Sigmatropic Rearrangements
Introduction
An interesting group of pericyclic reactions involves the migration of a sigma bond from one site to another within the same molecule. Such reactions are called sigmatropic rearrangements. A sigmatropic rearrangement of order [i,j] is a reaction in which a sigma bond that is flanked by one or more pi bonds migrates to a new position whose termini are i-1 and j-1 atoms removed from the origninal bonded loci. Phew!! What does that mean? Take a look at Figure 1, which illustrates the definition of a [3,3]-sigmatropic rearrangement.
Figure 1
A [3,3]-Sigmatropic Rearrangement
Notice that the reaction outlined in Figure 1 is stereospecific, i.e. the stereoisomer of the starting material produces exclusively the stereoisomer of the product shown. Stereospecificity is a characteristic of sigmatropic rearrangements. Orbital symmetry theory rationalizes the result shown in Figure 1 in terms of a disrotatory motion of the orbitals that are involved in the reaction. Figure 2 animates the process. As the Figure shows, the reaction involves simultaneous (concerted) rotation about the C2-C3, C3-C4, C5-C6, and C6-C7 bonds. Rotation about the C2-C3 bond proceeds in a clockwise direction, opposite to that around the C6-C7 bond. This is called a disrotatory motion. The concerted rotation around two bonds in the same direction, either clockwise or counterclockwise, is described as conrotatory.
Figure 2
Disrotation is Allowed (Datrotation ain't)
Exercise 1 The rotation about the C3-C4 bond in Figure 2 is clockwise counterclockwise . The rotation about the C5-C6 bond in Figure 2 is clockwise counterclockwise . The rotation about the C4-C5 bond in Figure 2 is disrotatory conrotatory .
Exercise 2 There are 3 variables to consider in evaluating whether a sigmatropic rearrangement is allowed or forbidden:
- the number of electrons, i.e. does it conform to Huckel's 4n+2 rule or not?
- if the reaction is thermally or photochemically induced
- is the reaction disrotatory or conrotatory?
A [3,3] sigmatropic rearrangement involves 6 electrons, i.e. a number that conforms to Huckel's Rule. It is thermally allowed when it occurs in a disrotatory fashion. Each time you change one of the three variables, an allowed reaction becomes forbidden. Determine whether the following sigmatropic reactions are allowed or forbidden:
- a thermal disrotatory [3,5] sigmatropic rearrangement allowed forbidden
- a photochemical conrotatory [5,5] sigmatropic rearrangement allowed forbidden
- a thermal distrotatory [1,5] sigmatropic rearrangement allowed forbidden
- a thermal conrotatory [1,3] sigmatropic rearrangement allowed forbidden
Other Orders
Figure 3 outlines sigmatropic rearrangements of order [1,3], [1,5], and [5,5]. The migrating bonds are highlighted in red.
Figure 3
Place Your Order Here
It is helpful in visualizing these transformations to use the curved arrow formalism to show the reorganization of electrons. Figure 4 illustrates this idea for the first reaction in Figure 3. The implication of the two arrows is that breaking the pi bond between C1 and C2, making the new pi bond between C2 and C3, breaking the sigma bond between C3 and D, and making the new sigma bond between D and C1 all occur simultaneously.
Figure 4
Throwin' a Curve
Exercise 3 Draw diagrams similar to that in Figure 4 depicting the electron reorganization in the [1,5] and [5,5] sigmatropic rearrangements in Figure 3.
The rest of this topic will focus on the last reaction in Figure 3, starting with the synthesis of the starting materials.
Interlude-Organic Synthesis
The synthesis of compounds 1 and 2 began with the dimerization of isoprene, 2-methyl-1,3-butadiene, as shown in Equation 1.
The Fe(acac)3, Et3Al, and 2,2'-Dipy form a catalyst package that serves as a template on which two molecules of isoprene dimerize to produce a mixture of 1,5-dimethyl and 2,5-dimethylcyclooctadiene. This type of chemistry is common in the petroleum industry.
