Catalytic Insertion Reactions of α‐Imino Carbenoids

Abstract

Over the past decade, α-imino carbenoids generated via transition metal (such as rhodium, nickel, copper, palladium, silver) catalyzed denitrogenative ring-opening of N-sulfonyl-1,2,3-triazoles have found an extensive account of applications in synthetic organic chemistry. Particularly, they have been widely utilized as a donor/acceptor carbene complex in a range of transformations leading to diverse nitrogen containing compounds and heterocycles. Along the same direction, 3-diazoindolin-2-imines were successfully applied as an alternative source of αimino carbenoid precursors for the development of a number of methodologies to access diverse indole derivatives. This review summarizes the insertion reactions of α-imino metal carbenes derived from N-sulfonyl-1,2,3-triazoles and 3-diazoindolin-2-imines.

Gold-Catalyzed N-Alkenylation of Isoxazolines and the Use of Alkenyl Gold Intermediates in the Synthesis of 2-Amino-1-pyrrolines

“…This reagent has previously been reported to trap α-imino gold carbene intermediates to give pyrroles, such as 10 , but no evidence of trapping was observed, and pyrroline 4a was the only product. To further probe the possibility of an α-imino gold carbene intermediate, diazoester 11 was prepared and treated with α,β-unsaturated ketone 12 under gold-catalyzed conditions previously reported to convert 11 to an α-imino gold carbene and with our optimized reaction conditions, but no evidence of 13 was observed, and 12 was recovered (Scheme C) . These experiments are inconsistent with the proposed fragmentation pathway for gold-catalyzed conversion of 1 and 2 to pyrrolines 4 ; however, they do not rule out cyclization by C–H insertion of a potential α-imino carbene intermediate .…”

“…The gold-catalyzed N -alkenylation of 1 and rearrangement to pyrroline 4 was initially proposed to proceed through either (1) [3,3′]-sigmatropic rearrangement of gold-coordinated addition product 3 via transition state TS3 or (2) fragmentation of gold-coordinated addition product 3 to gold carbene 7 and α,β-unsaturated ketone 8 followed by recombination via cycloaddition (Scheme A) . To determine whether fragmentation to a carbene intermediate was occurring, vinyl ether 9 was added to the reaction mixture of 1a and 2a (Scheme B) . This reagent has previously been reported to trap α-imino gold carbene intermediates to give pyrroles, such as 10 , but no evidence of trapping was observed, and pyrroline 4a was the only product.…”

Synthesis and reactivity of 1-sulfonylcyclooctatriazoles

“…The route to metal carbenes from 1-STs offers not only complementary reactivity to established carbene methodology but has allowed novel applications and this strategy has been applied to create value-added products, bioactive compounds and natural products. A small selection of 1-ST denitrogenation methodology includes (Scheme 1): functionalising C–H bonds with high yield and excellent selectivity ( 5 ), 2 heterocycle synthesis ( 6 , 10 ) 3 and 1,2-H shift ( 11 ). 4…”