Unusually Low Barrier to Carbamate C−N Rotation
The Journal of Organic Chemistry2002Vol. 67(11), pp. 3949–3952
Citations Over TimeTop 16% of 2002 papers
Martin J. Deetz, Christopher C. Forbes, Marco Jonas, Jeremiah P. Malerich, Bradley D. Smith, Olaf Wiest
Abstract
The barrier for rotation about an N-alkylcarbamate C(carbonyl)-N bond is around 16 kcal/mol. In the case of an N-phenylcarbamate, the rotational barrier is lowered to 12.5 kcal/mol, but with N-(2-pyrimidyl)carbamates the barriers are so low (<9 kcal/mol) that the syn and anti rotamers cannot be observed as separate signals by 500 MHz NMR spectroscopy at 183 K. X-ray and computational data show that the N-(2-pyrimidyl) carbamates have C(carbonyl)-N bonds that are on average 0.03 A longer than for related N-phenylcarbamates. The computational results trace the origin of the effect to increased single bond character for the C(carbonyl)-N bond due to the increased electron-withdrawing ability of the pyrimidyl ring.
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