Neopentyl-type complexes

While neopentyl-type and related cr-alkylpalladium complexes are stable toward dehy-dropalladation, all ordinary alkylpalladium complexes formed by carbopalladation of alkenes do undergo /S-hydride eUminalion more or less rapidly. However, under certain structural precondilions and also, to a certain extent, controlled by the external conditions, rearranganents, further carbopalladations (Sect. IV.3), transmetallations, car-bonylations, or nucleophilic substitutions (e.g., by hydride) can occur prior to /3-dehy-dropalladation. 

Organometallic complexes are one of the most common types of precursors used in catalyst preparation by anchoring/grafting. Among them the allyls and the carbonyls are most used, but other ligands such as methyl, w-butyl, benzyl, neopentyl (a ligands), cyclo-pentadienyl, arene (n ligands) can be also employed. 

Polymerization activity was obtained with a variety of catalyst compositions. The best stereospecific catalyst was the split pretreated type (357) in which one mole of VC14 was reduced by a stoichiometric amount of an alkyl metal (0.34 mole AlEt3) in heptane at room temperature and heated 16 hours at 90° C. to obtain the purple crystalline VC13-1/3 A1C13. This reduced transition metal component was then treated with two moles of (i-Bu)3Al tetrahydrofuran complex for 20 hours at room temperature to obtain a chocolate-brown catalyst consisting predominantly of divalent vanadium with 0.21 Al/V and 1.4 i-Bu/Al. Polymerizations at 30° C. gave crystalline polymers from methyl, ethyl, isopropyl, isobutyl, tert.-butyl, and neopentyl vinyl ethers. 

Thermolysis of ()) -cyclopentadienyl)bis(neopentyl)(tri-methylphosphine)vanadium(I) (82) in the presence of 1,2-bis(dimethylphosphuio)ethane leads to the formation of the Schrock-type see Schrock-type Carbene Complexes) alkylidene vanadium complex (83), which is supported by X-ray crystallographic analysis (Scheme 45). " The imido vanadium complex (84) is converted to the corresponding alkylidene complex (86) on treatment with ben-zylidene(triphenyl)phosphorane (85) via substitution of the phosphine ligand (Scheme 46). ... 

Another type of carbene complex is characterized by a high oxidation state of the central metal and an a-carbon atom that does not usually bear a hetero atom. It is called an alkylidene complex or Schrock-type complex, since R. R. Schrock first synthesized a tantalum complex of this type [14]. Formation of the tantalum carbon double bond is based on the a-elimination reaction of a neopentyl ligand as shown in eq.(2). 

A very active heterogeneous olefin metathesis catalyst was prepared by reaction of a reduced Philipps catalyst with Fischer-type molybdenum or tungsten carbene or carbyne complexes or with Schrock-type carbyne complexes [L3WsC(Bu )j (L = Cl, O(Bu ), neopentyl).Surface species of the type shown in Scheme 2... 

The alkylidene complex, called Schrock type carbene complex, was first isolated in an attempt of synthesis of pentakis(neopentyl)tantalum(V) (Eq. 7.27)... 

Feldman, J., Murdzek, J.S., Davis, WAl., Schrock, RR, Reaction of neopentyl-idene complexes of the type M(Ch-T-Bu)(N-2,6-C6h3-I-Pr2)(or)2(M = W, Mo) with methyl aciylatc and N,N-dimethyIacrylamide to give metallacyclobutane complexes, Organometallics 1989, 8 2260-2265. 

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