Complex metallacyclopentane

Thermolysis of Cp W(NO)(CH2Bu02 in neat hydrocarbon solutions transiently generates the neopentyli-dene complex, Cp W(NO)(=CHBu ), which subsequently activates solvent C H bonds. The trialkyl complexes Cp W(NBuO(CH2R)3 (R = Me, Et) undergo a similar thermal decomposition to afford the isolable alkyhdene derivatives, Cp W(NBu0(=CHR)(CH2R). Photolysis of the metallacyclopentane complex, p-Bu -calix[4]-(0)4 W(CH2)4 (46), affords the alkylidene complex, / -Bu -calix[4]-(0)4 W(=CHPr) (47, equation 15). The related alkylidene complex,... 

Puddephatt and co-workers (139, 140) also studied the title Pt(IV) metallacyclopentane derivatives (L = PMcaPh or PPhs) under a variety of conditions. Their results and mechanistic conclusions are summarized in Scheme 23 with two competing decay paths (A and B) for the excited molecules. No cyclobutane was formed in either case, in contrast to the production of cyclopropane upon irradiation of the corresponding metal-lacyclobutane complexes. However, as noted above, cyclopropane was a thermal rather than photochemical product. The gaseous photolysis products of the metallacyclopentane complexes were exclusively ethylene and... 

Many crystal structures of nickel-alkene complexes have been reported. As demonstrated in Scheme 55, bis(alkene) complexes may exist in equilibrium with the corresponding metallacyclopentane complex. However, several alkene complexes which have the potential to undergo oxidative cyclization to a metallacycle have been fuUy characterized. The substitution chemistry of bis(iY -cycloocta-l,5-diene)nickel(0) (2) is representative of most nickel(0)-alkene complexes, which are readily substituted by a variety of ligands. Bis(q -ethene)(tricyclohexylphosphine)nickel(0) has been prepared and fully character-ized,l " l and a variety of complexes of electron-deficient alkenes such as 69 have been prepared which tend to be more stable than the complexes of ethene (Scheme 56).l" " The alkene complexes may be prepared directly from bis(q -cycloocta-l,5-diene)nickel(0) (2)l" l or from nickel(ll) chloride " that is reduced by zinc metal. 

Metallacyclopentane complexes can form through the addition of an olefin to an olefin complex. Unsubstituted metallacyclopentane complexes have been observed in solutions of biphenolate complexes under ethylene [47]. One biphenolate complex, W(NArci)(Biphen)(C4Hg) [48], and one disiloxide, Mo(NAr)(C4Hg)(OSiPh3)2 [43], have been structurally characterized. Substituted metallacyclopentanes have not yet been identified. 

The metallacyclopentane complex is more stable than the metallacyclo-heptane complex, as shown by the thermal decomposition temperatures of 151°C and 56°C, respectively, as determined by a DSC method. The metal-lacycloheptane complex decomposes with the liberation of 1 -hexene. 

Wreford and his coworkers [94] have reported that (n-C4H5)2Ti-dmpe (dmpe = l,2-bis(dimethylphosphino)ethane) catalyzes the dimerization of ethylene. A mechanism involving formation of a metallacyclopentane complex is proposed ... 

This metallacyclopentane complex decomposes to give 1-butene. This is not necessarily by the p-elimination sequence. Intermolecular hydrogen transfer and a-elimination have been suggested as alternative decomposition paths leading to 1-butene. The kinetics of the reaction shows first-order dependence on the catalyst and the olefin. Formation of a monoolefin complex is the probable rate determining step. 

Interestingly, the analogous niobium system fails to dimerize ethylene to 1-butene because of its inability to form a metallacyclopentane complex. 

The small but significant quantities of undeuterated and dideuterated ethylene also produced can be explained by the formation and subsequent decomposition of a second metallacyclopentane complex. 

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