Rh-catalyzed asymmetric hydroboration of vinylarenes

Olefin hydroboration, which is the addition of a B-H bond across C=C bond, was first discovered by H. C. Brown in 1956 and Koster in 1958. Typically, the reaction does not require a catalyst and the simple borane reagent (e.g., BHg THF, BH3-SMe2, BH2Cl Et20, thexylborane, disiam-ylborane, and 9-BBN) or boranes bearing electron-withdrawing substituents (e.g.. Piers s borane B(C6p5)2 ) react rapidly even at room temperature to afford, after oxidation, the linear anti-Markovnikov products. The reaction can be remarkably C=C/C=0 chemoselective for terminal alkenes 

In this chapter, origin of enantioselectivity in the hydroboration/ oxidation of vinylarenes with catecholborane by cationic rhodium complexes is discussed based on computational work of Fernandez and Bo as 

Fernandez and Bo suggested that the key intermediate responsible for both regio- and enantioselectivity is pentacoordinated H-Rh-(ligand)-catecholborane-styrene complex, where styrene is coordinated trans to the naphthylpyridine moiety and the hydride is in axial position.i 3 The 

Chart 2.2 Isomers to produce different reaction products. 

What is the evidence that Ji2 interaction is stabilizing and Jt3 interaction is destabilizing The relative strength of each %-n interaction was evaluated by considering model systems (Chart 2.3). 

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