Showing papers by "Curt Wentrup published in 1998"

4,6-Dimethyl-o-quinone Methide and 4,6-Dimethylbenzoxete

Abstract: 4,6-Dimethyl-o-quinone methide (4) was produced by FVP of alcohol 3 or of the trimer 6 and matrix isolated in Ar at 7.6 K. Photolysis of 4 with long wavelength light (>345 nm) at this temperature afforded 4,6-dimethylbenzoxete (5), which was observable up to room temperature in the solid state in the absence of water. 5 can be converted back to 4 by UV irradiation at 254/190 nm. Quantum chemical calculations on the thermal interconversion of 4 and 5 indicate activation barriers of the order of 40 kcal/mol for 4 → 5, and 30 kcal/mol for 5 → 4. The dimer, trimer, and tetramer (8, 6, and 7) of 4 are characterized.

Facile 1,3-Shift of Chlorine in a Chlorocarbonylketene

Abstract: Chlorocarbonyl(phenyl)ketene (2) undergoes a degenerate 1,3-shift of chlorine, as determined by C-13 NMR spectroscopy. The two carbonyl signals (183 and 157 ppm) coalesce at -30 degrees C, and from this as well as line-shape analysis, an activation barrier for the 1,3-Cl shift interconverting 2a and 2a' of Delta G double dagger = 41.8 +/- 4 kJ mol(-1) (10.0 +/- 1 kcal mol(-1)) is calculated. These data are in excellent agreement with calculated (G2(MP2,SVP) and B3-LYP/6311+G(3df,2p)//6-31G* + ZPVE) 1,3-Cl shift and rotational barriers. Analogous 1,3-halogen shifts in acyl isocyanates are predicted.

Retro-Ene Reactions in Acylallene Derivatives

Abstract: Allenic esters and amides 4 undergo a retro-ene reaction to vinylketene (6) and an aldehyde or imine (5) under the conditions of flash vacuum thermolysis (FVT). The same products are obtained by FVT of cyclobutenones 7 via electrocyclic ring opening to alkoxy- or aminovinylketenes 3 and 1,3-rearrangement of ketenes 3 to allenes 4. All the intermediates and products were characterized by matrix isolation IR spectroscopy, and in the case of 4c the reaction was also monitored by online mass spectrometry. A lower temperature for the retro-ene reaction of 4c, eliminating an imine, than for 4a, eliminating formaldehyde, is in agreement with a lower calculated activation barrier (167 and 181 kJ mol-1, respectively, at the G2(MP2,SVP) level of theory). The allenic amide 11 undergoes an analogous retro-ene reaction to the (unobserved) vinylketene 13, the latter isomerizing to cyclohexenylacrolein 16 in a 1,5-H shift (calculated barrier 125 kJ mol-1; G2 (MP2, SVP)).

N-Mesityl-C-acylketenimines: 1,5-Sigmatropic Shifts and Electrocyclization to Quinolines.

Abstract: Flash vacuum thermolysis (FVT) of triazoles 6a-c generates alpha-oxoketenimines 10, the ester 10a being isolable. FVT of pyrroledione 8 generates the isomeric imidoylketene 9a. Ketenes 9 and ketenimines 10 undergo thermal interconversion by 1,3-shifts of methoxy and dimethylamino groups under mild FVT conditions (ca. 350-400 degrees C). Both 9 and 10 are directly observable by IR spectroscopy at either 77 K or on Ar matrix isolation at 12 K. On FVT at temperatures above ca. 400 degrees C, the ketenimines 10 undergo a 1,5-H shift to o-quinoid imines 12/13, followed by electrocyclization to dihydroquinolines 14 (unobserved) and 15 (observed by NMR). The latter are easily oxidized to alkylquinoline-3-carboxylates or quinoline-3-carboxamides 16 by atmospheric oxygen. Ab initio calculations on model compounds 18-23 predict an energy barrier of ca. 38 kcal mol(-1) (161 kJ mol(-1)) for the 1,5-H shift in N-(o-methylphenyl)ketenimines via the transition state TS19 followed by an electrocyclization barrier to dihydroquinoline 23a via TS22a of ca. 16 kcal mol(-1).

Dihydro-1,3-diazepinones and diazabicyclo[3.2.0]heptenones from pyridyl azides

Abstract: 2-Alkoxy-1H-1,3-diazepines 5 and 7, 2-dialkylamino-5H-1,3-diazepines 6, 2,3-dihydro-1H-1,3-diazepin-2-ones 8 and 2,4-diazabicyclo[3.2.0]hepten-3-ones 9 are obtained by photolysis of tetrazolo[1,5-a]pyridines or 2-azidopyridines 1/2, including the unsubstituted parent compounds; the relative stabilities of the 2-alkoxy- and 2-dialkylamino-1,3-diazepines 5 and 6 are in excellent agreement with ab initio energy calculations.

