No abstract

Amines are critical functional groups that are incorporated into many biologically active compounds and functional materials of importance to the biomedical, agrochemical and fine-chemical industries. [1] An idealized synthetic approach for the preparation of this important class of compounds would take advantage of the direct and byproduct free conversion of feedstock alkenes directly into unprotected amines with good regio-and stereoselectivity under mild reaction conditions. These goals could be realized with early transition metal catalyzed hydroaminoalkylation (Scheme 1), a CÀH functionalization reaction a to nitrogen that results in selective C À C bond formation. [2, 3] However, to date, all promising Group 4 and 5 metal complexes for this transformation demand harsh reaction conditions. [4,5] The identification of a system that can be used with mild reaction conditions is desirable. Here we show that by using a sterically demanding, N,O-chelating, electron-withdrawing phosphoramidate as an easily installed auxiliary ligand, room-temperature alkene hydroaminoalkylation can be achieved for the first time.Unlike late transition metal catalysts (Ir, Ru) [6,7] for this reaction, early transition metal catalysts (Ti, [4] Zr, [4a] Ta, [5] Nb, [5e-g] ) do not require a removable directing group or activated alkene substrates. Hydroaminoalkylation results in unprotected amines ready for further functionalization. This transformation gives selectively substituted amines in a single and atom-economic catalytic reaction, using inexpensive early transition metals of low toxicity. Thus, hydroaminoalkylation is an excellent reaction to target for advances in green chemistry.Ligand screening investigations by Herzon and Hartwig have shown that electron-withdrawing chloride ligands enhance reactivity, such that select substrate combinations yield products at 90 8C. [5b] Notably, the more challenging dialkylamine substrates required temperatures of 150 8C and thermally polymerizable styrene derivatives were not reported. [4b] Herein, we show that our phosphoramidate-ClTaMe 3 precatalyst is easily synthesized and can achieve roomtemperature hydroaminoalkylation with a broad range of substrates.

Eliminating small molecules from dioxolane rings affords isotactic poly(mandelic acid), with competing chain transfer overcome through dynamic vacuum polymerisation.

The mechanism of controlled radical polymerization of vinyl acetate using vanadium catalysts is investigated using a range of experimental and computational studies. Optimal control is achieved using the non-innocent bis(imino)pyridine ligand framework. [BIMPY]VCl 3 , where BIMPY = 2,6-[(2,6i Pr 2 C 6 H 3)N=C(Me)] 2 (C 5 H 3 N)), is one of only a few transition metal systems capable of mediating the polymerization of vinyl acetate. Initiation using AIBN at 120°C results in excellent control over poly(vinyl acetate) molecular weights and PDIs, to give vanadium-terminated polymer chains which can be readily converted to both proton-terminated poly(vinyl acetate) or poly(vinyl alcohol). Irreversible halogen transfer from the parent [BIMPY]VCl 3 complex to a radical derived from AIBN generates the active species, [BIMPY]VCl 2. This catalyst cannot use the halogen atom transfer equilibrium to control polymerization, but can act as a persistent radical and trap the propagating polymer chains through an OMRP reversible termination process. Computational studies support this novel two-step reaction pathway and reveal that the poor control exerted over styrene versus the excellent control observed for vinyl acetate under these conditions is not only dependent on radical reactivity, but also due to chelation of the carbonyl group of vinyl acetate to the vanadium center, making the trapping step more favorable. This correlates with an energy difference of just 4 kcal/mol between the reduced [BIMPY]VCl 2 , and [BIMPY]VCl 2 R species for vinyl acetate compared to over 20 kcal/mol for styrene. This [BIMPY]VCl 3 system can be extended to other vinyl ester monomers, with good control over molecular weights and PDIs obtained for vinyl propionate, vinyl pivalate and vinyl benzoate.

A synthetic protocol using atom-economic, catalytic hydroaminoalkylation and ring-opening metathesis polymerization (ROMP) has been developed for the versatile synthesis of a new class of aryl-substituted secondary aminecontaining polymers. This catalytic route minimizes waste generation and avoids protection/deprotection protocols, postpolymerization modification, and byproduct formation. Different amines can be readily incorporated to access variable hydrogen-bonding characteristics. Thermal and melt rheological characterization has shown the profound effect of hydrogen bonding on the bulk properties of these amine-containing norbornene polymers. ■ INTRODUCTIONThere is growing interest in developing approaches to synthesize amine-containing polymers due to their diverse range of applications including antimicrobial materials, 1−5 compatibilizers for polymer blends, 6−8 CO 2 uptake, 9−11 water purification, 12,13 and catalytic materials. 14−20 Current strategies for the synthesis of these high-value products are often plagued by multistep monomer syntheses with inefficient protection/ deprotection protocols 1,21−23 and/or the use of postpolymerization modifications. 24,25 These traditional routes often result in limited control over amine functionalization and poorly defined polymer microstructures. 26 In recent years, ROMP 20 has emerged as a powerful tool for the synthesis of functionalized polymers including polyamides. 27−30 Notably, ROMP conditions rarely tolerate free primary or secondary pendant amine groups, 31,32 presumably due to the fact that these nucleophilic and sterically accessible amines can coordinate and decompose catalytic Ru species. 33 More recently, select norbornene monomers with tertiary or secondary alkylamines have been shown to undergo ROMP using a newly reported cationic Mo alkylidene metathesis catalyst. 34 However, these reports involve multistep synthetic protocols and stoichiometric reagents to access the requisite norbornene substrates. Thus, to date, the controlled polymerization of unprotected amine-containing monomers has not been a practical and efficient strategy for accessing this broadly useful class of materials.Our approach for addressing synthetic challenges in aminecontaining polymer synthesis exploits our recently developed hydroaminoalkylation catalysts, 35,36 coupled with promising results in ROMP of functionalized monomers (Scheme 1). 37 Hydroaminoalkylation is an α-C−H alkylation reaction of secondary amines with alkene substrates. 36 By using our N,Ochelated tantalum phosphoramidate precatalyst 1, 35 amine functionalized, cyclic alkene monomers can be prepared in one solvent-free, atom-economic reaction of cyclic diene precursors (Scheme 2). Thus, these two catalytic strategies can be combined to access a flexible and modular synthetic approach while realizing optimized atom and step efficiency. Importantly, hydroaminoalkylation yields unprotected secondary amines that participate in hydrogen bonding to give amine-containing polymers with tunable physical and me...