Phenylpropene methyl hyperconjugation

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. 

Fig. 4.1 Diagram to show combined effects of phenyl conjugation (-K) and methyl hyperconjugation (+K) in 1-phenylpropene [6]...

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See also in sourсe #XX -- [ ]

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