First Unambiguous Characterization of a Novel RO-BH<sub>2</sub> Intermediate by <sup>1</sup>H{<sup>11</sup>B} and <sup>1</sup>H{<sup>13</sup>C,<sup>11</sup>B} NMR Spectroscopy. Proof of Boron−Carbonyl Interaction through Innovative Use of NMR Techniques

An unusual reduction of the aryl-substituted 3-hydroxyacrylic acid ester <b>1</b> by BH<sub>3</sub> results in the propenoic acid ethyl ester <b>3</b>. We were able to prove the presence of an intermediate <b>2</b> containing a cyclic O-BH<sub>2</sub> ← OC< moiety in <b>2</b> by using <sup>1</sup>H−<sup>11</sup>B and <sup>1</sup>H−<sup>13</sup>C{<sup>11</sup>B} NMR correlation experiments.