Enone

As enones ketones are known in organic chemistry, which (for the systematic names of substances is in the suffix -on ) contain, in addition to the C = O double bond or a C = C double bond ( suffix s). The substance group of such substances is shortened Linguistically called En -one. The carbon skeleton of enones can be circular ( cyclic) or acyclic; Combinations of these skeletons are possible.

An older term is " unsaturated ketones ". To the position (see permanence ) of the C = C double bond, a distinction is α, β -unsaturated ketones of β, γ - unsaturated γ, δ -unsaturated etc. ketones, wherein α stands for the carbon atom of the carbonyl group adjacent C- atom.

The α, β -unsaturated ketones, the two double bonds " conjugated ". Often β -unsaturated ketones are α, thermodynamically more stable than their double bond isomers. Therefore, the term " enones " is often limited to the former.

Nomenclature

Greek letters are, however, used in organic chemistry also to describe other structures, such as in stereochemistry, or to the appointment of crystal modifications. The positional parameters by Greek letters, however, are rooted in the history of organic chemistry, and are therefore unlikely to be " cut off ". But in the nomenclature of individual compounds should be avoided if possible.

With acyclic ketones are numbered the longest chain of carbon atoms as the alkane. This will allocate their Stellungsbezifferung and the suffix -on the carbonyl group. Then you wear the position of the C = C double bond, as in the numbering of the alkenes.

Case of cyclic ketones, the C = O group the highest rank and therefore is numbered 1, the numbers ( locants ) of the C = C double bond as possible should be low ( starting from the O = C- atom can be clockwise or counter- - clockwise count).

Many enones containing bicyclic or polycyclic carbon skeletons, for example Steroids. In these cases the specific numbering of these rings determines the position of the " on".

Traditionally, many enones are referred to by their common names, especially natural products. An instructive example is carvone: 2- Methyl-5- isopropenyl -2- cyclohexen- 1-one, more precisely 2 -methyl-5 -(1- methylethenyl ) -cyclohex -2 -en -1-one.

Special properties α, β - unsaturated ketones

While = not affect the non-conjugated enone C - C and C = O double bonds, generally occurs at enones to an electronic interaction between the two functional groups, which is described with the term conjugation. The prerequisite is that the four atoms of the enone system ( C = CC = O) lie in a plane, or only slightly deviate from this. In other words, the enone system should be in a planar conformation as possible.

The delocalization of the π - electrons can be represented by the following resonance structures:

The enone system is isoelectronic with the 1,3 -diene system (prototype 1,3-butadiene ); However, the polar boundary structure (right) obtained by a higher weight if the terminal carbon atom in the hydrocarbon is replaced by an oxygen atom. Because oxygen has a higher electronegativity than carbon, and " therefore attracts the electrons more closely to itself" ( loosely expressed ). From this consideration it is also understandable that - compared with "normal" ketones - the O = C bond is longer, and the formal single bond = CC = shorter.

A second way to describe the enone function, provides the molecular orbital theory, which has the simple Hückel approximation, it can identify critical trends. The π molecular orbitals ( π - MOs ) are polarized.

The special bonding in the enones cause, among other characteristic changes in the IR spectra and UV spectra compared with simple ketones.

Reactivity α, β - unsaturated ketones

Enones are bifunctional compounds; therefore, for the O = C and C = C function ( alkene and ketone ) characteristic reactions are possible with non-conjugated enones. In general, reactions take place with electrophilic agents at the C = C double bond, while nucleophilic attack on the carbon atom of the carbonyl group.

Wherein the α, β -unsaturated ketones, however, the π -electron system is delocalized. In terms of their reactivity, therefore, the enone function must be considered as a unit. Thus, the reactivity of these enones is changed compared to " normal" ketones. Although the addition of nucleophiles to the carbon atom of the carbonyl group is often observed, but another type of reaction is the association of a nucleophile with the β - carbon atom of the enone system, the so-called " conjugate addition '' (English conjugate addition). These include the Michael addition reaction, and many reactions with organometallic compounds. Looking at the polar interface structural formula (see above), it is plausible.

For the understanding of reactivity, however, the models of the molecular orbital theory have proved to be more suitable. In the framework of the theory of the frontier orbitals (HOMO -LUMO interactions) conjugated enones are distinguished by the LUMO, which are relatively low in the energy scale. In reactions with nucleophilic agents - apart from Coulomb forces - to a first approximation the importance of the interaction of the enone LUMO with the HOMO of a nucleophile. The MO coefficients at the carbonyl carbon and β - carbon atom differ in their size; soft nucleophiles, therefore, have a tendency to the latter anzugreifen.Dies is true of many radicals.

Synthesis α, β - unsaturated ketones

Numerous methods have been developed for the preparation of compounds of this class of compounds, of which, however, in this context only a selection may be discussed. For practical application, it is crucial that the starting materials, the raw materials are easily available and cheap as possible.

Ketones all may be considered products of the dehydrogenation (oxidation) of secondary alcohols. If the corresponding unsaturated alcohols are available, their oxidation is possible. Compared with the saturated alcohols it is done under " mild " conditions, such as with manganese dioxide. An example is the synthesis of 2 - cyclopentenone.

In principle, saturated ketones can be dehydrated but the selective dehydrogenation in the α, β -position is difficult. A "classic" detour is the halogenation of saturated ketones in α - position; from the α - halo ketones formed can be removed usually by bases hydrogen halide ( α, β - elimination ). As the halogen bromine is usually selected. For some α, α' - dibromo were so Dienones be obtained, 4,4-dimethyl- 2 ,5 -cyclohexanone.

The oldest principle of synthesis is the condensation of aldehydes or ketones with another ketone ( " aldol condensation "). So benzylideneacetone, dibenzylidenacetone, Benzylidenacetophenon or mesityl oxide were first obtained. The half- trivial names of the first three examples show today at the origin of these compounds.

One also knows intramolecular versions of this reaction: dicarbonyl Some provide cycloalkenones. For example, can be undecane -2 ,5 -dione cyclize to dihydrojasmone, an important fragrance.

309044
de