Synthesis of Polysubstituted Imidazoles and Pyridines via Samarium(III) Triflate‐Catalyzed [2+2+1] and [4+2] Annulations of Unactivated Aromatic Alkenes with Azides

Abstract

Samarium(III) triflate-catalyzed [2 + + 2 + + 1] and [4 + + 2] annulations have been identifiedf or the preparation of fully substituted imidazolesa nd 2,3,5trisubstituted pyridines from the readily available unactivated aromatic alkenes and azidomethyl aromatics.T hese reactions proceeded smoothly with one-or two-nitrogen synthons to afford ar ange of twot ypes of skeletally distinct N-heterocycles in good yields. Mechanistic studies revealedt hat the annulation reaction proceed via an initial samarium(III) triflate-induced 1,3 dipolar cycloaddition followed by 1,2-migrationt of orm an enamine as the key intermediate. Density functional theory (DFT)c alculationss uggested that the selectivity of 1,2-Ha nd 1,2-aryl migrationdepends on the structureoft he alkenes. YingchunW ang et al. Scheme 1. Sm(OTf) 3 -catalyzed [2 + + 2 + + 1] annulations of asymmetric stilbenes 1 with benzyla zide 2a. Scheme 3. Control experiments.Scheme4.Synthesis of 2,3,5-trisubstituted 5a and X-ray crystal structure of 5a.

One‐Pot Trimetallic Relay Catalysis: A Unified Approach for the Synthesis of β‐Carbolines and Other [c]‐Fused Pyridines

“…Table 3t hat the scope of this procedure is significant. [1] On the basis of our experimental observations and in conjunction with previous reports, [10,20] aplausible mechanism that explains the formation of 5a and 7a from 1a is proposed in Scheme 4a. Compound 7j possesses the complete carbon framework present in the b-carboline natural products dichotomide IX and tunicoidine D( see Scheme 1).…”

“…[20] Aziridine 9a undergoes deprotonation followed by ring opening to generate 10 a,w hich upon aromatization delivers 5a.T hus,t he Pd II complex plays ar emarkable role in the exclusive formation of 5a.T his mechanism also explains the marked yield enhancement in the presence of Pd II complexes (see Table 1). [20] Aziridine 9a undergoes deprotonation followed by ring opening to generate 10 a,w hich upon aromatization delivers 5a.T hus,t he Pd II complex plays ar emarkable role in the exclusive formation of 5a.T his mechanism also explains the marked yield enhancement in the presence of Pd II complexes (see Table 1).…”

One‐Pot Trimetallic Relay Catalysis: A Unified Approach for the Synthesis of β‐Carbolines and Other [c]‐Fused Pyridines

“…Compound 7j possesses the complete carbon framework present in the b-carboline natural products dichotomide IX and tunicoidine D( see Scheme 1). [1] On the basis of our experimental observations and in conjunction with previous reports, [10,20] aplausible mechanism that explains the formation of 5a and 7a from 1a is proposed in Scheme 4a. [21] Thus,s ilver(I)-catalyzed 5-exo-dig cyclization and protodemetalation of 1a generates the indoline intermediate 2a.Abismuth(III)-promoted cascade involving 1,3-allylic alcohol isomerization (1,3-AAI) and nucleophilic azidation then provides the azide 3a,w hich undergoes aH uisgen-type intramolecular azide-alkene [3+ +2] cycloaddition to form 4a.T ransformation of the triazole 4a into 8a and aromatization provides 5a.…”

“…On the other hand, the azide 3a is converted into the aziridine intermediate 9a in the presence of aPd II complex. [20] Aziridine 9a undergoes deprotonation followed by ring opening to generate 10 a,w hich upon aromatization delivers 5a.T hus,t he Pd II complex plays ar emarkable role in the exclusive formation of 5a.T his mechanism also explains the marked yield enhancement in the presence of Pd II complexes (see Table 1). [15] In the absence of aP d II complex, 3a forms 8a,and the reaction takes an interesting detour.…”

Catalytic Synthesis of 1,2,4,5‐Tetrasubstituted 1H‐Imidazole Derivatives: State of the Art

“…[164] Still in 2015, Wang, Pan and coworkers achieved the synthesis of 1-arylmethyl-2,4,5-triaryl-1H-imidazoles 93 in high yields via samarium triflate-catalyzed [2 + 2 + 1] annulation of symmetric stilbenes 91 with arylmethyl azides 92 in toluene at 110 °C under O2 atmosphere (Scheme 61). [166] Scheme 61. Sm(OTf)3-catalyzed synthesis of imidazoles 93…”