Regioselective synthesis of 3- and 5-aminopyrazoles
Just a bit of sodium ethoxide does the trick …
The Knorr pyrazole synthesis, broadly defined, involves the condensation of hydrazines with 1,3-dielectrophiles, e.g., 1,3-diketones, beta-ketoesters, alpha-cyanoketones, beta-alkoxyacrylonitriles, alkoxymethylenemalonates, etc. When a nitrile electrophile is involved, an aminopyrazole typically results, producing compounds that are very useful in the pharmaceutical field. The first two equations in the general scheme below use hydrazine as the dinucleophile, but what happens when an unsymmetrically-substituted hydrazine is involved? Wouldn’t it be nice to be able to produce either the 3- or 5-aminopyrazole regioisomer in a selective fashion?
This problem has been kicking around the literature for a long time. The 5-aminopyrazole is generally the major product, leaving the 3-aminopyrazole as a useful-yet-expensive poor cousin. Much effort has been spent trying to rationalize and predict the outcome of these processes, so a recent paper from Fandrick, et al., from Boehringer Ingelheim is a most welcome arrival.
Heating the nitrile 3 with the alkyl-substituted hydrazine 4 in ethanol produces the 5-aminopyrazole 7, presumably through the adduct 5, a result that is consistent with the prior literature. At first glance, this should be rather surprising, since it is known that alkylhydrazines are more nucleophilic at the most substituted nitrogen, i.e., k1 should be larger than k2, thus producing adduct 6 and the 3-aminopyrazole 8. (Arylhydrazines are different; they are generally more nucleophilic on the unsubstituted nitrogen, though it depends on the nature of the arene.) Fandrick and co-workers proposed that 6 is indeed the kinetically-formed adduct, but its cyclization to 8 (under typical neutral conditions) is slower than isomerization to the more stable adduct 5, producing 7 instead.
In order to obtain the more rare and often desirable 3-aminopyrazole 8, Fandrick simply introduced sodium ethoxide to the mix with the idea that the kinetically favored 6 might be transformed to 8 before it can isomerize to 5. It worked, producing 3-aminopyrazoles with selectivities of up to 99:1 depending on the exact example.
Increasing the size of the alkyl group on the hydrazine should erode the selectivity for 8 in the kinetically-controlled ethoxide reaction, since the internal nitrogen of the hydrazine, while electronically more nucleophilic, is less accessible due to steric hindrance. Indeed, moving to cyclohexylhydrazine and then t-butylhydrazine leads to diminished and even reversed selectivities of 72:28 and 5:95 for 8:7. Under neutral thermodynamic conditions, cyclohexyl- and t-butylhydrazine gave >99:1 ratios of 7:8, as expected.
Arylhydrazines are known to highly favor 5-aminopyrazoles, which was supported by the current work when employing neutral (thermodynamic) conditions. Interestingly, reasonable quantities of 1-aryl-3-aminopyrazoles 8 were formed under kinetically-controlled (ethoxide) conditions. Hence, with phenylhydrazine, a 1:1 ratio of 8:7 was formed, indicating that a good deal of 6 was formed kinetically. Using the more electron-rich p-methoxyphenylhydrazine under kinetically controlled conditions produced even more of the 3-aminopyrazole, giving a 78:22 ratio of 8:7. This presumably reflects and increase in the nucleophilicity of the internal hydrazine nitrogen due to the resonance donating effect of the p-methoxy substituent.