Regioselective Synthesis of Multisubstituted Furans via Metalloradical Cyclization of Alkynes with α-Diazocarbonyls: Construction of Functionalized α-Oligofurans

Cui, Xin; Xue, Xiangxin; Wojtas, Łukasz; Kim, Martin M.; Zhang, X. Peter

doi:10.1021/ja309446n

Cited by 182 publications

(73 citation statements)

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“…Considering that these ring-closing steps determine the enantioselectivity of the reaction, the computational results are in line with the experimental observations showing 2 H -chromene formation without asymmetric induction, despite using the chiral [Co II ( P3 )] catalyst (Figure 2) known to be a highly enantioselective carbene-transfer catalyst in other reactions. 6–9 …”

Section: Resultsmentioning

confidence: 99%

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Metalloradical Approach to 2H-Chromenes

et al. 2014

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Cobalt(III)–carbene radicals, generated through metalloradical activation of salicyl N-tosylhydrazones by cobalt(II) complexes of porphyrins, readily undergo radical addition to terminal alkynes to produce salicyl-vinyl radical intermediates. Subsequent hydrogen atom transfer (HAT) from the hydroxy group of the salicyl-moiety to the vinyl-radical leads to the formation of 2H-chromenes. The Co(II)-catalyzed process can tolerate various substitution patterns and produces the corresponding 2H-chromene products in good isolated yields. EPR spectroscopy and radical-trapping experiments with TEMPO are in agreement with the proposed radical mechanism. DFT calculations reveal the formation of the salicyl-vinyl radical intermediate by a metalloradical mediated process. Unexpectedly, subsequent HAT from the hydroxy moiety to the vinyl radical leads to formation of an o-quinone methide intermediate, which dissociates spontaneously from the cobalt center and easily undergoes an endo-cyclic, sigmatropic ring-closing reaction to form the final 2H-chromene product.

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Section: Resultsmentioning

confidence: 99%

“…6,7 In addition, the metalloradical-catalyzed approach was successfully applied for highly enantioselective alkyne cyclopropenation 8 as well as regioselective synthesis of furans. 9 …”

Section: Introductionmentioning

confidence: 99%

Metalloradical Approach to 2H-Chromenes

et al. 2014

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“…As a result several examples of reactions in which C-C and C-H bonds are formed have been shown to involve carbene radicals [32][33][34]. The most representative examples are cyclopropanation reactions mediated by metallo-radical cobalt(II) porphyrin complexes.…”

Section: Carbon-centered Radicals In Catalysismentioning

confidence: 99%

“…Besides cyclopropanation, other catalytic reactions involving carbene radicals have also been reported recently [33,34]. One of them involves formation of ketene intermediates by carbonylation of metal carbene radicals, followed by trapping of the ketene with nucleophiles such as amines or imines, forming amides and b-lactams [42].…”

Section: Carbon-centered Radicals In Catalysismentioning

confidence: 99%

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

et al. 2015

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In the context of homogeneous catalysis, openshell systems are often quite challenging to characterize. Nuclear magnetic resonance (NMR) spectroscopy is the most frequently applied tool to characterize organometallic compounds, but NMR spectra are usually broad, difficult to interpret and often futile for the study of paramagnetic compounds. As such, electron paramagnetic resonance (EPR) has proven itself as a useful spectroscopic technique to characterize paramagnetic complexes and reactive intermediates. EPR spectroscopy is a particularly useful tool to investigate their electronic structures, which is fundamental to understand their reactivity. This paper describes some selected examples of studies where EPR spectroscopy has been useful for the characterization of open-shell organometallic complexes. The paper concentrates in particular on systems where EPR spectroscopy has proven useful to understand catalytic reaction mechanisms involving paramagnetic organometallic catalysts. The expediency of EPR spectroscopy in the study of organometallic chemistry and homogenous catalysis is contextualized in the introductory Sect. 1. Section 2 of the review focusses on examples of C-C and C-N bond formation reactions, with an emphasis on catalytic reactions where ligand/substrate non-innocence plays an important role. Both carbon and nitrogen centered radicals have been shown to play an important role in these reactions. A few selected examples of catalytic alcohol oxidation proceeding via related N-centered ligand radicals are included in this section as well. Section 3 covers examples of the use of EPR spectroscopy to study important commercial ethylene oligomerization and polymerization processes. In Sect. 4 the use of EPR spectroscopy to understand the mechanisms of Atom Transfer Radical Polymerization is discussed. While this review focusses predominantly on the application of EPR spectroscopy in mechanistic studies of C-C and C-N bond formation reactions mediated by organometallic catalysts, a few selected examples describing the application of EPR spectroscopy in other catalytic reactions such as water splitting, photo-catalysis, photo-redox-catalysis and related reactions in which metal initiated (free) radical formation plays a role are included as well. EPR spectroscopic investigation in this area of research are dominated by EPR spectroscopic studies in isotropic solution, including spin trapping experiments. These reactions are highlighted in Sect. 5. EPR spectroscopic studies have proven useful to discern the correct oxidation states of the active catalysts and also to determine the effective concentrations of the active species. EPR is definitely a spectroscopic technique that is indispensable in understanding the reactivity of paramagnetic complexes and in conjunction with other advanced techniques such as X-ray absorption spectroscopy and pulsed laser polymerization it will continue to be a very practical tool.

