Complexities of Regioselective Ring-Opening vs Transcarbonylation-Driven Structural Metamorphosis during Organocatalytic Polymerizations of Five-Membered Cyclic Carbonate Glucose Monomers

Rigorous investigations
of the organobase-catalyzed ring-opening
polymerizations (ROPs) of a series of five-membered cyclic carbonate
monomers derived from glucose revealed that competing transcarbonylation
reactions scrambled the regiochemistries of the polycarbonate backbones.
Regioirregular poly­(2,3-α-d-glucose carbonate) backbone
connectivities were afforded by 1,5,7-triazabicyclo[4.4.0]­dec-5-ene
(TBD)-catalyzed ROPs of three monomers having different cyclic acetal
protecting groups through the 4- and 6-positions. Small molecule studies
conducted upon isolated unimers and dimers indicated a preference
for Cx–O2 vs Cx–O3 bond cleavage from tetrahedral intermediates
along the pathways of addition–elimination mechanisms when
the reactions were performed at room temperature. Furthermore, treatment
of isolated 3-unimer or 2-unimer, having the carbonate linkage in
the 3- or 2-position as obtained from either Cx–O2 or Cx–O3
bond cleavage, respectively, gave the same 74:26 (3-unimer:2-unimer)
ratio, confirming the occurrence of transcarbonylation reactions with
a preference for 3-unimer vs. 2-unimer formation in the presence of
organobase catalyst at room temperature. In contrast, unimer preparation
at −78 °C favored Cx–O3 bond cleavage to afford
a majority of 2-unimer, presumably due to a lack of transcarbonylation
side reactions. Computational studies supported the experimental findings,
enhancing fundamental understanding of the regiochemistry resulting
from the ring-opening and subsequent transcarbonylation reactions
during ROP of glucose carbonates. These findings are expected to guide
the development of advanced carbohydrate-derived polymer materials
by an initial monomer design via side chain acetal protecting groups,
with the ability to evolve the properties further through later-stage
structural metamorphosis.