Rhodium(II) Acetate Catalyzed Synthesis of Cyclic Enamides and Enamines via β-Hydride Elimination

Abstract

A series of cylic diazoamides and diazoamines were synthesized. Treatment of these cylic diazoamides and diazoamines with a catalytic amount of rhodium(II) acetate furnished the corresponding cyclic enamides and enamines, respectively. Interestingly, cyclic diazoamines produced stereoselectively Z-enamines.The utility of a-diazo carbonyl compounds in modern organic synthesis and their participation in a variety of transformations like cyclopropanation, insertion reaction and ylide formation is well documented. 1 The tandem processes involving rhodium carbenoids generated from diazo carbonyl compounds with a catalytic amount of rhodium(II) acetate have been used to construct various complex polycyclic systems. 2 The rhodium(II)-catalyzed reactions of cyclic and acyclic diazo ketoamides were skillfully applied to generate ylides followed by trapping with dipolarophiles to synthesize various alkaloids 3 and nitrogen containing heterocycles. 2,4 The study of b-hydride elimination reactions of rhodium carbenoid is scarcely reported in the literature, particularly to obtain cis-enoates, 5 trans-b-methoxy enones 6 and a,b-unsaturated esters. 7 However, to the best of our knowledge there is no report dealing with the b-hydride elimination chemistry of diazoamides and diazoamines. 1 In continuation of our interest on the reactions of diazo carbonyl compounds 8 and to expose the novel utilities of these diazo compounds, we herein wish to report the results of bhydride elimination reactions of cyclic diazoamides and diazoamines in the presence of rhodium(II) acetate as a catalyst.For this purpose, a-diazo carbonyl compounds 4a-f having b-hydrogen atoms were chosen for this study. The required cyclic diazoamides 4a-c can be easily prepared from the corresponding cyclic amides 1a-c as described below. The Michael addition reaction 9 of cyclic amides 1a-c and ethyl acrylate afforded the corresponding cyclic amides 2a-c in quantitative yields. To generate a diazo group adjacent to ester functionality, the formylation 10 reaction of cyclic amides 2a-c using sodium hydride/ethyl formate was carried out. The formylation took place smoothly only a to the ester functional group and provided compounds 3. Without the isolation of formyl compounds 3, the diazo transfer reaction was performed using mesyl azide to afford the cyclic diazoamides 4a-c in good yields (Scheme 1).We initially investigated the performance of diazoamide 4a using rhodium(II) acetate as a catalyst in the absence of any dipolarophile. Diazoamide 4a was dissolved in anhydrous benzene and refluxed with 0.3 mol% of rhodium(II) acetate for 25 h. The reaction was followed by TLC until the disappearance of diazoamide 4a. The chromatographic purification of the crude reaction mixture afforded two products which were characterized as cyclic enamides of type 5a (40%) and 6a (31%) with Z and E geometry, respectively (Scheme 1, Table). Scheme 1IR spectrum of compound 5a showed a band at 1632 cm -1 indicating the presence of a olefin functional group. The 1 H NMR spectrum of ...

Transition Metal‐Catalysed Intramolecular Carbenoid C−H Insertion for Pyrrolidine Formation by Decomposition of α‐Diazoesters

“…On the other hand, N ‐benzylsulfonamide 17 b showed a notably different behaviour depending on the transition‐metal catalyst employed. Thus, the use of Pd(0)‐catalysts (entries 6–7, Table ) resulted in the exclusive formation of alkene 20 , due to the 1,2‐H migration of the metal‐carbenoid intermediate,, while in the presence of the Ru(II)‐catalyst a complex reaction mixture was obtained, from which only ketone 21 was isolated (entry 9, Table ). The lack of the C−H insertion product in the latter case may be ascribed to the delocalization of the nitrogen lone‐pair into the sulfonyl group, which would hamper the stabilization required in the corresponding zwitterionic intermediate ( vide supra ).…”

Hydrogen-Bond-Directed Highly Stereoselective Synthesis of Z-Enamides via Pd-Catalyzed Oxidative Amidation of Conjugated Olefins

“…14 On the other hand, several catalytic routes have been also investigated, such as isomerization of N-allylamides by Fe, Rh, or Ru complexes 15 and Rh-catalyzed β-hydride elimination of diazoamides. 16 Additionally, metal-mediated C-N bond formation has been an area of great interest, and some protocols have been developed, including Ru-or Cu-catalyzed addition of amides to alkynes 17 and coupling of vinyl (pseudo)halides with amides using Pd 18 or Cu species. 19 Despite the numerous examples for the catalytic synthesis of enamides, such reactions often suffer from either low yield or difficulty in preparing necessary vinyl halides.…”

Rh2(OAc)4-catalyzed reactions of α-diazoimides: a simple and novel synthesis of mono- and bis(2,3-fused perhydrooxazol-4-one) systemsElectronic supplementary information (ESI) available: experimental procedure and spectral data for compounds 2a–p, 3, 5a–f. See http://www.rsc.org/suppdata/cc/b2/b211717g/

“…It is also known that in the absence of any added trapping agent, the dimerization 13 of carbonyl ylide dipoles takes place; we did not observe any such products from the isomünchnone dipoles. It is relevant to mention that the cyclic diazoimides 1 having an a-hydrogen atom to diazo functionality underwent 7 b-hydride elimination under similar experimental conditions.…”