Tetrafluoroborate complexes

Manufacture of rhodium precatalysts for asymmetric hydrogenation. Established literature methods used to make the Rh-DuPhos complexes consisted of converting (1,5-cyclooctadiene) acetylacetonato Rh(l) into the sparingly soluble bis(l,5-cyclooctadiene) Rh(l) tetrafluoroborate complex which then reacts with the diphosphine ligand to provide the precatalyst complex in solution. Addition of an anti-solvent results in precipitation of the desired product. Although this method worked well with a variety of diphosphines, yields were modest and more importantly the product form was variable. The different physical forms performed equally as well in hydrogenation reactions but had different shelf-life and air stability. 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

Similar acetone complexes can also be prepared from the PPh3 containing tetrafluoroborate complexes. 

Tris(2,3-butanedione dihydrazone)metal bis(tetrafluoroborate) complexes, [M(C4H10N4)3] [BF4]2, are prepared by dissolving 2.00 g (0.0175 mole) of 2,3-butanedione dihydrazone in 100 mL of hot absolute ethanol and adding a solution of 2.15 g (0.00586 mole) of the appropriate metal(II) tetrafluoro-borate hexahydrate salt (or 41% aqueous solution in the case of commercially available Fe(BF4)2) in 30 mL of absolute ethanol. After the solution is cooled, the product is filtered on a Buchner funnel, washed with absolute ethanol, and dried in vacuo. Yields range from 70 to 95%. All these complexes, except those of iron(II), slowly decompose when stored under ordinary conditions (i.e., at room temperature, bottled in air) and should be used within a week of preparation. 

The clathrochelate complexes, [M(ClgH30Ni2)] [BF4]2, are prepared by adding 5.0 g of 37% aqueous formaldehyde solution to a suspension of 5 g of the appropriate tris(2,3-butanedione dihydrazone)metal bis(tetrafluoroborate) complex in acetonitrile. The condensation reaction is catalyzed by the addition of 0.5 mL of concentrated tetrafluoroboric acid. An immediate darkening of the solutions is observed as the condensation reaction proceeds. The iron (II)-containing solution becomes a very intense purple the nickel(II) solution becomes very dark olive-brown. Precipitation of the clathrochelates requires the addition of approximately 30% by volume of diethyl ether and refrigeration. The products are filtered from the solution, washed with methanol, and air dried. The iron(II) complex is somewhat more soluble in acetonitrile than the other complexes, and increased yields can be obtained by the dropwise addition... 

During the following 15 years, only small advances were achieved in increasing catalyst efficiencies. Independently, Fenton [9a] at Union Oil and Nozaki [9b] at Shell Development Company (USA) discovered several related palladium chlorides, palladium cyanides, and zero-valent palladium complexes as catalysts. Sen and co-workers [10] reported that cationic bis(triphenyl-phosphine)-palladium tetrafluoroborate complexes in aprotic solvents such as dichloromethane, produced ethylene/carbon monoxide copolymers under very mild conditions. The reaction rates were, however, very low, as were the molecular weights. 

This ligand coupling method was quite efficient for acetophenone and rerf-butylketone derivatives. However, coupling of the TMS ether of cyclohexanone (121) failed. The oxidative coupling of the TMS ether of cyclohexanone (121) to give 2,2 -bicyclohexanone was successful only when this silyl enol ether was treated with the iodosobenzene-tetrafluoroborate complex.230,231... 

There are, however, indications that electronic effects may in some cases control the carbopalladation step. For example, the reaction of methyl 3-phenylpropynoate with an ionic cyclopalladated azobenzene tetrafluoroborate complex gives rise to the formation of the 2,4-diphenyl isomer of the cinnolinium salF (Scheme 12). In this case, the conjugating effect of the ester group apparently exerts more influence in directing the carbopalladation than the phenyl does. 

Rhodium(I) bis(l,5-cyclooctadiene) tetrafluoroborate complex [bis(l,5-cyclooctadiene)rhodium(I) tertra-... 

In toluene the activity and selectivity of these Ru-allenylidene catalysts is dependant on Ihe nature of the anion (Scheme 6). The triflate anion provides the best activity and selectivity for the RCM product a, whereas the tetrafluoroborate complex is less active and gives a mixture of the three products a, b, and c. 

The tetrafluoroborate complex, [Cr(BF4)(NO)2(T -Cp)], is converted into the more stable salt, [Cr(NO)2(MeCN)(Ti5-Cp)][BF4L on treatment with MeCN. 5 Similarly, the interaction of the isoelectroniccomplex, [Fe(CO)2(Ti5-Cp)l2, with p-toluenesulphonic add in acetonitrile leads to oxidative deavage of the Fe-Fe bond and formation of the acetonitrile solvento-complex, [Fe(CO)2(MeCN)(Ti5-Cp)][C)3SC6H4Me].8 For this reaction it was reported that the protic add was responsible for oxidation of the complex. However, the evolution of molecular hydrogen was not observed. 

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