Catalysis of Organic..

Disselkamp et al. 221A somewhat more involved reaction scheme is required for 14PD3OLhydrogenation, and is illustrated in Scheme 3 below.Scheme 3.In Scheme 3 for 14PD3OL, since this substrate is a di-olefin, there ishydrogenation (H-atom addition via reaction 1) to the first C4 alkyl radicalintermediate that serves as the first branching point for either further hydrogenation(reaction 2) or enol formation (reaction 5). The latter enol, again, is expected toundergo tautomerization to the potentially stable intermediate 1-penten-3-one(reaction 7) and from here eventually hydrogenate to 3-pentanone (reaction 9).Alternatively, the intermediate 1-penten-3-ol can gain a second H-atom (reaction 3)to a second C2 surface-bound alkyl radical and serve as a second branching point foreither additional hydrogenation to the saturated alcohol (3-pentanol, reaction 4), oranother unimolecular H-elimination process (reaction 6) to the enol and (via reaction8) form 3-pentanone. Thus because there are two surface bound alkyl radicals (i.e.,two branching points), there are two opportunities for 3-pentanone formation,whereas scheme 1 above for 3B2OL only had one opportunity for ketone formation.Some general observations and tentative conclusions can be made of the resultsof Table 2. First, the activity of the US compared to SS processing were at least 250-fold larger (e.g., compare for example B1/B3, B2/B4, or B5/B6). Second, variablecatalyst loading experiments for stirred/silent D 2 hydrogenation processing (e.g.,compare B6/B7 or P5/P6) indicated that mass transfer of deuterium gas to the Pdsurfacemay have played a role such that higher catalyst loading reduced surface D-atom concentrations and reduced saturated alcohol formation (e.g., via perhapsreduced D-atom addition to surface alkyl radicals). Third, for US processing theketone selectivities for experiments employing water compared to D 2 O indicated that3B2OL were twice as large (e.g., compare B1/B2), whereas for 14PD3OL they were