Intramolecular in chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 describes a process or characteristic limited within the structure of a single molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

, a property or phenomenon limited to the extent of a single molecule.

Examples

• intramolecular hydride transfer (transfer of a hydride ion from one part to another within the same molecule)
• intramolecular hydrogen bond (a hydrogen bond formed between two functional groups of the same molecule)

In intramolecular organic reaction

Organic reaction

Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...

s, two reaction sites are contained within a single molecule. This creates a very high effective concentration

Concentration

In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...

 (resulting in high reaction rate

Reaction rate

The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...

s), and, therefore, many intramolecular reactions that would not occur as an intermolecular reaction between two compounds take place.

Examples of intramolecular reactions are the Smiles rearrangement

Smiles rearrangement

The Smiles rearrangement is an organic reaction and a rearrangement reaction. It is an intramolecular nucleophilic aromatic substitution of the type:...

, the Dieckmann condensation

Dieckmann condensation

The Dieckmann condensation is the intramolecular chemical reaction of diesters with base to give β-ketoesters. It is named after the German chemist Walter Dieckmann . The equivalent intermolecular reaction is the Claisen condensation....

 and the Madelung synthesis

Madelung synthesis

The Madelung synthesis is a chemical reaction that produces indoles by the intramolecular cyclization of N-phenylamides using strong base at high temperature....

.

Molecular tethers

In a niche concept called molecular tethers, otherwise-intermolecular reactions can be made temporarily intramolecular by anchoring both reactions by a tether

Tether

A tether is a cord, fixture, or signal that anchors something movable to a reference point which may be fixed or moving. There are a number of applications for tethers: balloons, kites, tethered wind-energy conversion systems, anchors, tethered water-flow energy conversion systems, towing, animal...

 with all the advantages associated to it. Popular choices of tether contain a carbonate ester

Carbonate ester

A carbonate ester is a functional group in organic chemistry consisting of a carbonyl group flanked by two alkoxy groups. The general structure of these carbonates is R1OOR2 and they are related to esters R1OR and ethers R1OR2 and also to the inorganic carbonates.Carbonate esters are used as...

, boronic ester, silyl ether

Silyl ether

Silyl ethers are a group of chemical compounds which contain a silicon atom covalently bonded to an alkoxy group. The general structure is R1R2R3Si−O−R4 where R4 is an alkyl group or an aryl group. Silyl ethers are usually used as protecting groups for alcohols in organic synthesis...

, or a silyl acetal link (silicon tethers) which are fairly inert in many organic reactions yet can be cleaved by specific reagents. The main hurdle for this strategy to work is selecting the proper length for the tether and making sure reactive groups have an optimal orientation with respect to each other. An examples is a Pauson-Khand reaction of an alkene and an alkyne tethered together via a silyl ether
In this particular reaction, the tether angle bringing the reactive groups together is effectively reduced by placing isopropyl

Isopropyl

In organic chemistry, isopropyl is a propyl with a group attached to the secondary carbon. If viewed as a functional group an isopropyl is an organic compound with a propyl group attached at its secondary carbon.The bond is therefore on the middle carbon....

 groups on the silicon atom via the Thorpe-Ingold effect

Thorpe-Ingold effect

The Thorpe–Ingold effect or gem-dimethyl effect, or angle compression is an effect observed in organic chemistry where increasing the size of two substituents on a tetrahedral center leads to enhanced reactions between parts of the other two substituents...

. No reaction takes place when these bulky groups are replaced by smaller methyl groups.

Another example is a photochemical [2+2]cycloaddition

Cycloaddition

A cycloaddition is a pericyclic chemical reaction, in which "two or more unsaturated molecules combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity." The resulting reaction is a cyclization reaction.Cycloadditions are usually described by the...

 with two alkene groups tethered through a silicon acetal group (racemic, the other enantiomer

Enantiomer

In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

 not depicted), which is subsequently cleaved by TBAF yielding the endo-diol.

Without the tether, the exo isomer forms.