Alkyne Reactions

By James Ashenhurst

Alkyne Reactions – The “Concerted” Pathway

Last updated: November 15th, 2022 |

Alkyne Reaction Mechanisms That Pass Through A “Concerted” Pathway – Cyclopropanation, Hydrogenation, Hydroboration

  • Various reactions that proceed through a concerted mechanism such as hydrogenation, hydroboration, and even cyclopropanation are also effective for alkynes
  • Certain reactions that work well for alkenes don’t work well for alkynes
  • Among the reactions that don’t work are dihydroxylation and epoxidation

Table of Contents

  1. The “Concerted” Mechanistic Pathway For Alkynes
  2. Cyclopropanation of Alkynes
  3. What Reactions From The “Concerted Pathway” Don’t Work For Alkynes?
  4. Epoxidation of Alkynes With m-CPBA Doesn’t Work. Neither Does Dihydroxylation of Alkynes With OsO4
  5. Notes
  6. (Advanced) References and Further Reading

1. The “Concerted” Pathway for Alkynes: Hydrogenation and Hydroboration

How does the chemistry of alkynes compare to alkenes? As we’ve seen in some previous posts, there are some significant differences, but a lot of the chemistry “rhymes”, if you will. In the series on alkenes we broke down most of the reactions into three major categories according to their mechanisms – the “carbocation”, “3-membered ring”, and “concerted” pathways, and – you guessed it – since we’ve covered the carbocation and 3-membered ring pathways for alkynes in previous posts, this post concerns the “concerted” category. Recall that reactions that proceed through a “concerted” mechanism break the C-C π bond with concomitant formation of two new single bonds to the adjacent carbons, which form on the same face (“syn” addition). As we’ll see, there isn’t actually a lot of new ground in this post that we haven’t discussed before, except – interestingly –  for some reactions that are absent in alkyne chemistry.

So – what works, and what doesn’t?

Two major reactions in the “concerted” pathway that work for alkynes are hydrogenation and hydroboration. However, as we’ve already seen, each of these reactions comes with a twist when applied to alkynes.

With hydrogenation, treatment of an alkyne with a late metal catalyst such as Pd-C (or platinum on carbon, among others) in the presence of hydrogen leads not just to one hydrogenation, but two. The product is an alkane. It’s possible to get the reaction to stop “halfway” by using a less reactive catalyst such as “Lindlar’s catalyst” or by using nickel boride. This provides the cis alkene. Alternatively (although this doesn’t really count as a “concerted” mechanism, one can obtain the trans alkene through the use of sodium in ammonia (Na/NH3).
concerted path alkyne hydrogenation goes twice lindlar hydrogenation gives cis sodium ammonia gives trans alkenes

Hydroboration provides the anti-Markovnikov product just as it does for alkenes, although the resulting product after oxidation – the “enol” – is  usually unstable relative to its constitutional isomer, the “keto” form, with which it is in equilibrium  (an average stability ratio is about 5000:1 favoring the keto form) through a reaction known as “keto-enol tautomerism”.
hydroboration of alkynes with r2bh or bh3 gives anti markovnikov enol which tautomerizes to aldehyde

2. Cyclopropanation of Alkynes

There’s actually a third reaction that does work for alkynes, although it is rarely mentioned in this context. It is possible to form cyclopropenes through the “Simmons-Smith” reaction of alkynes with zinc-copper couple (Zn-Cu) and diiodomethane (CH2I2). Although this is an interesting result, and cyclopropenes are fun intermediates in advanced organic chemistry (and even are found in nature!) their application in introductory organic chemistry is limited and we shall speak no more of this reaction.

cyclopropanation of alkynes with zinc copper couple and diiodomethane ch2 gives cyclopropene

3. What Reactions That Pass Through A Concerted Mechanism Don’t Work For Alkynes?

An even more interesting question on this topic of “concerted” reactions is “What Doesn’t Work?

