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

Article abstract of DOI:10.1021/jo00180a022

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.