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The reaction is named after pinacol (also known as 2,3-dimethyl-2,3-butanediol or tetramethylethylene glycol), which is the product of this reaction when done with acetone as reagent. The reaction is usually a homocoupling but intramolecular cross-coupling reactions are also possible. Pinacol was discovered by Wilhelm Rudolph Fittig in 1859.
The reaction product he obtained instead he called paraceton which he believed to be an acetone dimer. In his second publication in 1860 he reacted paraceton with sulfuric acid (the actual pinacol rearrangement). Again Fittig was unable to assign a molecular structure to the reaction product which he assumed to be another isomer or a polymer.
Pinacol is a branched alcohol which finds use in organic syntheses. It is a diol that has hydroxyl groups on vicinal carbon atoms. A white solid that melts just above room temperature, pinacol is notable for undergoing the pinacol rearrangement in the presence of acid and for being the namesake of the pinacol coupling reaction .
The semipinacol rearrangement is a rearrangement reaction in organic chemistry involving a heterosubstituted alcohol of the type R 1 R 2 (HO)C–C(X)R 3 R 4. The hetero substituent can be a halogen (Cl, Br, I), a tosylate, a mesylate or a thiol group. This reaction proceeds by removal of the leaving group X forming a carbocation as
It has the formula [(CH 3) 4 C 2 O 2 B] 2; the pinacol groups are sometimes abbreviated as "pin", so the structure is sometimes represented as B 2 pin 2. It is a colourless solid that is soluble in organic solvents. It is a commercially available reagent for making pinacol boronic esters for organic synthesis.
Other key reactions encountered in this synthesis are a pinacol coupling and a Reformatskii reaction. In terms of raw materials the C20 framework is built up from L-serine (C3), isobutyric acid (C4), glycolic acid (C2), methyl bromide (C1), methyl iodide (C1), 2,3-dibromopropene (C3), acetic acid (C2) and homoallyl bromide (C4).
The overall reaction, therefore, is: (CH 3 ) 3 COH + HCl → (CH 3 ) 3 CCl + H 2 O Because tert -butanol is a tertiary alcohol, the relative stability of the tert -butyl carbocation in the step 2 allows the S N 1 mechanism to be followed, whereas a primary alcohol would follow an S N 2 mechanism.
Pinacol type rearrangements are often used for this type of contraction. [20] Like the expansion reaction this proceeds with an electron donating group aiding in the migration. Contraction reactions of one ring can be coupled with an expansion of another to give an unequal bicycle from equally sized fused ring.