摘要:

Thecisandtransisomers of 2-(3-phenylthioureido)cyclopentanecarbonitrile,1, and the respective carboxamides,3, and acids,4, have been prepared. Acid cyclization of both nitriles, faster with thecisisomer, gave the more stablecis-2-thiooxo-cyclopenta[d]pyrimidin-4-one,7. In basecis-1formed thecis4-imino-2-thiooxopyrimidine2which in aqueous alkali broke down via3to the acid4; while in the presence of 66% acetonitrile2rearranged to the 4-phenyliminopyrimidine5to give as final product the thioureido acid6carrying no phenyl group. The1H NMR data for imino and phenylimino derivatives2and5showed strong bias for conformationAwith 1-N pseudoaxial in the cyclopentane ring. Spectra of theEandZisomers of the iminopyrimidine2under slow exchange could be recorded in DMSO-d6. The phenylimino tautomer of5is observed in CD3OD and in CDCl3with theEandZisomers in a 1:1 ratio. In DMSO-d6the phenylamino tautomer5ais also detected. The first process in aqueous KOH, the conversion of nitrilecis-1into the imino intermediate2, reaches an equilibrium which shifts towards the nitrile at higher alkalinities because of ionization of the phenylthioureido group (Ke= [2]/[1] = 2.43 and pKAH= 12.74). The cyclization of1to2is first order in [OH-] while the slower breakdown of2is pH independent. The latter is 104times faster than the hydrolysis of acetonitrile evidencing substantial anchimeric assistance. The change in the reaction route towards the rearranged phenyliminopyrimidine5upon addition of acetonitrile can be caused by the lower dielectric constant favouring the elimination step leading to the intermediate isothiocyanate, and by increased activity of OH-accelerating the (presumably) second order elimination step as opposed to the pH-independent hydrolysis of the imino derivative2.Key words: anchimeric assistance, phenylthioureido nitriles, iminothiooxopyrimidine tautomers, alkaline hydrolysis, solvent effects.

ISSN:0008-4042    

DOI:10.1139/v99-216

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Certain types of aroyl compounds such as benzaldehyde, acetophenone, benzophenone, and anthraquinone produce intense gas-phase luminescence in excited nitrogen at atmospheric pressure. This luminescence was measured in pressure ranges of 1.00–1.67 atm and temperature ranges of 343–473 K. A novel, radioactively (Ni-63) stimulated, high-voltage(<=1750 V/mm), low-current (<=35 nA) discharge in high-purity nitrogen was used for gas chromatographic detection and spectral excitation. The gas-phase luminescence spectra of about sixty aroyl compounds - introduced as gas chromatographic peaks - could thus be measured and compared with literature spectra obtained by conventional excitation incondensedphases. Only a fewgas-phase spectra are available from the literature, and these did agree well with the spectra of this study. A speculative luminescence mechanism is proposed, in which ground-state N2becomes excited by collision with fast electrons. This is followed by efficient triplet-triplet energy transfer from N2(A3&Sgr;u+) to then–>&pgr;* excited aroyl compound.Key words: aroyl phosphorescence, excited-nitrogen excitation, aroyl triplet-triplet excitation, aroyl luminescence detection, gas-phase phosphorescence spectra, gas-phase aroyl detecti

ISSN:0008-4042    

DOI:10.1139/v99-203

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Phosphorus-31 1D NMR spectra of a stationary powder sample of a phosphole tetramer containing two phosphorus spin pairs have been obtained at 4.7 T and 9.4 T. In order to separate31P-31P spin-spin coupling from anisotropic chemical shielding, 2D spin-echo NMR spectra have been acquired. Phosphorus-31 CPMAS NMR experiments indicate that the two spin pairs of the tetramer are equivalent and each may be treated as an isolated spin pair. Within a given spin pair, the difference between the isotropic chemical shifts of two directly bonded phosphorus nuclei is 1.7 ppm. As well, they are spin-spin coupled by both the indirect and direct interactions,1J(31P,31P) = -362 Hz andRDD= 1.80 kHz, respectively. The principal components and relative orientation of the two phosphorus chemical shielding tensors have been determined using the dipolar-chemical shift technique; however, since the dipolar tensor is axially symmetric, ambiguities in the chemical shielding tensor orientation relative to the molecular framework result. Using ab initio calculations and simulations of the 2D spin-echo spectra, many of these ambiguities have been resolved. The spans and skews of the phosphorus shielding tensors for all four three-coordinate phosphorus nuclei are the same within experimental error, 115 ppm and 0.70, respectively. Combined experimental and theoretical results indicate that the phosphorus shielding tensor orientations are dictated by the local environment. For both shielding tensors, the most shielded component, &dgr;33, is approximately 78° from the P-P bond and in the phosphole ring plane. The relative orientation of the &dgr;33components is described by a dihedral angle of 82°, similar to the dihedral angle of approximately 76° defining the twist of the phosphole rings about the bridging P-P bond.Key words: solid-state31P NMR, phosphorus chemical shielding tensors, phosphole tetramer,31P—31P spin pairs, ab initio calculati

