Noteworthy Chemistry

February 27, 2012

Carbon dioxide enhances the solubility of copolymers in water. Y. Zhao and co-workers at the University of Sherbrooke (QU) discovered that CO2 can ionize an amine-containing polymer, poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). CO2 dissolves in water to form carbonic acid, which dissociates into HCO ions that interact with the amine groups in the repeat units to form ammonium hydrogen carbonates. This series of reactions makes the polymer soluble in water. Inert gases, such as argon, can be used to purge the dissolved CO2 from the solution and neutralize and desolubilize the polymer.

The authors prepared copolymers of PDMAEMA with poly(N-isopropylacrylamide) (PNIPAM) (1) and poly[2-(2-methoxyethoxy)ethyl methacrylate] (PMEO2MA) (2). Exposing the copolymers to dissolved CO2 enhances the water solubility of PNIPAM and PMEO2MA.

Ordinarily, PNIPAM dissolves in water at ≈31 °C, but the solution turns cloudy with increased temperature. With CO2 present, a PNIPAM copolymer containing 4% PDMAEMA dissolves in water at 40 °C. A copolymer of PMEO2MA with 6% PDMAEMA forms a clear aqueous solution at 25–40 °C in the presence of CO2.

Poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA) functions similarly to its PDMAEMA homologue. A copolymer (3) with 16% PDEAEMA makes PNIPAM miscible in CO2-saturated water. All of these changes can be reversed by purging with argon. (ACS Macro Lett. 2012, 1, 57−61; Sally Peng Li)

Old becomes new: Self-healing siloxane elastomers are rediscovered. In the context of self-healing materials, P. Zheng and T. J. McCarthy* at the University of Massachusetts (Amherst) shed new light on decades-old observations of siloxane equilibration (Kantor, S. W.; Grubb, W. T.; Osthoff, R. C. J. Am. Chem. Soc. 1954, 76, 5190–5197; Osthoff, R. C.; Bueche, A. M.; Grubb, W. T. J. Am. Chem. Soc. 1954, 76, 4659–4663) [Liz: Please link to abstracts.] They synthesized an elastomeric “living” siloxane network that contains cyclic oligomers and reactive end groups via the anionic polymerization of a mixture of octamethylcyclotetrasiloxane and bis(heptamethylcyclotetrasiloxanyl)ethane.

The authors demonstrated the self-healing nature of the silicone elastomer by fracturing it and then heating it at 90 °C for 24 h. Healing is catalyzed by the “living” chain ends. The healed material exhibits cohesive strength and fracture toughness comparable with the original silicone network. After healing and cooling, the elastomer did not fracture at the site of the original crack. The authors attribute the elastomer’s ability to reversibly imprint shapes within the siloxane platform to chemical stress relaxation within the network. (J. Am. Chem. Soc. 2012, 134, 2024–2027; LaShanda Korley)

Detect thiol-containing biomolecules with a fluorescent probe. Cysteine (Cys) and homocysteine (Hcy) are involved in many biological processes, and efficient systems for assaying them in physiological media are in demand. A research team at Shanxi University (Taiyuan, China) led by W. Guo developed a fluorescent bioprobe for detecting these thiol-containing biomolecules.

The researchers synthesized a naphthalimide-based glyoxal hydrazone (1). This fluorogen is weakly fluorescent because its C=N bond rapidly isomerizes in the excited state. Cyclizing its aldehyde unit with the thiol amino acids forms a thiazolidine (2) in the case of Cys and a 1,3-thiazinane (3) in the case of Hcy. The five-membered cyclic intramolecular hydrogen bond systems prevent C=N isomerization–induced fluorescence quenching and enhance the emission of light from the fluorogen. (Org. Lett. 2012, 14, 520–523; Ben Zhong Tang)

Molecular oxygen is trapped and reduced in a cryptand. Many examples of the reduction of molecular oxygen to the peroxide dianion (O22–) in biological systems exist, but the anion’s instability challenges chemists to develop practical soluble sources of O22–. C. C. Cummins, D. G. Nocera, and co-workers at MIT (Cambridge, MA) report the encapsulation of O22– in a hexacarboxamide cryptand (1) on a gram scale. The reversible reduction of O2 to O22– is facilitated by encapsulating O22–.

