Pyridine synthesis from oximes and alkynesviarhodium(iii) catalysis: Cp* and Cp^tprovide complementary selectivity

Abstract

The synthesis of pyridines from readily available α,β-unsaturated oximes and alkynes under mild conditions and low temperatures using Rh(III) catalysis has been developed. It was found that the use of sterically different ligands allows for complementary selectivities to be achieved.

Access to Sultams by Rhodium(III)‐Catalyzed Directed CH Activation

“…[9] In this context, rhodium(III)-catalyzed reactions [10] using a wide range of carbonyl-derived directing groups such as benzamides, [11] acetanilides, [12] imines, [13] ketones, [14] and carboxylic acids [15] were reported, thus illustrating the utility of this reactivity concept. Despite their critical importance, aryl sulfonamides have, to the best of our knowledge, not been explored with rhodium catalysts.…”

Rh(III)- and Ir(III)-Catalyzed C–C Bond Cross Couplings from C–H Bonds

“…In contrast, Rovis focused on the steric features of the Cp* ligand and synthesized a related Rh(III) complex with a cyclopentadienyl ligand bearing two tBu substituents. The increased hindrance of this catalyst resulted in improved or complementary selectivity in the alkyne insertion event for the dehydrogenative coupling between vinylic amides or oximes and alkynes [39,49]. Cp ligand design also recently enabled a diastereoselective [2 +1] annulation reaction [151].…”

“…Following these pioneering reports, several other N-O-and N-Nbased oxidizing directing groups (such as N-phenoxyacetamides, O-acyloximes, hydrazones, etc.) were designed and applied in redox-neutral couplings with alkynes [49][50][51][52][53][54]. In addition, an intramolecular version of this transformation was used as a key step allowing construction of indolizidine-based natural products like (AE)-antofine and (AE)-septicine [55] and (AE)-goniomitine, bearing a pyrido [1,2-a] indole core [56,57] (3).…”

Facile Rh(III)-Catalyzed Synthesis of Fluorinated Pyridines

“…Chiba 14 and Rovis 15 have established the utility of [Cp*RhCl 2 ] 2 /metal acetate salt catalyst systems for the synthesis of multi-substituted pyridines from α,β-unsaturated oximes and internal alkynes. 16-17 However, we found that the nucleophilic alcoholic solvents utilized in their protocols, MeOH or 2,2,2-trifluoroethanol (TFE), posed a problem for the construction of fluorinated analogues due to alcohol displacement of the fluorine under the basic reaction conditions (Table 1, entries 1–2).…”

“…Unsymmetrical internal alkynes also provided 3-fluoropyridines 3f , 3g , 3i and 3l in good yields, but with variable regioselectivities as has been previously reported for the preparation of non-fluorinated pyridines. 14-15 Attempts to incorporate internal alkynes with bulky t -butyl or TMS substituents were not successful.…”