Neutral and Cationic Main Group Lewis Acids - Synthesis, Characterization and Anion Complexation

The molecular recognition of fluoride and cyanide anions has become an increasingly pertinent objective in research due to the toxicity associated with these anions, as well as their widespread use. Fluoride is commonly added to drinking water and toothpastes to promote dental health, and often used in the treatment of osteoporosis, however, high doses can lead to skeletal fluorosis, an incurable condition. Cyanide is also an extremely toxic anion, which binds to and deactivates the cytochrome-c oxidase enzyme, often leading to fatality. The molecular recognition of these anions in water has proven to be challenging. For fluoride, the anion is small, and thus, efficiently hydrated (?H�hyd = -504 KJ/mol), making its complexation in aqueous environments particularly difficult. In addition to being small and efficiently hydrated like the fluoride anion, cyanide has a pKa(HCN) of 9.3 making its competing protonation in neutral water a further complication. Recent efforts to complex fluoride and cyanide have utilized triarylboranes, which covalently bind the anion. Monofunctional triarylboranes display a high affinity for fluoride with binding constants in the range of 105-106 M-1 in organic solvents, and chelating triarylboranes exhibit markedly higher anion affinities. These species, however, remain challenged in the presence of water. This dissertation focuses on the synthesis and properties of novel Lewis acids designed for the molecular recognition of fluoride or cyanide in aqueous environments. To this end, a group 15 element will be incorporated into a main group Lewis acidcontaining molecule for the purpose of: i) increasing the Lewis acidity of the molecule via incorporation of a cationic group, and ii) increasing the water compatibility of the host. Specifically, a pair of isomeric ammonium boranes has been synthesized. These boranes are selective sensors which selectively bind either fluoride or cyanide anions in water. The study of phosphonium boranes has revealed that the latent Lewis acidity of the phosphonium moiety is capable of aiding triarylboranes in the chelation of small anions. Finally, my research shows that Br�nsted acidic H-bond donors such as amides, when paired with triarylboranes, are capable of forming chelate complexes with fluoride

Metal-free stabilization of monomeric antimony(I) : a carbene-supported stibinidene

A diamidocarbene was coordinated to an antimony(III) dichloride Lewis acid. Subsequent reduction with magnesium gave a monomeric, formally antimony(I) fragment that is supported by the diamidocarbene. Spectroscopic, crystallographic, and computational analyses demonstrated that the carbene ligand engages the antimony(I) center in π‐backbonding resulting in a short (2.068(7) Å) SbC interaction that is comparable to those observed in known stibaalkenes

A Seven-Membered N,N '-Diamidocarbene

Condensation of N,N'-dimesitylformamidine with phthaloyl chloride afforded 1 center dot HCl, which, upon treatment with base, afforded 2,4-dimesitylbenzo[e][1,3]diazepin-1,5-dione-2-ylidene (1), a seven-membered N,N'-diamidocarbene (DAC), in high yield (85%). The free DAC was used to synthesize four new, late transition metal complexes: [Rh(cod)(1)Cl] (2a) (cod = 1,5-cyclooctadiene), [Ir(cod)(1)Cl] (2b), [Rh(CO)(2)(1)Cl] (3a), and [1-AuCl] (5). The Tolman electronic parameter (TEP) of 1 was calculated to be 2047 cm(-1) from the IR spectrum of 3a. This TEP value is approximately 10 cm(-1) than known DACs and 5 cm(-1) lower than known imidazol-2-ylidenes, indicating that DAC 1 is a relatively strong electron donor. Additionally, electrochemical analyses of 2a and 2b corroborated the IR data obtained on 3a and revealed E-1/2 values that were shifted cathodically by ca. 0.16 V when compared to analogous complexes supported by N-heterocyclic carbenes. The gold complex 5 was found to catalyze the hydration of phenylacetylene, affording acetophenone in yields up to 78% after 12 h at 80 C at a catalyst loading of 2 mol %. Treatment of 1 with 2,6-dimethylphenylisocyanide afforded N, N'-diamidoketenimine 4 as a thermally robust, crystalline solid

Ammonia N-H activation by a N,N '-diamidocarbene

The synthesis and characterization of N,N'-dimesityl-4,6-diketo-5,5-dimethylpyrimidin-2-ylidene is reported; this crystalline N,N'-diamidocarbene was found to split ammonia and engage in other reactions not exhibited by typical N-heterocyclic carbenes

Selective scission of pyridine-boronium complexes: mechanical generation of Bronsted bases and polymerization catalysts

Coupling two pyridine-capped poly(methyl acrylate) (PMA) chains of varying molecular weights to bis(pentafluorophenyl) boron chloride afforded the first examples of boronium-based polymers. Ultrasonication of CH3CN solutions of these polymers with number average molecular weights (M-n) > 40 kDa induced selective scission of a boron-pyridine bond, affording a two-fold reduction in M-n. The liberated pyridine was used to effect a colorimetric change, via a stoichiometric Bronsted acid-base reaction with an indicator, and to catalyze the polymerization of alpha-trifluoromethyl-2,2,2-trifluoroethyl acrylate. No reduction in M-n, colorimetric response, or polymerization activity were observed (i) in the absence of sonication, (ii) for polymers with M-n < 40 kDa, (iii) for a high molecular weight PMA (Mn 110 kDa) containing a terminal boronium species, or (iv) when the boron-pyridine adduct was not covalently linked to a PMA chain. Collectively, these results support the notion that the aforementioned scission processes were induced by an applied mechanical force

