Pinacons

Pinacons are colorless crystalline substances that are highly soluble in ethanol and chloroform and poorly soluble in water. Pinacons form associates with water and often crystallize from aqueous solutions in the form of hydrates, so the pinacon during crystallization from hot water drops out in the form of hexahydrate (pinahydrate).

The reactivity is similar to the reactivity of tertiary alcohols - they are alkylated, reduced with hydroiodic acid, etc.

A specific feature of the pinacons are reactions with rearrangement of the carbon skeleton.

So, when heated, pinacons can decompose with a breaking of the central bond into ketone and secondary alcohol, in the case of benzpinacone (tetraphenylethane-1,2-diol), this decomposition occurs even at the melting point:

Ph ₂ C (OH) -C (OH) Ph ₂${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   Ph ₂ CO + Ph ₂ CHOH

Under the action of strong acids, pinacones undergo a Wagner-Meerwein type rearrangement into ketones ( pinacolins ) with the migration of one of the substituents ( pinacoline rearrangement ):

Historically, the first method for the synthesis of pinacons is the condensation of ketones during their one-electron reduction, going through the intermediate formation of anion-radical intermediates - ketyls , this reaction is a classical method for the synthesis of pinacons and at present:

The reduction of ketones to pinacons is carried out in non-polar aprotic solvents ( benzene , toluene , tetrahydrofuran ), using divalent metals or magnesium amalgams ^[2] and aluminum as reducing agents, since the use of divalent metals increases the yield of pinacons due to the coordination of two ketyl radical anions at the cation metal.

A specific method for the synthesis of aromatic pinacons is the photochemical reduction of diarylketones with isopropanol under the influence of ultraviolet radiation , in the case of benzophenone, the yield of benzpinacone is 93-95% ^[3] :

2 Ph ₂ CO + (CH ₃ ) ₂ CHOH${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   Ph ₂ C (OH) -C (OH) Ph ₂ + (CH ₃ ) ₂ CO

Another method for the synthesis of pinacones is the interaction of Grignard reagents with α-diketones or readily available oxalic esters. Using α-diketones, pinacons are formed with various substituents on alcohol carbon atoms:

RCO-COR + R ¹ MgHal${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   RR ¹ C (OH) -C (OH) RR ¹ ,

in the case of the use of alkyloxalates, pinacons are formed with the same substituents on the alcohol carbon atoms:

ROCO-COOR + R ¹ MgHal${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   R ¹ ₂ C (OH) -C (OH) R ¹ ₂

Pinacons can also be synthesized from tetraalkylethylenes - as direct oxidation:

RR ¹ C = CRR ¹ + [O]${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   RR ¹ C (OH) -C (OH) RR ¹ ,

and halogenation followed by hydrolysis of tetraalkyl dihaloethanes:

RR ¹ C = CRR ¹ + Br ₂${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   RR ¹ CBr-CBrRR ¹

RR ¹ CBr-CBrRR ¹ + 2 OH ^-${\displaystyle <span\; class="mwe-math-element"><span\; class="mwe-math-mathml-inline\; mwe-math-mathml-a11y"\; style="display:\; none;">\to </span></span>}$ {\ displaystyle \ to}   RR ¹ C (OH) -C (OH) RR ¹ + 2 Br ^-