Butadiene complexes, stereospecific

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such cata-lysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of 7r-allyl compounds with Si02 and AUOj were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial ir-allyl compounds. n-Allyl complexes of Mo and W supported on silica were found to be active in olefin disproportionation (142a). 

As discussed in connection with olefin-coupling reactions and shown in Fig. 4, the coupling of vinyl Grignard reagents is stereospecific and dependent upon the transition metal catalyst used (32, 33). The dimerization of ethylene, shown in Fig. 6, was also shown to produce primarily the terminal olefin 1-butene (35). The size of the metal has also been shown to influence the course of the catalyzed oligomerization reactions of butadiene. When bis-(ir-allyl) metal complexes are used as... 

Scheme 51 Stereospecific butadiene polymerization based on a MMAO-activated neodymium methyl complex supported by a dianionic modification of neutral 2,6-diimino-pyridine [190]...

Stereospecific emulsion polymerization of butadiene has been achieved in the presence of soluble transition metal salts 350, 351). Polymer microstructure was controlled by varying the transition metal ion and its ligands. Although the initiation mechanism has not been determined, it is most likely to be of the coordinated radical type with steric control arising from the transition metal-diene complexes. 

The 7r-allylic bond is stable against hydrolysis in absence of oxygen (3), and thus the butadiene polymerization initiated by the catalysts mentioned above can be carried out in water. The nature of the halide in bis(7r-crotylnickel halide) affects the stereospecificity and reactivity of the complex in emulsion polymerization in the same way as in hydrocarbon media (cf., Table II). 

The interaction of bis(7r-crotylnickel halides) and Lewis acids (B, Al, Ti, V, Mo, W halides, etc.) results in formation of products which are insoluble in hydrocarbons. The compounds initiate stereospecific butadiene polymerization to polymers whose structure is unaffected by the nature of the halide in the initial complex or by the nature of metal in the Lewis acids. The reactivity of the catalysts is higher than that of corresponding bis(7r-crotylnickel halides) (cf., Table III). 

Substitution of complexed dienols (244) or dienol acetates with carbon or heteroatom nucleophiles, in the presence of a Lewis acid, occurs with retention of configuration (Scheme 69). (Alkyl aluminum reagents act as both nucleophile and Lewis acid in this process). This reaction is believed to proceed via stereospecific ionization, with anchimeric assistance from the iron, to generate the transoid pentadienyl cation (247) followed by attack of the weak nucleophile on the face opposite to iron. The cross-conjugated pentadienyl cation can also be generated the substitution of (2-acetoxymethyl-l,3-butadiene)Fe(CO)3 (193) has previously been discussed (Section 6.1.1). 

Recent developments in catalysis at the atomic level are described. The use of transition metal halides which form intermediate metal n-complexes can promote both the catalytic and stoichiometric coupling of aryl Grignard reagents, the stereospecific coupling of 1,2 disubstituted vinyl groups and the double coupling of ethylene to dimers of butadiene. 

Allyl complexes of cobalt in the oxidation states III and I have been identified as stmcturally defined catalysts for stereospecific butadiene polymerization. 

The synthesis and characterization of anionic allylneodymium(m) complexes and their use as catalysts for the stereospecific polymerization of butadiene have been reported. For the already known tetrakis(allyl) complex Li[Nd(7]3-C3Hs)4]-1.5dioxane a significantly improved method of preparation from anhydrous NdCl3 and LiC3HS dioxane in DME was found (Scheme 29).182... 

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