Abstract
PdX2L2/L/HA (A = weakly coordinating anion, L = phosphine) complexes are active catalysts in the hydroesterification of alkenes, alkynes, and conjugated dienes. Shell, the only major corporate player in the field, recently developed two very active catalyst systems tailored to the hydroesterification of either alkenes or alkynes. The hydroesterification of propyne with their Pd(OAc)2/PN/HA (PN = (2-pyridyl)diphenylphosphine, HA = strong acid with weakly coordinating anion, like methanesulfonic acid) catalyst has been declared commercially ready. However, despite the significant progress in the activity of Pd-hydroesterification catalysts, further improvements are warranted. Thus, for example, activity maintenance still seems to be an issue. Homogeneous Pd catalysts are prone to a number of deactivation reactions. Activity and stability promoters are often corrosive and add to the complexity of the system, making it less attractive. Nonetheless, the versatility of the process and its tolerance toward the functional groups of substrates should appeal especially to the makers of specialty products. Although hydroesterification yields esters from alkenes, alkynes, and dienes in fewer steps than hydroformylation does, the latter has some advantages at the current state of the art. (1) Hydroformylation catalysts, particularly some recently published phosphine-modified Rh systems, can achieve very high regioselectivity for the linear product that hydroesterification catalysts cannot match yet. By analogy with hydroformylation, bulkier ligands ought to be tested in hydroesterification to increase normal-ester selectivity. (2) Hydroformylation is proven, commercial. Hydroesterification can only replace it if it can provide significant economic incentives. Similar or just marginally better performance could not justify the cost of development of a new technology. (3) Hydroesterification requires pure CO while hydroformylation uses syngas, a mixture of CO and H2. The latter is typically more available and less expensive (for industrial applications CO is most often separated from syngas). (4) The acid component of the hydroesterification catalyst makes the process corrosive. It would be desirable to develop new hydroesterification catalysts that do not require acid stabilizer/activity booster. Clearly, any new hydroesterification technology will directly compete with the hydroformylation route. This is especially true for olefin feeds, since both processes add one CO to the olefin, yielding oxygenates that can be converted into identical products. For some niche applications, like the production of MMA from propyne, hydroesterification seems to have an advantage as compared to hydroformylation due to the high activity and selectivity of the Pd(OAc)2/(2-pyridyl)diphenylphosphine catalyst. Since hydroesterification is an emerging technology, it is reasonable to assume that the potential for improvement is greater than in the mature hydroformylation. It is therefore possible that hydroesterification will become competitive in the future; thus, continued effort in the field is warranted.
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