Cobalt complexes pentadienyl

Organocobalt complexes catalyze the cyelocotrimerization of acetylenes and nitriles, which affords pyridine and benzene derivatives (100). (Cyclo-pentadienyl)cobalt complexes such as CoCp(COD) favor pyridine formation (100), and modification of the Cp ligand has considerable influence on the activity of the catalyst and the chemo- and regioselectivity of the catalytic process (101). 

Protonation of (norbornadiene)cobalt complex 43 induces C-C bond cleavage of the norbornenyl ring to form cationic complex 44 [63,64]. Re-protonation of the reduced complex 45 induces a second cleavage of a non-strained cyclopen-tene ring to give an open r)5-pentadienyl complex 46. It is postulated that a three-center interaction of the highly electrophilic metal center with the a-electrons of the adjacent C-C bond is involved. 

The conjugation of glutamic acid to a 5 -aminohexyl-modified ODN has been used to prepare copper ion-directed triple helices. Using ODNs with a twofold axis of symmetry, the Glu-modified pyrimidine strand was shown to form a symmetrical triplex in the presence of Cu. Tetraphenylcyclobutadiene(cyclo-pentadienyl)cobalt complexes (170) and phenylene-ethynylene trimers (171)... 

A different skeletal rearrangement reaction of norbornadiene takes place on cobalt (Scheme 7.7) [9]. Protonation of the (norbornadiene)cobalt complex 19 induced C-C bond cleavage of the norbornenyl ring to form the cationic aUyl-cobalt complex 20. Reduction with LiBHEtj formed the neutral diene complex 21, which undergoes further C-C bond cleavage upon treatment with acid to give the T -pentadienyl complex 22. 

The Overman esterification is a palladium-catalyzed formation of enantioenriched allyl esters from carboxylic esters with primary allyl imidates. From a mechanistic point of view, it is related to the Tsuji-Trost reaction. It proceeds under quite mild conditions with high enantiomerically access if the COP ([Ti -(5)-2-(4-methylethyl)-oxazolinylcyclo-pentadienyl]-(T] -tetraphenylcyclobutadiene)cobalt) complex is used. For example, Kirsch and co-workers used the Pd-catalyzed Overman esterification " in their approach to 1,3-polyols starting from (Z)-allylic trichloroacetimidates to build up the stereogenic centers. By choice of the required enantiomer of COP-OAc catalyst, every possible diastereoisomer is accessible in high stereoselectivity (Experimental Procedure below). 

The catalyst-free 5-1-1-cycloaddition reaction between C, A(-cyclic A -acyl azome-thine imines (134) with isocyanides (135) formed imin-l,3,4-oxadiazin-6-one derivatives (136) in high to excellent yields (99%) under mild conditions (Scheme 42). The cobalt-mediated / -pentadienyl (137)/alkyne (138) 5-1-2-cycloaddition reaction yielded /, -cycloheptadienyl complexes (139) under kinetic control. The initial cycloadducts are isomerized to the final >/ -cycloheptadienyl complexes (140) (Scheme 43). The Ru(I)-catalysed intra- and intermolecular 5 4- 2-cycloaddition reactions of 3-acyloxy-l,4-enynes and alkynes produced adducts with highly functionalized seven-membered rings. The reaction sequence includes a 1,2-acyloxy migration. The catalyst-free 5 4- 2-cycloaddition reaction of 2-vinylaziridines with sulfonyl isocyanates in CH2CI2 yielded seven-membered cyclic ureas in high yields (up to 90%). ... 

This chapter includes those transition metal-pentadienyl cationic complexes that are quite stable, can be stored and handled easily, and are therefore useful as stoichiometric intermediates for organic synthesis. The dienyliron systems, which are readily available and inexpensive, have dominated this area of chemistry, and will occupy the larger part of the discussion. The chemistry of more expensive and less easily prepared dienylmetal complexes, such as those of manganese and cobalt, will be dealt with at the end of the chapter. 

Titanosilicates have been synthesized which have the large pore structure of UTD-1. These molecular sieves were prepared using the metal complex Cp 2Co (bis(pentamethylcyclo-pentadienyl)cobalt(III) ion) as the template. Ti-UTD-1 has been found to be an effective catalyst for the oxidation of alkanes, alkenes and phenols using hydrogen peroxide as well as the larger oxidant t-butylhydroperoxide. The channel structure defined by 14 membered rings in Ti-UTD-1 also allows the conversion of larger substrates such as 2,6-di-t-butylphenol. 

Dienes as well as cyclobutadienes may undergo exchange reactions between various complexes [equation (8.42)]. During flash vacuum pyrolysis of cyclo-pentadienyl(l,l-dioxo-2,5-dimethylthiophene)cobalt(I), SO2 elimination takes place and... 

Metallicenium salts contain the electron-rich medium transition metals iron or cobalt as central metal atoms M which are, in contrast to the early transition metals in the metallocene diacido complexes, coordinatively saturated when bound to two T) -cyclo-pentadienyl ligands. 

---