Showing papers by "Curt Wentrup published in 2009"

Interconversion of Nitrenes, Carbenes, and Nitrile Ylides by Ring Expansion, Ring Opening, Ring Contraction, and Ring Closure: 3-Quinolylnitrene, 2-Quinoxalylcarbene, and 3-Quinolylcarbene

Abstract: Photolysis of 3-azidoquinoline 6 in an Ar matrix generates 3-quinolylnitrene 7, which is characterized by its electron spin resonance (ESR), UV, and IR spectra in Ar matrices. Nitrene 7 undergoes ring opening to a nitrile ylide 19, also characterized by its UV and IR spectra. A subsequent 1,7-hydrogen shift in the ylide 19 affords 3-(2-isocyanophenyl)ketenimine 20. Matrix photolysis of 1,2,3-triazolo[1,5-c]quinoxaline 26 generates 4-diazomethylquinazoline 27, followed by 4-quinazolylcarbene 28, which is characterized by ESR and IR spectroscopy. Further photolysis of carbene 28 slowly generates ketenimine 20, thus suggesting that ylide 19 is formed initially. Flash vacuum thermolysis (FVT) of both 6 and 26 affords 3-cyanoindole 22 in high yield, thereby indicating that carbene 28 and nitrene 7 enter the same energy surface. Matrix photolysis of 3-quinolyldiazomethane 30 generates 3-quinolylcarbene 31, which on photolysis at >500 nm reacts with N2 to regenerate diazo compound 30. Photolysis of 30 in the presence of CO generates a ketene (34). 3-Quinolylcarbene 31 cyclizes on photolysis at >500 nm to 5-aza-2,3-benzobicyclo[4.1.0]hepta-2,4,7-triene 32. Both 31 and 32 are characterized by their IR and UV spectra. FVT of 30 yields a mixture of 2- and 3-cyanoindenes via a carbene–carbene–nitrene rearrangement 31 → 2-quinolylcarbene 39 → 1-naphthylnitrene 43. The reaction mechanisms are supported by density functional theory calculations of the energies and spectra of all relevant ground and transition state structures at the B3LYP/6–31G* level.

Microwave-Induced Molecular Rearrangements. Flash Thermolysis in the Gas-Phase and in Solution: Synthesis of Quinolones and Naphthyridones

Abstract: 3- and 4-Pyridyliminopropadienones were prepared by flash vacuum thermolysis of Meldrum’s acid derivatives and characterized by low temperature IR spectroscopy. They react with dimethylamine to afford 1,5-, 1,6-, and 1,7-naphthyridones. The same naphthyridones are also obtained by microwave irradiation of the Meldrum’s acid derivatives. Quinolones were obtained by microwave irradiation of phenylaminomethylene-Meldrum’s acids and 1-phenylpyrrole-2,3-diones.

Interconversion of nitrenes, azirenes, and diradicals: rearrangement of 3-isoquinolylnitrene to o-cyanophenylketenimine and 1-cyanoisoindole.

Abstract: Photolysis of tetrazolo[1,5-b]isoquinoline/3-azidoisoquinoline 22T/22A generates 3-isoquinolylnitrene 23, which has been characterized together with a diradical species (25) by Ar matrix ESR spectroscopy. Photolysis at λ > 300 nm generates azirene 24, characterized by IR spectroscopy, whereas further broad-band UV photolysis destroys the azirene to produce o-cyanophenylketenimine 17. The use of 15N-labeled tetrazole/azide 22T′/22A′ demonstrates rapid equilibration of two regioisomeric 15N-labeled azirenes 24′ and 24′′ prior to formation of 17. Flash vacuum thermolysis (FVT) of 22T/22A affords 1-cyano-2H-isoindole 27 in quantitative yield. FVT of 15N-labeled tetrazole/azide 22T′/22A′ causes scrambling of 15N label in the 1-cyano-2H-isoindole product. It is concluded that the interconversion of azirenes 24 takes place via the unobserved diazacycloheptatetraene/diazacycloheptatrienylidene 32/33, and that the rearrangement of azirene to ketenimine 17 and 1-cyanoisoindole 27 takes place via reversion to nitrene 23 followed by ring opening to diradical 25.

