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Regiospecific hydroboration with borane Hydroboration-oxidation of (E)-prop-1-en-1-ylbenzene Hydroboration-oxidation of 1-methyl-cyclohex-1-ene The net reaction is hydration. Because the addition of H-B to olefins is stereospecific, this oxidation reaction will be diastereoselective when the alkene is trisubstituted. [ 10 ]
Hydroboration–oxidation reaction is a two-step hydration reaction that converts an alkene into an alcohol. [1] The process results in the syn addition of a hydrogen and a hydroxyl group where the double bond had been. Hydroboration–oxidation is an anti-Markovnikov reaction, with
1,2-disubstituted Cycloalkene undergoing syn and anti addition. Syn addition is the addition of two substituents to the same side (or face) of a double bond or triple bond, resulting in a decrease in bond order but an increase in number of substituents. [3] Generally the substrate will be an alkene or alkyne.
Hydroboration occurs stereospecifically syn — on the same alkene face. The transition state for this concerted reaction can be visualized as a square with the corners occupied by carbon, carbon, hydrogen and boron, maximizing overlap between the olefin p-orbitals and the empty boron orbital.
With 1-methyl-4-t-butylcyclohexene, oxymercuration yields only one product – still anti addition across the double bond – where water only attacks the more substituted carbon. The reason for anti addition across the double bond is to maximize orbital overlap of the lone pair of water and the empty orbital of the mercuronium ion on the ...
The rhodium-catalyzed hydroboration reaction is thought to be initiated with the dissociation of a triphenylphosphine from the Rh(I) centre. Oxidative addition of the B-H bond of the borane reagent to this 14 e − species is then followed by coordination of the alkene to the 16e − Rh(III) hydride complex.
The same is true when an alkene reacts with water in an additional reaction to form an alcohol that involves carbocation formation. The hydroxyl group (OH) bonds to the carbon that has the greater number of carbon-carbon bonds, while the hydrogen bonds to the carbon on the other end of the double bond, that has more carbon–hydrogen bonds.
Disiamylborane is prepared by hydroboration of trimethylethylene with diborane. [1] The reaction stops at the secondary borane due to steric hindrance. Disiamylborane is relatively selective for terminal alkynes and alkenes vs internal alkynes and alkenes. Like most hydroboration, the addition proceeds in an anti-Markovnikov manner. [1]