Editing Hydroboration–oxidation reaction (section)

==Mechanism and scope==
===Hydroboration step===
{{main|hydroboration}}

In the first step, [[borane]] (BH<sub>3</sub>) adds to the [[double bond]], transferring one of the hydrogen atoms to the carbon adjacent to the one that becomes bonded to the [[boron]]. This [[hydroboration]] is repeated two additional times, successively reacting each B–H bond so that three alkenes add to each BH<sub>3</sub>. The resulting trialkylborane is treated with hydrogen peroxide in the second step.  This process replaces the B-C bonds with HO-C bonds.  The boron reagent is converted to [[boric acid]].  The reaction was originally described by [[H.C. Brown]] in 1957 for the conversion of [[1-Hexene|1-hexene]]  into [[1-Hexanol|1-hexanol]].<ref>{{Cite journal| doi = 10.1021/jo01360a626| title = Communications – Selective Conversion of Olefins into Organoboranes Through Competitive Hydroboration, Isomerization and Displacement Reactions| year = 1957| last1 = Brown | first1 = H.| last2 = Rao | first2 = B. C.| journal = Journal of Organic Chemistry| volume = 22| issue = 9| pages = 1137 }}</ref>

[[File:Hydroboration-oxidation-Brown-1957.svg|thumb|400px|center|Hexanol synthesis]]

Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the initial hydroboration step determines the regioselectivity. Hydroboration proceeds in an [[Markovnikov addition#Anti-Markovnikov reactions|anti-Markovnikov]] manner. The reaction sequence is also [[stereospecific]], giving [[syn addition]] (on the same face of the alkene): the hydroboration is syn-selective and the oxidation replaces the boron with hydroxyl having the same geometric position. Thus 1-methylcyclopentene reacts with diborane predominantly to give ''trans''-1-hydroxy-2-methylcyclopentane<ref>{{Cite journal| doi = 10.1021/ja01472a027| title = Amine Boranes. VIII. The Hydroboration of Terminal Olefins, Dienes and Terminal Acetylenes with Trimethylamine t-Butylborane| year = 1961| last1 = Hawthorne | first1 = M. F.| journal = Journal of the American Chemical Society| volume = 83| issue = 11| pages = 2541–2544 }}</ref>—the newly added H and OH are ''cis'' to each other.

Until all hydrogens attached to boron have been transferred away, the boron group BH<sub>2</sub> will continue adding to more alkenes. This means that one mole of hydroborane will undergo the reaction with three moles of alkene. Furthermore, it is not necessary for the hydroborane to have more than one hydrogen. For example, reagents of the type R<sub>2</sub>BH are commonly used, where R can represents the remainder of the molecule. Such modified hydroboration reagents include [[9-Borabicyclo(3.3.1)nonane|9-BBN]], [[catecholborane]], and [[disiamylborane]].

===Oxidation step===
In the second step of the reaction sequence, the nucleophilic [[hydroperoxide]] [[anion]] attacks the boron atom. Alkyl migration to oxygen gives the alkyl borane with [[Walden inversion|retention]] of stereochemistry (in reality, the reaction occurs via the trialkyl [[borate]] B(OR)<sub>3</sub>, rather than the monoalkyl borinic ester BH<sub>2</sub>OR).

[[File:Hydroboration-oxidation-mechanism.svg|thumb|400px|center|Hydroboration–oxidation mechanism]]
The 'H' atom in the reaction comes from B<sub>2</sub>H<sub>6</sub>, the 'O' atom comes from hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) whereas the O attached 'H' atom comes from the solvent (refer mechanism).