The syn-addition of hydroboranes to alkenes occurs with predictable selectivity, wherein the boron adds preferentially to the least hindered carbon. This selectivity is enhanced if sterically demanding boranes are used.
Coupling the hydroboration with a subsequent oxidation of the new formed borane yields anti-Markovnikov alcohols. The hydroboration/oxidation sequence constitutes a powerful method for the regio- and stereoselective synthesis of alcohols.
The product boranes may also be used as starting materials for other reactions, such as Suzuki Couplings (see Recent Literature).
Mechanism of the Brown Hydroboration
The selectivity of the first addition of borane can be relatively low:
The subsequent additions are more selective as the steric bulk increases, and anti-Markovnikov selectivity predominates in the end:
Oxidation with hydrogen peroxide leads to alcohols:
Sterically demanding boranes offer enhanced selectivity. One example of a sterically demanding borane (9-BBN) is generated by the double addition of borane to 1,5-cyclooctadiene:
The reactivity and selectivity of the borane reagent may be modified through the use of borane-Lewis base complexes.
Hydroboration. 97. Synthesis of New Exceptional Chloroborane-Lewis Base Adducts for Hydroboration. Dioxane-Monochloroborane as a Superior Reagent for the Selective Hydroboration of Terminal Alkenes
J. V. B. Kanth, H. C. Brown, J. Org. Chem, 2001, 66, 5359-5365.
A mild hydroboration of various 1-alkynes with pinacolborane was achieved in the presence of a catalytic amount of dicyclohexylborane at room temperature under neat conditions to afford the corresponding (E)-1-alkenylboronic acid pinacol esters in good yields.
K. Shirakawa, A. Arase, M. Hoshi, Synthesis, 2004, 1814-1820.
Highly Stereoselective Synthesis of cis-Alkenyl Pinacolboronates and Potassium cis-Alkenyltrifluoroborates via a Hydroboration/Protodeboronation Approach
G. A. Molander, N. M. Ellis, J. Org. Chem., 2008, 73, 6841-6844.
Concise Formation of 4-Benzyl Piperidines and Related Derivatives Using a Suzuki Protocol
S. Vice, T. Bara, A. Bauer, C. A. Evans, J. Fort, H. Josien, S. McCombie, M. Miller, D. Nazzareno, A. Palani, J. Tagat, J. Org. Chem, 2001, 66, 2487-2492.