Brown hydroboration reaction mechanism

Brown, H. C., Chandrasekharan, J. Mechanism of hydroboration of alkenes with borane-Lewis base complexes. Evidence that the mechanism of the hydroboration reaction proceeds through a prior dissociation of such complexes. J. Am. Chem. Soc. 1984,106,1863-1865. 

While there was initial debate between Pasto5 and Brown6 about the mechanism of the hydroboration reaction, Brown s proposed mechanism6 is now generally accepted as the most likely pathway. Brown suggested that the reaction proceeds via an equilibrium between BH3 THF (5) and free BH3 (6). The free BH3 then rapidly adds B-H across a 71-system of the olefin (7) in an anti-Markovnikov fashion via an asynchronous, 4-centered transition state (8) to afford hydroboration product 9. 

Now we pass on to another addition reaction, hydroboration, which is the addition of hydrogen and horon across a n hond. The mechanism is not simple, and it will take some effort for you to become comfortable with all the details. The mechanistic discussion does make several worthwhile points, so it merits your attention for that reason alone. But there is another, more compelling reason to master the hydroboration reaction—it is one of the most useful of synthetic reactions. Indeed H. C. Brown (1912-2004) won a Nobel Prize in 1979 for the development of hydroboration. Consider the following seemingly simple synthetic task outlined in Problem 9.13. 

Brown and co-workers have continued their extensive studies of the mechanisms of hydroboration. Some of this work has recently been reviewed. Kinetic studies have been reported of the reaction of 9-borabicyclo[3.3.1]nonane dimer (3) with alkenes, alkynes, aldehydes, and ketones, in various solvents and with various Lewis bases and Br0nsted acids. Earlier studies suggested that the reaction of (3) with alkenes in a variety of solvents proceeded via the initial dissociation as in equation (4) to the monomer (4) followed by the hydro-... 

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