Catalysis of Organic..

390 Carbonylation of ChloropinacoloneIncluded in Table 3 (entries 5-9) is a survey of the effect of the tricyclohexylphosphine level. Unlike the earlier screening runs, these highly concentrated, lowcatalyst level reactions demonstrated clearly discernible rate and selectivity effectswith respect to the phosphine:Pd ratio with the optimal rate being observed at a ratio(cy-hex 3 P):Pd ratio of ca. 7:1. Selectivity improvements increased consistently withrising phosphine levels. The presence of a rate optimum is consistent with originalexpectations for a carbonylation process requiring stabilizing ligands. In commercialpractice, a slightly longer reaction time would be acceptable if it leads to improvedselectivity since this approach conserves raw materials. As a consequence, theprocess would normally be operated at the higher phosphine levels. In the fastestoperations (entries 3,4,7-9, Table 3), the material accountability remains high (92-96% without accounting for impurities in the chloropinacolone).Table 3. Increasing Rate and Turnover Number using Reduced MethanolConcentrations to Attain High Turnover Process aSelectivityEntryPd(mmol)Cy-hex 3 P(mmol) bP(atm) Conversion MPA PA TON1 0.10 0.40 5.4 99% 86% 3.0% 21302 0.05 0.30 5.4 66% 91% 2.5% 30303 0.019 0.138 5.4 78% 91% 3.2% 94504 0.0095 0.069 5.4 47% 77% 2.6% 96205 0.019 0.038 8.5 73% 70% 2.0% 67606 0.019 0.088 8.5 84% 79% 3.0% 64107 0.019 0.138 8.5 89% 88% 3.7% 102808 0.019 0.188 8.5 77% 88% 2.8% 90209 0.019 0.238 8.5 74% 91% 2.4% 8970a Conditions: 33.0 mL (0.251 mol) chloropinacolone, 90 mL (0.378 mol) Bu 3 N, 31.0mL (0.766 mol) MeOH, 120°C, 3 h; catalyst: (cy-hex 3 P) 2 PdCl 2 + cy-hex 3 P; COpressure as indicated.b Includes tricyclohexyl phosphine contained in the (cy-hex 3 P) 2 PdCl 2 catalyst.As indicated earlier, the effect of CO pressure is often unpredictable incarbonylations. To optimize this process, the effect of CO pressure was measured at120°C and 130°C and the results appear in Table 4. With these highly active catalystsystems, there appeared to be an optimum CO pressure and excess CO pressures wasdeleterious to the reaction. While the presence of CO optima is not unknown incarbonylation chemistry, it is normally observed at significantly higher COpressures. It is likely that the optimum observed in this study represented thetransition from a mass transfer limited reaction to a chemically limited reaction. (Thecombination of a phosphine optima and rate reductions with increased CO likelyindicate a rate determining dissociative process along the reaction pathway.)Comparing data in Table 4 at 120°C (entries 1-4) with data at 130°C (entries 5-7) indicates that a decrease in selectivity occurred as the temperature was raised