Alkyne Reactions – The “Concerted” Pathway

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).

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”.

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.

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.

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

Notes

(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.
4. PALLADIUM CATALYST FOR PARTIAL REDUCTION OF ACETYLENES
   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.
6. (Z)-4-(TRIMETHYLSILYL)-3-BUTEN-1-OL
   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.
7. SYNTHETICALLY USEFUL REACTIONS WITH NICKEL BORIDE. A REVIEW
   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:
8. THE HYDROBORATION OF ACETYLENES – A CONVENIENT CONVERSION OF INTERNAL ACETYLENES TO CIS OLEFINS OF HIGH PURITY AND OF TERMINAL ACETYLENES TO ALDEHYDES
   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).
9. PALLADIUM-CATALYZED REACTION OF 1-ALKENYLBORONATES WITH VINYLIC HALIDES: (1Z,3E)-1-PHENYL-1,3-OCTADIENE
   Miayura, N.; Suzuki, A. Org. Synth. 1990, 68, 130
   DOI:15227/orgsyn.068.0130
   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:

10. 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
11. 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
12. 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:

13. 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
    DOI: 1021/ja047064k
    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.