Potassium borohydride-lithium chlorid

Esters are more difficult to reduce, and usually, no reaction takes place with sodium borohydride. However, the potassium borohydride/lithium chloride system was found to reduce esters under microwave conditions in a solvent-free reaction33. The reactions are generally completed in 2-8 min and provide the corresponding alcohols in 55-95% yield (Scheme 4.13). 

Keywords ester, potassium borohydride-lithium chloride, microwave irradiation, alcohol... 

White and Kittredge (2005) reported a microwave-assisted reduction of cyclohexanone by sodium borohydride that is supported on SiO. The reaction was completed in less than 3 min while Feng et al. (2001) reported the reduction of ester to the corresponding alcohols, using potassium borohydride/lithium chloride and microwave irradiation under solvent-free condition. The reactions were generally completed in 2-8 min, with good to excellent yields (55% to 95%). 

Potassium borohydride (1.0 g, 20 mmol), anliydrous lithium chloride (0.8 g, 20 mmol) were thoroughly mixed in a mortar and transferred to a flask (100 mL) connected with reflux equipment, then dry THF (10 mL) was added and the mixture was heated to reflux for 1 h. After cooling, the ester (10 mmol) was added and stirred for 0.5 h at room temperature, then the THF was removed under reduced pressure. After the mixture was irradiated by microwave for 2-8 min, the mixture was cooled to room temperature, water (20 mL) was added, extracted with ether (3 x 15 mL), dried with magnesium sulfate, and evaporated to give the crude product, which was purified by crystallization, distillation or column chromatography. 

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. 

Ethanol with catalytic HCI 2 Benzyl chloride 3 Potassium borohydride and lithium chloride 4 Cyclopropylmethyl bromide with NaH as base 5 Catalytic hydrogenolysis with Pd/C 6 and 7 analogous to propranolol (Scheme 8.1) 8 HCI. 

Sodium borohydride and potassium borohydride react regioselectively with the formyl group of alkyl 2-formylcyclopropanecarboxylates lithium aluminum hydride is unseleetive. Stereospecific reduction of the formyl group was observed when formyl-cyclo-propanes were subjected to yeast alcohol dehydrogenase. ei Furthermore, regiospecific reduction of the aldehyde function in a 2-cyclopropylprop-2-enal derivative was performed using a mixture of sodium borohydride and cerium(III) chloride. 

Related Reagents. Cerium(III) Chloride Nickel Boride Potassium Triisopropoxyborohydride Sodium Cyanoboro-hydride Sodium Triacetoxyborohydride Cobalt Boride Lithium Borohydride Lithium Aluminium Hydride Zinc Borohydride Tetramethylammonium Triacetoxyborohydride. 

Alkyl bromides and especially alkyl iodides are reduced faster than chlorides. Catalytic hydrogenation was accomplished in good yields using Raney nickel in the presence of potassium hydroxide [63] Procedure 5, p. 205). More frequently, bromides and iodides are reduced by hydrides [505] and complex hydrides in good to excellent yields [501, 504]. Most powerful are lithium triethylborohydride and lithium aluminum hydride [506]. Sodium borohydride reacts much more slowly. Since the complex hydrides are believed to react by an S 2 mechanism [505, 511], it is not surprising that secondary bromides and iodides react more slowly than the primary ones [506]. The reagent prepared from trimethoxylithium aluminum deuteride and cuprous iodide... 

REDUCTION, REAGENTS Bis(N-methylpi-perazinyl)aluminum hydride. Borane-Di-methyl sulfide. Borane-Tetrahydrofurane. Borane-Pyridine. n-Butyllithium-Diisobu-tylaluminum hydride. Calcium-Amines. Diisobutylaluminum hydride. 8-Hydroxy-quinolinedihydroboronite. Lithium aluminum hydride. Lithium 9-boratabicy-clo[3.3.1]nonane. Lithium n-butyldiisopro-pylaluminum hydride. Lithium tri-j c-butylborohydride. Lithium triethylborohy-dride. Monochloroalane. Nickel boride. 2-Phenylbenzothiazoline. Potassium 9-(2,3-dimethyl-2-butoxy)-9-boratabicy-clo[3.3.1]nonane. Raney nickel. Sodium bis(2-methoxyethoxy)aluminum hydride. Sodium borohydride. Sodium borohy-dride-Nickel chloride. Sodium borohy-dride-Praeseodymium chloride. So-dium(dimethylamino)borohydride. Sodium hydrogen telluride. Thexyl chloroborane-Dimethyl sulfide. Tri-n-butylphosphine-Diphenyl disulfide. Tri-n-butyltin hydride. Zinc-l,2-Dibromoethane. Zinc borohydride. 

Other reagents which have occasionally been used to cleave hydrazides include diborane (which also reduces the carbonyl groups), sodium naphthalenide, 0,0-diethyldithiophosphoric acid, (EtO)2PS2H, - and sulfur monochloride. Nickel-aluminum alloy in aqueous methanolic potassium hydroxide is a good reagent for reductively cleaving a number of N—N bonded compounds, such as A -methyl-A -phenylhydrazine and Af/Z-dimethylnitrosamine. - Nitrosamines have also been cleaved with titanium(IV) chloride-sodium borohydride and lithium aluminium hydride. 

Related Reagents. Calcium Hydride Iron(III) Chloride-Sodium Hydride Lithium Aluminum Hydride Potassium Hydride Potassium Hydride-5-Butyllithium-(V,(V,(V, (V -Tetra-methylethylenediamine Potassium Hydride-Hexamethylphos-phoric Triatnide Sodium Borohydride Sodium Hydride-copper(II) Acetate-Sodium t-Pentoxide Sodium Hydride-nickel(II) Acetate-Sodium t-Pentoxide Sodium Hydride-palladium(II) Acetate-Sodium t-Pentoxide Tris(cyclopenta-dienyl)lanthanum-Sodium Hydride Lithium Hydride Sodium Telluride. 

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