Borane, also known as borine, is an unstable and highly reactive molecule with the chemical formula BH3. The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong Lewis acid. Consequently, it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen. It normally dimerizes to diborane in the absence of other chemicals.
BH3 is a trigonal planar molecule with D3h symmetry. The experimentally determined B–H bond length is 119 pm.
In the absence of other chemical species, it reacts with itself to form diborane. Thus, it is an intermediate in the preparation of diborane according to the reaction:
BX3 +BH4− → HBX3− + (BH3) (X=F, Cl, Br, I)
2 BH3 → B2H6
The standard enthalpy of dimerization of BH3 is estimated to be −170 kJ mol−1.
The boron atom in BH3 has 6 valence electrons. Consequently, it is a strong Lewis acid and reacts with any Lewis base ('L' in equation below) to form an adduct:
BH3 + L → L—BH3
in which the base donates its lone pair, forming a dative covalent bond. Such compounds are thermodynamically stable, but may be easily oxidised in air. Solutions containing borane dimethylsulfide and borane–tetrahydrofuran are commercially available; in tetrahydrofuran a stabilising agent is added to prevent the THF from oxidising the borane. A stability sequence for several common adducts of borane, estimated from spectroscopic and thermochemical data, is as follows:
PF3 < CO< Et2O< Me2O< C4H8O < C4H8S < Et2S< Me2S< Py < Me3N< H−
BH3 has some soft acid characteristics as sulfur donors form more stable complexes than do oxygen donors. Aqueous solutions of BH3 are extremely unstable.
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La première partie du cours décrit les méthodes classiques de synthèse asymétrique. La seconde partie du cours traite des stratégies de rétrosynthèse basées sur l'approche par disconnection.
Organoboron chemistry or organoborane chemistry is the chemistry of organoboron compounds or organoboranes, which are chemical compounds of boron and carbon that are organic derivatives of borane (BH3), for example trialkyl boranes. . Organoboron compounds are important reagents in organic chemistry enabling many chemical transformations, the most important ones being hydroboration and carboboration. Reactions of organoborates and boranes involve the transfer of a nucleophilic group attached to boron to an electrophilic center either inter- or intramolecularly.
In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon (, , , and ). This chemical reaction is useful in the organic synthesis of organic compounds. Hydroboration produces organoborane compounds that react with a variety of reagents to produce useful compounds, such as alcohols, amines, or alkyl halides. The most widely known reaction of the organoboranes is oxidation to produce alcohols typically by hydrogen peroxide.
Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents. The structure of diborane has D2h symmetry. Four hydrides are terminal, while two bridge between the boron centers. The lengths of the B–Hbridge bonds and the B–Hterminal bonds are 1.
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