Allylic strain

The allylic strain (actually 1,3 -allylic strain or English 1,3 - Allylstrain ) is a term used in organic chemistry and has been defined by Francis Johnson in 1968. It refers to the interaction between the 1 - and 3- substituents ( R1 and R3 ) of allyl system. The also existing 1.2 -allylic strain, however, is of minor importance.

Description

An allylic system is a reactive functional group in an organic molecule. It is characterized by a double bond between two carbon atoms, followed by a single bond to another carbon atom. The simplest molecule with such a group is the propene ( C3H6 ). The methylene group adjacent to the double bond, is freely rotatable in this molecule, i.e., it makes up for the energy content of the molecule is no difference, such as the methyl group arranging space. There is no different conformations, only measured for the rotation of the hydrogen atoms rotational barrier of 12.6 kJ / mol (see conformer ).

Carrying the double bond of the neighboring carbon atom, however, instead of hydrogen atoms different substituents of various sizes so that the most stable conformation, in which the substituent is located in the lowest level of the double bond. For this substituent interacts with the π orbitals of the adjacent double bond in the interaction, so that the energy content of the molecule increases. This increased energy is referred to as allylic strain. Because each molecule seeks a state as small as possible energy content, the single bond between the carbon atoms in the molecule rotates now so that one substituent occurs in this interaction, with the allylic strain is lowest. It is thus a steric hindrance for (Z)- alkenes. You might also have electronic effects such as hyperconjugation to occur. In this case, the smallest residual sterically be turned by ± 30 ° from the plane of the double bond, so that an interaction between the σ - bond of the substituents having the HOMO or LUMO of the π - bond is allowed.

It has been calculated for the example of a very simple allylic following relative energy differences:

The 1,3 -allylic strain is comparable to the 1,3 -di- axial interaction of cyclic aliphatics and the Felkin -Anh rules for carbonyl systems.

Effects

This effect can be exploited for the stereoselective synthesis.

Thus, for example, hydroboration, wherein a new substituent is added to the double bond, these are added from the side, and the small substituent is at the side to the vinyl carbon.

Wearing one of R3 or R4 is a functionality that, directing acts for a reagent which = C bond attacks the C, it is called an active volume, as preferred in a reaction, the attack of this volume (of this page ) is coordinated to the functionality, and thus a stereochemical information from the 1-position is transferred to the 3-position.

Wearing one of R3 or R4 is not acting coordinating functionality, but works due to its steric bulk of a reagent = C bond attacks the C, more shielding, one speaks of an inert volume, as in a reaction, the attack of this volume is at a disadvantage (of this page). Thus, a stereo information from the 1-position is transferred to the 3-position.

Examples

By the bulkier residues, the steric demand and the increased diastereoselectivity increases, since the attack can take place only from the front ( beta site).

Meta-chloroperbenzoic acid ( mCPBA ) is coordinated by the benzyl alcohol, and the free alcohol and the molecule selectively engages from behind (alpha site) to form the epoxide of.

Other examples of stereoselective syntheses using the allylic strain are cycloadditions Iodlactonisierungen and epoxidation.