Organoboranes
Organoboranes are organic compounds of boron having one or more alkyl or aryl functional groups attached directly to the boron atom. Alkylboranes can be conveniently prepared by the addition of diborane, B2H6, to a carbon-carbon double or triple bond:
6 RCH=CH2 + B2H6 2 (RCH2CH2)3B
4 (CH3)2C=CHCH3 + B2H6 2 ((CH3) 2CHC(CH3H)2BH
2 (CH3)2C=C(CH3)2 + B2H6 2 (CH3)2CHC(CH3)2BH2
Alkylboranes can also be prepared from the reaction of boron halides and metal alkyls, such as alkyllithium compounds or Grignard reagents (RMgX, where R denotes an alkyl chain). Boron halides undergo metathesis (double decomposition) reactions with alkyllithium reagents according to the hard-soft acid-base principle. Boron is a softer acid than lithium, and carbon in these molecules is a very soft base. Metathesis generates the lithium halide and organoborane.
Hydroboration of most alkenes proceeds directly to the trialkylborane; however, it is possible to prepare dialkylboranes, R2BH, and monoalkylboranes, RBH2, if there is sufficient steric hindrance about the carbon-carbon double and triple bond. The partially substituted borane derivatives, R2BH and RBH2, form dimers with three- centered two-electron bonds between the borons and two bridging hydrogen atoms. Most alkylboranes spontaneously ignite in air. Controlled oxidation, followed by dissolution in water, yields dihydroxy(alkyl)boranes (RB(OH)2). The various aryl derivatives can be prepared from boron halides using the Grignard reaction.
The term organoborane is generally reserved for boron-containing compounds that have at least one carbon-boron bond. Boron is trivalent, and two of the chemical bonds can involve non-carbon atoms. Other important organoboranes include: alkoxydialkylboranes (R2BOR); dialkoxyalkylboranes (RB(OR)2); chlorodialkylboranes (R2BCl); dichloro(alkyl)borane (RBCl2); hydroxydialkylborane (R2BOH); and dihydroxy(alkyl)borane. Trialkoxyborane (B(OR)3) is at times listed as an organoborane even though it does not have the boron-carbon bond required by definition. Organoboranes undergo substitution reactions with complete retention of stereochemistry. This facilitates the manufacture of stereoisomers of pharmaceutically important organic compounds. Organoboranes are used as synthetic reagents in organic chemistry in the preparation of acylic and cylic saturated and unsaturated hydrocarbons, terpenes, amino acids, steriods, carbohydrates, and numerous other organic molecules.
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