Nitrile N-Selenide (RC⋮NSe) and Isoselenocyanate (RNCSe) Neutrals and Radical Cations by Selenation of Nitriles and Isonitriles: Tandem Mass Spectrometry and ab Initio Studies

Abstract: Cyanogen N-selenide radical cations, NCCNSe (4), have been produced by dissociative ionization of 3,4-dicyano-1,2,5-selenadiazole (3). By using a new hybrid tandem mass spectrometer, it is shown that these ions are excellent agents of selenation of nitriles and isonitriles, producing nitrile N-selenide ions (RCNSe) and isoselenocyanate ions (RNCSe), respectively. The connectivities of these ions have been established by collisional activation, and the stabilities of the corresponding neutrals were probed by neutralization - reionization experiments and ab initio G2(MP2,SVP) calculations. The neutral nittile N-selenides are found by experiment and theory to be observable species in the gas phase. All the cations are potential energy minima, and their calculated fragmentation energies are in accord with experimental observations. NCCNSe is found to be an effective Se transfer agent because of its low N-Se bond dissociation energy and exceptional low-lying LUMO. The effect of substituents (CH, NH, OH, F, Cl, Br, I, CN, and SCH) on the structures and stabilities of RCNSe neutrals is also reported.

Synthesis of aminoquinolones from triazoles via carboxamidoketenimine and amidinoketene intermediates

Abstract: 1-Aryl-1H-1,2,3-triazole-4-carboxamides 7d–f have been synthesized and converted to 2-dimethylamino-4-quinolones 12d–f by flash vacuum thermolysis (FVT). The reaction takes place via carboxamidoketenimine 9 and amidinoketene intermediates 10. The former are observed by FTIR spectroscopy at 77 K or in Ar matrices at 12 K.

On the Di-1-naphthylcarbene−Dibenzofluorene Rearrangement and the Ethylenization of Diarylcarbinols

Abstract: Solution-spray flash vacuum thermolysis of di-1-naphthyldiazomethane (9) results in the formation of dibenzo[c,g]fluorene (12), formed via a carbene- carbene rearrangement of di-1-naphthylcarbene (10). The isomeric dibenzo[α,i]fluorene (11) claimed by Franzen and Joschek (Liebigs Ann. Chem. 1960, 633, 7) is not formed, and no dibenzofluorene is formed on thermolysis in boiling naphthalene. 11 is formed on dehydration of di-1-naphthylcarbinol (14) with phosphoric acid, but the claimed tetra-1-naphthylethylene (13) is not formed, and the so-called ethylenization of diarylcarbinols is cast in doubt generally. The compound previously believed to be 13 is 13-[di(1- naphthyl)methyl]-dibenzo[α,i]fluorene (22). The alleged cleavage of 13 into two molecules of carbene 10 is unsubstantiated.

Carboxy(vinyl)ketene intermediates in the thermolysis of methylthio- and methoxy-substituted Meldrum’s acid derivatives

Abstract: Methylthio- and methoxy-substituted carboxy(vinyl)ketenes 10 and 16a have been identified by Ar matrix isolation FTIR spectroscopy following flash vacuum thermolysis (FVT) of Meldrum’s acid derivatives 7 and 13a. Methylthio(methyl)methyleneketene 9 and alkoxy(methyl)methyleneketene 15 are formed concurrently at high FVT temperatures. The alkoxy(methyl)methyleneketenes 15 do not isomerise to alkoxy(vinyl)ketenes 18, which have been generated and identified in other reactions. Ethoxy(methyl)methyleneketene 15b readily eliminates ethene in a retro-ene type reaction to produce acetylketene 22. Ketenes 15 react with alcohols and amines to produce 3-alkoxybutenoic acid derivatives 21.

Studies of Reactive Intermediates Using Matrix and Gas-Phase Techniques

Abstract: In the direct investigation of reactive intermediates it is particularly valuable to use a combination of several spectroscopic techniques. This commentary highlights recent examples, using primarily flash vacuum thermolysis for the generation of the intermediates, and matrix IR spectroscopy in conjunction with gas-phase mass spectrometric methods for their identification. The examples include nitrile imines, nitrile ylides, nitrile sulfides and selenides, dinitrogen sulfide and several novel cumulenes (X=C=C=Y, RN=C=C=C=X).

Retro‐Ene Reactions in Acylallene Derivatives.

Abstract: Allenic esters and amides 4 undergo a retro-ene reaction to vinylketene (6) and an aldehyde or imine (5) under the conditions of flash vacuum thermolysis (FVT). The same products are obtained by FVT of cyclobutenones 7 via electrocyclic ring opening to alkoxy- or aminovinylketenes 3 and 1,3-rearrangement of ketenes 3 to allenes 4. All the intermediates and products were characterized by matrix isolation IR spectroscopy, and in the case of 4c the reaction was also monitored by online mass spectrometry. A lower temperature for the retro-ene reaction of 4c, eliminating an imine, than for 4a, eliminating formaldehyde, is in agreement with a lower calculated activation barrier (167 and 181 kJ mol-1, respectively, at the G2(MP2,SVP) level of theory). The allenic amide 11 undergoes an analogous retro-ene reaction to the (unobserved) vinylketene 13, the latter isomerizing to cyclohexenylacrolein 16 in a 1,5-H shift (calculated barrier 125 kJ mol-1; G2 (MP2, SVP)).

Dihydro-1,3-diazepinones and Diazabicyclo[3.2.0]heptenones from Pyridyl Azides.

Abstract: 2-Alkoxy-1H-1,3-diazepines 5 and 7, 2-dialkylamino-5H-1,3-diazepines 6, 2,3-dihydro-1H-1,3-diazepin-2-ones 8 and 2,4-diazabicyclo[3.2.0]hepten-3-ones 9 are obtained by photolysis of tetrazolo[1,5-a]pyridines or 2-azidopyridines 1/2, including the unsubstituted parent compounds; the relative stabilities of the 2-alkoxy- and 2-dialkylamino-1,3-diazepines 5 and 6 are in excellent agreement with ab initio energy calculations.