After separation of the isomeric cyclooctadienes, the 2,5-dimethyl isomer was treated with a large excess of dibromocarbene. Equation 2 shows the preparation of dibromocarbene.
Carbenes are interesting molecular entities. They contain a divalent carbon atom with a lone pair of electrons. The carbon is electrically neutral, i.e. its formal charge is 0, but it is electron deficient , i.e. it has only six electrons in its valence shell. Consequently species such as dibromocarbene are highly reactive towards potential nucleophiles. They react as soon as they are formed. They add to double bonds and even insert into single bonds. The generation of dibromocarbene, as shown in Equation 2, is an example of a 1,1-elimination; both the H and the Br are eliminated from the same carbon atom.
If reaction 2 is conducted in the presence of 2,5-dimethylcyclooctadiene, the dibromocarbene adds to the double bonds as shown in Equation 3.
In order to convert all of the 2,5-dimethyl-1,5-cyclooctadiene to product, it was necessary to use a large excess of dibromocarbene. Interestingly, the formation of the products shown in Equation 3 was accompanied by the evolution of gas. Analysis of the gas by mass spectrometry established its molecular mass to be 28. Initially it was thought this gas might be atmospheric N2 that was forced from the reaction flask by the heat evolved in the reaction. However, when the flask was swept with argon and the gas collected, its molecular mass was still 28. Well, 16+12 equals 28, and it soon became apparent that the gas was CO, not N2! Fortunately the chemist survived to tell the tale and to rationalize how the carbon monoxide was being formed:
Apparently the dibromocarbene was reacting with the solvent before it could react with the diene. It is such a reactive species that it inserted into the O-H bond of the t-butyl alcohol. The resulting compound then underwent a fragmentation reaction, as indicated by the arrows numbered 1-4, that produced isobutene and formyl bromide. The formyl bromide spontaneously decomposed into hydrogen bromide and carbon monoxide.
Now for the rest of the tale. After moving his apparatus into a hood, our experimentalist treated the mixture of addition products he obtained from reaction 3 with methyl lithium as shown in Equation 4 for one of the two isomers formed in reaction 3.
The details of this transformation are secondary to the main story. What's important to note is that the product of this reaction is a 10-membered ring. The dots between the double bonds in the structure of the product represent carbon atoms. A cabon that is doubly bonded to two other carbon atoms is called an allene. The product of reaction 4, then, is called a bis-allene.
Exercise 4 What is the shape of allene, H2C=C=CH2, according to VSEPR theory?
Exercise 5 What is the dihedral angle between the planes defined by the H-C1-H and the H-C3-H fragments in allene?
Exercise 6 Draw a picture that indicates clearly the p orbital overlap in allene.
Heating this bis-allene to 225oC as shown in Equation 5 resulted in the formation of the two isomeric tetraenes first mentioned in Figure 1.
Compounds 1 and 2 could be separated by gc. They were not interconverted at 225oC. However, when heated at 350oC an equilibrium between these two compounds was established as shown in Equation 6.
Again, the reaction was monitored by gc. Figure 5 presents a schematic diagram of the apparatus that was used to perform this reaction.
Figure 5
A Flow Reactor
With this apparatus, as a compound elutes from the first gas chromatograph, it may be transferred into a gold-coated coil that constitutes the pyrolysis chamber via a 3-way valve that connects the first gas chromatograph to the pyrolysis reactor. Since the volume of the coil is precisely known, the residence time of the sample in the coil can be determined by simply measuring the flow rate of the He carrier gas. When a component of interest emerges from the reactor, it may be transferred via a second 3-way valve to another gas chromatograph for isolation.
In the case of reaction 6 there were 8 peaks observed in addition to the peaks corresponding to compounds 1 and 2! Figure 6 identifies those compounds besides 1 and 2 whose structures were determined. While the mechanism for the formation of most of the compounds in Figure 6 is not known, the driving force for the formation of more stable, aromatic isomers of 2 is obvious.
Figure 6
Go Figure
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