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“…In 2012, Zhang and co-workers reported a regioselective synthesis of multisubstituted furans via metalloradical cyclization of alkynes with a-diazocarbonyls. 18 In their procedure, Co(III)-carbene radicals generated from activation of a-diazocarbonyls by Co(II)-porphyrin complexes undergo a tandem radical addition reaction with alkynes and affords five-membered furan structures (Scheme 5.7). The Co(II) Reaction Procedure (Scheme 5.7): Catalyst [Co(P1)] (2 mol%) was placed in an oven-dried, resealable Schlenk tube, which was capped with a Teflon screw-cap, evacuated and backfilled with nitrogen.…”

Section: Cobalt-catalyzed Heterocycle Synthesismentioning

confidence: 99%

CHAPTER 5. Cobalt-Catalyzed Heterocycle Synthesis

2014

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Cobalt has the chemical configuration [Ar]4s 2 3d 7 and has oxidation states Co(II) and Co(III). Cobalt has many applications in a wide range of areas. In materials, cobalt is primarily used as the metal, for example, in the preparation of magnetic, wear-resistant and high-strength alloys. In biology, cobalt is the active center of coenzymes called cobalamins, the most common example of which is vitamin B 12 . As such, it is an essential trace dietary mineral for all animals. Cobalt in inorganic form is also an active nutrient for bacteria, algae and fungi. In chemistry, various cobalt compounds are used as oxidation catalysts in chemical reactions. Cobalt acetate is used for the conversion of xylene to terephthalic acid, the precursor to the bulk polymer poly(ethylene terephthalate). Typical catalysts are the cobalt carboxylates (known as cobalt soaps). They are also used in paints, varnishes and inks as 'drying agents' through the oxidation of drying oils. The same carboxylates are also used to improve the adhesion of steel to rubber in steel-belted radial tires. Cobalt-based catalysts are also important in reactions involving carbon monoxide. In this chapter, the applications of cobalt catalysts in the synthesis of heterocycles is discussed. Five-Membered HeterocyclesIn 1986, Pattenden and co-workers reported a cobalt-promoted oxidative freeradical cyclization of alkyl bromides, 1 by which functionalized butyrolactones were selectively formed in good yields. In 1990, a convenient method for the stereoselective preparation of trans-2-hydroxymethyltetrahydrofurans by the oxidative cyclization of 5-hydroxy-1-alkenes was developed by Inoki and Mukaiyama. 2 They used bis(1-morpholinocarbamoyl-4,4-dimethyl-1,3-pentanedionato)cobalt(II) [Co(modp) 2 ] as the catalyst with molecular oxygen as the RSC Catalysis Series No.

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Regioselective Synthesis of Multisubstituted Furans via Metalloradical Cyclization of Alkynes with α-Diazocarbonyls: Construction of Functionalized α-Oligofurans

Cited by 182 publications

References 56 publications

Metalloradical Approach to 2H-Chromenes

Metalloradical Approach to 2H-Chromenes

EPR Spectroscopy as a Tool in Homogeneous Catalysis Research

CHAPTER 5. Cobalt-Catalyzed Heterocycle Synthesis

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