Hydroboration/oxidation vs acid-catalyzed hydration of 1-phenylpropene I am trying to predict the products of both the hydroboration/oxidation and acid-catalyzed hydration of 1-phenylpropene:



Both C1 and C2 of the double bond are secondary carbons. However, I have the feeling that there are slight differences in the electron density on each carbon, which will affect the addition of the boron electrophile and the stability of the carbocation for the two reactions respectively.

Which regioisomers are predominantly formed from both reactions?
Hydroboration

With the simplest hydroborating agent $\ce{BH3.THF}$, the hydroboration/oxidation of 1-phenylpropene proceeds to give 1-phenylpropan-1-ol as the major product.[1] The isomer of the substrate used was not reported.



There is some ambiguity in the above source, as the labels 8 and 8′ (which the paper uses to refer to the two regioisomers) are consistently swapped; hence, I have taken the names and not the labels to be accurate. However, as additional proof, there are other examples of similar regioselectivity in the hydroboration of styrenes (here, an E/Z mixture of styrenes was used):[2]



Rationalising this selectivity is not very easy, but in one of Herbert Brown's papers where the same regioselectivity is seen with 9-BBN as the hydroborating agent, he writes:[3]


  The preferred site of hydroboration in 1-phenylpropene, the site to which boron becomes attached, is the 1-position. This is not what one would expect on the basis of steric effects. One possible explanation for this is the combined effects of phenyl conjugation (-K) and methyl hyperconjugation (+K) which act to decrease the amount of electron density at the 2-position and to increase it at the 1-position.


I'm not totally convinced by this, but I also don't have any better explanation. Further evidence for this explanation is the fact that the (Z)-isomer reacts with poorer regioselectivity than the (E)-isomer. This is because there is allylic strain in the (Z)-isomer which forces the phenyl group to twist out of conjugation with the C=C double bond (depicted below). Consequently, the electron-withdrawing effect of the benzene ring (via conjugation) is reduced, and the formation of the 2-hydroxy isomer is less disfavoured.





Acid-catalysed hydration

Examples of these in the literature are harder to find and I cannot find any generic examples of acidic hydration of $\ce{R^{1}-C=C-R^{2}}$, where $\ce{R^1}$ is an aryl group and $\ce{R^2}$ an alkyl. However, I see no particular reason to go against the conventional wisdom: the more stable carbocation intermediate is formed at the benzylic position, which should lead to the 1-hydroxy compound as the major product.



References


Camacho, C.; Uribe, G.; Contreras, R. Diphenylamine Borane, A New Stable Amine .Borane with Remarkable Hydroborating and Reducing Properties. Synthesis 1982, 1982 (12), 1027–1030. DOI: 10.1055/s-1982-30051.
Allegretti, M.; Bertini, R.; Cesta, M. C.; Bizzarri, C.; Di Bitondo, R.; Di Cioccio, V.; Galliera, E.; Berdini, V.; Topai, A.; Zampella, G.; Russo, V.; Di Bello, N.; Nano, G.; Nicolini, L.; Locati, M.; Fantucci, P.; Florio, S.; Colotta, F. 2-Arylpropionic CXC Chemokine Receptor 1 (CXCR1) Ligands as Novel Noncompetitive CXCL8 Inhibitors. J. Med. Chem. 2005, 48 (13), 4312–4331. DOI: 10.1021/jm049082i.
Brown, H. C.; Nelson, D. J.; Scouten, C. G. Hydroboration. 63. Kinetics and regiospecificity of the hydroboration of isomeric cis- and trans-alkenes via 9-borabicyclo[3.3.1]nonane (9-BBN). Effects of 1,1- and 1,2-dialkyl interactions. J. Org. Chem. 1983, 48 (5), 641–643. DOI: 10.1021/jo00153a003.