Olefin-urea complexes

Separation of mixtures of E- and Z-isomers and nearly quantitative recovery of the individual components has been accomplished by selective formation of E olefin-urea complexes 201). 

MTO [methyltrioxorhenium(VII), cf. Chapter 3.3.13] can be used as a catalyst for the epoxidation of olefins with urea hydroperoxide in [EMIMJBF4 [19]. The activity is reported to be comparable with the reaction in organic solvents but side reactions are suppressed. The use of an ionic liquid as a co-solvent in CH2CI2 for the enantioselective Mn-salen complex-catalyzed epoxidation of olefins with Na(OCl) was reported to result in enhanced reaction rates at no loss of enantioselectivity [20]. Cr-salen complexes can further be used for the asymmetric kinetic resolution of epoxides by ring-opening with azide [21]. 

Another possibility for enhaneing the selectivity toward epoxides is use of the urea-H202 (UHP) adduct. This enables the oxidation to be carried out in water-free solutions, thus avoiding formation of any diols and other side reactions. In the case of the oxidation of chiral allylic alcohols (see below) high diastereo-selectivities have been achieved [3]. The ability to transform olefins to epoxides diastereoselectively seems to indicate that the reaction proceeds through a peracid-like transition state. However, a drawback of the urea-H202 system is the insolubility of the polymeric complex. 

PNA occurs as both E- and Z-rotamers in un-complexed forms, but in duplexes it is uniformly oriented in the Z-form. Schutz et u/. have prepared olefinic PNA (OPA) in which the amide bond is replaced by the isostructural C-C double bond in both E- and Z-configurations (19, 20). Incorporation of either isomer into OPA resulted in a decrease in thermal stability against DNA with similar affinity. Homopolymers of OPA also did not form triplexes. OPA also binds preferentially in the parallel mode. A PNA oligomer to which has been attached diethylenetriamine (DETA) via a urea bond was shown to cleave an RNA target at micromolar concentrations under physiological conditions. ... 

Currently, carbon dioxide is used as a chemical feedstock for the production of carboxylic acids, carbonates, carbon monoxide, and urea (14—16). Despite the fact that numerous chemical reactions utilizing carbon dioxide are thermodynamically advantageous, there is often a substantial kinetic barrier to their occurrence. Transition metal compounds can serve to catalyze reactions of carbon dioxide, i.e., in the utilization of carbon dioxide in synthetic organic chemistry, transition metal complexes can simultaneously activate both carbon dioxide and other substrate molecules such as hydrogen or olefins. 

A method for the polymer supported epoxidation of olefins has been reported <05TL1643>. The resin supported phthalate, 11, was oxidized to peracid 12 through oxidation with or a urea-HjOj complex. The reaction was most conveniently carried out by mixing 11, the urea-HPj complex and the olefinic substrate. Simple filtration then provides a wide variety of epoxides in excellent yield. This reagent system provides all of the typical advantages of supported reagents as well as an improved safety profile of the supported peracid. 

Influence of Catalyst Preparation. Pebrine reported on the influence of the synthesis conditions of HZSM-5 on the selectivity toward light olefins. Synthesizing ZSM-5 in the presence of a tetra-urea-cobalt(II) complex resulted in an ethylene yield of 24.3 wt% of the hydrocarbon fraction at 43.7% methanol conversion, whereas the conventionally prepared ZSM-5 yielded only 18 wt% ethylene at the same conditions and conversion. Heering et al. mentioned that the conversion of dimethylether on ZSM-5 catalysts crystallized from a sodium-free gel with 1,6-dicunino-hexane as organic base was more selective toward both ethylene and propylene than on a conventionally prepared zeolite in the sodium form from a gel containing tetrapropylammonium. 

Dinuclear or polynuclear manganese complexes of salen-type ligands were also tested in the epoxidation of olefins with H2O2 as oxidant. Enantioselective epoxidation of several olefins with urea-H202 and a dinuclear Mn-salen type of complex has been reported by Kureshy and co-workers (130). Conversions of more than 99 % ee were obtained with chromenes and... 

From a practical point of view, isocyanates, together with carbamates and ureas (Chapter 3), are the most important organic products discussed in this book. Their synthesis from nitroarenes has indeed been the subject of many patents. There are also limited examples of aliphatic isocyanates obtained by this route. Organic mono- and diisocyanates may be prepared in a continues liquid phase method by treating the appropriate amine with phosgene. However, the reaction is rather complex [6] and, besides the use of the dangerous phosgene, the formation of the corrosive hydrochloric acid creates several problems. Aliphatic isocyanates can also be obtained from olefins with isocyanate ion in the presence of a salt of a coordination compound of palladium or platinum [7], from olefins with isocyanic acid in the vapour phase over Pt/ALOs [8], and from formamides, by oxidation over a silver catalyst [9]. Apparently only the last reaction seems to have some potential practical applications [10]. 

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