Oxazaborolidine, complex with borane

Treatment of the amino alcohol with borane provides the oxazaborolidine catalyst, which presumably complexes with borane to provide the reducing agent. 

Preparative Methods a solution of (l/ ,2S)-(—)-ephedrine (8.25 g, 50 mmol) in anhydrous THF (50 ml) was treated with Borane-Dimethyl Sulfide complex (50 mmol, 5 mL of 10 M solution). The reaction mixture was stirred at 25 °C for 1 h, at which time one equivalent of hydrogen had evolved. The volatiles were removed in vacuo to furnish a white solid, B NMR (3 8 ppm). The solid was gradually heated to 100 °C and maintained at that temperature until the second equivalent of hydrogen had evolved. The product was distilled under reduced pressure to provide the pure oxazaborolidine (86%). An alternative procedure is available. ... 

Substituted oxazaborolidines also react with borane (B2H6, H3B THF, or H3B SM62) to form an oxazaborolidine-borane complex (19) (eq 9). The oxazaborolidine-borane complex, by releasing the strain of the partial double bond between the ring boron and nitrogen, is more stable than the free oxazaborolidine, and in many cases exists as a stable crystalline solid. ... 

Oxazaborolidine catalyzed reductions are generally performed in an aprotic solvent, such as dichloromethane, THF, or toluene. When the reactions are run in a Lewis basic solvent, such as THF, the solvent competes with the oxazaborolidine to complex with the borane, which can have an effect on the enantioselectivity and/or rate of the reaction. The solubility of the oxazaborolidine-borane complex can be the limiting factor for reactions run in toluene, although this problem has been circumvented by using oxazaborolidines with more lipophilic... 

An analogous approach was used in a stereoselective synthesis of /Tamino-a-hydroxy phosphonates [33]. Reduction of corresponding a-keto phosphonate substrates with borane-dimethylsulfide complex aided by oxazaborolidine catalysis afforded a mixture of diastereomers, but significant diastereoselectiv-ity was achieved using catecholborane as the reductant in toluene at -60 °C. 

Catalyst (3.115) is often referred to as the CBS catalyst after the names of the original authors (Corey, Bakshi and Shibata). A catalytic cycle was proposed which explains the experimental observations. The oxazaborolidine interacts reversibly with borane, which then allows complexation of the ketone to give the key intermediate (3.122), as depicted in Figure 3.2. In this process the catalyst acts as both a Lewis acid and Lewis base activating the borane towards hydride delivery and the ketone towards reduction by interaction with the boron in the oxazaborolidine. This dual activation and enhanced steric bulk of the pyrrolidine moiety leads to... 

The use of boranes has attracted considerably more interest over the last several decades and particularly the use of chiral oxazaborolidine catalysts in combination with borane as a stoichiometric reducing agent (the well-known Corey-Bakshi-Shibata (CBS) reduction) (564, 565) represents one of the most important and versatile methods for the stereoselective reduction of prochiral ketones. It is not intended to give a detailed overview of the successful applications of this versatile methodology for complex (natural product) synthesis, as this would be far beyond the scope of this volume. Instead two examples are chosen below to highlight the potential of this method, especially when used for highly selective late-stage modifications (566,567). 

The reduction of dialkylketones and alkylaryl ketones is also conveniently accomplished using chiral oxazaborolidines, a methodology which emerged from relative obscurity in the late 1980s. The type of borane complex (based on (,V)-diphenyl prolinol)[39] responsible for the reductions is depicted below (10). Reduction of acetophenone with this complex gives (/ )-1 -phenylethanol in 90-95% yield (95-99% ee) [40]. Whilst previously used modified hydrides such as BiNAL-H (11), which were used in stoichiometric quantities, are generally unsatisfactory for the reduction of dialkylketones, oxazaborolidines... 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

The enantioselective reduction of ketones using oxazaborolidine-borane complexes is a useful synthetic route to chiral alcohols (equation 63). Additives such as simple alcohols have been found to enhance the enantioselectivity of the process, and the reaction has been used in the large-scale synthesis of an important drug with anti arrhythmic properties249. 

The CBS reduction has also proven to be an efficient method for asymmetric reduction of a,ft-unsaturated enones14 and ynones15 (Scheme 4.31). The asymmetric reduction of alkynyl ketones affords propargylic alcohols 30 with high levels of enantioselectivity and in moderate to good yields. Optimized reaction conditions for the reduction are the use of THF at — 30° C, 2 equivalents of chiral oxazaborolidine 28b, and 5 equivalents of borane methyl sulfide complex. 

The product is stored at room temperature protected from moisture. A nitrogen atmosphere is recommended for long term storage. Unlike the free oxazaborolidine that readily reacts with and is decomposed by atmospheric moisture, the oxazaborolidine-borane complex is significantly more stable, allowing it to be handled briefly in the open. 

C. The reported procedure provides a practical preparation of (S)-tetrahydro-i-methyl-3,3-diphenyl-lH,3H-pyrrolo[i,2-c][l,3,2]oxazaboroie and conversion to its more stable borane complex.13 The oxazaborolidine-borane complex has also been prepared by treatment of a toluene solution of the free oxazaborolidine with gaseous fiborane followed by recrystallization from a dichloromethane-hexane bilayer.14 This nd other chiral oxazaborolidines have been used to catalyze the enantioselective eduction of prochiral ketones.15 The yield and enantioselectivity of reductions using catalytic amounts of the oxazaborolidine-borane complex are equal to or greater than those obtained using the free oxazaborolidine.13... 

Free oxazaborolidine (16), by itself, will not reduce ketones. Furthermore, (16) is not particularly stable, reacting with moisture (H2O), air (O2), unreacted amino alcohol, other alcohols, or, depending on the substituents, with itself to form various dimers. This instability is due to the strain of a partial double bond between nitrogen and boron (eq 4). Formation of the oxazaborolidine-borane complex (17) tends to release some of this strain. As such, (16) and (17) are generally prepared and used in situ without isolation in many cases, they have not been fully characterized. ... 

The mode of addition and the reaction temperature both affect the enantioselectivity of the reaction. The best results are obtained when the ketone is added slowly to a solution of the oxazaborolidine (or oxazaborolidine-borane complex) and the borane source, at as low a temperature that provides a reasonable reaction rate. This is in contrast to a previous report that indicated that oxazaborolidine-catalyzed reductions lose stereoselectivity at lower temperatures . With unsubstituted (R = H) oxazaboro-... 

E.J. Corey and co-workers synthesized the cdc25A protein phosphatase inhibitor dysidiolide enantioselectively. In the last phase of the total synthesis, the secondary alcohol functionality of the side-chain was established with a highly diastereoselective oxazaborolidine-catalyzed reduction using borane-dimethylsulfide complex in the presence of the (S)-6-methyl CBS catalyst. Finally, a photochemical oxidation generated the y-hydroxybutenolide functionality. This total synthesis confirmed the absolute stereochemistry of dysidiolide. 

These points will be further illustrated in three specific cases (1) reductions of carbonyl compounds by chiral oxazaborolidines-borane complexes ( 2.3), (2) reactions of carbonyl compounds with dialkylzinc reagents in the presence of chiral ligands ( 2.5.4), and (3) dihydroxylation of olefins by OSO4 in the presence of cinchona alkaloids ( 2.9). 

An oxazaborolidine 65 prepared from a-pinene is a new system. More drastic ariations are the oxazaphospholidine oxides, such as 66, which have been examined for their effectiveness in catalyzing the asymmetric reduction.The corresponding oxazaphospholidine-borane complex catalyzes the reduction of imines with diminished aereocontrol. 

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