Organoborane

Organoborane or organoboron compounds are chemical compounds comprised of boron and carbon. organoboron chemistry or organoborane chemistry is the chemistry of these compounds.

Related Topics:
Chemical compounds - Boron - Carbon

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The parent borane, diborane is classified in organic chemistry as a strong electrophile and reacts rapidly to alkenes in a process called hydroboration. Although diborane as a pure compound is a dimer, BH3 forms a 1:1 complex with oxygen in for instance THF. In an ordinary electrophilic addition reaction the Markovnikov's rule determines regioselectivity but with boranes the mode of action is the exact opposite. The reason is that boron is less electronegative than hydrogen. When a positive charge develops in the alkene on most substituted carbon atom, that is where the partially negatively charged hydrogen atom adds to, leaving the least substituted carbon atom for the boron atom. The so called anti-Markovnikov addition is most pronounced when the boron compound has very bulky substtituents. One organoboron reagent that is often employed in synthesis is 9-borabicyclononane or 9-BBN which is generated from the reaction of cyclooctadiene and diborane .

Related Topics:
Borane - Diborane - Organic chemistry - Electrophile - Alkene - Hydroboration - THF - Electrophilic addition - Markovnikov's rule - Regioselectivity - Electronegative - Anti-Markovnikov addition - 9-BBN - Cyclooctadiene

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Hydroborations take place stereoselective in a syn mode, that is on the same face of the alkene. In this concerted reaction the transition state is represented as a square with the corners occupied by carbon, carbon, hydrogen and boron with maximum overlap between the two olefin p-orbitals and the empty boron orbital. Thus 1-methylcyclopentene reacts with diborane predominantly to the trans-alkane .

Related Topics:
Stereoselective - Concerted reaction - Transition state - Olefin - P-orbital

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Hydroboration reaction also takes place to alkynes. Again the mode of action is syn and secondary reaction products are cis-alkenes .

Related Topics:
Alkyne - Cis-alkene

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In organic synthesis the hydroboration reaction is taken further to generate other functional groups in the place of the boron group. The Hydroboration-oxidation reaction offers a route to alcohols by oxidation of the borane with hydrogen peroxide or to the carbonyl group with the stronger oxidizing agent chromium oxide. Amines can be obtained by action of chloroamine . Reaction with iodine or bromine afford the corresponding alkyl halides. A carboxylic acid simply replaces the borane group by a proton.

Related Topics:
Functional groups - Hydroboration-oxidation reaction - Alcohol - Hydrogen peroxide - Carbonyl - Oxidizing agent - Chromium oxide - Amine - Chloroamine - Iodine - Bromine - Carboxylic acid - Proton

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A second group of reactions that organoboron compounds are involved in create new carbon carbon bonds. Carbon monoxide is found to react very easily with a trialkylborane. What follows is a 1,2-rearrangement when a alkyl substituent on the anionic boron migrates to the adjacent electrophilic carbon of the carbonyl group. The carbonyl group can then be reduced to a alcohol group.

Related Topics:
Carbon monoxide - 1,2-rearrangement

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Organoboron compounds also lend themselves to transmetalation reactions with organopalladium compounds. This reaction type is exemplified in the Suzuki reaction.

Related Topics:
Transmetalation reaction - Organopalladium - Suzuki reaction

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