The far-ultraviolet photochemistry of alkylcyclopropenes in solution

Fahie, Brian J.; Leigh, William J.

doi:10.1139/v89-289

Cited by 23 publications

(53 citation statements)

References 28 publications

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“…For this particular problem, these measurements cast considerable light on the mechanism of this reaction (26). However, these results have much broader application in all of NMR.…”

Section: Discussionmentioning

confidence: 91%

“…The compound studied in this paper is 4-methyl-2-pentyne (I), which is one of the products of the solution phase photolysis of 1,3,3-trimethylcyclopropene (2) (26). To elucidate the mechanism of this reaction, compound 2 was synthesized with a 13C label at position 1, and the postulated mechanisms lead to this label being at position 2 or position 3 of the alkyne 1 (26).…”

Section: Resultsmentioning

confidence: 99%

“…To elucidate the mechanism of this reaction, compound 2 was synthesized with a 13C label at position 1, and the postulated mechanisms lead to this label being at position 2 or position 3 of the alkyne 1 (26). It was therefore crucial to determine the enrichment at the various sites in 1 while working with a restricted amount of material (approximately 3.5 mg).…”

Section: Resultsmentioning

confidence: 99%

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Improvements in the quantitation of NMR spectra by the use of statistical methods

Bain¹,

Fahie²,

Kozluk³

et al. 1991

Can. J. Chem.

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. Can. J. Chem. 69, 1189 (1991). In many applications of NMR spectroscopy to chemistry, quantitation plays a key role. This paper argues that peak heights not only can be used for quantitative work in NMR but that for careful work they should be used. Integrals are accurate, but because they depend on many subjective factors, their precision is suspect. Peak heights are very reproducible on modem spectrometers and so, given this precision, the accuracy can be obtained as well by using calibration methods. An example of the application of these methods is given in the case of a 13c labelling experiment that elucidates a photochemical transformation. Introduction NMR is certainly well known as one of the best methods for qualitative analysis: the identification of a molecule. Analysis of 13C and 'H spectra can often solve a structure completely, and the development of two-dimensional NMR methods has dramatically advanced this process (1, 2). These methods are based on spectral analysis, i.e., the positions of the lines in the spectrum, but quantitative analysis has also been an important part of NMR spectroscopy (3, 4). Counting protons using integrals is a very familiar part of proton NMR. Theory says that in the absence of saturation, the integral over a spectral line is directly proportional to the number of spins and the proportionality is the same for all lines in the spectrum. This simple quantitation can be extended and applied so that accurate and precise integration of NMR spectra is important in many fields (5-10). Measurements of enantiomeric and diastereomeric excess are crucial in the design of chiral synthesis. Particularly in isotopic labelling studies, such as the one described in this work, NMR is one of the few methods than can give a detailed picture of the mechanism of the reactions. The important data in this case are relative enrichments at different sites in the molecule (6, 7), so separations are impossible, and mass spectrometry is not selective enough to identify individual sites. However, the validity of these results depends crucially on the precision and accuracy of the quantitation of the NMR data.Normal methods of quantitation using integration are not without problems (3,4,11,12). In practice, the process involves several subjective decisions on the part of the operator. The choice of integration region, the phasing of the spectrum, and the correction of the baseline are all under the control of the operator, and each may profoundly affect the value of the integral. Careful work with good signal-to-noise ratios and standard conditions can minimize the variation of the results, but there will still be a random component due to the operator's particular choices as well as possible biases between operators (12). Typical estimates of the reliability of an integral are 3-5%

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“…For this particular problem, these measurements cast considerable light on the mechanism of this reaction (26). However, these results have much broader application in all of NMR.…”

Section: Discussionmentioning

confidence: 91%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Improvements in the quantitation of NMR spectra by the use of statistical methods

Bain¹,

Fahie²,

Kozluk³

et al. 1991

Can. J. Chem.

