Organoborane

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

The parent borane, diborane is classified in organic chemistry as a strong electrophile and reacts rapidly to alkenes in a process called hydroboration. This concept was discovered by Dr. Herbert Charles Brown at Purdue University with help from Georg Wittig. 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 substituents. One organoboron reagent that is often employed in synthesis is 9-borabicyclo[3.3.1]nonane or 9-BBN which is generated from the reaction of cyclooctadiene and diborane ^[1].

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 ^[2].

Hydroboration reaction also takes place to alkynes. Again the mode of action is syn and secondary reaction products are cis-alkenes ^[4].

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 ^[3]. Reaction with iodine or bromine afford the corresponding alkyl halides. A carboxylic acid simply replaces the borane group by a proton.

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 an alkyl substituent on the anionic boron migrates to the adjacent electrophilic carbon of the carbonyl group. The carbonyl group can then be reduced to an alcohol group.

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

TEB - Triethylborane was used to ignite the JP-7 fuel of the Pratt / Whitney J-58 ramjet engines powering the Lockheed SR-71 Blackbird.

External links

[National Pollutant Inventory - Boron and compounds]

References

^[1] Advanced Organic Chemistry, F.A. carey, R.J. Sundberg ISBN 0-306-41088-5
^[2] 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. [Abstract]
^[3] 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, William S. Murphy; J. Am. Chem. Soc 3565 1964. [abstract]
^[4] 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. [abstract]
The Roles of Boron and Silicon, Susan E. Thomas; Oxford Chemistry Primers No.1; 1991: Very good general book covering all the important reactions of boron and organoboranes in organic chemistry.

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