Superoxide (O2) or O2 can be used to provide the oxygen atoms for encapsulated O22– in 1. When KO2 is added to a slurry of 1 in DMF, the strong tendency of 1 to encapsulate O22– drives the disproportionation of O2 to O2 and O22–. Reducing O2 in situ by cobaltocene (CoCp2) also produces [(O2)⊂1]2– (2) the adduct of 1 and O22–.

The authors elucidated the structure of 2 by using 1H NMR and X-ray crystallography. O22– forms six strong hydrogen bonds (2.63–2.73 Å) with amide protons and six weak hydrogen bonds (3.16–3.23 Å) with aryl protons. The adduct is extremely stable in solution at room temperature and does not decompose even when heated at 50 °C for 100 min.

O2 can be released by treating a solution of 2 with 1.1 equiv 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The O2 yield is 88%. Applying a strong enough reduction potential to the solution via a glassy carbon electrode also releases O2.

Additional electrochemical and chemical redox studies led the authors to propose a two-electron reduction mechanism. The mechanism suggests that O2 from the one-electron reduction of O2 rapidly binds with 1. The bound O2 acquires one more electron from the working electrode or disproportionate to be reduced to O22–. This work is an example of driving chemical reactions via molecular recognition, and it has potential applications in technologies that require practical sources of O22–. (Science 2012, 335, 450–453; Xin Su)

How should sodium hydride be handled safely on a large scale? Three main operational aspects must be considered: the physical handling of NaH from reactor charging to quenching; control of hydrogen release during the reaction; and confirmation of complete quench at the end of the reaction. J. M. McCabe Dunn and coauthors at Merck (Rahway and Summit, NJ), Mochida Pharmaceutical (Shizuoka, Japan), and Chesilton (Colts Neck, NJ) recommend safety protocols for these operations:

  • Reactors should be operated in distillation mode to avoid any “wash back” of water that may remain in dead spots.
  • Hydrogen release should be controlled by adding the starting materials at an appropriate rate and diluting the produced hydrogen with nitrogen for venting.
  • Quench completion should be verified by an online mass spectrometer in the reactor vent that measures the H2/N2 ratio in the vent gas.

(Org. Process Res. Dev. 2011, 15, 1442–1446; Will Watson)

Use a dual catalyst system to prepare L-menthol. L-Menthol is an important chemical for the flavor industry. The five-step commercial synthesis requires an asymmetric catalytic isomerization with a 2,2’-bis(diphenylphosphino)-1,10-binaphthyl (BINAP)–rhodium complex. A shorter method has been developed, but one step is the wasteful rectification of a mixture of (E)- and (Z)-citral to obtain the pure (Z)-olefin needed for the subsequent asymmetric hydrogenation step to produce (R)-citronellal.

Y. Hori and co-workers at Takasago International Corporation (Kanagawa, Japan) developed a dual catalytic system that avoids the rectification step. Following literature reports that organocatalytic hydride reductions of (E)–(Z) mixtures do not require rectification, they used hydrogen gas as the hydride source, Pd/BaSO4 as a heterogeneous catalyst, and chiral amines as organocatalysts.

A screen of several amines and reaction conditions for reducing an (E)–(Z)-citral mixture (1) showed that (R)-2-diphenylmethylpyrrolidine (2, Ar = Ph) catalyzes the formation of (R)-citronellal (3) in 62% yield and 77% ee. Certain substituents on the aromatic rings or the pyrrolidine ring raise the enantiomeric excess to as high as 89%.

The authors believe that their synthesis of L-menthol will be the basis for new heterogeneous organo- and metal-catalyzed asymmetric hydrogenations of prochiral olefins, including conjugated dienes. (Chem. Commun. 2012, 48, 1772–1774; JosÉ C. Barros)

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