First of all, one of the more useful reactions of alkenes is their conversion to epoxides through the use of a peroxyacid like m-chloroperoxybenzoic acid (m-CPBA).

Try it on alkynes, though, and nothing happens!  It just doesn’t work.

Why not?

Ha! Learning organic chemistry – as you must know by now –  is a process of being continually surprised by the complex phenomena that can lurk behind the most innocuous seeming questions.  Two answers to this question are appropriate: one is “you don’t need to know yet”, which, to be honest, is an answer a lot of students are perfectly fine with.

The second answer is that it turns out that the product of the hypothetical reaction between m-CPBA and an alkyne is a molecule called an “oxirene” – which has a very interesting property known as “antiaromaticity“. (See article: Antiaromaticity)

For reasons we can’t get into right now, antiaromatic molecules are particularly unstable, and in fact oxirenes have only rarely been isolated – and even then, only at very low temperatures.

4. Epoxidation of Alkynes With mCPBA Doesn’t Work. Neither Does Dihydroxylation With OsO4.

alkyne epoxidation with mcpba does not work oxirene is antiaromatic dyhydroxylation of alkynes with oso4 does not work

Dihydroxylation with OsO4 is another useful reaction of alkenes that fails for alkynes (or at the very least, is not significant). In all my years I don’t recall seeing a single example of this reaction being effective on an alkyne, but if someone out there has, please feel free to correct me.

In any case, the fact that OsO4 is not a significant reaction for alkynes is useful to keep in mind – this will become important when we start to plan out sequences of reactions (synthesis!).

In the next post let’s circle back a bit an talk about an interesting way to make alkynes – and then we’ll finally get to the really good stuff: how to design sequences of reactions.

Next Post: Alkynes Via Elimination Reactions


(Advanced) References and Further Reading

Hydrogenation with Pd/C:

  1. Convergent and efficient palladium-effected synthesis of 5, 10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF)
    Edward C. Taylor and George S. K. Wong
    The Journal of Organic Chemistry 1989, 54 (15), 3618-3624
    DOI: 1021/jo00276a023
    The synthesis of compound 27 involves the hydrogenation of an alkyne with Pd/C.
  2. Preparation of chiral lactone from laevoglucosan; a key intermediate for synthesis of the spiroacetal moieties of the avermectins and milbemycins
    Raymond Baker, R. Hugh O. Boyes, D. Mark P. Broom, Mary J. O’Mahony, and Christopher J. Swain
    J. Chem. Soc., Perkin Trans. 1, 1987, 1613-1621
    DOI: 10.1039/P19870001613
    The synthesis of compounds 28a and 28b involves Pd/C catalyzed hydrogenation of an alkyne.Lindlar Hydrogenation:
  3. Ein neuer Katalysator für selektive Hydrierungen
    Lindlar, H. Chim. Acta 1952 35 (2), 446
    DOI: 10.1002/hlca.19520350205
    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.
    H. Lindlar, R. Dubuis Org. Synth. 1966, 46, 89
    DOI: 10.15227/orgsyn.046.0089
    This procedure by Lindlar also gives a detailed preparation of the catalyst.
  5. A density functional theory study of the ‘mythic’ Lindlar hydrogenation catalyst
    Garcı´a-Mota, J. Go´mez-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
    DOI: 10.1021/s00214-010-0800-0
    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
    DOI: 10.15227/orgsyn.068.0182
    The second reaction in this 2-step synthesis is a Lindlar hydrogenation to give the Z-alkene.
    Jitender M. Khurana, Amita Gogia
    Organic Preparations and Procedures International
    The New Journal for Organic Synthesis
    DOI: 1080/00304949709355171
    This is a review on the application of nickel boride in organic synthesis, which can be used in similar applications to Lindlar’s catalyst.Hydroboration of alkynes:
    Brown, H.C.; Zweifel, G. Am. Chem. Soc. 1959 81 (6), 1512
    DOI: 10.1021/ja01515a058
    The original paper describing the hydroboration of alkynes, by Nobel Laureate Prof. H. C. Brown (Purdue).
    Miayura, N.; Suzuki, A. Org. Synth. 1990, 68, 130
    A procedure by Nobel Laureate Akira Suzuki for the hydroboration of an alkyne with catecholborane. The resulting product can then be subsequently used in a Pd-catalyzed Suzuki coupling reaction.

