Heterocycles from ketenimines. 12. Pyrrolo[3,2-b]pyridines

Abstract

The cycloadducts formed from the reactions of pyridine-N-imines and 1,3-dipolarphiles usually are unstable and undergo rapid dehydrogenation to the aromatic pyrazolopyridine system 2.2 We now describe the reaction of pyridine-N-imines with triarylketenimines.From the reaction of equimolar concentrations of 1 and diphenylketene-ZV-p-tolylimine (3) was isolated a 54% yield of a bright orange solid 4. Structural assignment of 4 as 2,3-

Ketenimine Formation Catalyzed by a High-Valent Cobalt Carbene in Bulky Alkoxide Ligand Environment

“…Similar to ketenes, ketenimines exhibit dipolar reactivity featuring electrophilic behavior at the central carbon and nucleophilic behavior at the terminal carbon . Ketenimines have been shown to undergo a multitude of transformations, including nucleophilic and radical additions, electrocyclic ring closure reactions, and cycloadditions. , Due to their reactive nature, ketenimines can serve as highly useful synthons in the production of nitrogen-containing molecules . A variety of synthetic routes toward ketenimines were developed to enable their utilization, including Wittig reaction with isocyanates, silylation of enolizable nitriles, aza-Claisen rearrangement of N -sulfonyl- N -allylynamides, and others. , One of the earliest reported synthetic pathways toward ketenimines involved coupling of carbenes with isocyanides. , While this reaction is possible under thermal or photochemical conditions, , transition metal catalysis under ambient conditions generally enables higher yields and selectivity for the formation of the desired product.…”

An electron-donor–acceptor complex between two intermediates enables a N–N bond cleavage cascade process to access 2,3-difunctionalized pyridines

“…In certain cases, unselective C3-functionalized pyridines via 1,3-migration of N -iminopyridiniums were observed as a byproduct. 15,16 Therefore, it appears that the electron-donating group such as OR or NR 2 is critical to promote the N–N bond cleavage/ N -migration cascade process of N -aminopyridiniums in our case. To further understand the insight of the reaction mechanism, a series of control experiments were performed (Fig.…”

Lewis acid catalyzed [3 + 2] annulation of ketenimines with donor–acceptor cyclopropanes: an approach to 2-alkylidenepyrrolidine derivatives

“…1, 20.3, 27.5, 52.1, 52.2, 52.3, 64.4 (s), 65.5, 115.1, 124.5, 125.6, 126.6, 126.9, 127.3, 127.6, 128.3, 128.5, 129.4 (s), 130.1, 130.9 (s) 6.39 (1 H, dd, J 8.8, 2.8), 6.83-6.88 (7 H, m), 7.04-7.14 (7 H, m), 7.74 (1 H, d, J 8.8); 13 C NMR (100 MHz, CDCl 3 ): δ 23. 9, 31.7, 51.5, 53.1, 53.3, 54.7, 56.3, 66.5 (s), 67.7, 98.7, 105.7, 119.3 (s), 125.5, 125.9, 127.0, 127.3, 127.5, 127.6, 128.5, 129.7, 130.8, 131.5, 131.6, 132.7 (s) 1,2,3,3a,4,benzopyran [4,3-b]pyrrole 10a. Eluent for column chromatography: hexanes-diethyl ether (1 : 1, v/v); (0.45 g, 74%); mp 210 °C (from Et 2 O); ν max (Nujol)/ cm −1 1755, 1734; 1 H NMR (400 MHz, CDCl 3 ): δ 2.17 (3 H, s), 3.02 (3 H, s), 3.48 (1 H, td, J 11.2, 4.4), 3.67 (3 H, s), 3.73 (3 H, s), 3.86 (3 H, s), 4.30 (1 H, d, J 11.2), 4.48 (1 H, dd, J 10.0, 4.4), 4.66 (1 H, dd, J 11.2, 10.0), 6.52 (1 H, s), 6.67 (1 H, s), 6.84 (2 H, d, J 8.4), 6.95 (2 H, d, J 8.4), 7.01-7.09 (3 H, m), 7.18-7.26 (7 H, m); 13 C NMR (100 MHz, CDCl 3 ): δ 20.…”

“…2 Concerning the participation of ketenimines in dipolar [3 + 2] cycloadditions it has been established that the natural role of these heterocumulenes is that of the dipolarophilic 2-atom component, involving either its cumulated CvC or CvN double bond. A few examples of the intermolecular version of this type of cycloaddition have been reported, in which the 1,3-dipolar counterparts are N-aminopyridinium ylides, 3 isoquinolinium-2-yl aminides, 4 diazo compounds, 5 azides, 6 acylnitrenes, 7 aziridines, 8 thiiranes, 9 oxaziridines 10 or heterocyclic carbine-derived 1,3dipoles. 11 In addition to these intermolecular reactions, we recently disclosed examples of intramolecular [3 + 2] cycloadditions of ketenimines, via its CvC bond, with azide functions as 1,3-dipoles.…”