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  • 1
    Electronic Resource
    Electronic Resource
    13C NMR study of the substituent effects on the internal steric hindrances in amidines (1989)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 27 (1989), S. 577-581 
    Details
    ISSN: 0749-1581
    Keywords: 13C NMR ; Rotation barriers ; Amidines ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 13C NMR spectra of ten formamidines, five acetamidines and five butyramidines were recorded at low temperatures. The barrier to rotation around the C—N1 bond for N2-alkylformamidines is approximately 50 kJ mol-1 and for N2-benzylformamidines approximately 52 kJ mol-1, and is thus ca. 10 kJ mol-1 lower than for N2-phenylformamidines. The height of the barrier is related to Taft's substituent constants at N2, δG* = 49.25 + 8.08σ* -3.21Es. In the acetamidines the steric hindrance between the alkyl substituent at N2 with CFCH3 contributed to the decrease in δ* (lower than 40 kJ mol-1). The i-C3H7 substituent at CF in butyramidines forces isomerization and these amidines become Z isomers. MNDO calculations indicate no significant changes of bond length on E/Z isomerization, and the low rotational barrier in butyramidines is probably caused by the steric interaction of the aromatic ring at N2 with the N1 (CH3)2 group.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260270610
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  • 2
    Electronic Resource
    Electronic Resource
    A 13C NMR study of hindered rotation in N1,N1-penta- and N1, N1-hexa-methylene-N2-substituted-phenylformamidines (1987)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 25 (1987), S. 514-517 
    Details
    ISSN: 0749-1581
    Keywords: 13C NMR ; N1,N1-penta-and N1,N1-hexa-methylene-N2-substituted-phenylformamidines ; rotation barriers ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rates of rotations about the C-N1 partial double bond in a series of six N1,N1 penta- and six N1,N1-hexa-methylene-N2-substituted-phenyformamidines were determined from 13C DNMR line shape analysis.Electron accepting substituents at the phenyl ring increase the barrier to rotation, and electron donating substituents decrease the barrier, compared with the nonsubstituted compound. Linear relationships are shown to exist between ΔG≠ and the 13C chemical shifts of the functional carbon or the pKa of formamidines.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260250611
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  • 3
    Electronic Resource
    Electronic Resource
    Solvent effect on the 13C and 15N NMR chemical shifts and rotational barriers in N,1N1-dimethyl-and N,1N1-hexamethylene-N2 -phenylformamidines (1989)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 27 (1989), S. 1031-1038 
    Details
    ISSN: 0749-1581
    Keywords: 13C NMR ; 15N NMR ; Solvent effects ; Amidine rotation barriers ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Solvent-induced 13C NMR chemical shifts of N1,N1-dimethyl-N2-phenylformamidine can be correlated with the acceptor number of solvents, AN, and also with the π* and α parameters of Kamlet and Taft. No correlation was found for the height of the rotational barrier around the C—N1 bond. The self-association of formamidine and the influence of polar, aprotic solvents cause an increase in ΔG1 of approximately 1 kJ mol-1 whereas, contrary to expectations, the ΔG1 values found in hydrogen-bonding solvents are smaller than in cyclohexane. The barrier, however, increases with the strength of the hydrogen bond and on protonation. Downfield shifts of the N1 and upfield shifts of the N2 signals were observed in the 15N NMR spectra of formamidine when the strength of the hydrogen bond with the solvent increased.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260271105
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  • 4
    Electronic Resource
    Electronic Resource
    1H and 13C NMR study of hindered rotation in N1,N1-dimethyl-N2-substituted phenylacetamidines (1988)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 26 (1988), S. 601-607 
    Details
    ISSN: 0749-1581
    Keywords: 1H and 13C NMR ; Rotation barriers ; N1,N1-Dimethyl-N2-substituted phenylacetamidines ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Barriers to rotation about the partial double C—N1 bond were determined from line shape analysis of 1H and 13C dynamic NMR spectra of N1,N1-dimethyl-N2-substituted phenylacetamidines with twelve different substituents on the phenyl ring. The values of ΔGTc≠ are 51.2-58.7 kJ mol-1 and the correlation with Hammett's σ values indicates an important contribution from the substituent effects to the barrier height. The decrease of the rotational barrier of approximately 10 kJ mol-1 in acetamidines in comparison with the respective formamidines is probably mainly due to a steric interaction of the C—CH3 group with aromatic protons; this leads to non-planarity and a decrease of the conjugation of the aromatic ring with the acetamidine moiety.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260260713
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