Showing papers by "Curt Wentrup published in 1994"

α-Oxoketenes - Preparation and Chemistry

Abstract: This review describes the methods of generation of alpha-oxoketenes and their use in synthesis. While the ketenes are often generated in situ without direct proof for their existence, methods used for their direct observation are also emphasized. The most important classes of precursor molecules are 2-diazo-1,3-diketones, 1,3-dioxin-4-ones, 2,3-dihydrofuran-2,3-diones, and beta-ketoacid derivatives. Synthetically useful reactions are nucleophilic additions to give carboxylic acid derivatives which can be subjected to further functional group manipulation, [2 + 2] cycloadditions to give four-membered rings, and [2 + 4] cycloaddition chemistry with a wide variety of double-and triple-bonded dienophiles, resulting in numerous six-membered heterocyclic ring systems.

Nitrile Imines: From Matrix Characterization to Stable Compounds

Abstract: The concept of 1,3-dipolar cycloaddition was developed, starting in the late 1950s, largely by Rolf Huisgen and his students in Munich, and it has led to one of the most versatile methods for the construction of five-membered ring heterocycles. Although first known only as transient intermediates, nitrile imines have been at the heart of mechanistic studies of this type of cycloaddition reactions. Hundreds of mechanistic papers appeared in 1960s and 1970s; reliable spectroscopic observations were achieved in the early 1980s both at low temperatures and in the gas phase; finally, the first crystalline nitrile imine was reported in 1988. The unusual structures found by X-ray analyses as well as the facile rearrangements observed experimentally have fostered a new interplay between experiment and theory. The story of nitrile imines, from matrix characterization to stable compounds, nicely illustrates the role that main group elements can play in organic chemistry.

Iminoethenethiones, Rn=c=c=s - Characterization by Neutralization-Reionization Mass-Spectrometry and G2(Mp2) Theory

Abstract: (Methylimino)ethenethione (2) and iminoethenethione (4) are stable molecules on the microsecond time scale of neutralization-reionization mass spectrometry experiments. The corresponding radical cations were generated by fragmentation of thiazolopyrimidinedione molecular ions 1(.+) and 3(.+). Iminoethenethione (4) does not tautomerize to thioformyl cyanide (H-CS-CN, 5) under the wall-less conditions of the MS experiment, but it does so under FVP conditions when generated from isoxazolones 6. Thioformyl cyanide was unequivocally identified by IR and mass spectra. The structures and stabilities of 2, 4, and 4(.+) were investigated by ab initio calculations at the G2(MP2) level of theory. Both 2 and 4 are predicted to have a singlet ground state, in contrast to O double bond C double bond C double bond S, for which a triplet state is preferred. The singlet-triplet gaps are approximately 40 kJ mol(-1). In agreement with experimental findings, both iminoethenethiones are calculated to be thermodynamically and kinetically stable species, lying in energy wells with at least a 100 kJ mol(-1) barrier to dissociation into HNC (or CH3NC) + CS. The IR and UV spectra and ionization energies of 2 and 4 are predicted. The iminoethenethione radical cation (4(.+)) is found to be the global minimum on the C2HNS.+ potential energy surface and stable toward all possible fragmentations: the most favorable fragmentations into H-. + NCCS+ and HNC + CS.+ are in accord with the mass spectrometric observations.

Ketene-Ketene Rearrangement: Substituent Effects on the 1,3-Migration in .alpha.-Oxo Ketenes

Abstract: The mechanism of the 1,3 migration in a series of a-ore ketenes, RCOCH=C=O (R = H, CH3, NH2, OH, SH, SiH3, OCH3, SCH3, N(CH3)(2), and C6H5),was studied by high-level ab initio molecular orbital calculations up to the QCISD(T)/6-311+G(2d,p) + ZPVE level of theory, The ore ketene-oxo ketene rearrangement is predicted to proceed via a four-membered cyclic transition structure. The calculated activation barrier depends strongly on the nature of the migrating group (R), ranging from 20 to 200 kJ mol(-1). n-Electron donor substituents (e.g., NH2 and SH) are found to stabilize the four-centered transition structures. For the dimethylamino substituent, the 1,3-shift is predicted to be a facile process (barrier = 20 kJ mol(-1)), and a stable four-membered cyclic interemediate is found to exist on the reaction profile. The predicted 1,3-migratory aptitude is in the order N(CH3)(2) > SCH3 > SH > NH2 > OCH3 > OH > SiH3 > H > C6H5 > CH3. This calculated trend is consistent with experimental observations. The stabilization of the transition structure can be rationalized by the donor-acceptor interaction between an appropriate n- or pi-electron donor substituent and the vacant carbon p orbital of the ketene LUMO. The E/Z conformational preferences (gas phase and solution), rotational barriers, and IR spectra of alpha-ore ketenes are also reported.

