In a recent paper by Huanfeng Jiang a new practical synthesis of both pyrazoles and indazoles was described. By using a simple copper / oxygen catalytic system direct C-H bond amination was achieved.
Huanfeng started investigating this reaction by varying the copper catalyst, additive, solvent and reaction temperature to attempt to optimise the reaction conditions (table 1).
With the optimal condition of Cu(OAc)2 (10 mol %) and DABCO (30 mol %) in DMSO at 100°C under O2 (1 atm) the scope of this reaction was explored (table 2 and 3).
As can be seen in table 2 the reaction gave pyrazoles in a >80 % yield where R1 = aromatic / olefinic, R2 = aromatic / olefinic / alkyl and R3 = aromatic / olefinic / alkyl. A wide range of functionalities were tolerated including fluorides, chlorides, bromides, nitriles, trifluoromethyls, carbonyls, sulphonamides and ethers. The only functionality investigated that was not tolerated was a free NH2. Huanfeng states that the geometry of the hydrazone double bond in the starting material is not important and believes that at elevated temperature in the presence on DABCO this would isomerise.
After further optimisation of the reaction conditions it was found that the addition of 1 equivalent K2CO3 and an increased temperature of 120°C gave the best yields of indazoles. This reaction, as expected, tolerated the same functionalities as the pyrazole series. However, the potential to produce, presumably difficult to separate, mixtures of regioisomers makes this synthesis less desirable. It was noted that in the case of unsymmetrical aryl group that electron-rich substrates were favoured in the insertion step and that steric factors might also affect the regioselectivity.
The methodology to synthesise pyrazoles was then extended further to start from an aryl enone (scheme 1). Yields for this one-pot procedure (85 %) were comparable to the one step synthesis from a hydrazone (92 %).
To further the applicability of this chemistry Huanfeng functionalised a pyrazoles with NBS and used this building block in a Sonogashira reaction, Suzuki-Miyaura reaction and an amidation reaction (scheme 2).
To probe the mechanism of the C-H amidation several experiments were conducted using stochiometric quantities of an electron-transfer scavenger (1,4-dinitrobenzene), a radical clock (diallyl ether), or a radical inhibitor (hydroquinone, TEMPO) (scheme 3). The reaction proceeded in each case and when using diallyl ether no cyclised product was observed. These results suggest that this is not a radical mediated transformation.
Based on these preliminary mechanistic studies Huanfeng has postulated the following mechanism (scheme 4). Initially olefinic hydrazone 1 or aryl hydrazone 3 would react with Cu(OAc)2 to form an Cu-N adduct A. The nitrogen from this Cu-N adduct could then undergo an intramolecular electrophilic substitution followed by aromatisation via C to give the pyrazole / indazole. The reduced copper species (Cun-2) could then be reoxidised by oxygen to complete the catalytic cycle. Alternatively metallacycle B could be formed by electrophilic metalation or C-H bond activation followed by reductive elimination and aromatisation.
In conclusion this synthesis uses cheap and readily available reagents and has been shown to proceed in high yields even when preformed in air (72 % yield, table 1 entry 14). This methodology circumvents the need to prefunctionalise starting materials with (pseudo) halides or directing groups and therefore has the potential to reduce the step count and open up previously more challenging or unavailable pyrazoles or indazoles.