Synthesis of benzylboronic acids and benzylboronates
A catalytic amount of Mg as the only metal enables a reductive coupling between benzyl halides and pinacolborane. HBpin acts both as an electrophile and as a reducing agent to regenerate an organomagnesium species in situ. An hydride oxidation mechanism is proposed on the basis of DFT calculations.
C. Pintaric, S. Olivero, Y. Gimbert, P. Y. Chavant, E. Duņach, J. Am. Chem. Soc., 2010, 132, 11825-11827.
Aliphatic, aromatic, heteroaromatic, vinyl, or allylic Grignard reagents eact with pinacolborane at ambient temperature in tetrahydrofuran to afford the corresponding pinacolboronates. The initially formed dialkoxy alkylborohydride intermediate quickly eliminates hydridomagnesium bromide and affords the product boronic ester in very good yield. This reaction also can be carried out under Barbier conditions.
J. W. Clary, T. J. Rettenmaier, R. Snelling, W. Bryks, J. Banwell, W. T. Wipke, B. Singaram, J. Org. Chem., 2011, 76, 9602-9610.
A regioselective hydroboration of vinylarenes catalyzed by a Rh(I)-DPPB complex proceeded rapidly when DMAP was used as an additive to furnish the desired boronates in good to excellent yield with high regioselectivity.
K. Endo, M. Hirokami, K. Takeuchi, T. Shibata, Synlett, 2008, 3231-3233.
CsF with 1.1 equiv of H2O effects highly efficient protodeboronation on tertiary diarylalkyl boronic esters with essentially complete retention of configuration, whereas TBAF·3H2O can be used for tertiary aryldialkyl boronic esters. Furthermore, substituting D2O for H2O provides ready access to deuterium-labeled enantioenriched tertiary alkanes. A short synthesis of the sesquiterpene (S)-turmerone is described.
S. Nave, R. P. Sonawane, T. G. Elford, V. K. Aggarwal, J. Am. Chem. Soc., 2010, 132, 17096-17098.
The use of an equimolar amount of base with a diborylmethane enabled a chemoselective Pd-catalyzed Suzuki–Miyaura cross-coupling reaction for the synthesis of various benzylboronate derivatives in very good yields. Reactions of sterically hindered aryl bromides can give products in good yields.
K. Endo, T. Ohkubo, T. Shibata, Org. Lett., 2011, 13, 3368-3371.
In the presence of a chiral monodentate taddol-derived phosphoramidite ligand, catalytic enantiotopic-group-selective cross-couplings of achiral geminal bis(pinacolboronates) provide a route for the construction of nonracemic chiral organoboronates with high levels of asymmetric induction. Mechanistic experiments suggest that the reaction occurs by a stereochemistry-determining transmetalation with inversion of configuration at carbon.
C. Sun, B. Potter, J. P. Morken, J. Am. Chem. Soc., 2014, 136, 6534-6537.