Organolithien

The organolithiens of are composed of type Organométallique S, which presenting a simple connection C - Li.

Structure and reactivity

Polarity of the connection

The reactivity of the organolithiens (called also simply lithiens) comes from the polarity of the connection C-Li. Indeed, of share the difference of electronegativity between lithium and carbon.

Lithium being electropositive than carbon, the electronic density of the connection C-Li will be localized on carbon (following the example organomagnésiens). Moreover, as lithium is electropositive than the Magnésium, the ic character Ion will be marked more in the lithiens than in the magnesian ones.

This characteristic makes the organolithiens more reactive than the other organometallic ones

Alkalinity

The ic character Anion of the lithiens have make strong bases, stronger than the magnesian ones.

Some values of pKa (constant of acidity):

This basic character strongly implies precautions during the synthesis of these compounds (cf paragraph hereafter)

Displeased, it offers a way of synthesis to the LDA (DisiopropylAmidure de Lithium), bases strong very useful in Organic chemistry.

BuLi + (iPr) 2NH - > BuH + LDA

Synthesis

The synthesis of the organolithiens is similar to that of the organomagnésiens. Here are some ways.

Insertion

This method apperente with the magnesian synthesis, and requires two equivalents of lithium.

Here the reactional assessment:

R- X + 2 Li = R-Li + LiX

X represents a halogen, usually a chlorine or a bromine

Exchange halogen-metal

R- X + R'-Li - > R-Li + R'-X

X represents a halogen

The driving force of the reaction is obtaining a more stable compound.

The stability of a organolithien is given by the study of the associated carbanion. Indeed, more the carbanion is basic, less it is stable.

Metalation

The metalation is the substitution of a hydrogen atom of an organic molecule by a metal atom.

It is easy to carry out exchanges hydrogen-metal bus the butyllithium is a commercial product.

This method is thus a right track of access to the lithiens derived from alkyne compounds, the aromatic ones or vinyl compounds.

Here an example.

Reactions

Their reactivity is connected with that of the Organomagnésien S. Cependant, the strongest polarity of the connection and the absence of magnesium salts makes it possible to avoid reactions known as parasitic which take place during the use of the organomagnésiens.

Ketone synthesis

The reaction of a organomagnésien on a carboxylic acid is a acido-basic reaction. The reaction stops because the entity is not then any more rather reactive.

The use of a organolithien makes it possible to continue the reaction, by thus obtaining a ketone.

Epoxy opening

The opening of Époxyde by a Organomagnésien often leads a mixture of several products, of which brominated compounds and reduced compounds.

The use of compounds containing lithium makes it possible to avoid these many parasitic reactions.

Here the mechanism of opening of an epoxy. The addition is carried out on carbon less encumbered.

Addition in 1,2 on combined ketones

The addition of a Organomagnésien on alpha-énone produces a mixture of made up of addtion 1,4 and 1,2

The use of a organlithien makes it possible to obtain only the compound of addition 1,2.

Oxygen carries number 1 and then the indexing continues carbon and carbon. Addition 1,2 means addition of organometallic on carbon in alpha of oxygen.

Here the mechanism of the reaction

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