The Elusive Seven-Membered Cyclic Imino Ether Tetrahydrooxazepine

Cyclic imino ether
heterocycles are used as ligands in transition
metal catalysis, in various drugs and as reactive monomers in living
cationic ring-opening polymerization (CROP). While five- and six-membered
cyclic imino ethers, i.e. 2-oxazolines and 4,5-dihydro-1,3-oxazines,
have extensively been studied in these areas, their seven-membered
ring counterparts have remained unexplored. Herein, we report the
synthesis of 2-phenyl-4,5,6,7-tetrahydro-1,3-oxazepine allowing reassignment
of the earlier, incorrectly reported 4,5,6,7-tetrahydro-1,3-oxazepines
as their N-acylated pyrrolidine isomers. Finally,
we also report a comparison of the CROP reactivity of a homologous
series of cyclic imino ethers with a 2-carbon, 3-carbon, and 4-carbon
methylene bridge, revealing a remarkable ring size effect.