Nitronium fluoroborate

From these results it appears that the 5-position of thiazole is two to three more reactive than the 4-position, that methylation in the 2-position enhances the rate of nitration by a factor of 15 in the 5-position and of 8 in the 4-position, that this last factor is 10 and 14 for 2-Et and 2-t-Bu groups, respectively. Asato (374) and Dou (375) arrived at the same figure for the orientation of the nitration of 2-methyl and 2-propylthiazole Asato used nitronium fluoroborate and the dinitrogen tetroxide-boron trifluoride complex at room temperature, and Dou used sulfonitric acid at 70°C (Table T54). About the same proportion of 4-and 5-isomers was obtained in the nitration of 2-methoxythiazole by Friedmann (376). Recently, Katritzky et al. (377) presented the first kinetic studies of electrophilic substitution in thiazoles the nitration of thiazoles and thiazolones (Table 1-55). The reaction was followed spec-trophotometrically and performed at different acidities by varying the... 

Chlorosulfonation of benzotrichloride with chlorosulfonic acid (28) or with sulfur trioxide (29) gives y -chlorosulfonyl benzoyl chloride [4052-92-0] in high yield. Nitration with nitronium fluoroborate in sulfolane gives 68% y -nitro-benzotrichloride [709-58-0] along with 13% of the ortho and 19% of the para isomers (30). 

The term macroscopic diffusion control has been used to describe processes in which the rate of reaction is determined essentially by the rate of mixing of the reactant solutions. The nitration of toluene in sulpholane by the addition of a solution of nitronium fluoroborate in sulpholane appears to fall into this class (Ridd, 1971a). Obviously, if a reaction is subject to microscopic diffusion control when the reactants meet in a homogeneous solution, it must also be subject to macroscopic diffusion control when preformed solutions of the same reactants are mixed. However, the converse is not true. The difficulty of obtaining complete mixing of solutions in very short time intervals implies that a reaction may still be subject to macroscopic diffusion control when the rate coefficient is considerably below that for reaction on encounter. The mathematical treatment and macroscopic diffusion control has been discussed by Rys (Ott and Rys, 1975 Rys, 1976), and has been further developed recently (Rys, 1977 Nabholtz et al, 1977 Nabholtz and Rys, 1977 Bourne et al., 1977). It will not be considered further in this chapter. 

Ostman122 has determined the kinetic isotope effect in the nitration of thiophene by benzoyl nitrate in acetonitrile the observed effect ( T/fcH= 1.14 + 0.10) is opposite to that expected for a primary isotope effect. It seems likely that it is a secondary a-isotope effect, ascribable to a rate-determining nature of the bond-formation step and similar to that observed by Olah et al. in the nitration of benzene derivatives with nitronium fluoroborate in tetramethylenesulfone.124... 

The intensity of the band 2360 cm"1 is enhanced by the addition of nitronium fluoroborates, sulphuric acid, phosphorous pentoxide, or small amounts of acetic... 

Whereas pyrazoles are quite readily iV-nitrated by nitric acid in acetic anhydride or with nitronium fluoroborate, imidazoles are usually far too basic, and give the nitrate salts instead. A nitro substituent, however, decreases the basicity sufficiently to allow iV-nitration to occur. The A -nitro compounds are subject to thermal rearrangement to the 2- and 4-nitro isomers, pointing to an alternative route to such compounds [63]. 

Both nitronium and nitrosonium salts are effective initiators but with quite different results. Thus, propene and nitronium fluoroborate react to produce the secondary a-nitrocarbenium ion which undergoes Ritter reaction with acetonitrile to yield amide (118). Under similar conditions, nitrosonium fluoroborate leads to heterocyclic products. Intramolecular reaction of the nitroso and nitrilium groups, followed by prototropic shifts, affords the iV-hydroxy imidazolium salt (119). This may be either neutralized to produce the iV-oxide or reduced to the imidazole (120 Scheme SS). 

The nitrations are carried out under anhydrous conditions. This is of special advantage in dealing with compounds which under usual strong acidic nitration conditions may undergo hydrolysis or oxidation. Aryl nitriles, for example, are easily hydrolyzed under nitration conditions and no direct dinitration, requiring forceful conditions, was previously possible. The nitronium fluoroborate method enable us to carry out mono and dinitration of aryl nitriles with high yields without any hydrolysis of the -CN group. 

A useful feature of this reagent is that high ratios of ortho to para products are found for some substituted aromatics. Finally, it possible to use nitronium-ion salts for nitration. Nitronium fluoroborate has been extensively studied. The trifluoromethanesulfonate salt is also readily prepared and is an active nitrating... 

Treatmentof biphenylene with nitronium fluoroborate in acetonitrile proceeds by attack on the ring junction and ring-opening to (557). 

The reaction of nitronium fluoroborate with olefins in acetonitrile leads, via a Ritter reaction of the intermediate carbonium ion with solvent, to 1-nitro-2-acetamidoalkanes. The inherent instability of DMF towards aqueous base has been noted. ... 

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