Hydroboration-oxidation reaction 240990 186889900 2008-01-25T21:14:54Z 124.7.42.125 /* Hydroboration-oxidation */ In [[organic chemistry]], the '''hydroboration-oxidation reaction''' is a two-step [[organic reaction|organic chemical reaction]] that converts an [[alkene]] into a neutral [[alcohol]] by the net addition of water across the [[double bond]].<ref>{{cite book | author = Loudon, Marc G. | year = 2002 | title = Organic Chemistry | chapter = Addition Reactions of Alkenes. | edition = Fourth Edition | pages = 168-172 | publisher = Oxford University Press | location = New York | id = ISBN 0-19-511999-1 }}</ref><ref>Organic Chemistry Help. Hydroboration of alkenes. [http://www.chemhelper.com/hydroboration.html http://www.chemhelper.com/hydroboration.html]</ref><ref>Organic Chemistry Portal. Hydroboration (including recent literature). [http://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm http://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm]</ref> The hydrogen and [[hydroxyl]] group are added in a [[Syn and Anti addition|syn addition]] leading to [[cis-trans isomerism|cis]] [[stereochemistry]]. Hydroboration-oxidation is an [[Markovnikov's rule|anti-Markovnikov]] reaction, with the hydroxyl group attaching to the less-substituted carbon. The general form of the reaction is as follows: [[Image:Hydroboration-oxidation reaction.png|600px|center|Hydroboration-oxidation]] where THF is [[tetrahydrofuran]], the archetypal [[solvent]] used for this reaction. In the first step, [[diborane|borane]] (BH<sub>3</sub>) adds to the [[double bond]], transferring one hydrogen from itself to the adjacent carbon. The second step substitutes the boron group BH<sub>2</sub> with the hydroxyl group, creating the final product. ==Hydroboration mechanism== Borane exists as a toxic, colorless gas called [[diborane]] (B<sub>2</sub>H<sub>6</sub>). In diborane, two hydrogen atoms are each bonded to both boron atoms by single pairs of electrons ("[[three-center two-electron bond]]s"). This [[delocalization]] satisfies the octet around each boron and reduces the [[electrophilicity]]. That said, even diborane is intensely [[Lewis acid]]ic, because of its vacant [[p orbital]]s. Because [[dimerization]] happens instantaneously, it is not possible to isolate pure borane. However, when diborane is treated with an [[ether]] or [[amine]], a stable complex is formed, as the [[lone pair]] from the Lewis basic oxygen or nitrogen atom is donated to the borane. These complexes act chemically like borane. Solutions of BH<sub>3</sub> complexes in [[THF]] or [[diethyl ether]] are commercially available and more easily handled than diborane gas, and so are the more common form found in laboratories. For simplicity in illustration, borane will be used instead of the borane-ether complex in this article. [[Image:Diborane-to-borane.jpg|center|Diborane to borane]] The addition of BH<sub>3</sub> to the alkene is a [[concerted reaction]], with multiple bond formation and breaking occurring simultaneously. The intermediate step can be visualized more clearly by a theoretical [[transition state]]. [[Image:Hydroboration.jpg|center|Hydroboration reaction mechanism]] Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the first step is the stereospecific-determining step. The hydroborane will add to the alkene so that the boron always ends up on the lesser substituted carbon. In the transition state, the more substituted carbon bears a partial positive charge (a partial [[carbocation]]). As a general rule, carbocations that are more substituted tolerate positive charge better than those that aren't. Had the hydroborane attacked with the opposite orientation, the lesser substituted carbon will bear the positive charge, which is electronically unfavorable. 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 equivalent of hydroborane will conduct the reaction with three equivalents of alkene. Furthermore, it is not necessary for the hydroborane to have more than one hydrogen. Therefore, BH<sub>3</sub> can be better represented as R-BH, where R can represents the remainder of the molecule. A widely used hydroboration reagent is [[9-BBN]] which has just one hydrogen at boron and the same applies for [[catecholborane]]. Hydroborations also take place [[stereoselective]] in a [[syn addition|syn]] mode, that is on the same face of the alkene. Thus 1-methylcyclopentene reacts with diborane predominantly to the trans-alkane <ref>''Hydroboration. IX. The Hydroboration of Cyclic and Bicyclic Olefins-Stereochemistry of the Hydroboration Reaction '' Herbert C. Brown, George Zweifel; [[J. Am. Chem. Soc.]]; '''1961'''; 83(11); 2544-2551. {{DOI|10.1021/ja01472a027}}</ref>. ==Hydroboration-oxidation== In the second '''hydroboration-oxidation''' step, 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 monoalkly borinic ester BH<sub>2</sub>OR). [[Image:Hydroboration3.gif|center|Hydroboration oxidation mechanism]] 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 disiamyl (di-sec-iso-amyl) borane is used. <ref>''Catecholborane (1,3,2-benzodioxaorole) as a new, general monohydroboration reagent for alkynes. Convenient synthesis of alkeneboronic esters and acids from alkynes via hydroboration'' Herbert C. Brown, S. K. Gupta; [[J. Am. Chem. Soc.]]; '''1972'''; 94(12); 4370-4371. {{DOI|10.1021/ja00767a072}} </ref> [[Amine]]s can be obtained by action of [[chloramine]].<ref> ''The Reaction of Organoboranes with Chloramine and with Hydroxylamine-O-sulfonic Acid. A Convenient Synthesis of Amines from Olefins via Hydroboration'' Herbert C. Brown, Wolfgang R. Heydkamp, Eli Breuer, and William S. Murphy [[J. Am. Chem. Soc.]]; '''1964'''; 86(17) pp 3565 - 3566; {{DOI|10.1021/ja01071a036}}</ref> Reaction with [[iodine]] or [[bromine]] afford the corresponding alkyl halides. A [[carboxylic acid]] simply replaces the borane group by a [[hydrogen ion|proton]]. ==See also== * [[Oxymercuration reaction|Oxymercuration-reduction]] is another reaction that converts an alkene into an alcohol. Unlike hydroboration-oxidation, oxymercuration-reduction is not stereospecific. However, oxymercuration-reduction is a Markovnikov reaction. Therefore, oxymercuration-reduction and hydroboration-oxidation are complementary because they add with opposite regiochemistry. ==References== <div class="references-small"><references/><div> [[Category:Addition reactions]] [[ar:تفاعل أكسدة هيدروبورونية]] [[de:Hydroborierung]] [[es:Hidroboración]] [[ja:ヒドロホウ素化]]