Chromium complexes halogens

Addition of halogens and pseudohalogens to the cyclopropylthiocarbene chromium complexes 122 affords the 1,4-dihalo-1-phenylthio-l-alkenes 123 stereoselectively [65]. Electrophilic halogen is likely to activate the carbene complexes, followed by the homo-Michael addition of halide anion. (Scheme 44)... 

Full details have now appeared of the stereoselective synthesis of 1,5,9-triphosphacyclododecane systems by oxidative liberation from molybdenum and chromium complexes of the macrocycle, obtained by coordination-template controlled reactionsMolybdenum complexes have also been used in the coordination-template dependent synthesis of the macrocyclic P,S system 151. A non-template synthesis of the 14-membered P2S2 macrocycle, 152, isolated in two isomeric forms, has been described. Halogenation of the cyclometallated phosphine 153 leads to a rearrangement, with the formation of the di-phosphinobiphenyl system 154, from which the free diphosphine can be liberated... 

The decomposition of mononuclear chromium complexes with HX leads to the formation of halogen-allyl compounds ... 

The chromium complexes are CrCl3L3 and CrCl2L2(NO)2, wherein the ligand L are pyridine and tri-n-butylphosphine in conjunction with ethylaluminum dichloride, effect simple dimerization of ethylene at 50°C [133-135]. A conversion of 4700 g butenes per g of chromium complex is achieved with the catalyst CxCX i -Etpy),. The butene fraction consists of 1-butene (50%), trans-2-butene (32%), ds-2-butene (18%), and isobutene (0.1%). Cr or Cr species may be involved in the reaction. Here chromium atoms are probably associated with the organoalu-minum halides to form bridged chromium-halogen-aluminum species. 

Similar investigations have been carried out and similar conclusions reached with the reaction of chromium(ii) complexes with alkyl halides (Castro, 1963 Kochi and Davis, 1964 Kochi and Mocadlo, 1966 Kray and Castro, 1964). The main argument in favour of the halogen-atom-transfer mechanism in this case was the order of reactivity of the halides tertiary > secondary > primary. 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

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