摘要:

An analytical expression for the two-dimensional reaction surface appropriate for discussing a concerted reaction has been derived for the quartic approximation to a reaction coordinate, and shown to be consistent with Marcus theory. By analysis of literature data for SN1 solvolyses and carbanion formation we have been able to reproduce the qualitative behavior of eliminations involvingtBuBr,iPrBr, and EtBr in EtOH/EtO−, and to predict the rates of these reactions within the uncertainties of the input data.Keywords: concerted reactions, Marcus theory, eliminations, reaction surfaces.

ISSN:0008-4042    

DOI:10.1139/v90-256

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Rates of ketonization of the enol of acetophenone, generated by flash photolytic photohydration of phenylacetylene, were measured in aqueous sulfuric and perchloric acid solutions over the concentration range 1–50 wt.% acid; rates of enolization of acetophenone, monitored by bromine scavenging, were also measured in aqueous perchloric acid solutions over the same concentration range. The results suggest that the curvature observed in a previousXacidity function correlation of the rate of enolization in sulfuric acid solutions was an artifact produced by insufficiently efficient scavenging, and that introduction of the activity of water in the correlating expression, used previously to eliminate the curvature and believed to reflect covalent involvement of water in the enolization reaction, is unnecessary. The present results also show that the keto–enol equilibrium constant for acetophenone decreases with increasing acidity in these concentrated sulfuric and perchloric acid solutions.Key words:acetophenone, enolization, ketonization, keto–enol equilibrium, concentrated acid solutions.

ISSN:0008-4042    

DOI:10.1139/v90-257

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

3,3,6,9,9-Pentamethyl-2,10-diazabicyclo[4.4.0]dec-1-ene has been synthesized and its proton transfer reactions, together with those of the bases 2,2,6,6-tetramethylpiperidine and 1,2,2,6,6-pentamethylpiperidine, have been studied with the carbon acids 4-nitrophenylphenylcyanomethane and 2-methyl-4-nitrophenylphenylcyanomethane. Equilibrium constants, rate constants, primary deuterium isotope effects, and activation parameters are reported for the reactions in acetonitrile solvent. Values of the pKain acetonitrile have been measured for the conjugate acid of each base and the Brønsted β value of 0.56 in acetonitrile has been determined for the proton transfer from 4-nitrophenylphenylcyanomethane.Keywords: proton transfer reactions, kinetic isotope effects, cyclic nitrogen bases.

ISSN:0008-4042    

DOI:10.1139/v90-258

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The reduction ofp-benzoquinone (BQ) by 1-benzyl-1,4-dihydronicotinamide (BNAH) proceeds by a free-radical chain mechanism initiated by single electron transfer (SET). In dry deoxygenated acetonitrile, the chain, whose propagation steps contain a SET–hydrogen atom transfer sequence, could be inhibited by dinitrobenzene and initiated by di-tert-butylperoxyoxalate. The reduction does not follow second-order reaction kinetics. The fractional order of each reactant was found to be 0.80 and 1.36 for BNAH and BQ, respectively. The AG0values for both the initiation and the propagation steps were evaluated electro-chemically (CV) and were found to be of a reasonable magnitude to sustain a free-radical chain process.Keywords: 1-benzyl-1,4-dihydronicotinamide,p-benzoquinone, free radical, reduction.

ISSN:0008-4042    

DOI:10.1139/v90-259

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The photoinitiated bromination of 1-chloropentane withN-bromosuccinimide (NBS) in the presence of Br2is compared with that of NBS and 1,1-dichloroethene to generate the succinimidyl radical (S•) and with that of Br2 + K2CO3to generate the bromine atom (Br•) as the chain carriers. The relative reactivities of intermolecular H-abstraction (r-values) shown by the NBS + Br2system decrease to levels lower than those shown by either S•or Br•chain propagations at lower temperatures under appropriate conditions: these observations indicate the presence of a new chain propagating species that is assumed to be the bromine radical complex (BRC) modified from S•or Br•reversibly under the experimental conditions. Supporting evidence for the equilibria of S•, Br•, and BRC in the photodecomposition of NBS + Br2is discussed. The unusual H-abstraction reactivity of Br•toward the methyl hydrogens of 1 -chloropentane was noted; this serves as independent support for the presence of a new radical propagation species.Keywords:N-bromosuccinimide, succinimidyl radical, photobromination, bromine radical complex.

