Dimethylsulfide complexes

In addition to the boron trifluoride-diethyl ether complex, chlorotrimcthylsilanc also shows a rate accelerating effect on cuprate addition reactions this effect emerges only if tetrahydrofuran is used as the reaction solvent. No significant difference in rate and diastereoselectivity is observed in diethyl ether as reaction solvent when addition of the cuprate, prepared from butyllithium and copper(I) bromide-dimethylsulfide complex, is performed in the presence or absence of chlorotrimethylsilane17. If, however, the reaction is performed in tetrahydrofuran, the reaction rate is accelerated in the presence of chlorotrimethylsilane and the diastereofacial selectivity increases to a ratio of 88 12 17. In contrast to the reaction in diethyl ether, the O-silylated product is predominantly formed in tetrahydrofuran. The alcohol product is only formed to a low extent and showed a diastereomeric ratio of 55 45, which is similar to the result obtained in the absence of chlorotrimethylsilane. This discrepancy indicates that the selective pathway leading to the O-silylated product is totally different and several times faster than the unselective pathway" which leads to the unsilylated alcohol adduct. A slight further increase in the Cram selectivity was achieved when 18-crown-6 was used in order to increase the steric bulk of the reagent. 

B. (Z)-l-Iodo-l-heptene.2 A solution of 8.52 g of (112 mmol) of borane-dimethylsulfide complex (Note 13) in 100 mL of ether is added to a flame-dried, three-necked, 300-mL, round-bottomed flask equipped with stirbar, temperature probe, and N2 inlet. The solution is cooled to 5°C with an ice-bath. Cyclohexene (18.4 g, 224 mmol) (Note 14) is then added by syringe over 10 min while keeping the temperature below 15°C. The mixture is stirred at 5°C for 15 min. A white solid precipitates either towards the end of the addition or during the subsequent stirring period. The reaction mixture is allowed to warm to room temperature and is stirred for 1 hr. The non-homogeneous solution is cooled to 2-3°C... 

M Borane-dimethylsulfide complex (2.0 mL) was added to the resulting solution. The mixture was cooled to 0-5 °C with an ice-bath, and then a solution of 2,3-butadione monoxime trityl ether (1.72 g) in dry tetrahydrofuran (5 mL) was added dropwise via a syringe pump over 1 hour at that temperature. 

SYNTHESIS A solution of 5.50 g 2,5-dimethoxy-4-ethylthio-B-nitrostyrene (see under 2C-T-2 for its preparation) was made in 80 mL boiling anhydrous THF. On cooling, there was some separation of a fine crystalline phase, which was kept dispersed by continuous stirring. Under an inert atmosphere there was added 3.5 mL of a 10 M borane dimethylsulfide complex, followed by 0.5 g sodium borohydride as a solid. There was a slight exothermic response, and the color slowly faded. Stirring was continued for a week. There was then added 40 mL H,0 and 20 mL... 

Hydroboration-oxidation of enecarbamates with borane-dimethylsulfide complex gives reasonable yields of /i-hydn > xycarbarn ales (equation 28) with some diastereoselectivity, depending upon what other functional groups are present in the starting material152. 

Wierenga and co-workers investigated the use of BH3.THF and the dimethylsulfide complex for the reduction of 3-methyl-3-thiomethyl-2-oxindoles and 3-alkyl-3-hydroxy-2-... 

The benzoyl groups of 43 were reduced using a borane-dimethylsulfide complex to give the corresponding iV-benzy 1 -diazocinc 44, which, upon hydrogenolysis with 20% Pd(OH)2 followed by acid hydrolysis, afforded the NH diazocine 30 (R = H) (Scheme 10) <1998CPB674>. 

Nitro-4-(trifluoromethyl)-phenol 42 (Scheme 17) in reaction with 2-bromo-2-methyl-propionamide in the presence of cesium carbonate and cesium iodide in acetonitrile afforded 2-hydroxy-2-methylpropionamide 43, apparently via derivative 44 as the intermediate [32]. Amide 44, prepared on a circuitous route, on reduction with borane-dimethylsulfide complex, gave amine 45 as the only isolated product. The parent 2-hydroxy-2-methyl-W-(2-... 

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. 

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. 

Allylation of a-thio-35), a-seleno-35) and a-silyl- 35,77) cyclopropyllithiums was not very successful35) but addition at —78 °C of 0.5 equivalent of copper (I) iodide-dimethylsulfide complex 35,106, W7> prior to the allylhalide leads 35,106,107) to a very high yield of homoallyl cyclopropyl sulfides or selenides (Scheme 24). Similar observations have been made on cyclobutyl derivatives3S). It is not clear at present whether a cuprate is involved in the process but we have evidence ( Se-NMR) that a new species is transiently being formed, at least in the seleno series. The synthesis of homoallyl cyclopropylsilanes was also reported 78) and involves the allylation of a postulated cuprate formed by the addition of lithium dibutyl cuprate to a-lithiocyclopropylsilane (Scheme 26). 

Borane is typically used as a THF (BHs-THF) or dimethylsulfide complex (BH3 SMe2). Although the reactivity of the two complexes is similar, the boron dimethylsulfide species is more stable over longer periods of time. Borane will chemoselectively reduce a carboxylic acid in the presence of an ester or nitrile. Reviews (a) Seyden-Penne, J. Reductions by the Alumino- and Borohydrides in Organic Synthesis, Wiley-VCH New York, 1997, 2" edition, (b) Brown, H. C. ... 

The preparation of each is easy and this is the route to 87/88 as described by Evans11 in Organic Syntheses. Reduction of the free acid 91 with the borane-dimethylsulfide complex is easy if smelly and the rest is straightforward. The same auxiliary 87 can be used with any acid acylation with the acid chloride gives the chiral derivative 93 ready for enolate formation. 

Segment Cl was synthesized from (tf)-malic acid as a starting material in the following three steps (Scheme 27) (I) diesterification with acetyl chloride in methanol, (II) selective reduction of a-hydroxy ester with boran dimethylsulfide complex and sodium borohydride in THF, and (III) acid catalyzed lactonization of 145 (90). 

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