Catalysis of Organic..

220 Cavitating Ultrasound HydrogenationSubstrate Conditions M/g-cat. % Sel. ketone k (min. -1 )3B2OL – B1 H 2 /H 2 O – US 67. 16. 2.73B2OL – B2 H 2 /D 2 O - US 67. 9. 4.33B2OL – B3 H 2 /H 2 O – SS 67. 42. 0.00383B2OL – B4 H 2 /D 2 O - SS 67. 32. 0.0103B2OL – B5 D 2 /H 2 O – US 67. 17. 3.33B2OL – B6 D 2 /H 2 O – SS 67. 36. 0.0123B2OL – B7 D 2 /H 2 O – SS 20. 68. 0.03014PD3OL – P1 H 2 /H 2 O – US 100. 17. 3.514PD3OL – P2 H 2 /D 2 O - US 100. 16. 2.314PD3OL – P3 H 2 /H 2 O – SS 100. 28. 0.006814PD3OL – P4 D 2 /H 2 O – US 100. 18. 2.314PD3OL – P5 D 2 /H 2 O – SS 100. 32. 0.007014PD3OL – P6 D 2 /H 2 O – SS 20. 42. 0.042Table 2. Summary of 3-buten-2-ol (3B2OL) and 1,4-pentadien-3-ol (14PD3OL)experiments are given. The abbreviations US and SS are defined as cavitatingultrasound and stirred/silent processing, respectively. The percent selectivity to finalketone plus saturated alcohol sum to 100%.Based on the generally accepted model of olefin hydrogenation of Horiuti andPolayni [14], it is straightforward to postulate the reaction mechanism leading toformation of ketone and saturated alcohol for our two substrates. The proposedreaction mechanism for 3B2OL is given as scheme 1 in the Inert Dopant section. Inscheme 1 it is seen that initially a single H-atom addition to the substrate occursleading to the C3 alkyl radical intermediate. This intermediate, in turn, serves as asource of the two products (i.e., a branching point) in that either a second H-atomadds to the surface bound alkyl radical leading to 2-butanol (via reaction 2), or that aunimolecular C2 H-atom elimination occurs from the substrate to yield the enol thatthen tautomerizes to 2-butanone (via reaction 4). Based on scheme 1 it isstraightforward to predict, at least qualitatively, the effect of H/D isotope substitutionon the various reaction steps. For example, employing D 2 O instead of water as asolvent will result in deuteration of the (substrate) alcohol throughout the reactionsequence. Thus, aside from general solvent effects (which are assumed to affectactivity more than selectivity) for all reaction steps, this scheme predicts thatprimarily the enol tautomerization step will be most effected as the olefin/O-Hversus olefin/O-D bond rearrangement processes can be expected to be mostdifferent. Similarly, when comparing H 2 versus D 2 hydrogenation, the selectivity isexpected to be different since the H-addition step (via reaction 2) may differ betweenH-addition and D-addition, however the unimolecular H-elimination step (viareaction 3) is identical as it remains unchanged upon differing hydrogenation gasesemployed. So here it can be concluded that differences will arise solely fromdifferences in H-addition (via reaction 2).