Borohydride ion

The authors believe hydrolysis to be rate-determining but do not account for the presence of ferricyanide in the transition state. 

---

The electron counts of nitrogen in ammonium ion and boron in borohydride ion are both 4 (half of eight electrons in covalent bonds) Because a neutral nitrogen has five electrons in its valence shell an electron count of 4 gives it a formal charge of +1 A neutral boron has three valence electrons so that an electron count of 4 in borohydride ion corresponds to a formal charge of -1... 

Although the nature of the general polar effect suggested by Kamernitzsky and Akhrem " to account for axial attack in unhindered ketones is not clear, several groups have reported electrostatic interactions affect the course of borohydride reductions. Thus the keto acid (5a) is not reduced by boro-hydride but its ester (5b) is reduced rapidly further, the reduction of the ester (6b) takes place much more rapidly than that of the acid (6a). Spectroscopic data eliminate the possibility that in (5a) there is an interaction between the acid and ketone groups (e.g. formation of a lactol). The results have been attributed to a direct repulsion by the carboxylate ion as the borohydride ion approaches. " By contrast, House and co-workers observed no electrostatic effect on the stereochemistry of reduction of the keto acid (7). However, in this compound the acid group may occupy conformations in which it does not shield the ketone. Henbest reported that substituting chlorine... 

At no point is H2 involved. The reducing agent is borohydride ion (BFLi ). 

Alkylborohydrides are also used as reducing agents. These compounds have greater steric demands than the borohydride ion and therefore are more stereoselective in situations in which steric factors come into play.72 These compounds are prepared by reaction of trialkylboranes with lithium, sodium, or potassium hydride.73 Several of the compounds are available commercially under the trade name Selectrides .74... 

Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable insight into the mechanism of this reaction.186 The salient conclusion is that the hydride is delivered from a borohydride ion, not directly from the silane. Although the borane forms a Lewis acid-base complex with the ketone, its key function is in delivery of the hydride. 

By analogy with the reduction of nickel salts with borohydride ion, a mechanism involving hydride, or H, ions as the purported reducing species was advanced by Hersch [41] and elaborated on by Lukes [42], and is shown here in part for acidic solutions ... 

Figure 5.18 Weak ooh->- obh back-transfer involving (left) H-bonded and (right) free B—H antibonds of the borohydride ion. Note that the nodal plane of the <7BH(b) precludes significant net overlap with ctQh in the L-shaped geometry.

Most of the attempted asymmetric reductions have used sodium borohydride in conjunction with quaternary ammonium catalysts. Recently, the solution structures of ion pairs formed by quaternary ammonium ions derived from quinine with borohydride ion have been characterized by nuclear magnetic resonance methods in CDC13.1741... 

The two-phase reduction of cyclohexanones by sodium dithionite in the presence of a stoichiometric amount of Adogen gave higher yields of the cyclohexanols than those obtained by the standard procedure using sodium dithionite in a water dioxane system (Table 11.9). A marked improvement in yield was also observed with the reduction of sterically hindered 2,6-dimethylcyclohexanone and there was a greater degree of stereoselectivity, which was comparable to that noted for the corresponding reduction with the borohydride ion [4]. 

With this repertoire of bonding possibilities at our disposal, we car construct the molecular structures of various boron-hydrogen compounds, both neutral species and anions. The simplest is the tetrahydroborate126 or borohydride ion, BH. Although borane is unstable with respect to dimerization, the addition of a Lewis base, H , satisfies the fourth valency of boron and provides a stable entity. Other Lewis bases can coordinate as well. 

Rather than continue to progress from less complex to more complex boron-hydrogen compounds, it will be more convenient to jump to a complex but highly symmetric borohydride ion. lB,H 2f. It may be synthesized bv the pyrolvsis of the BjHJ ion ... 

The electron counts of nitrogen in ammonium ion and boron in borohydride ion are both 4 (one half of 8 electrons in covalent bonds). 

---