Trimethylamine, complex with borane

Borane, which is used as a complex with tetrahydrofuran [992] or dimethyl sulfide [611, 992] or generated in situ from lithium borohydride with boron trifluoride etherate [646] or sodium borohydride with aluminum chloride [184], reacts with 3 mol of an alkene to form a tertiary borane. The oxidation with alkaline hydrogen peroxide [183, 992, 1201] or with trimethylamine oxide [991, 992] yields an alcohol (equations 598 and 599). 

Because of its similarity to trimethylamine oxide, the ylid may be considered as a Lewis base, and, as such, it should coordinate with various Lewis acids. Complexes with lithium bromide and trimethylbromostannane have already been mentioned and it would be expected that coordination with boranes and alanes would also give stable products. 

Evidence for the reverse process, donation of electron density from the nucleophilic dimer atom to an electron-deficient molecule, also exists. Konecny and Doren theoretically found that borane (BH3) will dissociatively adsorb on Si(100)-2x1 [293]. While much of the reaction is barrierless, they note an interaction between the boron atom and the nucleophilic atom of the Si dimer during the dissociation process. Cao and Hamers have demonstrated experimentally that the electron density of the nucleophilic dimer atom can be donated to the empty orbital of boron trifluoride (BF3) [278]. XPS on a clean Si(100)-2 x 1 surface at 190 indicates that BF3 dissociates into BF2(a) and F(a) species. However, when BF3 is exposed on a Si(100)-2 x 1 surface previously covered with a saturation dose of trimethylamine, little B-F dissociation occurs, as evidenced by the photoelectron spectrum. They conclude that BF3 molecularly adsorbs to the nucleophilic dimer atom and DFT calculations indicate that the most energetically favorable product is a surface-mediated donor-acceptor complex (trimethylamine-Si-Si-BF3) as shown in Figure 5.19. 

Alcohols are converted to alkyl iodides by reaction with A A-diethyl-aniline/borane/iodine. Reductive iodination is observed when this system reacts with ketones or with carboxylic acids934,935. Direct reduction of aromatic aldehydes to benzyl bromide is reported by Le Corre and coworkers, who have brominated benzaldehydes and acetophenones in presence of a trimethylamine/borane complex and have obtained benzyl bromides and a-bromoethylbenzenes, respectively936. See also Reference937. 

NSiQHn, tert-Butylamine, iV-(trimethyl-silyl)-, 25 8 N2, Dinitrogen iron complexes, 24 208, 210 osmium complex, 24 270 N2BC4H, Borane, cyano-compd. with trimethylamine (1 1),... 

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. ° ... 

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