From 2000 to Mid-2010: A Fruitful Decade for the Synthesis of Pyrazoles

Molecular and Solid State Structure of the Doubly-Substituted B-Diketone 3,3'-Bis(ferrocenylmethyl)pentane-2,4-Dione#

“…β-Diketones are an important class of organic compounds frequently encountered in synthetic chemistry, 1,2 where in addition to their established role in the synthesis of various heterocyclic derivatives which have been widely studied and reviewed, [3][4][5][6] they have been used as ligands for the coordination of transition metals. 7,8 On the other hand, β-diketones are attractive building blocks for the construction of polydentate Schiff bases, and both the parent derivative 2,4-pentanedione (CH 3 COCH 2 COCH 3 ), [9][10][11][12] and its organometallic counterpart ferrocenoylacetone (FcCOCH 2 COCH 3 , 13 Fc = (h 5 -C 5 H 5 )Fe(h 5 -C 5 H 4 )) have been widely employed to prepare tridentate Schiff base complexes, also called half-units, upon mono-condensation with primary diamines.…”

Synthesis of Chiral Pyrazoles: A 1,3‐Dipolar Cycloaddition/[1,5] Sigmatropic Rearrangement with Stereoretentive Migration of a Stereogenic Group

“…[10] For these reasons, and in spite of the variety of existing methods, the development of new methodologies for the efficient regioselective synthesis of polysubstituted pyrazoles is still an area of very active research. [11,12] A particularly challenging type of pyrazoles are chiral pyrazoles with a stereogenic carbon attached at N-position. Limited examples of this type of chiral pyrazoles with biological activity have been reported, [13] but their usefulness have been hampered by the lack of general methods for their preparation.…”

Nickel-Catalyzed Oxidative Cyclotrimerization of α-Amino Ketones: Selective Synthesis of Pyrazoles

“…Extensive screening revealed that (C 5 H 5 )Ni(II)Cl(PPh 3 ) displayed the highest catalytic reactivity (entry 5). It is noteworthy that Ni(PPh 3 ) 4 , a zerovalent Ni catalyst, also effected the reaction (entry 6). Interestingly, benzoic acid was found to favor the reaction: The yield of 2a was enhanced to 75% when one equivalent PhCO 2 H was added (entry 7).…”

“…3 Many elegant methods have been developed for their synthesis, [4][5][6][7][8] including, (i) the cyclocondensation of hydrazines with 1,3-dielectrophiles (1,3-dicarbonyl compounds or α,β-unsaturated aldehydes and ketones), 5 (ii) the intermolecular 1,3-dipolar cycloaddition of diazoalkanes and nitrilimines with unsaturated compounds (such as alkenes or alkynes), 6 and (iii) the introduction of substituents onto a pre-existing aromatic ring (often onto the nitrogen atom). 7 However, these methods suffer from the poor reactivity, somewhat limited substrate scope, and the potential hazardousness and detonation of the substrates; moreover, regio-and chemoselectivity are usually unsatisfactory in many cases.…”

“…12 Indeed, imine 6a could be cyclotrimerized leading to product 2a in the presence of Ni catalyst, such as (C 5 H 5 )Ni(II)Cl(PPh 3 ), Ni(cod) 2 or Ni(Ph 3 ) 4 , albeit with lower activity of the latter two Ni catalysts (equation 2 in Scheme 3), suggesting that this present cyclotrimerization reaction may proceed via the first generation of an imine intermediate 6a.…”