Iridium Complex-Catalyzed Intramolecular Ene-Type Reaction of 1,6-Enynes Accelerated in Ionic Liquid

Abstract

Iridium complex catalyzes an intramolecular ene-type reaction of 1,6-enynes to give cyclic 1,4-dienes. The reaction proceeds more efficiently in an imidazolium salt than in toluene and the ionic liquid can be reused.Transition metal-catalyzed cycloisomerization is an atomeconomical and powerful protocol for the synthesis of carbo-and heterocyclic systems. 1 Especially, an intramolecular ene-type reaction of enynes has been comprehensively studied and various transition metal complexes like Pd, 2 Pt, 3 Ti, 4 and Ru 5 ones are efficient catalysts. In these years, a series of Rh complex-catalyzed ene-type reaction of 1,6-enynes, possessing Z-olefinic moiety, has been reported. 6,7 We here disclose an Ir complex-catalyzed intramolecular ene-type reaction, where 1,6-enynes, having E-olefinic moiety, are more reactive substrates than Z-olefinic moiety. Moreover, an imidazolium salt was found to be a superior reaction media to conventional organic solvents.We chose allyl propargyl ether 1a, where the E/Z ratio of the olefinic moiety is 2:1, as a model 1,6-enyne and examined a cycloisomerization using iridium complex [IrCl(cod)] 2 in toluene (Table 1, entry 1). As a result, the substrate was completely consumed in eight hours at 90°C and an ene-type reaction proceeded to give cyclic 1,4-diene 2a in good yield. 8 Unlike Ir complex-catalyzed carbonylative coupling 9 and [2+2+2] cycloaddition, 10 the addition of phosphine ligands deactivated the Ir-catalyst in the ene-type reaction. For an example, Ir-DPPP complex 9b took longer reaction time to give product 2a in lower yield (entry 2). When the reaction was quenched at 3 hours, 30% of enyne 1a was recovered and the Z-isomer of 1a was dominant (entry 3). These results imply that the E-isomer reacted more promptly than the Z-isomer. Actually, enyne 1a, having E-olefinic moiety, was consumed only within three hours and the 1,4-diene 2a was obtained in higher yield of 82% (entry 4). 11 On the contrary, enyne 1a, having Z-olefinic moiety, gave much lower yield under the same reaction conditions (entry 5). Compound 1a-Z was completely consumed by longer reaction time, however, the yield did not reach that by 1a-E. The higher reactivity of the E-isomer of enyne than the Z-isomer in ene-type reaction was opposite to the case in the Rh complex-catalyzed reaction. 6,12

Cycloisomerization of 1,n‐Enynes: Challenging Metal‐Catalyzed Rearrangements and Mechanistic Insights

“…[140] In the course of their studies on tandem dienyne cycloisomerizations [Eq. (112)], [135] the research group of Malacria observed a change in reactivity on introduction of an ester-protected alcohol at the propargylic position. Indeed, when substrate 354 was treated with PtCl 2 , enol ester 355 was obtained in 88 % yield [Eq.…”

Iridium-Catalyzed Formation of Carbon-Carbon and Carbon-Heteroatom Bonds

“…Cycloisomerization of 20 to the 1,4-diene product proceeded more efficiently in an imidazolium salt than in toluene (Scheme 35). 86 An electrophilic cationic iridium catalyst has been shown to activate the alkyne moiety of enynes to give cyclopropanes. Nitrogen-bridged 1,6-enyne 21 underwent cycloisomerization to give the chiral cyclopropane by the catalyst system [Ir(cod)Cl] 2 /TolBINAP/AgOTf (Scheme 36).…”

Transition Metal-Catalyzed Intramolecular Enyne Cyclization Reaction

“…Contrary to most late transition metal-catalyzed 1,n-enyne cyclization reaction through a cyclometallation reaction pathway, the disubstituted alkene with a trans-configuration reacts faster than that with a cis-configuration (Scheme 36) [33]. Contrary to most late transition metal-catalyzed 1,n-enyne cyclization reaction through a cyclometallation reaction pathway, the disubstituted alkene with a trans-configuration reacts faster than that with a cis-configuration (Scheme 36) [33].…”