Synlett

New solution-processable chrysene-basked deep-blue fluorescent dendrimers consisting of chrysene as a core end-capped by carbazole dendrons were designed, synthesized, and characterized. These dendrimers exhibit strong deep-blue emissions in solution or thin-film states with decent hole mobility and high thermal and electrochemical stability. They can be effectively used as nondoped emitters in organic light-emitting diodes (OLEDs). The nondoped OLEDs, which have a simple structure, showed good electroluminescence (EL) performance (luminance: 2334–2400 cd m–2; external quantum efficiency: 1.88-2.51%; turn-on voltage: 3.8–4.0 V) and deep-blue EL spectra (CIE y: 0.065–0.075) with a narrow full width at half maximum of 61–65 nm.

Heteroarenes are important units in organic chemistry and are ubiquitous in natural products, pharmaceuticals, and numerous artificial molecules. Despite great efforts devoted to accessing heteroarenes, the development of new methods to efficiently produce heteroarenes remains a long-term interest. Recently, the strategy of radical-mediated heteroaryl migration has supplied a robust toolkit for the synthesis of a diversity of heteroaryl-containing compounds. This Account summarizes our recent achievements in this field and provides insight into the incorporation of heteroarenes into organic skeletons.1 Introduction2 Radical-Mediated Heteroarylation of Alkanes and Alkenes via Intramolecular Heteroaryl Migration2.1 C(sp3)–H Heteroarylation via Intramolecular Heteroaryl Migration2.2 Difunctionalization of Alkenes via Intramolecular Heteroaryl Migration3 Intermolecular Difunctionalization of Alkenes via ‘Docking-Migration’ Strategy3.1 Sulfone-Based Bifunctional Reagents for Difunctionalization of Alkenes by Docking Migration3.2 Sulfone-Based Reagents for the Synthesis of N-Fused Heteroarenes by Docking Migration3.3 Tertiary Alcohol Based Bifunctional Reagents for Difunctionalization of Alkenes by Docking Migration3.4 Diaryl Ether Based Bifunctional Reagents for Difunctionalization of Alkenes by Docking Migration3.5 Conclusion

Here we report a microfluidic system for photochemical cycloadditions fabricated using silicon micro processing technologies. The system was optimized to yield residence times of just a few minutes for a range of photochemical [2+2]-cycloaddition reactions facilitated using high power UV-LEDs at 375 nm and triplet photosensitizers, which removed the need for the low wavelengths typically required for these types of transformations. Adducts using different excitable olefins with different linear, carbocyclic, and heterocyclic coupling partners were explored to demonstrate the feasibility of performing photochemistry in microflow in an academic research environment. Finally, a reaction leading to a novel dihydrooxepin-2(3H)-one scaffold and a mechanistic proposal for its formation are reported.

A direct access to O-heterocycles by Bi(OTf)3-catalyzed intermolecular cyclization of phenols with diols is described. Catalytic dehydrative tandem formation of C–O and C–C bonds was achieved by using a Bi(OTf)3 catalyst without any additives. The catalytic intermolecular cyclization of phenols with various diols proceeded to give the corresponding chromanes in up to 91% yield and/or pyran derivatives in up to 66% yield.

A facile approach for the synthesis of benzimidazolones via a Ph3P–I2 promoted reaction of hydroxamic acids is reported. Upon Lossen-type rearrangement of the O-activated hydroxamic acids, the in situ generated isocyanates undergo an intramolecular attack by ortho N-nucleophiles producing the cyclized products in good yields under mild conditions. The method allows the direct preparation of a single regioisomer of N-monosubstituted derivatives using readily accessible starting materials and low-cost reagents with broad substrate scope.

A facile and efficient method has been developed for the synthesis of fluorenes by Pd-catalyzed C–H alkylation of biphenyl-2-yl trifluoromethanesulfonates. The trifluoromethanesulfonates are more readily available and more environmentally benign than biphenyl iodides, and are advantageous substrates for traceless directing-group-assisted C–H activation. The reaction generates C,C-palladacycles as the key intermediates that form two C(sp2)–C(sp3) bonds through reaction with CH2Br2. The reaction tolerates various functional groups, permitting easy access to a range of fluorene derivatives.

Carbohydrates have played an important role in organic synthesis. Since they contain many stereocenters, they have been widely used as chiral-pool starting materials. Herein, we report the asymmetric formal synthesis of (–)-swainsonine, which exhibits anticancer and immunosuppressive activities and inhibits lysosomal α-mannosidase activity, from d-mannose and d-arabinose. The synthesis utilized Zn-mediated Bernet–Vasella reaction, Horner–Wadsworth–Emmons olefination, and Grubbs olefin metathesis as key reactions.

1,4,2-Dioxazol-5-ones are known to undergo decarboxylation under thermal conditions followed by Lossen’s rearrangement to give isocyanates. Described herein is the in situ trapping of the isocyanates by indoles to give indole-3-carboxamides in good to excellent yields.

The mechanically induced symmetry-allowed disrotatory ring openings of cis- and trans-gem-dichlorocyclopropane (gDCC) diesters are demonstrated through sonication and single-molecule force spectroscopy (SMFS) studies. In contrast to the previously reported symmetry-forbidden conrotatory ring opening of alkyl-tethered trans-gDCC, we show that the diester-tethered trans-gDCC primarily undergoes a symmetry-allowed disrotatory pathway even at the high forces (>2 nN) and short-time scales (ms or less) of sonication and SMFS experiments. The quantitative force-rate data obtained from SMFS data is consistent with computational models of transition-state geometry for the symmetry-allowed process, and activation lengths of 1.41 ± 0.02 Å and 1.08 ± 0.03 Å are inferred for the cis-gDCC diester and trans-gDCC diester, respectively. The strong mechanochemical coupling in the trans-gDCC is notable, given that the directionality of the applied force may appear initially to oppose the disrotatory motion associated with the reaction. The stereochemical perturbations of mechanical coupling created by the ester attachments reinforce the complexity that is possible in covalent polymer mechanochemistry and illustrate the breadth of reactivity outcomes that are available through judicious mechanophore design.

Palladium-catalyzed direct arylation of α,β-unsaturated carbonyl compounds is an efficient and attractive strategy to access arylated α,β-unsaturated carbonyl compounds through the construction of carbon–carbon bonds. This reaction has several challenges, especially in terms of the control of regioselectivity between α- and γ-arylation and the selectivity for monoarylation and multiple arylation. Herein, we discuss the recent development of γ-arylation of α,β-unsaturated carbonyl compounds and present the ligand-controlled, site-selective α- and γ-arylation of α,β-unsaturated carbonyl ketones with (hetero)aryl halides. The site selectivity of the reaction is switchable by simply changing the phosphine ligand.1 Introduction2 Reaction Development and Mechanistic Investigation3 Conclusion and Outlook