A variety of other reagents were developed by H. C. Brown for hydroboration, including catecholborane, 9-BBN, and disiamylborane. The advantage with these reagents is that they will undergo monoaddition to alkynes, whereas borane will add twice. Representative references for the reaction of these reagents with alkynes are below:

  1. Catecholborane (1,3,2-Benzodioxaborole) as a New, General Monohydroboration Reagent for Alkynes. A Convenient Synthesis of Alkeneboronic Esters and Acids from Alkynes via Hydroboration
    Brown, H. C.; Gupta, S. K. Am. Chem. Soc. 1972 94 (12), 4370
    DOI: 10.1021/ja00767a072
  2. 50. Hydroboration of Representative Alkynes with 9-Borabicyclo[3.3.1]nonane-a Simple Synthesis of Versatile Vinyl Bora and gem-Dibora Intermediates
    Brown, H. C.; Scouten, C. G.; Liotta, R. J. Am. Chem. Soc. 1979 101 (1), 96
    DOI: 10.1021/ja00495a016
  3. XI. The Hydroboration of Acetylenes-A Convenient Conversion of Internal Acetylenes into cis-Olefins and of Terminal Acetylenes into Aldehydes
    Brown, H. C.; Zweifel, G. J. Am. Chem. Soc. 1961, 83 (18), 3834
    DOI: 10.1021/ja01479a024

This paper describes the use of disiamylborane for the selective monohydroboration of alkynes.

Cyclopropenation of alkynes:

  1. A New Chiral Rh(II) Catalyst for Enantioselective [2 + 1]-Cycloaddition. Mechanistic Implications and Applications
    Yan Lou, Manabu Horikawa, Robin A. Kloster, Natalie A. Hawryluk, and E. J. Corey
    Journal of the American Chemical Society 2004, 126 (29), 8916-8918
    This paper by Nobel Laureate Prof. E. J. Corey (Harvard) describes a chiral Rh complex that can be used for asymmetric addition of a CH2 unit to alkynes – in essence, facially selective addition.


Comment section

8 thoughts on “Alkyne Reactions – The “Concerted” Pathway

    1. Alkynes are generally less electron-rich than alkenes, owing to the larger electronegativity of sp hybridized carbon atoms. Electrophilic reagents like ozone, HX and OsO4 tend to react more slowly with alkynes.

      Regarding why OsO4 doesn’t work, I’m not entirely sure.

    1. That is new to me.

      This didn’t appear in my copy of March, nor any other book in my collection, and I don’t have Scifinder at my fingertips anymore to really dig into the literature.

      I’d need to see the primary literature to know the exact conditions for the reaction. It’s very possible to selectively dihydroxylate an alkene in the presence of an alkyne; reactions with alkynes are much slower.

      Thanks for adding this.

      1. For what it’s worth, I cannot find the first reaction (oxidation of diphenylacetylene to benzil with osmium tetroxide) on Reaxys. Quite a variety of oxidants have been used for this particular substrate, including KMnO4 which is perhaps more familiar to undergrads (JOC 1989 5182), but not OsO4.

        The second substrate is even more dubious. There is only one report of this transformation (TL 2004 8575) using trifluoro DMDO, not OsO4, and the authors report that the 1,2-diketone was only a side product, obtained in 18% yield (the main product being that with the hydroxyls oxidised to carbonyls). I don’t know where the “OsO4/KClO3” comes from, but it’s not in the primary literature.

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