Boron fluoride, for catalysis

Boron fluoride, for catalysis, 1 23 compound with hexamethyldi-silazane, 5 58 high-purity, 1 2l Boron fluoride-trimethylamine, 5 26... 

It might be thought that the question whether a particular alkyl halide is a co-catalyst for a particular monomer-catalyst combination could be settled easily, by adding some of the compound in question to a non-reacting mixture of monomer and catalyst. This approach has been used [36, 44], but it must be carried out in a polar solvent which is itself not a co-catalyst, or only a very weak one. The ideal solvent for this kind of work remains to be found it may be that S02 or even CS2 (which behaves like a polar solvent) will provide the answer. If one wants to use alkyl chloride solvents without being troubled by the possibility of solvent co-catalysis, boron fluoride should be used as catalyst, since the ion BF3C1 is not formed under the conditions generally used for polymerisations. 

The first study with an oxygen compound which was sufficiently rigorous to provide evidence on the question of co-catalysis was that of Farthing and Reynolds [61]. They showed that 3,3-bischloromethyl oxetan could be polymerised in methyl chloride solution by boron fluoride only in the presence of water. Tater, Rose [62] obtained kinetic evidence for the need for a co-catalyst in the system oxetan—boron fluoride—methyl chloride, and he interpreted the low reaction rate when no water was added as due to residual water he also showed that water and a hydroxyl-terminated polymer could act as co-catalysts. 

At the time this mechanism was proposed it was considered that boron fluoride catalysis showed rather similar kinetics and activation energy, differing from the stannic chloride only in that chain transfer could occur but termination apparently was very slow or absent (13). Subsequent work with boron fluoride suggests however (14) that the apparent fractional order dependence on monomer may be spurious and thus tends to discredit the data for stannic chloride in spite of the fact that the rate expressions give an excellent account of the rate of disappearance of epoxide both within and between the individual experiments. The difference in behaviour between stannic chloride und boron fluoride remains one of the unsolved problems in this field. 

Triflates of aluminum, gallium and boron, which are readily available by the reaction of the corresponding chlorides with triflic acid, are effective Fnedel-Crafis catalysis for alkylation and acylation of aromatic compounds [119, 120] Thus alkylation of toluene with various alkyl halides m the presence of these catalysts proceeds rapidly at room temperature 111 methylene chloride or ni-tromethane Favorable properties of the triflates in comparison with the correspond mg fluorides or chlorides are considerably decreased volatility and higher catalytic activity [120]... 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

The related direct oxidation of trialkylboranes has been studied 178), as well as the brominolysis and iodinolysis of benzeneboronic acid in aqueous acetic acid (50%) and nt-chlorobenzeneboronic acid in aqueous solution. The latter reveals a pattern involving tetracovalent boron 179). In water solutions, and acetate buffers at constant ionic strength with the w-chloro derivative, plots of log k vs. pH were linear with unit slope (from pH 2 to 5), suggesting specific lyate ion catalysis. In both cases catalysis by fluoride ion was observable, and catalysis by hydroxy acids or diols, which form coordination complexes with boron, was seen in the latter case. Indeed in water solvent, the catalytic constant for fluoride ion is some 6000 times that of the uncatalyzed case. 

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