Science Concentrates POLYMERS MOLECULAR ELECTRONICS
One-pot method recycles unwanted polycarbonates
▸ N-heterocyclic carbenes aminate silicon surfaces
In the classification scheme for recycling polymers, polycarbonates typically fall into the forsaken “other” category. That means millions of tons of polycarbonates produced each year for compact disks, headlight lenses, and other applications can’t be easily recycled by conventional methods. Gavin O. Jones, Jeannette M. Garcia, and coworkers at IBM O Research-Almaden H have come up with O O O OH a process that could n help avoid relegatPolycarbonate ing polycarbonates O O to landfills. Their S method allows the easy conversion K2CO3 F F of polycarbonates into poly(aryl ether O O sulfone)s in the presS ence of a carbonate H salt and a bis(aryl O O OH fluoride) (Proc. n Natl. Acad. Sci. USA Polysulfone 2016, DOI: 10.1073/ pnas.1600924113). In this reaction, a carbonate salt degrades The carbonate salt polycarbonate, with the reactive intermediates initiates depolymercondensed by a sulfone-containing aryl fluoride to ization of the polyform value-added poly(aryl ether sulfone). carbonate, and the sulfone-containing aryl fluoride condenses the resulting reactive phenoxide intermediates into poly(aryl ether sulfone)s, with the loss of carbon dioxide as a by-product. The researchers performed computational studies to determine that the carbonate salt’s role is twofold: It decomposes the polycarbonate by nucleophilic attack and promotes the reaction of subsequently formed phenolate dimers with the aryl fluoride.—CELIA ARNAUD
A synthetic approach based on N-heterocyclic carbenes provides a way to covalently link amines and aminals in close proximity to silicon surfaces. The technique could be useful for modulating the electronic properties of silicon chips used in the semiconductor industry. Jeremiah A. Johnson of Massachusetts Institute of Technology and coworkers developed the strategy in which they functionalize silicon by inserting the persistent carbene into Si–H surface bonds (J. Am. Chem. Soc. 2016, DOI: 10.1021/ jacs.6b04962). Silicon chips could already be aminated in other ways. But the carbene-insertion technique is the first to attach amine groups only one carbon away from the surface, rather than through a typically long spacer group. A one-carbon separation positions nitrogen close enough to the silicon surface to modify how readily electrons can escape, potentially easing customization for specific microelectronics applications. In addition, the well-controlled reactivity of carbenes reduces problematic side-reactions, and the technique derivatizes surfaces with better site-selectivity than some existing amine-insertion techniques. Johnson says his group plans to investigate the performance of these novel carbene-derived monolayers on silicon in solar cells and to explore substrates beyond silicon.—STU
BORMAN
REACTION MECHANISMS
▸ Arylation reaction fails ascorbic acid test Ascorbic acid is familiar to many organic chemists as a useful reducing agent to help promote reactions. For example, researchers have thought that ascorbic acid is needed in conjunction with tert-butyl nitrite to carry out C–H activation and radical arylation of heterocyclic N-oxides to make biaryl compounds. Richard A. J. Horan of GlaxoSmithKline and coworkers published a paper in Organic Process Research & Development last year expanding on the selective metal-free coupling reaction. But it wasn’t until the researchers later ran control experiments omitting ascorbic acid as part
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C&EN | CEN.ACS.ORG | JULY 11, 2016
of their ongoing investigations that they discovered they could get the same results without it. “This was a complete surprise, as ascorbic acid is a well-precedented additive,” Horan says. “As a result, we thought it appropriate to retract our paper at the galley proof stage and investigate the mech-
Contrary to previous assumptions, ascorbic acid isn’t needed to promote this cross-coupling reaction. NH2 O O +
OH O
OH OH HO OH L-Ascorbic acid ONOC(CH3)3
O
O O
+N
O–
nism in greater depth” (Org. Process Res. Dev. 2015, DOI: 10.1021/acs.oprd.5b00231). The team has now published a new version of the paper, with a warning for fellow chemists to not take the role of ascorbic acid for granted (Org. Process Res. Dev. 2016, DOI: 10.1021/acs.oprd.6b00117). “This has highlighted the importance of mechanistic understanding in organic chemistry, and we hope that our work will encourage further research in this area,” Horan says. “In my mind, this case shows how science should work,” adds Kai Rossen, a research director at Sanofi and editor-in-chief of OPR&D. “The next +N question will be how the – O common assumptions about this chemistry will be corrected.”—STEVE RITTER
