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

Abstract

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.

Glycopolymers based on carbohydrate or vinyl backbones and their biomedical applications

“…As shown in Fig. 3b, the majority of current studies on ROP monomers focus on several five- and six-membered rings with high ring strain, such as the cyclic carbonates/phosphates reported by the Wooley group, 60–69 the cyclic ethers reported by the Buchard group 8–10,70,71 and the cyclic lactam reported by the Grinstaff group. 72–75 However, the field has developed slowly because the construction of a new ring often requires tedious protection and deprotection processes.…”

“…As shown in Fig. 3b, the majority of current studies on ROP monomers focus on several five-and six-membered rings with high ring strain, such as the cyclic carbonates/phosphates reported by the Wooley group, [60][61][62][63][64][65][66][67][68][69] the cyclic…”

“…Bicyclic glucose-based carbonate monomers for the ROP of polycarbonates could be obtained by the cyclisation reactions of glucose diols with phosgene derivatives such as triphosgenge 60 and bis ( pentafluorophenyl) carbonate, 61 as widely reported by the Wooley lab. Five-membered cyclic carbonate monomer 13 could be directly synthesized by the carbonylation reaction of hydroxyl groups at the 2 and 3 positions of commercially available methyl 4,6-O-benzylidene-α-D-glucopyranoside, 66,69 while six-membered monomers 14, 15 and 16 could be prepared in three steps. Specifically, the hydroxyl groups at the 2 and 3 positions of methyl 4,6-O-benzylidene-α-D-glucopyranoside were first protected by methyl groups, 60,61 benzyl groups 62 or acyl groups 64,67 in more than 90% yield.…”

“…70 Wooley et al reported the preparation of some regioregular and regiorandom poly(D-glucose carbonate) derivatives with different protecting groups from 15a-f and provided an explanation for the regioselectivity of the TBD-catalyzed ROP. 61,64,67,69 Specifically, poly(D-glucose carbonate) (PGC) with methoxy protecting groups exhibited a regiorandom property when the polymers modified by alkyl groups with different lengths were regioregular. 61,64,67 To explore the regioselectivity during the initiation and propagation stages, the unimers and dimers of 15e and 15f with carbonate side chains were obtained by decreasing the equivalent weight of initiator 4-MeBnOH and temperature to −78 °C for limiting chain growth, respectively (Fig.…”

Upgrading of Biobased Glycerol to Glycerol Carbonate as a Tool to Reduce the CO2 Emissions of the Biodiesel Fuel Life Cycle

“…As a building block, GC has been included in diverse classes of polymeric materials, such as acrylates [ 29 , 30 ], polyesters [ 31 , 32 , 33 ], isocyanate-free polyurethanes [ 33 , 34 , 35 , 36 , 37 ], hybrid structures [ 38 , 39 ], and others [ 40 , 41 ]. Finally, GC can also be used as a carbonate carrier able to transfer the same function to other polyols, including sugars [ 42 , 43 ], through transcarbonylation processes. The carbonate function can also be considered as a precursor to highly electrophilic (and valuable) epoxy function [ 44 , 45 ].…”