# 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

1. 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.

2. 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.

3. 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.

Note: $H_3O^+$ looks like $H_3O^-$ due to thin width of vertical bar in $+$

During HBO, the carbon nearer to the phenyl ring is more sterically hindered than the other one next to it, otherwise these two are similar in many respects exscluding some, this can be understood by imagining the intermediate $R_3B$ by taking R as branched through both carbons one by one, maybe then you'll appreciate the fact of difference in steric hindrance.

In case of acid catalyzed hydration, the carbon nearer to it has more carbocation stabilization than the one next to it, because of the conjugation with the benzene/phenyl ring. Even if the carbocation is formed on the $\beta$ wrt phenyl, it'll rearrange to $\alpha$/allylic position.Thus the product.

Notes:

Herbert C. Brown (1912–) won the Nobel Prize in 1979 for his invention and development of hydroboration, mostly carried out at Purdue University, which made this new chemistry familiar to the practising organic chemist. The prize was shared with Georg Wittig, showing how important organic chemistry with main group elements had become. He vigorously opposed ‘non-classical’ carbocation theory.

For most substrates, the addition in hydroboration is stereospecific and syn, with attack taking place from the less-hindered side. Brown, H.C.; Zweifel, G. J. Am. Chem. Soc. 1961, 83, 2544; Bergbreiter, D.E.; Rainville, D.P. J. Org. Chem. 1976, 41, 3031; Kabalka, G.W.; Newton, Jr., R.J.; Jacobus, J. J. Org. Chem. 1978, 43, 1567.

The only special case known (atleast to me) is the treatment of iPrCHCHMe with borane which gave 57% of product with boron on the methyl-bearing carbon and 43% of the otherBrown, H.C.; Zweifel, G. J. Am. Chem. Soc. 1961, 83, 1241.

In all cases, the boron goes to the side of the double bond that has more hydrogens, whether the substituents are aryl or alkylFor a thorough discussion of the regioselectivity with various types of substrate and hydroborating agents, see Cragg, G.M.L.Organoboranes in Organic Synthesis Marcel Dekker, NY, 1973, pp.63–84, 137–197. See also, Brown, H.C.; Vara Prasad, J.V.N.; Zee, S. J. Org. Chem. 1986, 51, 439.

• Apparently, ordinary hydroboration doesn't have the desired regioselectivity for 1,2-disubstituted alkenes when one of the substituents is a phenyl group (see this reaction scheme). Ordinary borane adds the boron atom predominantly at the benzylic position in those instances. I think it's possible to overcome this by using a sterically hindered borane (e.g., 9-BBN), however. – Greg E. Nov 3 '14 at 5:20
• @GregE. You are right about the expected regioselectivity of the hydroboration. Can you upgrade your comment to an answer as the current one is not correct? I wouldn't think 9-BBN will change considerably this selectivity since it is mainly electronically driven. – K_P Nov 3 '14 at 9:00
• @K_P, thanks for the comment. As far as the regioselectivity, I understand that electronic factors are decisive. Admittedly, I'm speculating, but I think with a sufficiently bulky borane (like 9-BBN, disiamylborane, catecholborane, etc.) it might be possible for sterics to override electronics and direct the addition to the less hindered carbon. There is ample literature indicating superior regioselectivity when using bulky boranes, but I can't find any reference specific to phenyl-substituted alkenes, so I can only conjecture. I'd like to research more before I write an answer. – Greg E. Nov 3 '14 at 9:28
• @Aditya, I recognize the text you've added from March's Advanced Organic Chemistry, and nothing in the excerpts you've used specifically addresses this issue because the degree of substitution at the two carbons of the alkene in 1-phenylpropene is the same, and sterics and electronics likely have opposite influences on regiochemistry in this instance. Perhaps this is covered in the primary literature March cited, but I don't have access to it at the moment to check. In the meantime, see scheme (d). – Greg E. Nov 4 '14 at 20:46
• (-1) Regioselectivity of hydroboration is wrong, as has been pointed out in previous comments. By the way, 9-BBN doesn't reverse the regioselectivity (cf. ref 3 in my answer). – orthocresol Sep 15 '18 at 14:05