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Isotactic poly(vinyl ether)s (PVEs) have recently been identified as a new class of semicrystalline thermoplastics with a valuable combination of mechanical and interfacial properties. Currently, methods to synthesize isotactic PVEs are limited to strong Lewis acids that require a high catalyst loading and limit the accessible scope of monomer substrates for polymerization. Here, we demonstrate the first Bronsted acid catalyzed stereoselective polymerization of vinyl ethers. A single-component imidodiphosphorimidate catalyst exhibits a sufficiently low pKa to initiate vinyl ether polymerization and acts as a chiral conjugate base to direct the stereochemistry of monomer addition to the oxocarbenium ion reactive chain end. This Bronsted acid catalyzed stereoselective polymerization enabled an expanded substrate scope compared to previous methods, the use of chain transfer agents to lower catalyst loading, and the capability to recycle the catalyst for multiple polymerizations.

Oxetanes are important motifs for drug discovery and are valuable templates in organic synthesis. Much of their use as synthetic intermediates exploits their inherent strain, often resulting in chain extensions at the expense of the heterocycle. Modifications on the carbon alpha to the oxygen of oxetanes, such as the C═O of β-lactones, extend the modes of reactivity. Nevertheless, the outcomes are still largely predictable. On the other hand, other alpha modifications, such as a ═CH2, a spiro-oxiranyl moiety, or a spiro-cyclopropyl group, increase strain and open pathways not available to simple oxetanes or β-lactones. Methods in generating 2-methyleneoxetanes, 1,5-dioxaspiro[3.2]hexanes, and 4-oxaspiro[2.3]hexanes have been developed by us and others. To date, reactions of these systems have sometimes been predictable, but often the outcomes have been unexpected. This has provided fertile ground for thinking about what controls reactivity and what other reaction pathways might be accessible to these strain-heightened oxetanes.This Account summarizes the published literature on the most straightforward approaches to 2-methyleneoxetanes, dioxaspirohexanes, and oxaspirohexanes and on their reactivity. In contrast to simple oxetanes, reactions of 2-methyleneoxetanes with nucleophiles at C4 release an enolate rather than an alkoxide. Also, 2-methyleneoxetanes can be converted to homopropargyl alcohols or undergo a silicon accelerated isomerization/electrocyclic ring opening, processes accessible only because of the exocyclic double bond. In addition, oxetane oxocarbenium ions, derived from protonation of the enol ether, can react with nucleophiles to provide 2,2-disubstituted oxetanes. Oxaspirohexanes are readily prepared by Simmons-Smith cyclopropanation of 2-methyleneoxetanes. These unusual systems undergo a variety of substituent dependent rearrangements in the presence of the Lewis acid BF3·Et2O. In addition, upon treatment with Zeise's dimer, oxaspirohexanes are transformed to synthetically useful 3-methylenetetrahydrofurans. Dioxaspirohexanes are easily accessed by dimethyldioxirane oxidation of 2-methyleneoxetanes. Predictably, dioxaspirohexanes react with many nucleophiles to give α-functionalized-β'-hydroxy ketones. Unexpectedly, 2,2-disubstituted oxetanes can also be selectively produced. This latter pathway has led to further unusual transformations, illuminating computational studies, and novel routes to biologically relevant molecules.