Nickel-Catalyzed Cycloadditions of Unsaturated Hydrocarbons, Aldehydes, and Ketones

Abstract

The nickel-catalyzed cycloaddition of unsaturated hydrocarbons and carbonyls is reported. Diynes and enynes were used as coupling partners. Carbonyl substrates include both aldehdyes and ketones. Reactions of diynes and aldehydes afforded the [3, 3] electrocyclic ring-opened tautomers, rather than pyrans, in high yields. The cycloaddition reaction of enynes and aldehydes afforded two distinct products. A new carbon-carbon bond is formed, prior to a competitive β-hydrogen elimination of a nickel alkoxide, between the carbonyl carbon and either one of the carbons of the olefin or the alkyne. The steric hindrance of the enyne greatly affected the chemoselectivity of the cycloaddition of enynes and aldehydes. In some cases, dihydropyran was also formed. The scope of the cycloaddition reaction was expanded to include the coupling of enynes and ketones. No β-hydrogen elimination was observed in cycloaddition reaction of enynes and ketones. Instead, C-O bond-forming reductive elimination occurred exclusively to afford dihydropyrans in excellent yields. In all cases, complete chemoselectivity was observed; only dihydropyrans where the carbonyl carbon forms a carbon-carbon bond with a carbon of the olefin, rather than of the alkyne, were observed. All cycloaddition reactions occur at room temperature and employ nickel catalysts bearing the hindered 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or its saturated analogue, 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolin-2-ylidene (SIPr).

Studies on the coordination chemistry of methylated xanthines and their imidazolium salts. Part 1: benzyl derivatives

“…When 1a reacted with Pd(OAc) 2 [75][76][77][78][79][80], in an attempt to synthesize the Pd-NHC complex, a solvent dependant behavior was observed. A CDCl 3 solution of 1a reacted in situ (NMR timescale) with Pd(OAc) 2 , showed a shift in the signals (downfield) assigned to the ligand.…”

“…When the product is analyzed in strongly coordinating solvents such as DMSO-d 6 , the carbene can be stabilized, and detected spectroscopically. The behavior observed is possible, since the coordination of DMSO restrains the free rotation around the M-NHC complex due to its anisotropy [75][76][77][78]. In fact, in chloroform, the possibility of stabilizing the species through coordination of the solvent does not exist, and rotation becomes possible, explaining the double amount of signals observed on the NMR for the ligand in this complex (see Experimental section, synthesis of 4).…”

“…In fact, in chloroform, the possibility of stabilizing the species through coordination of the solvent does not exist, and rotation becomes possible, explaining the double amount of signals observed on the NMR for the ligand in this complex (see Experimental section, synthesis of 4). In this sense, in solution, the interconversion between the carbene and the N-coordinated species probably depends on the anisotropy of 1a as NHC [75][76][77][78].…”

Rhodium‐Catalyzed Asymmetric [2+2+2] Cyclization of 1,6‐Enynes and Aldehydes

“…[4][5][6][7] In 2008, two research groups, including ours, independently discovered the transition-metal-catalyzed [2+2+2] cyclization of 1,6-enynes and carbonyl compounds. [4][5][6]8] Our research group developed the cationic rhodium(I)/H 8 -binap complex catalyzed asymmetric [2+2+2] cyclization of 1,6-enynes and electron-deficient ketones, which produced bicyclic heterocycles in high yields with high enantiomeric excess (ee; Scheme 1). [4] The Louie group developed the nickel-catalyzed [2+2+2] cyclization of 1,6-enynes and electron-rich carbonyl compounds.…”

“…[4] The Louie group developed the nickel-catalyzed [2+2+2] cyclization of 1,6-enynes and electron-rich carbonyl compounds. [6] In this reaction, not only ketones, but also aldehydes are able to react with 1,6-enynes. We also examined the reaction of 1,6-enyne 1 a and aldehyde 2 a in the presence of a cationic rhodium(I)/rac-binap complex.…”