The Pummerer rearrangement is an

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, mechanistic organ ...

whereby an

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...

rearranges to an α- acyloxy–

thioether In organic chemistry, a sulfide (British English sulphide) or thioether is an organosulfur functional group with the connectivity as shown on right. Like many other sulfur-containing compounds, Volatile organic compound, volatile sulfides have ...

(mono

thioacetal In organosulfur chemistry, thioacetals are the sulfur (''thio-'') analog (chemistry), analogues of acetals (). There are two classes: the less-common monothioacetals, with the formula , and the dithioacetals, with the formula (symmetric dithio ...

-ester) in the presence of

acetic anhydride Acetic anhydride, or ethanoic anhydride, is the chemical compound with the chemical formula, formula . Commonly abbreviated , it is one the simplest organic acid anhydride, anhydrides of a carboxylic acid and is widely used in the production of c ...

. The stoichiometry of the reaction is: :RS(O)CHR'₂ + Ac₂O → RSC(OAc)R'₂ + AcOH

Synthetic implementation

Aside from

trifluoroacetic anhydride Trifluoroacetic anhydride (TFAA) is the acid anhydride of trifluoroacetic acid. It is the perfluorinated derivative of acetic anhydride. Preparation Trifluoroacetic anhydride was originally prepared by the dehydration of trifluoroacetic acid with ...

and

trifluoromethanesulfonic anhydride Trifluoromethanesulfonic anhydride, also known as triflic anhydride, is the chemical compound with the formula (CF3SO2)2O. It is the acid anhydride derived from triflic acid. This compound is a strong electrophile, useful for introducing the tri ...

have been employed as activators. Common nucleophiles besides acetates are arenes, alkenes, amides, and phenols. The usage of α-acyl sulfoxides and

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...

s, such as TiCl₄ and SnCl₄, allow the reaction to proceed at lower temperatures (0 °C).

Thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately Volatility (chemistry), volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a Halogenation, chlorinating reagen ...

can be used in place of acetic anhydride to trigger the elimination for forming the electrophilic intermediate and supplying chloride as the nucleophile to give an α-chloro-thioether: Other

anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid. In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...

s and acyl halides can give similar products. Inorganic acids can also give this reaction. This product can be converted to

aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

Mechanism

The mechanism of the Pummerer rearrangement begins with the

acylation In chemistry, acylation is a broad class of chemical reactions in which an acyl group () is added to a substrate. The compound providing the acyl group is called the acylating agent. The substrate to be acylated and the product include the foll ...

of the

(

resonance structure In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

s 1 and 2) by

to give 3, with

acetate An acetate is a salt formed by the combination of acetic acid with a base (e.g. alkaline, earthy, metallic, nonmetallic, or radical base). "Acetate" also describes the conjugate base or ion (specifically, the negatively charged ion called ...

as byproduct. The acetate then acts as a catalyst to induce an

elimination reaction An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one- or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 r ...

to produce the cationic- thial structure 4, with

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

as byproduct. Finally, acetate attacks the thial to give the final product 5. The activated thial

electrophile In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively Electric charge, charged, have an ...

can be trapped by various intramolecular and intermolecular

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

s to form

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

–carbon bonds and carbon–heteroatom bonds. The intermediate is so electrophilic that even neutral nucleophiles can be used, including

aromatic ring In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The e ...

s with electron donating groups such as

1,3-benzodioxole 1,3-Benzodioxole (1,2-methylenedioxybenzene) is an organic compound with the formula CHOCH. The compound is classified as benzene derivative and a heterocyclic compound containing the methylenedioxy functional group. It is a colorless liquid. Alt ...

: It is possible to perform the rearrangement using selenium in the place of sulfur.

Pummerer fragmentation

When a substituent on the α position can form a stable

carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...

, this group rather than the α-hydrogen atom will eliminate in the intermediate step. This variation is called a Pummerer fragmentation. This reaction type is demonstrated below with a set of sulfoxides and

(TFAA): The organic group "R2" shown in the diagram above on the bottom right is the

methyl violet Methyl violet is a family of organic compounds that are mainly used as dyes. Depending on the number of attached methyl groups, the color of the dye can be altered. Its main use is as a purple dye for textiles and to give deep violet colors in pain ...

carbocation, whose pK_R+ of 9.4 is not sufficient to out-compete loss of H⁺ and therefore a classical Pummerer rearrangement occurs. The reaction on the left is a fragmentation because the leaving group with pK_R+ = 23.7 is particularly stable.

History

The reaction was discovered by , who reported it in 1909.

References

{{DEFAULTSORT:Pummerer Rearrangement Rearrangement reactions Name reactions

Synthetic implementation

Mechanism

Pummerer fragmentation

History

See also

References