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Alkene Reactions

By James Ashenhurst

Alkene Addition Pattern #3: The “Concerted” Pathway

Last updated: January 21st, 2020 |

“Concerted” Mechanisms In Alkene Addition Reactions: Hydroboration, Hydrogenation, Epoxidation, Dihydroxylation, And Simmons-Smith Cyclopropanation.

In contrast to alkene addition reactions in the Carbocation Pathway and the 3-Membered Ring Pathway, we saw in the last two posts that hydroboration of alkenes is anomalous. The regioselectivity of the reaction is “anti-Markovnikov” and the stereochemistry of the addition is “syn“.

We also saw that the “syn” stereochemistry is due to the concerted nature of the mechanism proposed for this reaction.

So… are there any other reactions of alkenes that produce similar outcomes to those of hydroboration?

That is, products of syn addition as a result of a concerted mechanism?

There sure are!

Table of Contents

  1. Concerted Mechanisms In Alkene Addition Reactions
  2. Hydrogenation Of Alkenes With H2 And A Metal Catalyst Such As Pd-C
  3. Epoxidation Of Alkenes With Peroxyacids Such As meta-Chloroperoxybenzoic Acid
  4. Dihydroxylation Of Alkenes With Osmium Tetroxide (OsO4)
  5. Cyclopropanation Of Alkenes With The Simmons-Smith Reagent (Zinc-Copper Couple)
  6. Dichlorocyclopropanation Of Alkenes With Chloroform And Base (Giving Dichlorocarbene)
  7. Summary: The Concerted Pathway In Alkene Addition Reactions
  8. Notes

1. Concerted Mechanisms In Alkene Addition Reactions

Just to review the previous post, here’s a drawing of the transition state for the hydroboration of alkenes showing the concerted mechanism that results in syn addition.

drawing of hydroboration mechanism 1-methylcyclohexene with bh3 transition state anti markovnikov product

Several other reactions of alkenes that proceed through a concerted transition state are the following:

  • Hydrogenation (Pd-C, H2)
  • Dihydroxylation (OsO4)
  • Epoxidation (RCO3H ; meta-chloroperoxybenzoic acid, m-CPBA is a common reagent in this family)
  • Cyclopropanation (CH2I2, Zn-Cu)
  • Dichlorocyclopropanation (CHCl3, KOH)

Although the exact mechanism of each reaction is not necessarily the same,  each of these reactions does proceed through a concerted transition state and the stereochemistry of the addition is syn. One important thing to note here is that, unlike hydroboration, each of the reactions is adding identical atoms to each carbon of the alkene, so the issue of “regioselectivity” is moot.

The fact that the reaction products have these characteristics in common (if not the exact mechanism) still allows us to group them together as a loosely connected “family” – the “Concerted Pathway”, if you will.

Let’s go through them one by one:

2. Hydrogenation Of Alkenes With H2 And A Metal Catalyst Such As Pd-C

hydrogenation of alkenes with h2 and pd-c gives syn stereochemistry

Treatment of alkenes with hydrogen gas and a “noble” metal catalyst such as palladium (Pd) or platinum (Pt) [nickel, rhodium, ruthenium and other metals also find use] results in the addition of two atoms of hydrogen to the same face of the alkene.  Under these conditions, the alkene and hydrogen gas are both “adsorbed” on to the surface of the metal.

In the transition state for this reaction, each of the two hydrogen atoms are delivered to the same face of the alkene. The rate of the reaction is surface area dependent: dispersing the metal on finely divided carbon (charcoal) drastically improves the reaction rate, hence the use of charcoal (finely divided carbon).*[Note 1]

3. Epoxidation Of Alkenes With Peroxyacids Such As meta-Chloroperoxybenzoic Acid (m-CPBA)

epoxidation of alkenes with peroxyacid like mcpba gives epoxide with syn stereochemistry

Treatment of an alkene with a peroxyacid such as m-CPBA results in formation of  an epoxide (“oxirane”). This also occurs through a concerted transition state:
transition state for epoxidation of alkenes with mcpba showing partial charges butterfly model concerted reaction

Note that as the (weak) O–O bond breaks, the proton from the peroxy acid is picked up by the (former) carbonyl oxygen.

4. Dihydroxylation Of Alkenes With Osmium Tetroxide (OsO4)

dihydroxylation of alkenes with oso4 gives syn diol

Osmium tetroxide, OsO4, will add to alkenes in a concerted process to form two new C-O bonds. The stereochemistry is also syn.

An intermediate in this reaction is a cyclic compound containing osmium, called an osmate ester. The second step shown in grey (KHSO3, H2O) results in breakage of the O-Os bonds and formation of the alcohols. This is called “hydrolysis”. KHSO3 is a reducing agent and aids in the workup of the toxic osmium.

concerted transition state for dihydroxylation of alkenes with oso4 gives osmate ester which is then cleaved through hydrolysis syn diol

5. Cyclopropanation Of Alkenes With The Simmons-Smith Reagent (Zinc-Copper Couple)

cyclopropanation of alkenes with ch2i2 and zn-cu simmons smith gives cyclopropane syn stereochemistry

In a reaction sometimes known as the “Simmons-Smith reaction”, diiodomethane (CH2I2) and zinc-copper couple (“Zn-Cu’) form a “carbene” (actually, a carbenoid to be more precise). Alkenes add to this species to give cyclopropanes. The stereochemistry of the addition is syn. Here is the transition state generally drawn for this reaction:

transition state for cyclopropanation of alkene with zinc carbenoid giving syn cyclopropane product

6. Dichlorocyclopropanation Of Alkenes With Chloroform And Base (Giving Dichlorocarbene)

dichlorocyclopropanation with chcl3 and naoh gives syn dichlorocyclopropane product

When treated with strong base, chloroform (CHCl3) is deprotonated to give its conjugate base. Loss of chloride ion from this species results in Cl2C:  , otherwise known as a “dichlorocarbene”. As in the reaction above, alkenes can add to this carbene to give a cyclopropane. The reaction proceeds through this transition state (empty p orbital and orbital lobe containing lone pair of electrons not shown)

transition state for cyclopropanation of alkene with dichlorocarbene giving dichlorocyclopropane

7. Summary: The Concerted Pathway In Alkene Addition Reactions

To summarize, each of the reactions in this post proceed through a concerted transition state to give products of syn stereochemistry. The stereochemistry of the alkene is preserved in the stereochemistry of the product (that is, they are all stereospecific reactions). The regiochemistry is not relevant in any of these cases except for hydroboration, which is anti-Markovnikov.

So how do we go about drawing arrow-pushing mechanisms for these reactions? An excellent question! More on that dilemma in the next post.

NEXT POST: An Arrow Pushing Dilemma In Concerted Addition Reactions


Notes

*Note 1 : this assumes the use of “heterogeneous” catalysts, such as Pd/C, Pt/C, etc which, rather than dissolve in solution, are suspended in it. There are also “homogeneous” catalysts for hydrogenation, such as Wilkinson’s catalyst. Since this reagent involves mechanisms of the d-block metals, this blog is not going to get into that. [Mike does, though!]

Comments

Comment section

3 thoughts on “Alkene Addition Pattern #3: The “Concerted” Pathway

    1. The first step, addition of hydrogen and palladium to multiple bonds is concerted (hydropalladation). And the second step (reductive elimination) preserves the stereochemistry at the carbon. Since hydropalladation is usually beyond the scope of introductory organic chemistry, it is not too much of a stretch to say that hydrogenation is concerted.

  1. Great website. I have been a regular reader for some time now. Extremely useful and lucid content. Loved it Sir!

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