Editing Hydroboration–oxidation reaction

{{Short description|Chemical reaction that converts an alkene to an alcohol}}
'''Hydroboration–oxidation reaction''' is a two-step [[hydration reaction]] that converts an [[alkene]] into an [[Alcohol (chemistry)|alcohol]].<ref>{{cite book| author = Marc G. Loudon| year = 2002| title = Organic Chemistry| chapter = Addition Reactions of Alkenes| edition = fourth| pages = 168–172| publisher = Oxford University Press| location = New York| isbn = 0-19-511999-1}}</ref> The process results in the [[Syn and anti addition|syn addition]] of a hydrogen and a [[hydroxyl]] group where the [[double bond]] had been. Hydroboration–oxidation is an [[Markovnikov's rule|anti-Markovnikov]] reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction thus provides a more [[stereospecific]] and complementary [[regiochemistry|regiochemical]] alternative to other hydration reactions such as [[Acid catalysis|acid-catalyzed]] addition and the [[Oxymercuration reaction|oxymercuration–reduction]] process. The reaction was first reported by [[Herbert C. Brown]] in the late 1950s<ref>{{Cite journal| doi = 10.1021/ja01510a059|title = A Sterepspecific Cis Hydration of the Double Bond in Cyclic Derivatives| year = 1959| last1 = Brown | first1 = H. C.| last2 = Zweifel | first2 = G.| journal = Journal of the American Chemical Society| volume = 81| pages = 247}}</ref> and it was recognized in his receiving the [[Nobel Prize in Chemistry]] in 1979.

The general form of the reaction is as follows:
[[File:Hydroboration-oxidation reaction.png|400px|center]]

[[Tetrahydrofuran]] (THF) is the archetypal [[solvent]] used for hydroboration.

==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).

==Alkyne hydroboration==
A hydroboration reaction also takes place on [[alkyne]]s.  Again the mode of action is ''syn'' and secondary reaction products are [[aldehyde]]s from terminal alkynes and [[ketone]]s from internal alkynes. In order to prevent hydroboration across both the pi-bonds, a bulky borane like [[disiamylborane|disiamyl (di-sec-iso-amyl) borane]] is used.<ref>{{Cite journal| doi = 10.1021/ja00767a072| title = Catecholborane (1,3,2-benzodioxaorole) as a new, general monohydroboration reagent for alkynes. Convenient synthesis of alkeneboronic esters and acids from alkynes via hydroboration| year = 1972| last1 = Brown | first1 = H. C.| last2 = Gupta | first2 = S. K.| journal = Journal of the American Chemical Society| volume = 94| issue = 12| pages = 4370}}</ref>
[[Image:Alkyne hydroboration en.png|thumb|400px|center|Hydroboration–oxidation of terminal alkyne]]

==Alternative oxidations==
Use of other oxidants instead of hydrogen peroxide can lead to carbonyl products rather than alcohols from alkenes. [[N-Methylmorpholine N-oxide|''N''-Methylmorpholine ''N''-oxide]] with catalytic [[tetrapropylammonium perruthenate]] converts the alkylborane into a carbonyl, thus a ketone or [[aldehyde]] product depending on what other groups were attached to that carbon in the original alkene.<ref>{{cite journal |title= One-pot conversion of olefins to carbonyl compounds by hydroboration / NMO–TPAP oxidation |first1= Matthew H. |last1= Yates |journal= Tetrahedron Lett. |volume= 38 |pages= 2813–2816 |year= 1997 |doi= 10.1016/S0040-4039(97)00476-0 }}</ref> Various [[dichromate]]s or related [[chromium]](VI) reagents give ketones as well, but give [[carboxylic acid]]s instead of aldehydes for terminal alkenes.<ref>{{cite journal |title= Organoboranes for Synthesis. 14. Convenient Procedures for the Direct Oxidation of Organoboranes from Terminal Alkenes to Carboxylic Acids |first1= Herbert C. |last1= Brown |first2= Shekhar V. |last2= Kulkarni |first3= Vijay V. |last3= Khanna |first4= Virendra D. |last4= Patil |first5= Uday S. |last5= Racherla |journal= Journal of Organic Chemistry |year= 1992 |volume= 57 |issue= 23 |pages= 6173–6177 |doi= 10.1021/jo00049a024 }}</ref>

==Other oxidation substrates==
Aside from boranes, the oxidation of [[silanes]] and disilanes can also yield [[Hydroxy group|hydroxy groups]]. A major difference is that while silyl groups like the phenyldimethylsilyl group are converted to the hydroxy group after acid or other [[electrophile]] treatment followed by oxidation by hydrogen peroxide, disilanyl groups are converted after [[Tetra-n-butylammonium fluoride|TBAF]] treatment followed by peroxide oxidation. This allows for selective oxidation of either group.<ref>{{Cite journal |last=Suginome |first=Michinori |last2=Matsunaga |first2=Shin-ichiro |last3=Ito |first3=Yoshihiko |date=September 1995 |title=Disilanyl Group as a Synthetic Equivalent of the Hydroxyl Group |url=http://www.thieme-connect.de/DOI/DOI?10.1055/s-1995-5150 |journal=Synlett |volume=1995 |issue=09 |pages=941–942 |doi=10.1055/s-1995-5150|url-access=subscription }}</ref> 

==References==
{{Reflist}}

==External links==
* Organic Chemistry Portal. Hydroboration (including recent literature). [https://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm https://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm]
{{Organic reactions}}
{{DEFAULTSORT:Hydroboration-oxidation reaction}}
[[Category:Addition reactions]]