Transition metal, tetrakis borates

These rod-shaped ligands share a sterically efficient terminal N-donor and their divalent Co chemistry is well established. They will be discussed here only with selected examples. [Co (NCMe)6](TFPB)2 (TFPB- = tetrakis(3,5-bis(trifhioromethyl)phenyl)borate)) has been synthesized and characterized in the solid state along with a number of other divalent transition metal analogs.357 As a result of the extremely poor coordinating ability of the anion and facile loss of MeCN ligands from the cation, the salt is an excellent source of naked Co2+ ions. Thermolysis up to 100 °C leads to the loss of one MeCN and formation of a r -bound nitrile, whereas above 130 °C decomposition occurs with loss of MeCN and abstraction of fluoride from the anion to form CoF2. 

F. BIS[TETRAKIS(l-PYRAZOLYL)BORATO]MANGANESE(II)— TRANSITION-METAL POLY(l-PYRAZOLYL)BORATES... 

Dialkyl metallocenes and other dialkyl Group 4 transition metal complexes are useful as precatalysts in combination with co-catalysts such as tris(perfluoro-aryl)boranes or tetrakis(periluoroaryl)borate salts [18]. Recently, an expedient procedure for the production of dimethyl metallocenes and Cp-amido dimethyl metal complexes in high yields and purity has been reported. The direct synthesis of Group 4 dimethylmetallocenes [19] consists of the one-pot reaction between the r-ligand, a 2-fold excess of MeLi, and MtCU. This simple method produces the dimethylated complexes in higher overall yield, and saves on reaction time and solvents, compared to the classic two-step route, which consists in the synthesis of the metallocene dichloride followed by its methylation with 2 equiv. MeLi. 

Indeed, Ishihara and co-workers succeeded in the first preparation of sPS through activation of a transition metal complex with MAO (36,37). Typically, Group IV metallocene complexes have been used as catalysts for the polymerization of sPS. Of these, the monocyclopentadienyl-type complexes of titanium have been found to give the highest pol5nnerization activity based on transition metal (38,39). Subsequent to the development of MAO as the sPS cocatalyst, it has been foimd that highly electrophilic activators, such as the tetrakis(pentafluorophenyl) borate type, can be used as cocatalysts for the production of sPS (40,41). 

Transition metal-catalysed methods for carbenoid insertion into C-H bonds remain well documented. The asymmetric intramolecular Cu(II)-catalysed C-H insertion reactions of (i) a-diazo-/ -keto esters and phosphonates and (ii) a-diazo sulfones have been described. One can note that the optimal reaction conditions have been found to be quite similar regardless of the nature of the carbenoid precursor the best conditions featured CUCI2 as Cu(II)-source, bis(oxazoline) (68) as chiral ligand and sodium tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (i.e., NaBARF) as additive. Under the so-optimized reaction conditions, each of these carbenoid sources have been eonverted into five-membered cyclopentanone-based derivatives (69), whereas a-sulfonyl diazo esters (70) have led to six-membered cyclic compounds (71), thus featuring a distinct but well-known selectivity. In a related work, the asymmetric C-H insertion cyclization of (70) to (71) has also been achieved under Rh(II)-catalysis, using a combination of Rh2(5-pttl)4 (72) as chiral catalyst and menthyl ester as chiral auxiliary. As already mentioned in the previous section, allene-containing substrates (49) have been shown to undergo an intramolecular C-H insertion process under Rh(II)-catalysis. ... 

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