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?
Why, yes! Several reactions of alkenes that fit the bill 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:
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]
Note that as the (weak) O–O bond breaks, the proton from the peroxy acid is picked up by the (former) carbonyl oxygen.
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.
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:
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)
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.
*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!]