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In organic chemistry, the Doering–LaFlamme allene synthesis is a reaction of alkenes that converts them to allenes by insertion of a carbon atom. [1] This name reaction is named for William von Eggers Doering and a co-worker, who first reported it.
H 2 C=CH 2 + HBr → H 3 C-CH 2 Br. Bromoethane is inexpensive and would rarely be prepared in the laboratory. A laboratory synthesis includes reacting ethanol with a mixture of hydrobromic and sulfuric acids. An alternate route involves refluxing ethanol with phosphorus and bromine; phosphorus tribromide is generated in situ. [4]
2-Phenethyl bromide (C 6 H 5 CH 2 CH 2 Br) is produced via this route from styrene. Hydrogen bromide can also be used to convert alcohols to alkyl bromides. This reaction, that must be done under low temperature conditions, is employed in the industrial synthesis of allyl bromide: HOCH 2 CH=CH 2 + HBr → BrCH 2 CH=CH 2 + H 2 O
A halogen addition reaction is a simple organic reaction where a halogen molecule is added to the carbon–carbon double bond of an alkene functional group. [1] The general chemical formula of the halogen addition reaction is: C=C + X 2 → X−C−C−X (X represents the halogens bromine or chlorine, and in this case, a solvent could be CH 2 ...
Insertion takes place in a suprafacial fashion, but the dihedral angle between the alkene and palladium-carbon bond during insertion can vary from 0° to ~90°. After insertion, β-hydride elimination affords the product and a palladium(II)-hydrido complex IV, which is reduced by base back to palladium(0). [5] (2)
One attractive feature of the Peterson olefination is that it can be used to prepare either cis- or trans-alkenes from the same β-hydroxysilane. Treatment of the β-hydroxysilane with acid will yield one alkene, while treatment of the same β-hydroxysilane with base will yield the alkene of opposite stereochemistry.
In the transition state for cyclization, the small substituent points toward the alkene. This model also explains the greater reactivity of cis alkenes relative to trans alkenes in [2+2] ketene cycloadditions. [5] (2) The configuration of the olefin is retained in the cycloaddition product.
Wittig reactions are most commonly used to convert aldehydes and ketones to alkenes. [1] [2] [3] Most often, the Wittig reaction is used to introduce a methylene group using methylenetriphenylphosphorane (Ph 3 P=CH 2). Using this reagent, even a sterically hindered ketone such as camphor can be converted to its methylene derivative.