Self Cite

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“…Determination of the 13 C isotopomeric distribution was carried out by 13 C NMR spectroscopy and MS analysis. For this purpose two pyrolysis runs of both pure 15(2-13 C) and 5(2-13 C) were used for each 13 [15,52].…”

Section: Methodsmentioning

confidence: 99%

“…[22,54] Pyrolysis of 5: Products identified by NMR spectroscopy. 17: see above, 8: published spectrum, [15,23,52,57] 32: published spectrum, [15,23,52,57] (E)-31: published spectrum, [15,23,57] (Z)-31: published spectrum, [15,23,52,57] 27: published spectrum, [23,50] 29: published spectrum. [23,57] Pyrolysis of 17: Products identified by NMR spectroscopy.…”

Section: Methodsmentioning

confidence: 99%

Gas-Phase Kinetic and Mechanistic Studies of some Interconverting Alkylcyclopropene Pairs: Involvement of Dialkylvinylidene Intermediates and Their Quantitative Behaviour

Schulenburg¹,

Hopf²,

Walsh³

2000

Chem. Eur. J.

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The pyrolyses of two isomeric pairs of alkylcyclopropenes, namely 1,3-dimethyl-(15) and 1-ethyl-cyclopropene (16), and 1,3,3-trimethyl-(5) and 1-isopropyl-cyclopropene (17), have been studied in the gas phase. Complete product analyses at various conversions up to 95% were obtained for the decomposition of each compound at five temperatures over a 40 degrees C range. The time-evolution data showed that the isomerisation reactions 15<==>16 and 5<==>17 were occurring. Kinetic modelling of each system allowed the determination of rate constants for these and all other decomposition processes. Tests confirmed that all reactions were unimolecular and homogeneous. Arrhenius parameters are reported for overall reactions and individual product pathways. Further kinetic analysis allowed us to extract the propensities (at 500 K) for 1,3-C-H insertion of the dialkylvinylidene intermediates involved in the rearrangements as follows: k(prim):k(sec): k(tert)= 1:16.5:46.4. Additional experiments with 13C-labelled cyclopropenes yielded alkyl group migration aptitudes for the dialkylvinylidenes (from the pattern of 13C in the alkyne products) as follows: Me:Et:iPr=1:3.1:1.5. Explanations for these trends are given. Another important finding is that of the dramatic rate enhancements for 1,3-diene product formation from the 1-alkylcyclopropenes; this can be explained by either hyperconjugative stabilisation of the vinylcarbene intermediates involved in this pathway, or their differing propensities to 1,2 H-shift. The observed large variations in product distribution amongst these four cyclopropenes is interpreted in terms of these specific effects on individual pathways.

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Alkyl-Wanderungstendenzen bei der Vinyliden-Acetylen-Umlagerung und Isotopeneffekt in der Vinylidenbildung aus einem deuterierten Cyclopropen

1999

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Das Geheimnis des Mechanismus von Vinyliden‐Umlagerungen wurde durch die Verwendung von spezifisch markierten Cyclopropenen unter milden, thermischen Bedingungen gelüftet (siehe Gleichungen). 13C‐Markierung liefert die überraschende 1,2‐Alkyl‐Wanderungstendenz Et>iPr>Me. Durch D‐Markierung gelang die erste Bestimmung des primären Isotopeneffektes der Ringöffnung eines Cyclopropens zu einem Vinyliden.

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The far-ultraviolet photochemistry of alkylcyclopropenes in solution

Cited by 23 publications

References 28 publications

Improvements in the quantitation of NMR spectra by the use of statistical methods

Improvements in the quantitation of NMR spectra by the use of statistical methods

Gas-Phase Kinetic and Mechanistic Studies of some Interconverting Alkylcyclopropene Pairs: Involvement of Dialkylvinylidene Intermediates and Their Quantitative Behaviour

Alkyl-Wanderungstendenzen bei der Vinyliden-Acetylen-Umlagerung und Isotopeneffekt in der Vinylidenbildung aus einem deuterierten Cyclopropen

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