Antimony(v) cations for the selective catalytic transformation of aldehydes into symmetric ethers, α,β-unsaturated aldehydes, and 1,3,5-trioxanes

1-Diphenylphosphinonaphthyl-8-triphenylstibonium triflate ([2][OTf]) was prepared in excellent yield by treating 1-lithio-8-diphenylphosphinonaphthalene with dibromotriphenylstiborane followed by halide abstraction with AgOTf. This antimony(V) cation was found to be stable toward oxygen and water, and exhibited exceptional Lewis acidity. The Lewis acidity of [2][OTf] was exploited in the catalytic reductive coupling of a variety of aldehydes into symmetric ethers of type L in good to excellent yields under mild conditions using Et3SiH as the reductant. Additionally, [2][OTf] was found to selectively catalyze the Aldol condensation reaction to afford α-β unsaturated aldehydes (M) when aldehydes with 2 α-hydrogen atoms were used. Finally, [2][OTf] catalyzed the cyclotrimerization of aliphatic and aromatic aldehydes to afford the industrially-useful 1,3,5 trioxanes (N) in good yields, and with great selectivity. This phosphine–stibonium motif represents one of the first catalytic systems of its kind that is able to catalyze these reactions with aldehydes in a controlled, efficient manner. The mechanism of these processes has been explored both experimentally and theoretically. In all cases the Lewis acidic nature of the antimony(V) cation was found to promote these reactions

N,N '-Diamidoketenimines via Coupling of Isocyanides to an N-Heterocyclic Carbene

Treatment of an N-heterocyclic carbene that features two amide groups N-bound to the carbene nucleus with various organic isocyanides afforded a new class of ketenimines in yields of up to 96% (isolated). DFT analyses revealed that the carbene exhibits a unique, low-lying LUMO, which may explain the atypical reactivity observed

Amino-acrylamido carbenes : modulating carbene reactivity via decoration with an α,β-unsaturated carbonyl moiety

Two 6-methoxy-4-oxo-1,3-diaryl-3,4-dihydropyrimidinium salts (2a and 2b) have been prepared as precursors to novel amino-acrylamido carbenes. Treatment of 2a (where the aryl groups are mesityl groups) with one equivalent of sodium hexamethyldisilazide in aromatic hydrocarbons affords the persistent amino-acrylamido carbene 3a, which has been characterized spectroscopically. This novel carbene has been trapped with Ir(I) transition metal fragments as well as electrophilic carbon disulfide and nucleophilic isocyanides. Infrared spectroscopic studies carried out on 3a-Ir(CO)2Cl (5a) indicated that this carbene exhibits a Tolman electronic parameter (TEP) of 2049 cm–1, a value that suggests that 3a is a stronger donor than both diamidocarbenes (DACs) and a recently reported amino-ureido carbene (DAC TEPs ≈ 2057 cm–1; amino-ureido TEP = 2058 cm–1), but similar σ-donating properties to a monoamido-amino carbene (TEP = 2050 cm–1). This result has been corroborated by DFT analyses carried out on all four species, which indicated that the HOMO and LUMO energies of 3a are comparable to the amino-ureido and monoamido-amino carbenes, whereas the DAC was shown to be more electrophilic with a much lower energy LUMO than the other three carbenes. Surprisingly, deprotonation of 2b (where the N-substituents are 2,6-diisopropylphenyl groups) does not afford the anticipated carbene. Indeed, 1H NMR spectroscopic analysis indicates the formation of a novel bent allene or carbodicarbene (3b), which decomposes rapidly in solution at room temperature

Photochemically Switching Diamidocarbene Spin States Leads to Reversible Büchner Ring Expansions

The discovery of thermal and photochemical control by Woodward and Hoffmann revolutionized how we understand chemical reactivity. Similarly, we now describe the first example of a carbene that exhibits differing thermal and photochemical reactivity. When a singlet ground-state <i>N,N</i>′-diamidocarbene <b>1</b> was photolyzed at 380 nm, excitation to a triplet state was observed. The triplet-state electronic structure was characteristic of the expected biradical σ¹p_π¹ spin configuration according to a combination of spectroscopic and computational methods. Surprisingly, the triplet state of <b>1</b> was found to engage a series of arenes in thermally reversible Büchner ring expansion reactions, marking the first examples where both cyclopropanation and ring expansion of arenes were rendered reversible. Not only are these photochemical reactions different from the known thermal chemistry of <b>1</b>, but the reversibility enabled us to perform the first examples of photochemically induced arene exchange/expansion reactions at a single carbon center