Fast and selective oxidation of primary alcohols to aldehydes or to carboxylic acids and of secondary alcohols to ketones mediated by oxoammonium salts under two-phase conditions

General Oxygenation Procedure. An apparently heterogeneous mixture of an olefin (cyclohexene, 1-pentene, or styrene, 1 g), NaBH, (300 mg, 7.9 mmol), (OEP)RhnxCl (4.0 mg, 6 pmol; [Rh] = 0.6 mM), and an internal standard (p-xylene, mesitylene, or durene, appropriate amount) in dry THF (10 mL) exposed to dry air was stirred a t 20-25 "C. The oxygenation of 1-methylcyclohexene was carried out by using the rhodium catalyst in an amount 2 or 20 times as much as that used above ([Rh] = 1.2 or 12 mM). The electronic spectra of the reaction mixture underwent no significant change even after 100 h. The formation of oxygenation products was monitored by gas chromatography. Similarly was carried out the oxygenation of 1,5-cyclooctadiene and acetylenes (1-heptyne and 3-heptyne) by using substrate (300 mg), NaBH, (300 mg), and (OEP)RhmC1 or (TPP)RhInC1 (4.0 mg) in THF (20 mL). Reaction products, after conversion if necessary to silylated derivatives, were identified by gas chromatography on the basis of coinjection with authentic samples, and their yields determined also by gas chromatography. 2-Methylcyclohexanol as a mixture of stereoisomers arising from the oxygenation of 1-methylcyclohexene was purified by preparative gas chromatography. The stereoisomer distribution was determined by 'H NMR spectroscopy by taking advantage of the characteristic signals for hydroxymethine protons a t 6 3.1 (for E isomer) and 3.75 (for 2 isomer). The following control runs were carried out by using cyclohexene as substrate: (1) without rhodium porphyrin catalyst, (2) without 02, (3) without NaBH,, and (4) with NaBH(OCHJ3 in place of NaBH,. In neither case was detected oxygenation of substrate to any significant extent. Another control run using cyclohexene oxide in place of cyclohexene under otherwise identical oxygenation conditions did not give cyclohexanol. Borane Transfer. A mixture of (0EP)Rh"'Cl (40 mg, 0.06 mmol), NaBH4 (100 mg, 2.64 mmol), and 1-pentene (70 mg, 1.0 mmol) in T H F (2 mL) in a vessel sealed with a rubber septum was degassed by freezepumpthaw cycles and was stirred a t room temperature for 19 h. The electronic spectrum of the mixture showed A, a t 395,514, and 545 nm, indicating the formation of (OEP)RhH? Following the standard procedure for the analysis of organoboranes,28 the mixture was then subjected to gas chromatography at 170 OC on a column of silicone SE-30 (2 m), which had been treated with Silyl-8 (Pierce Chemical Co.) to mask protic sites with trimethylsilyl groups. The product was readily identified as tripentylborane on the basis of coinjection with the authentic sample prepared by hydroboration of olefin with diborane under standard conditions. The mixture was exposed to air, stirred for 20 min, and then analyzed by gas chromatography to show the formation of 1-pentanol and 2-pentanol (94:6, in a total yield of 45% based on mol of Rh complex used). Oxidation of Alkylborane. A T H F solution of (E)-bis(2methy1cyclohexyl)borane" was prepared by the hydroboration of 1-methylcyclohexene (96 mg, 1.0 mmol) with borane-THF (1 M) (0.5 mL, 0.5 mmol) in THF (1 mL) under nitrogen. To this was added 1 N aqueous NaOH (0.5 mL), and the mixture was stirred under air atmosphere for 20 h. Gas chromatographic analysis using silicone DCQF-1 showed the formation of 2methylcyclohexanol with the stereoisomer ratio of E / Z = 7624. Another control run for the oxidation of alkylborane with O2 was carried out in the presence of NaBH, (38 mg, 1.0 mmol) instead of aqueous NaOH under otherwise identical conditions and gave the isomer ratio of E / Z = 81:19. A solution of (E)-bis(2-methylcyclohexyl)borane in THF (0.21 mL) was prepared as above starting from the olefin (15.4 mg, 0.16 mmol). This solution was added to (OEP)RhH15 (100 mg, 0.16 mmol) under nitrogen. The mixture was then allowed to contact with a gentle stream of THF-saturated air for 20 h. Gas chromatography coupled with 'H NMR analysis indicated almost exclusive formation of (E)-2-methylcyclohexanol.