62497-98-7Relevant academic research and scientific papers
Alternative method for alkylation of arylpolyhalomethanes with trialkylborane in the presence of magnesium
Condon, Sylvie,Nédélec, Jean-Yves
supporting information; experimental part, p. 32 - 35 (2010/09/18)
Reduction of benzal halide derivatives and α,α,α,-trichloromethylbenzene by magnesium powder in DMAc affords α-halocarbanions which then react with triethylborane to give alkylated products. After oxidation with H2O2-NaOH, secondary
Reactions of silyl-stabilised sulfur ylides with organoboranes: Enantioselectivity, mechanism, and understanding
Howells, Dean,Robiette, Raphael,Fang, Guang Y.,Knowles, Luke S.,Woodrow, Michael D.,Harvey, Jeremy N.,Aggarwal, Varinder K.
scheme or table, p. 1185 - 1189 (2008/10/09)
The reaction of trimethylsilyl-substituted sulfonium ylides with organoboranes (Ph3B, Et3B) has been studied and although homologated products were obtained in good yield (after oxidation to the corresponding alcohols), the enantiomeric excesses were low with our camphor-based chiral sulfide (up to 40% ee, cf. corresponding phenyl-substituted sulfonium ylides gave >95% ee). Cross-over experiments were conducted to ascertain the nature of this difference in selectivity. Thus, aryl- and silyl-substituted sulfonium ylides (1 equiv.) were (separately) reacted with Et3B (1.5 equiv.) followed by Ph3B (1.5 equiv.) The experiments were repeated changing the order of addition of the two boranes. It was found that the aryl-substituted sulfonium ylide only trapped the first borane that was added indicating that ate complex formation was non-reversible and so was the selectivity determining step. In contrast the silyl-substituted sulfonium ylide only trapped Ph3B (it is more reactive than Et 3B) indicating that ate complex formation was reversible and so 1,2-migration was now the selectivity determining step. The reactions have been studied computationally and the experimental observations have been reproduced. They have further revealed that the cause of reversibility in the case of the silyl-substituted sulfonium ylides results from ate complex formation being less exothermic and a higher barrier to 1,2-migration. This journal is The Royal Society of Chemistry.
Hydroboration. 66. Addition of Lithium Triethylborohydride to Substituted Styrenes. A Simple, Convenient Procedure for the Markovnikov Hydroboration of Aromatically Conjugated Olefins and the Synthesis of Unusual Mixed Trialkylboranes
Brown, Herbert C.,Kim, Suk-Choong
, p. 1064 - 1071 (2007/10/02)
The addition of lithium trialkylborohydride to substituted styrenes and the synthetic applications of the products are described.Styrene, α- and β-methyl-, p-chloro-, and p-methoxystyrene readily undergo the addition reaction in refluxing tetrahydrofuran or in diglyme at 100 deg C. 1,1-Diphenylethylene reacts readily, even at 0 deg C.The Substituents that decrease the electron density at the double bond increase the rate of the addition.For the system, p-XC6H4CH=CH2, the rate is in the order X=Cl>H>OMe. trans-β-Methylstyrene and α-methylstyrene react slower than styrene itself.Lithium tri-n-butylborohydride also undergoes these reactions.However, hindered trialkylborohydrides, such as lithium triisobutylborohydride and lithium tri-sec-butylhydride, exhibit a slower reaction, which fails to go to completion, even with styrene.The resulting addition products, lithium tetraalkylborates, are transformed into the corresponding aromatic hydrocarbons by hydrolysis.More significantly, the protonolysis of these borates with strong acids yields mixed trialkylboranes by selective protonation of the ethyl group.Oxidation of these boranes gives only α-ols, indicating that the boron atom is attached exclusively to the α-carbon.The observed regiochemistry and electronic effects suggest that the reaction involves a nucleophilic attack of R3BH- on the styrene double bond to form the carbanion ArCHMe-, which is trapped by R3B to form the tetraalkylborate product.Consequently, the present method provides a Markovnikov hydroboration of substituted styrenes with exceptional regioselectivity.
