Spotlights Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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SYNTHESIZING POLYPEPTIDES THROUGH PHOTOCHEMISTRY IN FLOW Polypeptides are long, linear chains of amino acids linked by amide bonds. These biopolymers take on a plethora of biological roles, for example as potent and selective signaling molecules, hormones, ion channel ligands, and neurotransmitters. Polypeptides can be synthesized through native chemical ligation (NCL), a method that assembles long chains through the convergent fusion of smaller peptide fragments. Now, Richard J. Payne and co-workers have greatly increased the efficiency of polypeptide synthesis by adapting NCL to a flow platform and developing a new photochemical desulfurization method (DOI: 10.1021/jacs.8b03115). Through optimization of a model system, Payne and his team have discovered that two appropriately functionalized peptides can be ligated in flow. This flow process takes less than 12 min, representing a rate of ligation 4- to 50-fold faster than the analogous batch reactions using existing methods. With the ligation products in hand, the researchers have also identified a new photochemical desulfurization method and adapted this reaction to a flow platform capable of directly desulfurizing the NCL products. They apply this in-line, flowbased, ligation−photodesulfurization protocol to the rapid and efficient synthesis of the clinically approved HIV entry inhibitor enfuvirtide and the peptide diagnostic agent somatorelin. Elizabeth Meucci
could be expanded to access a variety of compounds derived from refractory materials. Christen Brownlee
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UNCOVERED: A LINK BETWEEN POLYMER ARCHITECTURE AND MECHANOCHEMISTRY In mechanochemistry, a physical force such as grinding solids together is used to drive chemical reactions. Researchers sometimes look to polymers as scaffolds to facilitate such transformations. Polymer mechanochemistry involves tethering the mechanophorethe molecular unit that will react in response to a mechanical triggerto the polymer backbone. Although studies have explored the ways the polymer backbone itself can influence the mechanochemical reaction, fundamental questions remain, such as, what role does the side chain size play? Tae-Lim Choi and co-workers are studying the effect of polymer architecture on mechanochemical kinetics for a class of macromolecular nanoscale structures known as dendronized polymers, or denpols (DOI: 10.1021/jacs.8b05110). The team synthesizes a class of polyphenylene-based denpols with varying both degrees of polymerization and generations of the dendron, resulting in side chains of different sizes. The researchers observe that the rates of ultrasound-induced scission correlate with side chain size. They use “master curves” from this degradation data to establish the rate constant for each polymer series, which can in turn be used to predict degradation rate constants for related polymers. The findings suggest that polymer mechanophores can be further optimized to expand and better tune their performance in various applications. Christine Herman, Ph.D.
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SMALL STEPWISE DEINTERCALATION ENABLES GIANT STEP TOWARD MBenes To prepare material phases that appear inaccessible via hightemperature synthesis methods, researchers are turning to lower temperature “soft chemistry” approaches. One example uses etchants to produce so-called MXenes, which are metal carbide and nitride analogs of graphene, from MAX phases, which are refractory materials with the formula Mn+1AXn (M = early transition metal, A = aluminum or silicon, and X = carbon or nitrogen). These researchers have been equally intrigued by the properties of MBenesboron analogs of MXenesbut the same soft chemistry approaches have not been successful in producing them. In a step toward synthesizing these unrealized compounds, Raymond E. Schaak, Nasim Alem, and co-workers explore aluminum deintercalation from MoAlB crystals (DOI: 10.1021/jacs.8b04705). Treating these refractory materials with etchants, they find that deintercalation happens in an unusual stepwise manner: as atoms disappear from aluminum double layers, the remaining aluminum distributes evenly between the two adjacent molybdenum boride layers. Longer etching times result in a variety of intergrowth phases, including Mo2AlB2, Mo3Al2B3, Mo4Al3B4, and Mo6Al5B6. The first of these, the authors suggest, has potential as a precursor to MoB, a MBene. They add that this deintercalation approach © XXXX American Chemical Society
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PALLADIUM CATALYSIS LEADS TO DIRECT SELECTIVE DIENYLATION Dienes are ubiquitous motifs in many pharmaceuticals, natural products, and materials. These conjugated π-systems are most often constructed through the sequential introduction of single alkene units, a time-intensive strategy that is necessary for them to be introduced regio- and stereoselectively. Direct incorporation of dienyl units would simplify the synthesis of conjugated dienes, but existing dienylation methods suffer from poor selectivity, yield, and scope. Oleg V. Larionov and co-workers have overcome these limitations by developing a new palladium-catalyzed method for the selective dienylation of aryl bromides (DOI: 10.1021/jacs.8b05421). At the heart of the new method is the use of readily available and bench-stable sulfolenes as cross-coupling dienylation reagents. Larionov and his team find that, under basic conditions, these sulfolenes undergo selective ring opening to form sulfinates, which subsequently participate in a desulfita-
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DOI: 10.1021/jacs.8b07151 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Spotlights
tive palladium-catalyzed coupling with aryl bromides to afford the desired dienes. The researchers achieve greater than 30:1 E/Z ratio when unsubstituted or 2-substituted sulfolenes are employed; however, a reversal in selectivity and greater than 30:1 Z/E ratio is achieved with 3-substituted sulfolenes. The authors believe the wide scope and high selectivity of this simple and scalable method will make it an invaluable tool for the synthesis of conjugated aryl polyenes and dienes. Elizabeth Meucci
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DOI: 10.1021/jacs.8b07151 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
