Sodium trialkylborohydrides

Sodium or tetramethylammonium triacetoxyborohydride has become the reagent of choice for diastereoselective reduction of P-hydroxyketones to antidiols. Trialkylborohydrides, eg, alkaH metal tri-j -butylborohydrides, show outstanding stereoselectivity in ketone reductions (39). 

Alkyl- or 2-aryl-substituted allyl halides 1, when treated first with a borane and then with a base, give substituted cyclopropanes in moderate to excellent yield. For the hydroboration step tetraalkyldiboranes 2 (method A) are superior to diborane (B2H ) itself (methods B or C, respectively), and sodium tetraalkylborates 4a or trialkylborohydrides 4b are preferred bases. 

The reduction of metal salts with borohydride or trialkylborohydride is a widely used colloid synthesis method. The preparation of platinum microcrystals having a mean diameter of 28 A by the reduction of chloroplatinic add with sodium borohydride has been reported as a reprodudble standardized preparation. [52] PVP stabilized copper sols have been prepared by borohydride reduction of copper salts. [53, 54] In some cases, however, the formation of metal... 

Alkali metal trialkylborohydrides are attractive reducing agents in addition to other numerous synthetic applications [1], Brown and coworkers [2] have also synthesized alkali metal di- and monoalkylborohydrides. Consequently, the reaction of lithium, sodium, and potassium hydrides with 9-BBN, ChXjBH, Sia BH, IPCjBH, ThxBHj, and IPCBHj afford the corresponding hydrides, which are examined with respect to rate, stoichiometry, and products. 

In the IR of trialkylborohydride [4], a characteristic absorption of B-H stretch occurs around 2,000 cm . The exact frequency depends mainly on the solvent used, nature of the alkyl group, and the cation [5]. Dialkylborohydride and monoalkylborohydride exhibit B-H absorption around 2,100 and 2,200 cm, respectively. The absorption in sodium borohydride occurs at 2,300 cm" . The change in the B-H stretching frequency is attributed to the inductive effect of the alkyl substituent (Table 22.3) [2]. 

The reaction of potassium hydride in THF with trialkylboranes has been found to proceed smoothly at 25 °C to produce the corresponding potassium trialkylborohydrides. A very complete study concerning steric effects in the reaction of a series of trialkylboranes with lithium and sodium hydrides in THF has been carried out. Sodium hydride exhibited greater reactivity than lithium hydride and the reactivity of the organoboranes decreased drastically as the steric hindrance increased. ... 

The substitution of hydride ligands in [BH ] and [A1H4]" anions by other groups is an effective approach to the modification of the reducing abilities of these reagents. Many substituted hydrides have been synthesized by replacement of H- in the [BH4] anion. For example, alkali metal trialkylborohydrides and trialkoxyborohydrides, as well as sodium and lithium cyanoborohydrides O" and dicyanoborohydrides, 2,l5 been... 

---