkenes is like flying into O’Hare: as we just saw in the previous series, this will give us plenty of subsequent options in synthesis, as there is an extremely rich variety of alkene addition reactions. Look at this reaction map of alkenes for some inspiration.
As it turns out, because the second π bond of alkynes is not quite as strong as the first [approx 46 kcal/mol vs 68 cal/mol], conditions have been found that allow for the partial reduction of alkynes.
For our purposes there are two ways to do this.
The first, catalytic hydrogenation, operates on the same principle as described above: treatment of the alkyne with hydrogen gas and a metal catalyst. The trick, however, is to modify the behavior of the catalyst such that it is powerful enough to reduce the first π bond but not reactive enough to affect the second. In other words, “poisoning” its reactivity. In practice, this is done by combining palladium on carbon with lead carbonate (PbCO3) and quinoline (an aromatic amine). The resulting mixture, known as “Lindlar’s catalyst” after its inventor, is effective for the partial reduction of alkynes.
Note the stereochemistry! Just as in conventional alkene hydrogenation, both hydrogen atoms are delivered in syn fashion to provide us with the “cis” (Z) alkene.
There’s another way to reduce alkynes that doesn’t involve catalytic hydrogenation. As described in this old Reagent Friday post, sodium metal in ammonia (Na/NH3) can also reduce alkynes to alkenes. This process is called dissolving metal reduction. It’s a different process than catalytic hydrogenation. In this reaction, electrons from Na metal sequentially add to the alkyne, resulting in an anion that is protonated by the NH3 solvent. An interesting facet of this reaction is, again, the stereochemistry: due to electronic repulsion, the geometry of the resulting alkene is trans [for the full mechanism see this post].
5. The Stereochemistry of Partial Reduction of Alkynes To Alkenes Has Important Consequences In Synthesis
So the bottom line here is that through using different reducing agents, we can obtain alkenes of different geometries. This might not seem like such a big deal at the moment, but it will have very important consequences for subsequent reactions – stay tuned. I’ve said this before and no doubt I’ll repeat the same comment: stereochemistry is one of the key testable concepts in Org 1, and the reactions of alkynes are a key component.
* Modern advances in C-H activation chemistry excepted, of course
- Reduction of Organic Compounds by Lithium in Low Molecular Weight Amines. 11. Stereochemistry. Chemical Reduction of an Isolated Nonterminal Double Bond
ROBERT A. BENKESER, GESE SCHROLI, AND DALE & I. SALVE
J. Am. Chem. Soc. 1955, 77, 12, 3378-3379
Lithium and low-molecular weight amines (methylamine, ethylamine, isopropylamine) can also be used for dissolving metal reduction of alkynes. The advantage is that these reagents are easier to handle over sodium/ammonia.
- Reactions involving electron transfer. V. Reduction on nonconjugated acetylenes
Herbert O. House and Edith F. Kinloch
The Journal of Organic Chemistry 1974 39 (6), 747-755
Study of the product distribution of metal reduction of internal and terminal alkynes using Na in HMPA-THF.
- Dissolving Metal Reduction of Acetylenes: A Computational Study
The Journal of Organic Chemistry 2006 71 (24), 9165-9171
This is a computational investigation using DFT (density functional theory) which studies the stability of proposed intermediates in the dissolving metal reduction of acetylene, both in the gas phase and with explicit ammonia solvation.
- Ein neuer Katalysator für selektive Hydrierungen
Helv. Chim. Acta 1952 35 (2), 446
The original paper by Lindlar describing the development of a new catalyst for the selective hydrogenation of alkynes to Z-alkenes during Vitamin A synthesis.
- PALLADIUM CATALYST FOR PARTIAL REDUCTION OF ACETYLENES
Lindlar, R. Dubuis
Org. Synth. 1966, 46, 89
This procedure by Lindlar also gives a detailed preparation of the catalyst.
- A density functional theory study of the ‘mythic’ Lindlar hydrogenation catalyst
Garcı´a-Mota, J. Gomez-Dı´az, G. Novell-Leruth, C. Vargas-Fuentes, L. Bellarosa, B. Bridier, J. Pe´rez-Ramı´rez, N. Lo´pez
Theor. Chem. Acc. 2011, 128, 663
This is a computational investigation using DFT (density functional theory) which studies how the various components in the Lindlar catalyst (Pd, Pb, quinoline) pack together and how that contributes to hydrogenation selectivity.
L.E. Overman, M. J. Brown, S. F. McCann
Org. Synth. 1990, 68, 182
The second reaction in this 2-step synthesis is a Lindlar hydrogenation to give the Z-alkene.
- SYNTHETICALLY USEFUL REACTIONS WITH NICKEL BORIDE. A REVIEW
Jitender M. Khurana, Amita Gogia.
Organic Preparations and Procedures International The New Journal for Organic Synthesis
This is a review on the application of nickel boride in organic synthesis, which can be used in similar applications to Lindlar’s catalyst.