Simple enols. 1. The generation of vinyl alcohol in solution and its detection and characterization by NMR spectroscopy

Abstract

Vinyl alcohol has been generated by hydrolysis of methoxy(vinyloxy)methyl chloroacetate and acetate (<b>1</b> and <b>3</b>), bis(vinyloxy)methyl dichloroacetate and trichloroacetate (<b>7</b> and <b>6</b>), dimethyl vinyl orthoformate (<b>4</b>), dimethyl vinyl orthoacetate (<b>5</b>), and ketene methyl vinyl acetal (<b>8</b>) in aqueous [<sup>2</sup>H<sub>3</sub>]acetonitrile or aqueous [<sup>2</sup>H<sub>7</sub>]dimethylformamide. It was characterized by <sup>1</sup>H and <sup>13</sup>C NMR spectroscopy and by conversion into acetaldehyde. On replacement of OH by OD the <sup>13</sup>C NMR spectrum shows isotope shifts of −0.12 and −0.09 ppm for the <i>α</i>- and <i>β</i>-carbons under conditions where no such shifts were detectable for the corresponding carbons of ethyl vinyl ether. Vinyl alcohol with an OH group was generated under conditions of slow hydroxyl-proton exchange with <b>8</b> as the precursor in CD<sub>3</sub>COCD<sub>3</sub> (99 v %)/H<sub>2</sub>O (1 v %) at −10 °C and the following coupling constants were evaluated: <sup>3</sup><i>J</i><sub>trans</sub> = +14.0, <sup>3</sup><i>J</i><sub>cis</sub> = +6.3, <sup>2</sup><i>J</i><sub>gem</sub> = −0.8, <sup>3</sup><i>J</i>(HOC<sub><i>α</i></sub>H) = +9.8, <sup>4</sup><i>J</i>(HOC<sub><i>β</i></sub>H<sub>anti</sub>) = +0.4, <b>|</b><sup>4</sup><i>J</i>(HOC<sub><i>β</i></sub>H<sub>syn</sub>)<b>|</b> < 0.2 Hz. Under the conditions used vinyl alcohol was converted into acetaldehyde ca 100 times faster than ethyl vinyl ether but could nevertheless be kept in solution for several hours below ca −10 °C. The postulated intermediate, methyl vinyl hemiorthoformate, could not be detected in the hydrolysis of <b>1</b>, <b>3</b>, and <b>4</b> nor could methyl vinyl hemiorthoacetate be detected in the hydrolysis of <b>5</b> and <b>8</b>. However divinyl hemiorthoformate was easily detected in the hydrolysis of <b>6</b> and <b>7</b>. It was concluded that this resulted from the weaker "push" of the vinyloxy group which remained attached to the pro-acyl carbon atom compared to a methoxyl group.