Fluoroquinolones from Imidoylketenes and Iminopropadienones, R–N=C=C=C=O

Abstract: 3-Fluoro-, 4-fluoro-, and 2,3,4-trifluorophenyliminopropadienones have been generated by flash vacuum thermolysis (FVT) of 5-[(fluoroarylamino)methoxymethylene]-2,2-dimethyldioxan-4,6-dione (Meldrum’s acid) derivatives. Their reaction with methanol affords interconverting imidoylketenes and oxoketenimines, which are employed in a synthesis of fluoroquinolones. The same quinolones are obtained from methyl 1-fluoroaryl-1,2,3-triazole-4-carboxylates, which on FVT eliminate N2 to generate oxoketenimines. Rearrangement of the oxoketenimines to imidoylketenes and cyclization afford the quinolones.

The Australian Journal of Chemistry – Its New Publishing Concept

Abstract: Curt Wentrup was born in Denmark and educated at the University of Copenhagen (Cand. Scient. 1966 with Prof. K. A. Jensen; DSc 1976), and the Australian National University, Canberra (PhD 1969 with Prof. W. D. Crow). After postdoctoral work with Prof. H. Dahn at the Universite de Lausanne, Switzerland, and a junior faculty position at the same institution, he was appointed professor of organic chemistry at the Universit t Marburg (1976-85) before taking up the Chair of Organic Chemistry at The University of Queensland in 1985. In 2008 he was appointed Emeritus Professor at The University of Queensland. He is a Fellow of the Royal Australian Chemical Institute, was elected Fellow of the Australian Academy of Science in 2000, and received the Centenary Medal of the Australian Commonwealth in 2001. He serves or has served on the editorial or advisory boards of a number of journals and has been the organizer or co-organizer of numerous national and international conferences and symposia, including the well-known Heron Island Conferences on Reactive Intermediates and Unusual Molecules. He collaborates extensively with groups in Australia, Austria, Belgium, Denmark, France, Germany, Singapore, and Switzerland. His research interests are in the field of reactive intermediates, particularly nitrenes, carbenes, zwitterions, and ylides (R-: N:, R(2)C:, and R-CN(+)-X-) and cumulenes (ketenes, ketenimines, and iminopropadienonesRN=C=C=C=O). This research employs flash vacuum thermolysis (FVT), photochemistry, matrix isolation, and in recent years has included microwave-induced thermal chemistry as an alternative to FVT. This technique offers much potential for the application of reactive intermediates in organic synthesis.

Highly Twisted C=C Double Bonds in 4-Methyleneisoxazolones

Abstract: As determined by X-ray crystallography, isoxazol-5(4H)-one derivatives 12–19 feature dihedral angles around the exocyclic C4=C6 double bonds of 26–90°. In the most highly twisted bis-tert-butylamino derivatives 18 and 19, the C4–C6 bonds are essentially single bonds. Density functional theory calculations at the B3LYP/6-31G(d) level with inclusion of a simulated solvent field, which helps stabilize zwitterionic structures, are in good agreement with the experimental crystallographic data. A good correlation between bond lengths and calculated π/π* orbital occupation quotients is observed. A good correlation between the twisting angle and the charge separation, measured by the calculated negative charge on the isoxazolone moiety, is also observed. Low barriers to rotation about the twisted exocyclic double bonds C4=C6 in compounds 13, 20, and 21 (ΔG ‡ = 15–16 kcal mol–1 (63–67 kJ mol–1)) were determined by dynamic 1H NMR coalescence measurements. The rotational barrier for 17 was estimated to be less than 10 kcal mol–1. The rotational barriers for compounds 10 and 18 were calculated to be ~8 kcal mol–1.

Ring opening and ring expansion of 8-cyano-tetrazolo[1,5-alpha]pyridine with secondary amines. Reactions of azides, tetrazoles and nitrenes with nucleophiles, part 2

Abstract: 8-Cyanotetrazolo[1,5-a]pyridine 6T undergoes photochemical ring expansion to afford 1,3-diazepine 7 with diisopropylamine, but with stronger nucleophiles such as dimethylamine a rapid, quantitative ring opening reaction affords dienyltetrazoles 8 and 9 in the dark ( an Addition of Nucleophile - Ring Opening reaction).