Allylic strain

[2] The "strain energy" of a molecule is a quantity that is difficult to precisely define, so the meaning of this term can easily vary depending on one's interpretation.

This term gives information about the relative stabilities of the involved conformers and the effect allylic strain has one equilibrium.

Heats of formation can be determined experimentally though calorimetric studies; however, calculated enthalpies are more commonly used due to the greater ease of acquisition.

In 3-methyl-1-butene, the interactions between the hydrogen and the two methyl groups in the allylic system cause a change in enthalpy equal to 2 kcal/mol.

For example, when examining 4-methyl-2-pentene which contains an additional allylic methyl group compared to 3-methyl-1-butene, the enthalpy of rotation for the highest energy conformer increases from 2 kcal/mol to 4 kcal/mol.

In order to relieve strain caused by interaction between the two methyl groups, the cyclohexanes will often exhibit a boat or twist-boat conformation.

[1] The size of the substituents interacting at the 1 and 3 positions of an allylic group is often the largest factor contributing to the magnitude of the strain.

This causes the transition state to be in a much more favorable position when the polar group is not interacting in a 1,3 allylic strain.

Rather than the strain that would normally occur in the close group proximity, the hydrogen bond stabilizes the conformation and makes it energetically much more favorable.

[2] In cases where an enolization is occurring around an allylic group (usually as part of a cyclic system), A1,3 strain can cause the reaction to be nearly impossible.

[13] This same enolization occurs much more rapidly under basic conditions, as the carboxylic group is retained in the transition state and allows the molecule to adopt a conformation that does not cause allylic strain.

Second, the TMS group conferred a stereoelectronic effect on the molecule by adopting an anti conformation to the directing orbitals of the alkene.

This is because the acidity of the proton is significantly reduced because for the deprotonation to occur, it will have to go through a developing allylic strain in the unfavored conformation.

In an intramolecular Diels-Alder reaction, asymmetric induction can be induced through allylic 1,3 strain on the diene or the dienophile.

In the model studies to synthesize chlorothricolide,[18] an intramolecular Diels Alder reaction gave a mixture of diastereomers.

But by installing the a bulky TMS substituent, the reaction gave the desired product in high diastereoselectivity and regioselectivity in good yield.

In the seminar paper on the total synthesis of (+)-monensin,[19] Kishi and co-workers utilized the allylic strain to induce asymmetric induction in the hydroboration oxidation reaction.

Allylic strain in an olefin.
Calculated rotational energies for different conformations of 3-methyl-1-butene. [ 7 ] Allylic 1,3-strain is most prevalent in 1c , making this conformation the highest in energy.
Various strain interactions shown in red (other hydrogens left off for simplicity).
1,8-dimethyl naphthalene and 4,5-dimethyl phenanthrene
The strength of allylic strain increases as the size of the interacting substituents increases.
A represents the conformation that would occur in most situations due to allylic strain. However, as shown in B a hydrogen bond can form that is energetically favorable and cancels the disfavorable allylic strain. Thus, B is the most stable conformation.
Molecule A undergoes cyclization after treatment with perchloric acid. Due to the allylic strain in molecule B, the molecule was found not to exhibit conjugation stabilization and was stabilized in the formation shown in C. Thus, allylic strain outweighs the stabilization effects of a conjugated system.
The keto form (left) cannot undergo enolization under normal acidic conditions, due to the formation of allylic strain (middle). Thus, under normal thermodynamic conditions, the molecule cannot undergo bromation (right).