Unusually low barrier to carbamate C-N rotation.
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.
Deetz
Martin J
MJ
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46656, USA.
Forbes
Christopher C
CC
Jonas
Marco
M
Malerich
Jeremiah P
JP
Smith
Bradley D
BD
Wiest
Olaf
O