Phenylpropene hydroboration

Subtle ligand changes have pronounced effects in hydroboration of 1,1-disubstituted alkenes. Addition of HBcat across the C=C bond in 2,3,3-trimethyl-l-butene is catalyzed by [Rh(COD)Cl]2/DIOP at —5°C affording 2-t-butyl-l-propanol in 69% ee upon oxidation. In the case of 2-phenylpropene, [Rh(C2H4)2Cl]2 combines with either DIOP or BINAP to provide higher enantiomeric excesses than with [Rh(COD)Cl]2.46... 

The bulkier groups, because of greater steric requirements, preferably direct hydroboration at the less hindered carbon (Chart 4.1) [6] except in the case of 1-phenylpropene. Also the directive influence is less in tram isomers of alkenes. 

However, 1-phenylpropene undergoes hydroboration at the C-1 position. This is not what one would expect based on the steric effect of a phenyl group. This is because of the combined effects of phenyl conjugation (-K) and methyl hyperconjugation (-tK), which act to decrease the amount of electron density at the C-2 position and to increase it at the C-1 position (Fig. 4.1) [6]. Evidently, these mesomeric effects are strong enough to override the steric effects of the phenyl group. 

Rotation of phenyl group out of the plane of n-electron cloud, which disrupts phenyl conjugation in cis-1-phenylpropene, is supported due to the greater hydroboration at the C-2-postion (17.5%), than in the tram isomer (3.2%). However, the rate-retarding steric effects of phenyl rotation prevail. 

Because of nonpolar geometry in czs-l-phenylpropene, it displays enhanced steric effects as compared with fra s-l-phenylpropene. Rotation of the phenyl group out of the plane of the Tt system in cz s-1-phenylpropene also leads to a greater amount of hydroboration at the C-2 position (17.5%) than in the trans isomer (3.2%). 

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