Iminopropadienones, RN=C=C=C=O: Theory and experiment

Abstract: Ab initio molecular orbital calculations at the MP2/6-31G* level have been used to examine the structures and infrared spectra of a new class of compounds, the iminopropadienones, RN=C=C=C=O (R = H, CH3, and Ph). The agreement between calculated and experimental IR spectra of PhNCCCO, Ph15NCCCO, and PhNCC13CO is excellent. Inclusion of electron correlation is found to be important for the correct prediction of the relative intensities of the cumulenic stretching vibrations. All three iminopropadienones are predicted to have a slightly bent NCCCO backbone (∠CCC ≈ 176°). As with carbon suboxide, these cumulenes are calculated to have an extremely flat CCC bending potential. The parent compound, HNCCCO (4a) is calculated to be thermodynamically stable toward dissociations into (i) HNCC + CO, (ii) HNC + CCO, (iii) HN + CCCO, and (iv) H• + NCCCO•. Rearrangement of 4a to the more stable cyanoketene isomer requires a sizeable barrier of 402 kJ mol-1 [G2(MP2)]. The calculated stability of 4a is consistent with its experimental observation in neutralization-reionization mass spectrum. The adiabatic ionization energy and heat of formation of HNCCCO are predicted to be 9.51 eV and 175 kJ mol-1, respectively.

Nitrilimine: von der Charakterisierung in Matrix zu stabilen Verbindungen

Abstract: Aus dem Konzept der 1,3-Dipolaren Cycloaddition, das zum grosten Teil auf Rolf Huisgen und seine Schule in Munchen zuruckgeht und das aus den spaten funfziger Jahren stammt, hat sich eine der vielseitigsten Methoden fur den Aufbau funfgliedriger Heterocyclen entwickelt. Obwohl zuerst nur als labile Zwischenstufen bekannt, standen Nitrilimine im Mittelpunkt mechanistischer Untersuchungen dieser Art von Cycloadditionen, wie Hunderte von Publikationen aus den sechziger und siebziger Jahren zeigen. Spektroskopisch nachgewiesen sowohl bei niedrigen Temperaturen als auch in der Gasphase wurden diese Verbindungen in den fruhen achtziger Jahren; 1988 schlieslich wurde das erste kristalline Nitrilimin vorgestellt. Die ungewohnlichen, durch Rontgenbeugung ermittelten Strukturen und auch die Leichtigkeit, mit der diese Verbindungen umlagern, haben das Zusammenspiel von Experiment und Theorie gefordert. Die Geschichte der Nitrilimine von der Charakterisierung in Matrix bis zu stabilen Verbindungen veranschaulicht in hervorragender Weise die Rolle, die Hauptgruppenelemente in der Organischen Chemie spielen konen.

Iminoethenone Radical Cations (RN=C=C=O.cntdot.+): Tandem Mass Spectrometry and ab Initio MO Studies

Abstract: Radical cations of iminoethenone, RN=C=C=O-.+ (R = H and CH3), have been generated by dissociative ionization of isoxazolopyrimidine precursors and structurally characterized by collisional activation mass spectrometry. Contrasting with the behavior of their sulfur analogues, vertical electron transfer in a neutralization experiment does not result in the formation of observable RN=C=C=O neutrals. This finding is in excellent accord with high-level ab initio calculations at the G2(MP2) lever of theory. Although both HNCCO (2) and CH3NCCO (4) are calculated to be stable equilibrium structures, dissociation into HNC (or CH3NC) + CO require activation energies of only similar to 10 kJ mol(-1). Both 2 and 4 are predicted to have a singlet ground state, 78 and 83 kJ mol(-1), respectively, below the triplet. Iminoethenone radical cation (2(.+)) is predicted to be the lowest-energy C2HNO+ ion and is stable toward all possible fragmentations: the most favorable fragmentations into H-.+ NCCO+ and HNC + CO.+ are in accord with the mass spectrometric observations.

