Hydroboration, reductive

Thermal decarboxylation of 119 provided the cyclohexene derivative 120, which gave compound 121 by lithium aluminum hydride reduction. Hydroboration - oxidation of 121, followed by acetylation, gave carba-sugar derivatives (122 and 123) in equal yields. 

Keywords Catalysis Electrochemical reduction Hydroboration Hydrogenation Hydrosilylation Iron hydride complex Photochemical reduction... 

The hydroboration of heteroallyl derivatives often produces boron-heteroatom rings. These reactions are very closely related to the internal hydroborations discussed in Section 4.17.9.1.2. The hydroboration of 2-vinylpyrrole affords boron-nitrogen heterocycle 189 (Equation 29) <1997JOM181>. Two important hydroborations of allyl methyl sulfide <1971JA2823> and the allene 190 <1989JOC5814> are shown in Equations (30) and (31), respectively. The reduction/ hydroboration of 191 probably occurs via the intermediacy of 192 (Scheme 23) <1992JOC5288>. 

Redox equations, balancing, 265 Reduction of aromatic compounds, 200 Reductive amination, 404 Reductive hydroboration, 95 Reformatsky reaction, 321 Regioselective reactions, 97 Reimer-Tiemann reaction, 438 Resonance, 23 in benzene, 192 energy, 24 structures, 29 Resorcinol, 327, 432 Retroaldol condensation, 399 Ring activation, 2(fiff Robinson inaction, 398 Rosenmund reduction, 306... 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Asymmetric a-alkylation of amines was achieved applying an immobilized chiral sulfonamide (Scheme 12.29) [13, 40]. To synthesize auxiliary 76, a tertiary alcohol (73) was converted into Grignard reagent 74. Addition of sulfur dioxide and subsequent chlorination furnished the sulfinyl chloride 75. Reaction with (S)-2-amino-l,l,2-triphenylethanol, followed by reduction, hydroboration and... 

The reduction-hydroboration of alkyldihalogenoboranes provides a more general approach to unsymmetrical dialkylboranes. The reduction of RBX2 (X = Br, Cl) with LiAlH followed by the addition of a terminal olefin gives RR BX. Redistribution does not occur, as proved by clean conversion of these dialkylhalogenoboranes into ketones via DCME reaction. However, if the olefin is present in the reaction mixture prior to the reduction, redistributed products are formed ... 

Carbonylation. Dialkylchloroboranes have also been used in the preparation of asymmetric ketones via reduction-hydroboration-carbonylaton protocol. In the presence of 1.0 equiv of trimethyl silane, IpcBCl2 is reduced to IpcBHCl, which selectively hydroborates terminal olefins faster than internal olehns, furnishing the dialkyl chloroborane. Upon further reduction of the R2BCI with LiAlH4, the internal olefin undergoes hydrobora-... 

Three types of reactions are used to convert alkynes to alkenes and alkanes catalytic reduction, hydroboration-protonolysis, and dissolving-metal reduction. 

Scheme 6.65. A potential pathway for the production of Z-2,3-[ H2]-2-butene by reductive hydroboration of 2-butyne.

Both combinations afford either the syn- (85, 86) or the anti-isomers (83, 88) as about 95 5 mixtures. As an important addition to hydrogenation and reduction, hydroboration has gone through an impressive development in the last decades. 

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