What is sigmatropic rearrangement and What is suprafacial and antara facial process ?

What is sigmatropic rearrangement ?

Many thermal or photo chemical rearrangements involve the shifting of a σ-bond , flanked by one or more π-electron systems, to a new position [ i , j ] within the molecule in an uncatalyzed  intra molecular process .
Since it is rearrangement  of a σ-bond , these reactions are called sigmatropic rearrangements  of order [ i, j ].

FMO  method:

The  terms sigmatropic rearrangement  is used  to describe a chemical change in which a sigma bond along with the group attached to this, flanked by one or more π-electron system, migrates in an uncatalysed  process  to a new position in the molecule.

Sigmatropic  rearrangements are given the order [ I,j ] which express the number of atoms , i and j across which the σ-bond migrates.

In  sigmatropic  rearrangement the migrating group usually H-atom is σ-bonded through the same atomic centre then such rearrangements  are also referred to as [ I,j ] order of sigmatropic rearrangement .

When the value of  [ i+j ] even then it is also represent  the number of electrons delocalized in the transition state.

What is suprafacial and antarafacial process ?

Sigmatropic  reaction involves the migration of a σ-bond across the π electronic system. When the migrating σ-bond moves across the same face of the conjugated system , it is called a suprafacial process.

On the other hand, when the migrating σ-bond is reformed on the opposite π electron face of the conjugated  system, then it is called antarafacial process.

The  following example of [ 1,5 ] sigmatropic shift  exhibits both these process and their stereo chemical consequences. 

     

Due to steric reasons, suprafacial migration are more common than antarafacial shifts.  However, with lengthening of the conjugated system. Sometimes it is possible for a σ-bond to migrate to the opposite π electron face.

Explanation of sigmatropic rearrangements :

One of the ways to explain the sigmatropic rearrangements is to assume that the migrating bond undergoes hemolytic  cleavage resulting in the formation of a pair of radicals.

As bonding characters are to be maintained throughout the course of the rearrangement , the most important bonding interaction will be between the HOMO’s of the two species produced by this cleavage.

 This is to be expected as it is these orbitals that contain the unpaired electrons. We shall illustrate this explanation by examining a suprafacial [1,5] sigmatropic shift of hydrogen in which the hemolytic cleavage results in the production of a hydrogen atom and pentadienyl  radical.

The ground state electronic configuration of  pentadienyl radical is Ѱ₁²Ѱ₂² Ѱ₃. Since HOMO ( Ѱ₃ ) of this radical has similar sign on the terminal lobes, [ 1,5 ] suprafacial migration will be a thermally allowed process.

The first excited state of the pentadienyl radical has the configuration Ѱ₁²Ѱ₂² Ѱ₄ and the symmetry  characteristics of HOMO ( Ѱ₄ ) are thus reversed ( C₂ symmetry).

[ 1,5 ] suprafacial migration is no longer possible and the shift has to be an antarafacial process.

A similar analysis of such systems has led the formation of selection rules which state that if a sigmatropic reaction of the order [i ,j ] has i+j =4n+2 , then thermal reaction is suprafacial and photo chemical reaction will be antarafacial.

However for those cases in which i+j=4n the rules are reversed and the thermal reactions are antarafacial while the photo chemical reaction will be suprafacial. These rules are summarized as,

Application of PMO  method to sigmatropic reactions:

Sigmatropic reactions can be treated in the same fashion as above and similar conclusions are arrived at as by other approaches.

For instance, the [ 1,3 ] suprafacial shift occurs via an transition state with  zero node and  4 electrons ( antiaromatic ) and thus is an photo chemical allowed process, whereas the [ 1,3 ]

antarafacial shift can occur via a transition state with one node, 4 electrons ( aromatic ) and is thus thermally allowed process . This two process are  as follow,

The selection rules for sigmatropic reactions of order [1,j ] by this method are shown below.

Selection rules for sigmatropic change by PMO method.

 Summary :

What is sigmatropic rearrangement ?

What is suprafacial and antara facial process ?

Explanation of sigmatropic reaction .

Application of PMO method to sigmatropic reactions.

Click here: Definition of pericyclic reactions.