Propylene oxide oxidation— kinetic scheme

Based on the landmark studies of Jacobsen and coworkers, who employed chiral (salen)CoX complexes for the asymmetric ring opening and kinetic resolution of aliphatic epoxides [18-20], Lu and coworkers synthesized highly isotactic copolymer from rac-propylene oxide and carbon dioxide (Scheme 5) [21]. 

Scheme 5 Kinetic resolution of rac-propylene oxide during its copol3anerization with CO2 in the presence of a chiral (salen)CoX complex...

In the presence of chiral polymerization catalysts, enantiomeric monomers are consumed at different rates (Scheme 75). Enantiomer-selective polymerization of racemic propylene oxide catalyzed by a diethylzinc-(-f)-bomeol system is a classical example of such kinetic resolution H 176). The polymeric product has an [a]D of +7.4°. The mechanism... 

From a few well chosen experiments in an integral reactor of technical dimensions with side-stream analysis both reaction schemes and the effective heat transfer and kinetic parameters of a reaction model for propylene oxidation could be deduced, from which valuable information for both catalyst development and optimization of the reaction conditions could be obtained. 

The Co(salen) catalyst is remarkably insensitive to the steric properties of terminal epoxide substrates, as substituents ranging from methyl to cyclohexyl to ferf-butyl groups are accommodated in the kinetic resolution. Propylene oxide presented an impressive illustration of catalyst enantiocontrol, where a kj i exceeding 400 was estimated for this substrate. This epoxide served to further emphasize the synthetic utiHty of this process, as the HKR of 1 mole of propylene oxide proceeded efficiently with catalyst that had been recycled from previous kinetic resolutions (Scheme 18) The HKR of propylene oxide has also been effected on a multi-hundred kilogram scale in the pilot plant at ChiRex. 

In the earlier kinetic scheme, we did not consider the asymmetric characteristic of the PO molecule. Propylene oxide could generate a primary or secondary alcohol when ring opening occurs as a consequence of a nucleophilic attack on the methylene (a) or methyne group (b),... 

SCHEME 24.7 Attempted kinetic resolution of racemic propylene oxide using 4. 

Tsuruta found that the optically pure complex [(/ -salcy)Co] (15) was active for epoxide polymerization (Scheme 24.15) when activated with ALEt3. Although the system exhibited no enantioselectivity for the polymerization of propylene oxide, it was moderately selective (r = 1,5) for the kinetic resolutions of tert-butyl ethylene oxide and epichlorohydrin (Scheme 24.15). 

In this paper, we describe a computational study of the cracking reactions of propane and the production of olefins, especially propene, as a preUminary step in our research of gas-phase production of propylene oxide. The purpose of the smdy is twofold. On the one hand, we aim to compare different computational schemes applied to a subset of reactions for which experimental data exist On the other hand, we want to obtain precise estimates of the thermochemistry and kinetics of the radical chain initiation, propagation and termination reactions involved in the mechanism. Previous computational and experimental studies on this area of research have been performed by several authors, which results we will use to compare to our own. 

Rincon-Rubio, L.M., Fayolle, B., Audouin, L., Verdu, J. A general solution of the closed-loop kinetic scheme for the thermal oxidation of pol) propylene. Polym. Degrad. Stab. 74, 177-188 (2001)... 

Oxidation of propylene to form acrolein depends on the first order of propylene and is independent of oxygen on multicomponent bismuth molybdate catalysts under the usual reaction conditions. The observed kinetics is the same with simple bismuth molybdates and suggests that the oxidation of propylene proceeds via the similar reaction scheme reported for simple molybdates, the slow step being the abstraction of allylic hydrogen (9-15, 19, 20). However, the reaction sometimes depends on the partial pressure of oxygen under lower temperature and lower oxygen pressure (41, 42). 

Heterogeneous Zn tartrate catalyzes nucleophilic ring opening by thiols of meso cyclohexene oxide in up to 85% optical yield (Scheme 6) (14). Racemic propylene sulfide may be resolved kinetically (30% optical yield) by using Cd tartrate (15). 

The calculated values are those obtained by assuming the ratedetermining step to be abstraction of an allylic hydrogen with an isotope effect equal to the discrimination effect obtained from the retention of deuterium in the products. The observed kinetic isotope effects show clearly that this first abstraction is the slow step. The good agreement with the discrimination values gives excellent confirmation for the stepwise mechanistic scheme proposed. In the tracer experiments using deuterium it was found that appreciable propylene isomerization occurred unless ammonia was present. The oxidation rate was unaffected by the ammonia. This was taken to indicate that the initial oxidative abstraction reaction had no carbonium ion characteristic. However, this conclusion does not apply in the case of cuprous oxide, where ammonia very severely inhibited the oxidation reaction. 

In recent years the nonsteady state mode has been used to an increasing extent because it permits accessing intermediate steps of the overall reaction. Very complete reviews of this topic are presented by Mills and Lerou [1993] and by Keil [2001]. Specific reactors have been developed for transient studies of catalytic reaction schemes and kinetics. One example is the TAP-reactor ( Transient Analysis of Products ) that is linked to a quadrupole mass spectrometer for on line analysis of the response to an inlet pulse of the reactants. The TAP reactor was introduced by Cleaves et al. in 1968 and commercialized in the early nineties. An example of appUcation to the oxidation of o.xylene into phthalic anhydride was published by Creten et al. [1997], to the oxidation of methanol into formaldehyde by Lafyatis et al. [1994], to the oxidation of propylene into acroleine by Creten et al. [1995] and to the catalytic cracking of methylcyclohexane by Fierro et al. [2001], Stopped flow experimentation is another efficient technique for the study of very fast reactions completed in the microsecond range, encountered in protein chemistry, e.g., in relaxation techniques an equilibrium state is perturbed and its recovery is followed on line. Sophisticated commercial equipment has been developed for these techniques. 

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