This perspective summarizes the chemistry of boroxines and outlines progress towards incorporating these ring structures into functional materials and macromolecular architectures. Special attention is paid to the dynamic covalent chemistry of boroxine ring construction and how these processes lead to novel molecular architectures and functional materials. Also highlighted in this perspective is the rich chemistry surrounding boroxine-ligand interactions and how these interactions flavor many areas of boroxine chemistry.
Boronic acid end-functionalized polycaprolactone (PCL) polymers were synthesized by ring-opening polymerization using a pinacol boronate ester-containing (Bpin) initiator. The polymerization provides access to boron-terminated polymers (i.e. Bpin-PCL-OH) with narrow molecular weight distributions (PDI ¼ 1.09). Postsynthetic manipulation of the polymer's terminal hydroxyl group by copper-catalyzed azide-alkyne cycloaddition chemistry provides a series of bis end-functionalized polymers with significant structural diversity at the termini. Deprotection of the boronate ester end group was accomplished with an acidic solid phase DOWEX resin. The boronate ester deprotection methodology does not result in hydrolysis of the polymeric backbone. The boronic acid-tipped polymers were converted into star polymer assemblies using thermal dehydration and ligand-facilitated trimerization. Thermal dehydration of (HO) 2 B-PCL-OAc to the corresponding boroxine-based star polymer assembly was inefficient and lead to degradation products. Ligand-facilitated trimerization using either pyridine or 7-azaindole as the Lewis base was efficient and mild. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5767-5774, 2010
One of the most well-known, highly utilized reagents for ether cleavage is boron tribromide (BBr3), and this reagent is frequently employed in a 1:1 stoichiometric ratio with ethers. Density functional theory calculations predict a new mechanistic pathway involving charged intermediates for ether cleavage in aryl methyl ethers. Moreover, these calculations predict that one equivalent of BBr3 can cleave up to three equivalents of anisole, producing triphenoxyborane [B(OPh)3] prior to hydrolysis. These predictions were validated by gas chromatography analysis of reactions where the BBr3:anisole ratio was varied. Not only do we confirm that sub-stoichiometric equivalents may be used for ether demethylation, but the findings also support our newly proposed three cycle mechanism for cleavage of aryl methyl ethers.
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