The competitive and non-competitive hydroformylation of conjugated dienes starting with tetrarhodium dodecacarbonyl. An in-situ high-pressure infrared spectroscopic study

It is well known that the liquid-phase homogeneous unmodified rhodium catal ysed hydroformylation of alkenes is poisoned by the presence of trace quant ities of conjugated dienes. Nevertheless, some hydroformylation of conjugat ed dienes is possible with unmodified rhodium, and this reaction is in gene ral slower than alkene hydroformylations at comparable reaction conditions. In the present contribution, we examined (A) the catalytic behaviour of al kenes in the presence of trace conjugated diene impurities and (B) the cata lytic behaviour of a variety of dienes using Rh-4(CO)(12) in n-hexane solve nt at 293 K under 1.0-4.0 MPa CO and 0.5-2.0 MPa H-2. The analytic method w as in-situ high-pressure infrared spectroscopy. It was observed that (I) in the hydroformylation of poisoned alkenes, most of the rhodium reacts with the trace quantity of conjugated dienes and not the alkenes in this competi tive situation and (II) the metal carbonyl spectra of the hydroformylation of a variety of dienes are very similar. The primary absorbance maxima obse rved in the hydroformylations of conjugated dienes occur at circa 2109, 209 1, 2087, 2064, 2049, 2037, 2030, 2020, 2012, 1999, and 1990 cm(-1). Given t he known chemistry of Rh-4(CO)(12) under syngas, and the very well document ed chemistry of Rh-4(CO)(12) under alkene hydroformylation conditions, the lack of bridging carbonyls in the present experiments strongly suggested th at the new infrared vibrations are due to mononuclear rhodium species. Prel iminary analysis suggests the presence of at least three new species. In pa rticular, the formation of observable eta(3) allyl rhodium tricarbonyl spec ies, sigma allyl rhodium tetracarbonyl species and even acyl rhodium tetrac arbonyl species RCORh(CO)(4) (R = alkenyl and/or formylalkyl) seems probabl e. Characteristic wavenumbers of 2108, 2064, 2037, 2020 and 1700 cm(-1) are tentatively assigned to the latter. The reduced hydroformylation activity in the competitive hydroformylation of alkenes arises due to the much highe r affinity of rhodium complexes for conjugated dienes than for alkenes unde r otherwise similar reaction conditions. (C) 2000 Elsevier Science S.A. All rights reserved.