Introduction
The previous post looked briefly at some interesting, albeit ordinary, addition reactions. It was kind of boring, so in this post we are looking at some addition reactions that simply do not make sense, because the reactants all seem to be highly stable. The addition reactions we looked at previously all possessed heteroatoms which conferred reactivity onto the compound.
For these types of addition reactions we are about to discuss, they are special in that there is effectively nothing which makes them reactive (we will discuss an exception to this, the Michael addition, later). Of course, we should already have guessed by now what class of compounds we are referring to. Hydrocarbons are the only compounds which lack heteroatoms.
That being said, how would alkanes, which have completely no reactivity (as compared to alkenes and alkynes, which contain nucleophilic π electrons), be able to react to form addition compounds? This is a question that we will answer indirectly as we go along, looking at the different unique types of reactions.
In this article, we will be looking at reactions of hydrocarbons with other hydrocarbons. We begin first with addition reactions of alkanes to alkenes and alkynes, followed by addition of alkenes and addition of alkynes. Note that in all of these reactions, none of the reactants possess any sort of heteroatom.
Alkanes to Alkenes
We begin first with the addition of alkanes. Where hydrogen and carbon add to opposite ends of a molecule, there is the rare but interesting reaction, the addition of alkanes to alkenes. As we would expect, any reaction involving alkanes is sure to involve drastic and harsh reaction conditions, because alkanes are not reactive on their own and some reactivity must be introduced to them so that they may react.
For the addition of alkanes, two ways are possible. The first way is heating of the reactants to high temperatures and pressures. In this case catalysis is not required. This reaction follows the free radical mechanism and it is relatively simple, with no complexities that we have not experienced so far. In fact, we should be able to draw out one of many possibilities of free radical addition.
The second method calls for catalysis by an acid, and surprisingly quite low temperatures are required, even as low as negative degrees Celsius. The reaction in this case does not take place by a free radical mechanism, and instead resembles a Friedel-Crafts type reaction, which we have previously seen for benzene rings. In this case it occurs to an alkene instead, which is an aliphatic.
In the first step of such a reaction, the alkene reacts with a proton (provided by the acid catalyst), forming a carbocation. In the second step, the carbocation reacts with an alkane RH, abstracting a hydrogen from it. This forms the R+ carbocation, which is highly reactive. This is also comparable to Friedel-Crafts type reactions, where an alkyl carbocation is similarly formed.
The alkyl carbocation then attacks another alkene, forming a bond to it, and in the final step this carbocation abstracts a hydrogen from another alkane, forming the final product. Note that the reaction bears similarity to a typical free radical addition or substitution reaction.
Alkanes to Alkenes (Michael-type)
We have not yet considered the Michael addition (Fig. 1), which is such an important reaction that it has spawned an entire class of addition reactions, known as Michael-type additions. This is because Michael reactions are very versatile and many different reagents may react; we will soon see why this is so.
Fig. 1: Michael addition.
The reagents required for the Michael addition include any compound with electron-withdrawing groups (usually bonded to a carbon atom), as well as an alkene with a vinylic electron-withdrawing substituent and a base. The reaction involves conjugate addition.
The first step of the mechanism is deprotonation of the compound with the electron-withdrawing group (not the alkene). This forms a carbanion that can then add to the alkene. We may note that the mechanism of the Michael addition does bear some similarity with deprotonation at the alpha carbon of carbonyls followed by the formation of the reactive carbanion.
Why does the compound without the π bond need to have electron-withdrawing groups? Normally, these compounds are not acidic as after donating the proton, an unstable carbanion is formed. The electron-withdrawing group makes the compound more acidic by stabilizing the conjugate base formed.
Alkanes to Alkynes
Yet another interesting reaction sees the addition of two different alkyls to a terminal alkyne. We may expect the product to be an alkane, as the addition of one alkyl should break a π bond and addition of the other should break another, however the product is an alkene instead. The reagents required are a Normant reagent (Fig. 2), an alkyl iodide, as well as triethyl phosphite (EtO)2P in ether-HMPA.
Fig. 2: Normant reagent (at the top).
The reaction is versatile and can be applied for many different R groups to be added to the alkyne. In the reaction, the Normant reagent reacts first with the alkyne, breaking a π bond. The R group adds to one side of the alkyne while the CuMgBr2 adds to the other side.
Finally, the alkyl iodide is used to introduce another R group, by replacing the CuMgBr2 with the alkyl group from the iodide. If the alkyl iodide is not used, other functional groups may also be introduced, such as carboxylic acids with addition of CO2.
Alkenes to Other Alkenes
A similar reaction to the above involves the addition of alkenes to other alkenes. The two alkenes may be similar, and this case is actually desired because less side products result. If the alkenes are similar, the reaction is also known as a dimerization. In the presence of a metal catalyst, such as nickel, the alkenes are able to add to each other, producing a variety of products if the alkenes are different.
The reaction is rather simple and follows a mechanism not all that different from the mechanism above. The intramolecular reaction has also been observed, allowing for ring-closing reactions and the formation of cyclic alkenes to occur.
This reaction is not widely used in organic chemistry but it has been noted in the landmark synthesis of lanosterol from squalene (Fig. 3), which involves multiple ring-closing reactions in the same molecule. As much as four rings may be closed at one time, as is demonstrated by the Johnson polyene cyclization. High yields have been reported from this reaction.
Fig. 3: Biosynthesis of lanosterol.
Alkenes to Alkenes (Ene Reaction)
This next reaction we will discuss also involves the reaction between two alkenes. Although the product is still an alkene, it is nevertheless distinctly different from the reaction we discussed above. It is known as the ene reaction (Fig. 4).
Fig. 4: Ene reaction.
It is rather comparable to the Diels-Alder reaction, however a simple alkene is employed instead of the diene. Despite this it still involves the dienophile. The reaction likely follows a concerted pericyclic mechanism, which occurs in a single step; however, stepwise mechanisms have also been suggested.
The reaction does not require any additional reagents or catalysts; however, high temperatures are required. Catalysis may be performed by Lewis acids.
Alkynes to Alkynes
Given that we can add alkanes to alkenes and alkenes to alkenes, would it be fair to consider the addition of alkynes to other types of alkynes? This is a worthy consideration. This type of addition has indeed been observed, and a specific case is noted for two molecules of acetylene.
Addition between the two molecules occurs in the presence of cuprous and ammonium chloride. Other catalysts have also been noted. Likewise for alkenes, the intramolecular reaction has also been noted but this is difficult because the geometry about the alkyne should be linear.
This means that there must be a large enough gap between the two alkynes in order for the expected reaction to occur. Only larger chains of diynes have been reported to undergo this type of dimerization reaction, as expected. Conjugated dienes undergo similar reactions to alkynes as can be seen in reactions such as the Nazarov cyclization.
P.S. There has been a little bit of a hiatus in terms of the posting of articles, due to a short break I am taking.