Potassium tris- borohydride

Replacement of hydrogen by alkyl groups gives compounds like lithium triethylborohydride (Super-Hydride ) [100], lithium tris sec-butyl)borohydride [101] (L-Selectride ) and potassium tris sec-butyl)borohydride (K-Selectride ) [702], Replacement by a cyano group yields sodium cyanoborohydride [103], a compound stable even at low pH (down to 3), and tetrabutylammonium cyanoborohydride [93],... 

REDUCTION, REAGENTS Aluminum amalgam. Borane-Dimethyl sulfide. Borane-Tetrahydrofurane. t-Butylaminoborane. /-Butyl-9-borabicyclo[3.3.1]nonane. Cobalt boride— f-Butylamineborane. Diisobutylaluminum hydride. Diisopropylamine-Borane. Diphenylamine-Borane. Diphenyltin dihydride. NB-Enantrane. NB-Enantride. Erbium chloride. Hydrazine, lodotrimethylsilane. Lithium-Ammonia. Lithium aluminum hydride. Lithium borohydride. Lithium bronze. Lithium n-butylborohydride. Lithium 9,9-di-n-butyl-9-borabicyclo[3.3.11nonate. Lithium diisobutyl-f-butylaluminum hydride. Lithium tris[(3-ethyl-3pentylK>xy)aluminum hydride. Nickel-Graphite. Potassium tri-sec-butylborohydride. Samarium(II) iodide. Sodium-Ammonia. Sodium bis(2-mcthoxyethoxy)aluminum hydride. 

In contrast, despite its steric bulk and its effectiveness with cyclic ketones (see p 4031), potassium tris(l-methylpropyl)borohydride (K-Selectride) is remarkably unsuccessful. Where direct comparisons have been made, it has been shown to be substantially less selective than L-Selectride4"5, and significantly less so than lithium triethylborohydride5. 

Trialkyl borohydrides such as Lithium Tri-s-butylborohydride and Potassium Tri-s-butylborohydride are superior reagents for the chemoselective 1,4-reduction of enones. On the other hand, 1,2-reduction can be obtained by using NaBHj in the mixed solvent MeOH-THF (1 9), or with NaBHj in combination with CeCH or other lanthanide salts. ... 

The simplest products are formed when Nu=H, but this poses a problem of regioselectivity in the nucleophilic attack step a nucleophilic hydride equivalent that selectively undergoes conjugate addition to the enone is required. This is usually achieved with extremely bulky hydride reagents such as lithium or potassium tri(sec-butyl)borohydride (often known by the trade names of L- or K-Selectride, respectively). In this example, K-Selectride reduces the enone to an enolate that is alkylated by methyl iodide to give a single regioisomer. 

The opposite effect is exerted by alkyl substituents. Potassium tri-sec-butyl-borohydride effects 1,4-reduction of cyclohexenones (36) exclusively. 

Cyclohept-2-enone is reduced by potassium tri-s-butyl borohydride in THF to a mixture of cycloheptanone and cyclohept-2-enol in a ratio of 35 55, respectively. 

Cyclohexenones which are unsubstituted at the p-vinylic carbon have been found to undergo 1,4-reduction and reductive alkylation with potassium tri-s-butylborohydride in tetrahydrofuran at 195 K with a high selectivity for the less stable isomer, as found for other sterically hindered reductants 2-cyclohexenone gave 95 % yield of the substituted cyclohexanone. If more than two equivalents of the borohydride are present and the reation is quenched with water at 195 K, only saturated alcohols result. 

The use of 2,4,6-tri-isopropylbenzenesulphonylhydrazides (in place of the tosylhydrazide) allows milder conditions to be used for the preparation of aromatic aldehydes by the McFadyen-Stevens reaction. Addition of hydrazine hydrate and traces of cupric sulphate suppresses formation of aromatic ester and alcohol by-products. Reduction of oxazolinium salts by sodium borohydride, as part of the well known sequence for the conversion of carboxylic acids into aldehydes, can lead to ring cleavage to form amino-alcohols. It is now reported that over-reduction is much less marked if potassium tri-s-butylborohydride is used as reducing agent (Scheme 2) ... 

The utility of lithium and potassium tri-s-butylborohydrides and lithium triethyl-borohydride as reagents for the conjugate reduction and reductive alkylation of unsaturated ketones and esters has been surveyed. ... 

Henkel-Loctite appears to have been careful to avoid the patent minefield laid by 3M and Dow. Their patent strategy in this field has involved mainly the use of metal alkali borohydrides [113,114], or tetralkylboranes [115-117]. In particular, alkali metal trialkyl borohydrides are used, the alkali metal salt being selected from lithinm triethylborohydride, sodium triethylborohydride, potassium triethylborohydride, lithium tri-sec-butylborohydride, sodium tri-sec-butylborohydride, potassium tri-sec-butylborohydride, and lithium triethylborodeuteride. 

Cyclooctadiene) (pyridine) (tri-cyclohexylphosphine)iridium(I) hexa-fluorophosphate, 88 Palladium catalysts, 230 Potassium-Graphite, 252 Sodium borohydride-Molybdenum(VI) oxide, 279... 

Numerous reducing agents were tried at this point unsuccessfully. For example, lithium aluminum hydride destroyed the substrate, whereas DIBAH or lithium borohydnde in THF and sodium borohydride in ethanol led to reduction of the quinoline system. On the other hand, both potassium borohydride (either with or without 18-crown-6) and zinc borohydride (with or without ethanol) produced no reaction at all. Lithium triethylborohydride resulted in de-methoxylation, and sodium borohydride in refluxing THF gave a 45% yield of diol 16 together with overreduced product. 

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