Protonated Forms of Iminopropadienones, RN:C:C:C:O, and Cyanoketenes: Combined ab initio MO and Mass Spectrometry Studies

Abstract: The structures and unimolecular fragmentation reactions of protonated iminopropadienones, RN=C=C=C=O (R=H and CH3), were investigated by a combination of mass spectrometry based experiments (collisional activation (CA), neutralization-reionization (NR), and chemical ionization (CI) mass spectrometry and flash vacuum pyrolysis) and high-level ad initio molecular orbital calculations, at the QCISD(T)/6-311+G(2d,p) level of theory. The C- and N-protonated forms, RN(+)=C-CH=C=O and R(HN+=C=C=O, were separately generated from a variety of precursors. The experiments and calculations indicate that the most favorable site of protonation of iminopropadienones is the central carbon atom (C-2), and the resulting ions are best regarded as N-alkylated or N-protonated cyanoketenes. The N-protonated iminopropadienone is close in energy to the C-protonated one, while the O-protonated form is significantly higher in energy. The proton affinities (298 K) of HNCCCO, CH3NCCCO, NCCECO, and NCC(CH3)CO are predicted to be 861, 920, 784, and 798 kJ mol(-1), respectively.

An improved synthesis of 5-alkyl-2,3-dihydro-furan-2,3-diones

Abstract: The synthesis of 5-tert-butyl-2,3-dihydrofuran-2,3-dione (2a) from ethenyloxysilane (3) and oxalyl chloride is reinvestigated. Apart from (2a), two other reaction products, namely furan-3-yl oxalate (6) and difuran-3-yl oxalate (7) are isolated, explaining the low yield of (2a) in this process. An improved method for the preparation of (2a), and its 5-methyl analog (2b) by cyclization of acylpyruvic acids (8a,b) is described

Synthesis and flash vacuum pyrolysis of isoxazolo- and isothiazolo[5,4-d]pyrimidines

Abstract: The multi-step synthesis of isothiazolo[5,4-d]pyrimidines (1c,d) and isoxazolo[5,4-d]pyrimidine (1b) is described. Flash vacuum pyrolysis of lb leads to phenyliminopropadienone, Ph-N=C=C=C=O, which was identified by Ar matrix infrared spectroscopy.

Nitrile Imines: From Matrix Characterization to Stable Compounds

Abstract: The concept of 1,3-dipolar cycloaddition was developed, starting in the late 1950s, largely by Rolf Huisgen and his students in Munich, and it has led to one of the most versatile methods for the construction of five-membered ring heterocycles. Although first known only as transient intermediates, nitrile imines have been at the heart of mechanistic studies of this type of cycloaddition reactions. Hundreds of mechanistic papers appeared in 1960s and 1970s; reliable spectroscopic observations were achieved in the early 1980s both at low temperatures and in the gas phase; finally, the first crystalline nitrile imine was reported in 1988. The unusual structures found by X-ray analyses as well as the facile rearrangements observed experimentally have fostered a new interplay between experiment and theory. The story of nitrile imines, from matrix characterization to stable compounds, nicely illustrates the role that main group elements can play in organic chemistry.

Characterization of new diimines of carbon suboxide by tandem mass spectrometry

Abstract: Bis-aryliminopropadiene radical-cations, [ArN=C=C=C=NAr']+., have been generated by dissociative ionization of isoxazolopyrimidine precursors and structurally characterized by collisional activation mass spectrometry; the corresponding neutral cumulenes have also been produced in neutralization-reionization experiments.

Contribution of Photoelectron-Spectroscopy to the Study of the Flash Vacuum Pyrolysis of Amino-Substituted Meldrum Acid-Derivatives

Abstract: The photoelectron spectroscopic study of the thermolysis of amino-substituted Meldrum's acids confirms the formation of several intermediates in the gas phase: their structure depends on the amine function being either secondary or tertiary. In the first case, imidoylketene is clearly characterized, while in the second case cyclic end-products are identified. In both cases, the electronic structure characterization of unstable methylketene is not obvious from PE data.

(2 + 4) Cycloaddition Reactions of Neat Dipivaloylketene.

Abstract: Various [2+4] hetero-Diels-Alder cycloddition reactions of dipivaloylketene (2) with polarized multiple bond systems are described. Aldehydes and ketones react with2 to furnish 1,3-dioxin-4-ones7, imines and heterocumulenes (X=C=N) form 1,3-oxazin-4-ones9 and11, respectively. Analogous reactions of2 with ethyl vinyl ether, ethoxyacetylene, diethylcyanamide, and adamantanethione leading to [2 + 4] adducts12, 14, 15 and16 are also reported. All these cycloadditions proceed in high yield at very mild reaction conditions.

Synthesis and Flash Vacuum Pyrolysis of Isoxazolo- and Isothiazolo(5,4- d)pyrimidines.

Abstract: The multi-step synthesis of isothiazolo[5,4-d]pyrimidines (1c,d) and isoxazolo[5,4-d]pyrimidine (1b) is described. Flash vacuum pyrolysis of lb leads to phenyliminopropadienone, Ph-N=C=C=C=O, which was identified by Ar matrix infrared spectroscopy.