Borane covalent bond formation

Covalent bond formation in borane, BHj. (a) Lewis structure for borane. (b) Orbital overlap picture of borane. 

STEP 1. Reaction of a nucleophile and an electrophile to form a new covalent bond. The addition of borane to an alkene is initiated by coordination of the vacant 2p orbital of boron (an electrophile) with the electron pair of the pi bond (a nucleophile). Chemists account for the stereoselectivity of hydroboration by proposing the formation of a cyclic, four-center transition state. Boron and hydro-... 

In the reaction with 1-hexene, borane reacts as a Lewis acid and the alk-ene is a Lewis base. Donation of two electrons from the x-bond to the boron is accompanied by cleavage of the x-bond and formation of a new C-B a-covalent bond. In addition, a hydrogen atom from boron must be transferred to carbon, forming a new C-H o-covalent bond as the H-B bond is broken. So far, this is similar to the reaction of an alkene with HBr. However, experiments have shown that there is no intermediate, so there is no carbocation. If there is no intermediate, the C-H o-bond must be formed almost simultaneously with the C-B o-bond. Based on the experimental evidence that there is no intermediate, the reaction of borane with an alkene is said to be a concerted reaction. Formally, it classified as concerted asynchronous rather than concerted synchronous. A reaction that has no intermediate is called a concerted reaction. If the bond-making and bond-breaking events do not occur simultaneously, the reaction is said to be asynchronous. 

The [B2oHj7SH] - ion has the potential to react with intracellular protein substituents through a variety of mechanisms. The ion has the potential to form disulfide bonds in a manner similar to that proposed, but not observed, for the BSH ion (Figure 12.10). Formation of the disulfide bonds would covalently bond the polyhedral borane anion within the tumor cells. 

A tetrahedraUy coordinated benzazaborine is generated as a major product 51 in 40% yield by the reaction of three equivalents of an aryUithium with a trialkyl borate (Scheme 22) (20000M206). In a plausible mechanism, the treatment ofB(OiPr)3 with two equivalents of the aryUithium generates diarylborane 49 which upon treatment with a third equivalent of the aryl-Hthium does not generate a triarylborane, but instead gives the tetrahedraUy coordinated borane species 50. This is probably due to the increased acidity of the a-protons on the tertiary amine as a result of the coordinate covalent B- N bond. Subsequent loss of an alkoxide leads to the formation of 51, and its structure has been confirmed by X-ray crystaUography and NMR spectroscopy. 

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