Rep:Xlt15 FURTHER

Further Work: Electrocyclic Reaction

Figure 1: Reaction scheme of 4π electrocyclic reaction of 1,3-butadiene derivative.

A 4π electrocyclic reaction in Figure 1 is investigated as a further work to the computational lab. An electrocyclic reaction is the formation or the breaking of σ bonds across the end of a conjugated π system ^[1]. The equilibrium favoured towards left hand side because of the ring strain in the 4-membered ring.

Method Used In Optimization and Analysis
Method 3 was employed in locating the transition state by which the product, cyclobut-1-ene derivative was drawn and optimized to minimum at PM6 level. With the optimized product, the C-C single bond formed during the pericyclic reaction was deleted, froze at 2.20 Å and optimized to minimum at PM6 level to identify the frozen guess transition state. The guess TS structure was then optimized at PM6 level and the PM6 optimized TS was used to run a IRC calculation. For reactant, 1,3-butadiene derivative, it was obtained from the first frame of IRC calculation and optimized to minimum at PM6 level.

Optimized Reactant, Transition State, and Product at PM6 Level

Table 1: Optimized Reactants, Transition State and Product at PM6 Level.

1,3-Butadiene Derivative

Transition State

3,4-Dimethylcyclobut-1-ene

The geometry of reactants, transition structure and product are checked to properly converge with their respective stationary points found in log files. In addition, transition structure has only one imaginary frequency and that imaginary frequency is then visualized to ensure a correct transition state structure is obtained. There is no imaginary frequency obtained in reactants and products.

IRC Calculation

Table 3. IRC Calculation of PM6 Optimized Transition Structures
Reaction Progress	IRC Output	Discussion
		The ring closing process of 1,3-butadiene derivative clearly proceed via conratation where both hydrogens of terminal alkene carbon rotate in the same direction, clockwise as shown in the IRC. This results in the trans-arrangement of the methyl groups in the product.

Conrotation or Disrotation Analysis Using MO

Table 4. MO of Reactant, Transition State and Product.

Molecular Orbital

1,3 Butadiene Derivative

Transition State

3,4-Dimethylcyclobut-1-ene

MO18

LUMO

MO17

HOMO

Table 5. Orbital Correlation Diagram of Reactant, TS and Product.
Orbital Correlation Diagram	Explanation
Figure 2: Simplied MO of TS and orbital correlation diagram between reactant and product. Figure 3: Conrotation and disrotation under thermal and photochemical condition.	The orbital correlation diagram in Figure 2 shows the simplified version of the MOs observed in reactant, transition state structure and product, with their actual HOMO and LUMO displayed in Table 4. The MOs are labelled symmetric (S) or antisymmetric (A) using the persistent symmetry element, C₂-rotational axis. The type of bonding or antibonding interaction in product is also shown and coloured in orange. The conservation of orbital symmetry introduced by Woodward and Hoffmann states that an orbital of a given symmetry in the reactant is converted smoothly to a product's orbital with the identical symmetry ^[2] ^[3]. Hence, the orbitals in reactant and product can correlate to one another as shown in the diagram on the left. Also, they suggested that the symmetry properties of HOMO in the open-chain conjugated π system control the stereochemical outcome of the electrocyclic reaction ^[4]. The pericyclic reaction calculated using Gussian at PM6 level assumed a thermal reaction condition. By examining the HOMO (MO17) of 1,3-butadiene derivative, under thermal condition, the conrotatory motion (anticlockwise direction) of the methyl groups allows an in-phase and constructive orbital interaction to form a σ bond between the terminal Cs. In contrast, disrotation brings the lobes of of the opposite phases for antibonding formation with a high energy transition structure and hence it is symmetry forbidden reaction path. Hence, under thermal condition, the 4π electron conjugated system reacts with itself antarafacially via a Mobius aromatic transition state and is an orbital symmetry allowed process ^[4]. A C₂ axis of symmetry is preserved during this reaction. On the other hand, under photochemical condition, the electron in MO17 gets excited into MO18, leading to the HOMO to be considered now is MO18. The disrotatory motion of the methyl group on the terminal Cs of 1,3-butadiene derivative brings the lobes of the same phase for bonding interaction and hence it has low activation energy and is an orbital symmetry allowed process ^[4]. This results in the cis-arrangement of methyl groups in the cyclobut-1ene product. In contrast, the conrotatory motion leads to an antibonding interaction and is an orbital symmetry forbidden pathway ^[4]. A summary diagram for conrotation of MO17 under thermal condition and disrotation of MO18 under photochemical condition is shown in Figure 3.

(Very good work with the correlation diagram. However your labelling of the symmetries is a bit off. It is unexpected but a particular MO may switch between symmetric and antisymmetric along the reaction path as it passes the Mobius transition state Tam10 (talk) 10:35, 26 February 2018 (UTC))

Woodward-Hoffmann Rule For Electrocyclic Reaction ^[1].

Table 4. Orbital Symmetry Allowed Reaction by Woodward-Hoffmann Rule.
Number of π Electon	Thermal Condition	Photochemical Condition
4n	Conrotatory	Disrotatory
4n	Antarafacial	Suprafacial
4n+2	Disrotatory	Conrotatory
4n+2	Suprafacial	Antarafacial

Log File For IRC Calculation

IRC Calculation of PM6 Optimized TS: File:XLT15SUETS IRC.LOG

References in Further Work

↑ ^1.0 ^1.1 J. Clayden, N. Greeves, S. Warren, P. Wothers, Organic Chemistry, Oxford University Press Inc., New York, 2001. Cite error: Invalid <ref> tag; name ":0" defined multiple times with different content
↑ E. V. Anslyn, D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books, Sausalito, United States, 2006.
↑ R. Hoffmann, R. B. Woodward, Acc. Chem. Res, 1968, 1(1), 17-22, DOI:10.1021/ar50001a003
↑ ^4.0 ^4.1 ^4.2 ^4.3 B. Dinda, Essentials of Pericyclic and Photochemical Reactions, Springer International Publishing, Switzerland, 2017.

[:0-1] 1.0 ^1.1 J. Clayden, N. Greeves, S. Warren, P. Wothers, Organic Chemistry, Oxford University Press Inc., New York, 2001. Cite error: Invalid <ref> tag; name ":0" defined multiple times with different content

[:1-2] E. V. Anslyn, D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books, Sausalito, United States, 2006.

[:2-3] R. Hoffmann, R. B. Woodward, Acc. Chem. Res, 1968, 1(1), 17-22, DOI:10.1021/ar50001a003

[:3-4] 4.0 ^4.1 ^4.2 ^4.3 B. Dinda, Essentials of Pericyclic and Photochemical Reactions, Springer International Publishing, Switzerland, 2017.

[1]

[2]

[3]

[4]

Further Work: Electrocyclic Reaction

Optimized Reactant, Transition State, and Product at PM6 Level

IRC Calculation

Conrotation or Disrotation Analysis Using MO

Log File For IRC Calculation

Exercise 1

Exercise 2

Exercise 3

References in Further Work