Semipinacol rearrangement
Encyclopedia
The semipinacol rearrangement is a rearrangement reaction
in organic chemistry
involving a heterosubstituted alcohol of the type R1R2(HO)C-C(X)R3R4. The hetero substituent can be a halogen
(Cl, Br, I), a tosylate, a mesylate
or a thioxy group. This reaction proceeds by removal of the leaving group
X forming a carbocation
as electron deficient center. One of the adjacent alkyl groups then migrates to the positive carbon in a 1,2-shift. Simultaneously with the shift, a pi bond
forms from the oxygen to carbon, assisting in driving the migrating group off its position. The result is a ketone
or aldehyde
. . In another definition all semipinacol rearrangements "share a common reactive species in which an electrophilic carbon center, including but not limited to carbocations, is vicinal to an oxygen-containing carbon and can drive the 1,2-migration of a C-C or C-H bond to terminate the process, generating a carbonyl group " .
The rearrangement reaction can be classified into 4 types. Type 1 concerns all 2-heterosubstitted alcohols. Substrates in type 2 rearrangements are allyl alcohol
s. The carbocation is formed by electrophilic addition
to the alkene group with electrophiles such as halonium ion
s, Brønsted acids and Lewis acid
s. In type 3 the substrates are epoxide
s, notably 2,3-epoxy-alcohols and type 4 concerns the reactions of alpha hydroxyketones
and alpha hydroxy imines. Reactions of type 4 are also called acyloin rearrangements.
While similar to the pinacol rearrangement
, the semipinacol rearrangement differs from the pinacol rearrangement
in that the cation is not formed from a vicinal 1,2-diol. With diazoalcohols the reaction is known as the Tiffeneau-Demjanov rearrangement
.
Rearrangement reaction
A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule...
in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
involving a heterosubstituted alcohol of the type R1R2(HO)C-C(X)R3R4. The hetero substituent can be a halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
(Cl, Br, I), a tosylate, a mesylate
Mesylate
In chemistry, a mesylate is any salt or ester of methanesulfonic acid . In salts, the mesylate is present as the CH3SO3− anion. When modifying the International Nonproprietary Name of a pharmaceutical substance containing the group or anion, the correct spelling is mesilate .Mesylate esters are a...
or a thioxy group. This reaction proceeds by removal of the leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
X forming a carbocation
Carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
as electron deficient center. One of the adjacent alkyl groups then migrates to the positive carbon in a 1,2-shift. Simultaneously with the shift, a pi bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
forms from the oxygen to carbon, assisting in driving the migrating group off its position. The result is a ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
or aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
. . In another definition all semipinacol rearrangements "share a common reactive species in which an electrophilic carbon center, including but not limited to carbocations, is vicinal to an oxygen-containing carbon and can drive the 1,2-migration of a C-C or C-H bond to terminate the process, generating a carbonyl group " .
The rearrangement reaction can be classified into 4 types. Type 1 concerns all 2-heterosubstitted alcohols. Substrates in type 2 rearrangements are allyl alcohol
Allyl alcohol
Allyl alcohol is an organic compound with the structural formula CH2=CHCH2OH. Like many alcohols,it is a water soluble, colourless liquid, but it is more toxic than typical small alcohols. Allyl alcohol is used as a raw material for the production of glycerol, but is used as a precursor to many...
s. The carbocation is formed by electrophilic addition
Electrophilic addition
In organic chemistry, an electrophilic addition reaction is an addition reaction where, in a chemical compound, a π bond is broken and two new σ bonds are formed...
to the alkene group with electrophiles such as halonium ion
Halonium ion
A halonium ion in organic chemistry is any onium compound containing a halogen atom carrying a positive charge. This cation has the general structure R-X+-R where X is any halogen and R any organic residue and this structure can be cyclic or an open chain molecular structure...
s, Brønsted acids and Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
s. In type 3 the substrates are epoxide
Epoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
s, notably 2,3-epoxy-alcohols and type 4 concerns the reactions of alpha hydroxyketones
Acyloin
Acyloins are a class of organic compounds in organic chemistry sharing a common functional group consisting of a hydroxyl group placed on the α-position of a carbonyl group.- Nomenclature :Common types of ketols include:...
and alpha hydroxy imines. Reactions of type 4 are also called acyloin rearrangements.
While similar to the pinacol rearrangement
Pinacol rearrangement
The pinacol rearrangement or pinacol-pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. This 1,2-rearrangement takes place under acidic conditions...
, the semipinacol rearrangement differs from the pinacol rearrangement
Pinacol rearrangement
The pinacol rearrangement or pinacol-pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. This 1,2-rearrangement takes place under acidic conditions...
in that the cation is not formed from a vicinal 1,2-diol. With diazoalcohols the reaction is known as the Tiffeneau-Demjanov rearrangement
Tiffeneau-Demjanov rearrangement
The Tiffeneau-Demjanov rearrangement is the chemical reaction of a 1-aminomethyl-cycloalkanol with nitrous acid to form an enlarged cycloketone....
.