(Triorganosilyl)pinacolboranes were prepared
by reaction of triorganosilyllithium reagents with pinacolborane or isopropoxypinacolborane in high yield. The
reaction also is applicable to synthesis of 2-(triorganosilyl)-4,4,6-trimethyl-1,3,2-dioxaborinane. A new germylborane derivative was similarly prepared from the
We report here novel catalytic asymmetric synthesis of optically active 1-arylalkanols (up to 96% ee) through asymmetric hydroboration of styrenes catalyzed by a chiral cationic rhodium complex, which complements the uncatalyzed asymmetric hydroboration with chiral alkylboranes derived from -pinene that has been successfully used for internal alkenes.1,2Since Mannig and Noth reported in 1985 that rhodium complexes catalyze the hydroboration with catecholborane,3 a few reports have appeared on application of the catalyzed hydroboration for organic synthesis, i.e., control of regio-and stereochemistry in the hydroboration of allylic alcohol derivatives4 and catalytic asymmetric hydroboration of 1,2-and 1,1-disubstituted alkenes.5We found that the use of a certain cationic phos-
Catalytic asymmetric hydrogenations of prochiral unsaturated compounds, 1 olefin, 2 ketone, 3 and imine, 4 have been intensively studied and are considered as a versatile method of creating a chiral carbon center. 5 However, no highly enantioselective hydrogenation of heteroaromatic groups has so far been reported except that of 2-methylquinoxaline to our knowledge. 6 Resonance stability of heteroaromatic compounds might impede the enantioselective hydrogenation, 7 which may find potentially wide applicability in stereoselective organic synthesis. 8,9 Herein, we describe the highly enantioselective hydrogenation of heteroaromatic compounds, indoles.We recently disclosed that the rhodium complex generated from Rh(acac)(cod) and PPh 3 is a good catalyst for the hydrogenation of five-membered heteroaromatic compounds. 10 Thus chiral rhodium complexes prepared in situ from Rh(acac)(cod) and various commercially available chiral bisphosphines (1 mol %) were examined for asymmetric hydrogenation of N-acetyl-2-butylindole (1a) at 60°C for 2 h with 5.0 MPa of H 2 in 2-propanol (eq 1), resulting in non-enantioselective hydrogenation (0-1% ee). 11 Fortunately, the successful asymmetric hydrogenation has been achieved by use of a trans-chelating chiral bisphosphine ligand, (S,S)-(R,R)-PhTRAP, 12,13 giving (R)-N-acetyl-2-butylindoline (2a) with 85% ee (77% conversion). No reduction of the fused aromatic ring of 1a was observed.On further investigation into the asymmetric hydrogenation, [Rh(nbd) 2 ]SbF 6 was found to be superior to Rh(acac)(cod) as catalyst precursor (Table 1). It is noted that addition of base is necessary for achievement of high enantioselectivity as well as high catalytic activity. The [Rh(nbd) 2 ]SbF 6 -(S,S)-(R,R)-PhTRAP catalyst scarcely promoted the hydrogenation in the absence of base, giving a trace of 2a with only 7% ee (S) (entry 1). Addition of 10 mol % of Et 3 N or Cs 2 CO 3 brought remarkable improvement of the enantioselectivity and catalytic activity (100% conversion, 94% ee (R)) (entries 2 and 3). 14 Both the enantioselectivity and catalytic activity were significantly dependent upon base: K 2 -CO 3 gave (R)-2a with 76% ee, and pyridine did not activate the cationic PhTRAP-rhodium complex at all (entries 4 and 5). The amount of Cs 2 CO 3 did not affect the selectivity: 20 mol %, 94% ee; 1 mol %, 93% ee. It is possible to carry out the asymmetric hydrogenation at lower pressure (1.0 MPa) without significant decrease of the selectivity and reaction rate (entry 6). The amount of PhTRAP-rhodium complex can be reduced to 0.1 mol %, and the reaction was completed within 20 h to give (R)-2a of 93% ee in 92% isolated yield (entry 7).Although 2-propanol has frequently been used as a hydrogen source in the transfer hydrogenation of unsaturated compounds (1) For reviews, see: (a) Takaya, H.; Ohta, T.; Noyori, R. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH Publishers: New York
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