Imaging of Aliphatic Polycarbonates with Photogenerated Base

Davies, Jack D.; Daly, William H.; Wang, Zhikai; Kutal, Charles

doi:10.1021/cm9504504

Cited by 13 publications

(5 citation statements)

References 12 publications

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“…Halide salts are the most common end products, and none exhibit any appreciable lipophilicity . The racemic tetraphenylborate salt 1 3+ 3B(C 6 H 5 ) 4 – was prepared some time later and was noted to be soluble in methanol and acetone. We sought to access related fluorinated tetraarylborate salts, particularly with the “barf” anion, B(3,5-C 6 H 3 (CF 3 ) 2 ) 4 – (BAr f – ).…”

Section: First-generation Catalystmentioning

confidence: 99%

Launching Werner Complexes into the Modern Era of Catalytic Enantioselective Organic Synthesis

2020

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Metrics & MoreArticle Recommendations CONSPECTUS: Reactions catalyzed by transition metal complexes almost always entail binding of one or more reactants to the metal center, and nearly every corner of the "chiral pool" has been picked over in efforts to develop enantioselective catalysts. As reported by Alfred Werner in 1911−1912, salts of the formally D 3symmetric [Co(en) 3 ] 3+ trication (en = ethylenediamine) were among the first chiral inorganic compounds to be resolved into enantiomers. These air-and water-stable complexes are substitution-inert, so for 100 years they languished without application in organic synthesis. We then showed that when they are rendered soluble in organic media by lipophilic anions such as fluorinated tetraarylborates BAr f − , they become potent catalysts for a variety of carbon−carbon and carbon−heteroatom bond forming reactions. These involve substrate activation by hydrogen bonding to the coordinated NH 2 units (pK a ca. 15), a "second coordination sphere" mechanism. Only modest enantioselectivities are obtained with [Co(en) 3 ] 3+ 3BAr f − or related chromium, rhodium, iridium, and platinum salts. However, high enantioselectivities are achieved when the three en ligands are replaced by the 1,2-diphenyl analogues (S,S)-or (R,R)-H 2 NCHPhCHPhNH 2 . Here only one BAr f − anion is required to solubilize the trication, so a number of mixed-salt catalysts (2X − BAr f − ) have been evaluated. Alternatively, a dimethylamino group can be tethered to the backbone of one en ligand, providing bifunctional catalysts that obviate any need for an external base. Interestingly, the counteranions modulate the enantioselectivities somewhat. However, catalysts with chiral anions do not significantly outperform benchmark catalysts with achiral anions. Cagelike chiral hexaaminecobalt(III) complexes known as sepulchrates and sarcophagines, which feature secondary NH donor atoms, can also serve as catalysts, but the enantioselectivities are very low. In a spinoff application, certain salts are found to be superb "chiral solvating agents", leading to distinct sets of NMR signals for enantiomers of chiral analytes with Lewis basic functional groups. Loadings of 10−25 mol % generally suffice, providing the best way of assaying the enantiomeric purities of a host of compounds. Also, mixtures of several chiral compounds can be simultaneously analyzed. It is not surprising that complexes that perform well in chiral recognition phenomena also excel as enantioselective catalysts. In this Account, the stereochemical properties of the preceding complexes are treated, as well as arcana generally known only to specialists in the field. These include the use of charcoal for equilibrating configurations of the cobalt stereocenter and Sephadex for separating enantiomers and diastereomers. Other types of metal-containing hydrogen-bond-donor catalysts are briefly surveyed (noncoordinating NH units can also be effective), including several developed by other groups. However, the mechanisms of enantioselection in all o...

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Section: First-generation Catalystmentioning

confidence: 99%

Launching Werner Complexes into the Modern Era of Catalytic Enantioselective Organic Synthesis

2020

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“…One factor inhibiting progress may be the lack of solubility in organic solvents. The racemic tetraphenylborate salt [Co(en) 3 ](BPh 4 ) 3 has been reported and is only soluble in methanol and THF. , We recently found that the more lipophilic B(3,5-C 6 H 3 (CF 3 ) 2 ) 4 “barf” (BAr f ) salt could be easily prepared by adding a CH 2 Cl 2 solution of NaBAr f to an aqueous solution of enantiopure Δ-[Co(en) 3 ]I 3 . , The trication rapidly transferred into the organic phase, and solvent removal afforded the hydrate Δ-[Co(en) 3 ](BAr f ) 3 ·14H 2 O. This substance was soluble in a wide range of organic solvents, and the water is believed to be associated with a “second” or “outer” coordination sphere involving hydrogen bonding with the NH protons.…”