ISSN:0008-4042    

DOI:10.1139/v99-229

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

The Mn(OAc)3-mediated hydroxylation of a series of &agr;-carbomethoxy-&ggr;-lactones by molecular oxygen is described. The reaction is regio- and stereoselective and gives &agr;-carbomethoxy-&agr;-hydroxy-&ggr;-lactones.Key words: manganese acetate, &ggr;-lactones, hydroxylation, molecular oxygen.

ISSN:0008-4042    

DOI:10.1139/v99-227

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

A short and efficient synthesis of both enantiomers of enterolactone, a mammalian lignan, is described. The overall yield for the natural enterolactone, over seven steps, was 19% and for its enantiomer 27%.Key words: enterolactone, 4-diphenylmethyl-2-oxazolidinone, succinate, enantioselective synthesis.

ISSN:0008-4042    

DOI:10.1139/v99-220

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

By using steady state and time resolved spectroscopic techniques, investigations were made on the nature and mechanisms of different nonradiative processes involved within the excited donor (or acceptor) and ground state acceptor (or donor) in nonpolarn-heptane (NH), polar protic ethanol (EtOH), and polar aprotic acetonitrile (ACN) solvents at the ambient temperature as well as in EtOH rigid glassy matrix at 77 K. The bicyclic molecule 1,2,3,4-tetrahydroquinoline (THQ) was used as a donor (D) in the present investigation whereas 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) were chosen as electron acceptors (A). When the donor chromophore was excited in presence of an acceptor, highly exothermic electron transfer (ET) reactions seemed to occur from observed large negative driving energy (deltaG0) values, measured by electrochemical techniques, Förster's type energy transfer, static quenching, etc. An attempt was made to estimate separately the contributions of static and dynamic quenching modes in the overall quenching mechanism of donor fluorescence by using a model of modified Stern-Volmer (SV) relation. From this relation it seemingly indicated that the major contribution in quenching originated from the static mode. When excited acceptors react with the ground state donor THQ it is primarily ET reactions that seem to occur. Observations of the transient absorption spectra, by laser flash photolysis techniques, of contact ion-pairs of the present D-A systems along with the triplet absorption spectra of the monomeric acceptor (9FL and 2N9Fl) corroborate our views regarding the occurrence of photoinduced ET reactions within the present reacting systems. At 77 K the combined effect of Förster type energy transfer and static quenching seems to be responsible for observed lowering of donor fluorescence emission intensity in presence of acceptors (9FL or 2N9FL). However, the phosphorescence lifetime measurements reveal that the triplet donor might be involved in photoinduced ET reaction with the unexcited acceptor in rigid glassy matrix at 77 K and this possibly causes the reduction in the phosphorescence band intensity of the donor THQ in presence of the latter one.Key words: electron transfer, static quenching, fluorescence quenching, phosphorescence, energy transfe

ISSN:0008-4042    

DOI:10.1139/v99-224

出版商:NRC Research Press    

年代:2000

数据来源: NRC

摘要:

Apparent molar heat capacitiesCp,φand apparent molar volumesVφfor aqueous diethanolamine (HOC2H4)2NH, diethanolammonium chloride (HOC2H4)2NH2Cl,N,N'-dimethylethanolamine (HOC2H4)(CH3)2N, andN,N'-dimethylethanolammonium chloride (HOC2H4)(CH3)2NHCl were determined from 283.15 to 328.15 K with a Picker flow microcalorimeter and vibrating tube densimeter. The experimental results have been analyzed in terms of Young's Rule with the Guggenheim form of the extended Debye-Hückel equation and appropriate corrections for chemical relaxation effects. These calculations lead to standard partial molar heat capacities and volumes for the neutral amines, (HOC2H4)2NH(aq) and (HOC2H4)(CH3)2N(aq), and the ions (HOC2H4)2NH2+(aq) and (HOC2H4)(CH3)2NH+(aq) over the experimental temperature range.Key words: standard partial molar volumes, standard partial molar heat capacities, diethanolamine, dimethyethanolamine, aqueous alkanolamine ionization

ISSN:0008-4042    

DOI:10.1139/v99-232

出版商:NRC Research Press    

年代:2000

数据来源: NRC