Boron hydrides, reactive intermediate

Its unique reactivity comes from the fact that borane first forms a Lewis acid-base complex with the acid and then a boron-carboxylate intermediate which increases the reactivity of the boron hydride and delivers the hydride by an intramolecular reaction. As such it provides a selective way to reduce acids and produce alcohols in the presence of most other functional groups. 

Tetrahydrofuran treated a few min. with diborane and iodine at room temp, and the intermediate boric acid ester treated with water 4-iodobutan-l-ol. Y 90%. — Similarly Anisole phenol. 1 mole diborane cleaves 6 moles ether. Large excesses of reagents are unnecessary and should be avoided. Metal boron hydrides are comparable in reactivity to diborane. Bromine and interhalogen compounds such as iodine chloride react more vigorously but are in general less convenient. L. H. Long and G. F. Freeguard, Nature 207, 403 (1965). 

In the chemistry of polyhedral boron hydrides, boron-centered cations were postulated to be key intermediates of an electrophile-induced nucleophilic substitution mechanism that is responsible for the formation of a variety of boron-substituted derivatives [14], Such boron-centered cations can be easily generated by abstraction of a hydride by the treatment of polyhedral boron hydrides with Lewis or Bronsted acids [15], Similar to the classical chelate-restrained borinium cations based on 3-coordinate boron, these species, which we called quasi-borinium cations, have an unstabilized p orbital and are strong electrophiles (Scheme 6.1). Such quasi-borinium cations are highly reactive and react with even weak nucleophiles, such as ether or nitrile solvent molecules giving the corresponding oxonium and nitrilium derivatives whose properties are close to those of similar complexes of transition metals [15-17]. 

It is necessary for the intermediate cation or complex to bear considerable car-bocationic character at the carbon center in order for effective hydride transfer to be possible. By carbocationic character it is meant that there must be a substantial deficiency of electron density at carbon or reduction will not occur. For example, the sesquixanthydryl cation l,26 dioxolenium ion 2,27 boron-complexed imines 3, and O-alkylated amide 4,28 are apparently all too stable to receive hydride from organosilicon hydrides and are reportedly not reduced (although the behavior of 1 is in dispute29). This lack of reactivity by very stable cations toward organosilicon hydrides can enhance selectivity in ionic reductions. 

Transfers of hydride from boron or lithium to carbon usually occur in the context of addition of the complete M—H moiety to polar or non-polar unsaturation. Additions of boranes to alkenes have been extensively reviewed (Brown et al., 1983a), but the experimental characterization of the hydroboration transition state remains problematic. Dialkylboranes, including 9-borabicyclo[3.3.1]nonane (Wang and Brown, 1980), borinane (Brown et al., 1984), and disiamylborane (Chandrasekharan and Brown, 1985) have now been shown to be dimeric in hydrocarbon and ethereal solvents. With unreactive alkenes, their additions are first order in alkene and half order in the dimer. With reactive terminal alkenes, the reactions are first order only in dimer, with intermediate behaviour between these extremes. A reaction scheme (10) involving reaction of monomeric borane with the alkene satisfies the data, with the observed order depending on the ratio k i/k2. 

Dihydro-1,4-benzothiazines are generally more reactive, and direct N-alkylation has been reported, but this reaction is not always straightforward.21 Funke et al. could not alkylate 2-phenyldihydro-l,4-benzothiazine with co-chloramines, even under forcing conditions.90 However, later work showed that this reaction was possible in toluene solution,37 and other workers have also reported direct alkylations.143 The l,4-benzothiazin-3-ones are, however, more easily alkylated, and reduction of the N-alkyl derivatives of these compounds, usually with lithium aluminum hydride, affords the corresponding N-alkyldihydro-1,4-benzothiazines.52,56 70 90,154 These products can also be prepared in one step from the corresponding 1,4-benzothiazines, e.g., Ill - 112, presumably via intermediate dihydro-1,4-benzothiazines, by sodium borohydride in the presence of a carboxylic acid. Boron derivatives, such as Na[(RCOO)3BH] and Na[(RCOO)4B] are suggested as the species responsible for N—C bond formation.155... 

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