USING HYDROGEN-BONDING TO CONTROL CARBAMATE C-N ROTAMER EQUILIBRIA

In chloroform solution, the syn/anti rotamer ratios for N-(2-pyridyl)c arbamates, 3, and N-phenylcarbamates, 4, are close to 0.05. Addition o f the double hydrogen bonding acetic acid moderately stabilizes the sy n rotamer of 4, but has no measurable effect on the syn/anti ratio for 3. Conversely, the hydrogen bond donor-acceptor-donor triad in 2,6-bi s(octylamido)pyridine, 1, strongly stabilizes the syn rotamer of 3, bu t has no effect on the synlanti ratio for 4. The K-a for syn-3:1 is 10 (3)-10(4) times higher than the K-a for anti-3:1. This implies that th e alkoxy oxygen in anti-3 is a much poorer hydrogen bond acceptor than the carbonyl oxygen in syn-3, most likely because of a combination of steric and electrostatic factors.