Acetonitrile exchange reactions

Allylic Sn(II) species are believed to be involved in reactions of allylic trialkyl stannanes in the presence of SnCl2. These reactions are particularly effective in acetonitrile, which appears to promote the exchange reaction. Ketones as well as aldehydes are reactive under these conditions.171... 

Rate constants for complex formation between Mn2+, Co2+, Ni2+, Cu2+ and Zn2+ and 5,10,15,20-tetraphenylporphyrin in acetonitrile correlate with rate constants for acetonitrile exchange at these cations, though of course the reactions with the porphyrins are much (104 to 106 times) slower than the corresponding rates of solvent exchange (269). 

The trifluoromethyl group is found in a wide variety of pharmaceuticals. Besides its effect on the electron density, its high lipophilicity improves transport characteristics in vivo and facilitates lower dosage rates. F for F isotopic exchange reaction has been used to synthesize [l- F]l,l-difluoro-2,2-dichloroethyl-arylethers. The substitution proceeds in good to excellent yields (> 85 %) and is strongly dependent on the reaction conditions. Substitution occurs even at room temperature in DMSO or in acetonitrile containing up to 20% of water (Scheme 61) [247]. 

Colorimetry. A variety of colorimetric techniques have been used to measure ions such as NH4, SO4-, and NO7 in ambient particles. For example, nitrate can be measured by reduction to nitrite using hydrazine in the presence of a copper catalyst, followed by its conversion to a colored azo dye, which can be measured by its absorbance at 524 nm (Mullin and Riley, 1955). Sulfate has been determined using an exchange reaction between sulfate and a barium-nitrosulfo-nazo(III) chelate in aqueous acetonitrile the chelate has an absorbance peak at 642 nm and hence the decrease in this peak can be followed as a measure of the amount of sulfate present that has exchanged with the chelate (Hoffer et al., 1979). Similarly, NH4 can be measured by the indophenol blue method (Weather-burn, 1967). 

Polymer-supported tetraphenylphosphonium bromide is a recyclable catalyst for halogen-exchange reactions. The reaction of 1 equivalent of chloro-2,4-dimtrobenzene with 1 5 equivalents of spray-dried potassium fluoride and 0.1 equivalent of this catalyst in acetonitrile at 80 °C for 12 h gives 2,4-dmitro-fluorobenzene in 98% yield An 11% yield is obtained without the catalyst [3 /]. 

The trimethylsilyl group is exchanged for a hydrogen atom using PfSA in acetonitrile, a reaction referred to as a protodesilylation Protonation of vinylsilane 7 leads in the process first to cation 25, which is stabilized by the p-effect of silicon.10 A nucleophile subsequently abstracts the TMS group, producing olefin 8. Vinylsi-lanes can be transformed into olefins not onl> with acid.10 but also with fluoride1011 or with catalytic amounts of iodine m the presence of water... 

The title compounds 135 have been synthesized176 in up to 85% yields by a facile 18F-for-19F exchange reaction (equation 87) carried out in DMSO or in wet acetonitrile. The [18F]fluoride ion has been prepared also as TBAF salt or as a polymer-bound species177. The possible uses of aryl ethers for synthesis of radiopharmaceuticals are under investigation176. 

Bis[trifluoromethyl] lellurium difluoride and excess trifluoroacetic anhydride reacted at 20° to liberate gaseous trifluoroacetyl fluoride and produce bis[trifluoromethyl tellurium bis[trifluoroacetate]. In acetonitrile as reaction medium the exchange of fluoride for trifluoroacctale did not go to completion2. 

Complexes having nitrogen ligands such as ammonia, acetonitrile, pyridine and dinitrogen readily undergo exchange reaction with RNC ... 

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