Section: Introductionmentioning

confidence: 99%

New Media for Classical Coordination Chemistry: Phase Transfer of Werner and Related Polycations into Highly Nonpolar Fluorous Solvents

et al. 2013

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Optimized procedures for the previously reported conversions of 1,3-diiodobenzene and perfluorohexyliodide (Rf6I; copper, DMSO, 140 °C) to 1,3-C6H4(Rf6)2 (3; 86-70%) and 3 to Br(3,5-C6H3(Rf6)2 (2; NBS, H2SO4/CF3CO2H; 88-75%) are described. The latter is converted (t-BuLi, BCl3) to the "fluorous BArf" salt NaB(3,5-C6H3(Rf6)2)4 (1 or NaBArf6; 77-70%), as given earlier. When orange aqueous solutions of [Co(en)3]Cl3 (en = ethylenediamine) are treated with perfluoro(methylcyclohexane) (PFMC) solutions of 1 (1:3 mol ratio), the aqueous phase decolorizes and [Co(en)3](BArf6)3 can be isolated from the fluorous phase (96%). Similar reactions with the trans-1,2-cyclohexanediamine analogue [Co(R,R-chxn)3]Cl3 and [Ru(bipy)3]Cl2 give [Co-(R,R-chxn)3](BArf6)3 (92%) and [Ru(bipy)3](BArf6)2 (95%). All of these salts are isolated as hydrates and exhibit toluene/PFMC partition coefficients of ≤1:≥99, establishing that the anion BArf6(-) can efficiently transport polar polycations into highly nonpolar fluorous phases. When equal volumes of CH2Cl2 and PFMC are charged with the "nonfluorous" BArf (B(3,5-C6H3-(CF3)2)4) salt [Co(en)3](BArf)3 and 3.0 equiv of the fluorous salt NaBArf6, the cobalt trication partitions predominantly into the fluorous phase (64:36). The arene 2 crystallizes in a polar space group (tetragonal, I4, Z = 8) with fluorous and nonfluorous domains and all eight bromine atoms located essentially on one face of the unit cell.

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“…Photobase generators attract interest because they can solve the erosion problem caused by photoacid generators and are used in many fields such as coating, inks and photoresists 5–13. Cobalt–amine complexes,14 acyl oximes,15 carbamates,16 formamides17 and quaternary ammonium arylalkylborate salts10, 11 have been used as photobase generators. Unlike other PBGs, quaternary ammonium arylalkyl borate salts can generate not only base but also free radical when exposed to UV radiation.…”

Section: Introductionmentioning

confidence: 99%

Behavior of quaternary ammonium salt photobase generators on initiating free radical photo‐polymerization

Zhong

Yang

Zeng

et al. 2006

Polymer International

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Series of quaternary ammonium tetraphenyl borates salt photobase generators (PBGs) were synthesized using p-methoxyphenacylmethylene and 2-naphthoymethylene as chromophores, and triethylene diamine, pyridine or 3-methyl pyridine as tertiary amine. The kinetics for polymerization of trimethylolpropane triacrylate (TMPTA) monomer using PBGs as free radical photo-initiator was monitored by differential photo-calorimeter (DPC). It was found that all the quaternary ammonium tetraphenyl borate salt photobase generators synthesized could initiate free radical polymerization of the acrylate monomer TMPTA by exposure to UV irradiation, but the activity was relatively low. Addition of a small amount of polar solvent to the system could largely increase the polymerization rate and final conversion. Photo-polymerization was also improved by increasing light intensity or raising reaction temperature. PBGs with p-methoxyphenacylmethylene as chromophore had higher absorbance at around 280 nm and showed higher activity in initiating photo-polymerization than those with N-(2-naphthoylmethyl) as chromophore.

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Imaging of Aliphatic Polycarbonates with Photogenerated Base

Cited by 13 publications

References 12 publications

Launching Werner Complexes into the Modern Era of Catalytic Enantioselective Organic Synthesis

Launching Werner Complexes into the Modern Era of Catalytic Enantioselective Organic Synthesis

New Media for Classical Coordination Chemistry: Phase Transfer of Werner and Related Polycations into Highly Nonpolar Fluorous Solvents

Behavior of quaternary ammonium salt photobase generators on initiating free radical photo‐polymerization

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