ISSN:0008-4042    

DOI:10.1139/v90-260

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The pKBH+values for protonation of the ureido groups of the title acids, and for protonation at the 2-carbonyl groups of the product hexahydropyrimidine-2,4-diones, have been determined by standard UV and NMR methods. A bell-shaped dependence of the observed rate constants on H2SO4concentration was found for both the ring-closure and ring-opening directions. This medium dependence is satisfactorily reproduced by means of rate equations based on the excess acidity treatment, providing account is taken of unproductive protonation equilibria. The derived slope parameters (m≠) for the ring-closure direction indicate that the slow step involves proton transfer from nitrogen rather than to oxygen. For the ring-opening or hydrolysis reactions, the corresponding slope parameters (m≠m*) are characteristic of similar reactions known to proceed with slow C—N scission concerted with proton transfer.Keywords: mechanism, hydrolysis, cyclization, acid catalysis.

ISSN:0008-4042    

DOI:10.1139/v90-261

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The mechanism of the photochemical cycloaddition reaction betweenN-benzoylindole,1, and cyclopentene to give cyclobutane adducts 2 and 3 has been examined. The triplet excited state lifetime and quantum yield of intersystem crossing were determined for1as (2.8 ± 0.3) × 10−8 s and 0.39 ± 0.01, respectively, using the triplet counting procedure. In addition, the dependence of the quantum yield of cycloadduct formation upon the concentration of cyclopentene and upon the concentration of excited state quenchers has been determined. The results are used to propose a mechanistic model in which the triplet excited state of1reacts with cyclopentene to give a triplet 1,4-biradical intermediate. Following spin inversion the biradical intermediate reverts to the ground state starting materials or proceeds to the products2and3; this partitioning, along with the quantum yield of intersystem crossing, gives rise to a limiting quantum yield of cycloaddition at infinite alkene concentration of 0.061. It is calculated that 84% of the biradical intermediates revert to the starting materials and 16% proceed to cycloadducts. The quantum yield data are also used to calculate two independent values of the rate constant for reaction of the triplet excited1with alkene; the values are (1.8 ± 0.1) × 107M−1 s−1and (4.0 ± 0.8) × 106 M−1 s−1'. Some evidence for self quenching of the triplet excited state of1by ground state1was also observed. The quantum yield of intersystem crossing and the triplet excited state lifetime of1were found to vary with the solvent used; this is discussed in terms of the possible existence of a charge transfer triplet excited state.Keywords: indole, photocycloaddition, mechanism.

ISSN:0008-4042    

DOI:10.1139/v90-262

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

4′-Bromo-1′-acetonaphthone (BAN) and 4′-bromo-1′-(1-dodecano)naphthone (BLN) exhibit strong phosphorescence in degassed solutions in benzene. We report rate constants (kq) for phosphorescence quenching determined by phosphorescence quenching measurements for the following series of quenchers: pyrene, anthracene, tetracyanoethylene,p-dicyanobenzene,N,N-dimethylaniline (DMA), andN,N-dimethylamino-4-toluidine (DMAT). The first three quenchers quench at the diffusion-controlled rate. DMAT is 20-fold more effective as a quencher than DMA (kq = 1.4 × 106 M−1 s−1), but what is noteworthy is that both activation energies are very small, and quenching by DMAT is favoured over DMA mainly by entropie effects (ΔS≠ = −105 vs.−120 J mol−1 K−1, respectively).Keywords: aromatic ketone photochemistry, triplet exciplex.

ISSN:0008-4042    

DOI:10.1139/v90-263

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Utilizing thujone-derived intermediates, efficient routes to the natural polygodial family of antifeedants as well as novel analogues within this family are described. One of the key features of this study involves the insertion of an oxygen function into a tertiary carbon center adjacent to a cyclopropane ring system thus providing convenient intermediates for subsequent elaboration. An interesting ring expansion of five-membered to six-membered ring systems by means of tri-n-butyltin hydride is also described.Keywords: antifeedants, polygodial, thujone.

ISSN:0008-4042    

DOI:10.1139/v90-264

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

1-(1-Methyl-2-pyrrolyl)-2,2,2-trifluoroethylp-nitrobenzoate (3) reacts by carbocation formation with anmvalue for the dependence of rate on the solvent polarity parameterYOTsof 0.56, and a rate 41 times slower than (1-methyl-2-pyrrolyl)methylp-nitrobenzoate. The products from3include significant amounts of material derived from solvent attack at the 5-position of the pyrrole ring. The results indicate a high degree of charge delocalization by the strongly donating pyrrolyl group, resulting in a lowk(H)/k(CF3) rate ratio and nucleophilic attack on the ring. 1,1, l-Trifluoro-2-(1-methyl-2-pyrrolyl)propan-2-ol (10) reacted with CF3CO2H via a tertiary carbocation, which led to dimeric products resulting from electrophilic substitution on the pyrrole ring.Keywords: pyrrole, trifluoromethyl substituents, carbocations.

ISSN:0008-4042    

DOI:10.1139/v90-265

出版商:NRC Research Press    

年代:1990

数据来源: NRC