Recent Advances in Dearomative Partial Reduction of Benzenoid Arenes

Dearomative partial reduction is an extraordinary approach for transforming benzenoid arenes and has been well-known for many decades, as exemplified by the dehydrogenation of Birch reduction and the hydroarylation of Crich addition. Despite its remarkable importance in synthesis, this field has experienced slow progress over the last half-century. However, a revival has been observed with the recent introduction of electrochemical and photochemical methods. In this Minireview, we summarize the recent advancements in dearomative partial reduction of benzenoid arenes, including dihydrogenation, hydroalkylation, arylation, alkenylation, amination, borylation and others. Further, the intriguing utilization of dearomative partial reduction in the synthesis of natural products is also emphasized. It is anticipated that this Minireview will stimulate further progress in arene dearomative transformations.

Redox Reorganization: Aluminium Promoted 1,5-Hydride Shifts Allow the Controlled Synthesis of Multisubstituted Cyclohexenes

An efficient synthesis of cyclohexenes has been achieved from easily accessible tetrahydropyrans via a tandem 1,5-hydride shift-aldol condensation. We discovered that readily available aluminium reagents, e.g. Al2 O3 or Al(Ot Bu)3 are essential for this process, promoting the 1,5-hydride shift with complete regio- and enantiospecificity (in stark contrast to results obtained under basic conditions). The mild conditions, coupled with multiple methods available to access the tetrahydropyran starting materials makes this a versatile method with exceptional functional group tolerance. A wide range of cyclohexenes (>40 examples) have been prepared, many in enantiopure form, showing our ability to selectively install a substituent at each position around the newly forged cyclohexene ring. Experimental and computational studies revealed that aluminium serves a dual role in facilitating the hydride shift, activating both the alkoxide nucleophile and the electrophilic carbonyl group.

Base-Modulated 1,3-Regio- and Stereoselective Carboboration of Cyclohexenes

While chain-walking stimulates wide interest in both polymerization and organic synthesis, site- and stereoselective control of chain-walking on rings is still a challenging task in the realm of organometallic catalysis. Inspired by a controllable chain-walking on cyclohexane rings in olefin polymerization, we have developed a set of chain-walking carboborations of cyclohexenes based on nickel catalysis. Different from the 1,4-trans-selectivity disclosed in polymer science, a high level of 1,3-regio- and cis-stereoselectivity is obtained in our reactions. Mechanistically, we discovery that the base affects the reduction ability of B2 pin2 and different bases lead to different catalytic cycles and different regioselective products (1,2- Vs 1,3-addition). This study provides a concise and modular method for the synthesis of 1,3-disubstituted cyclohexylboron compounds. The incorporation of a readily modifiable boronate group greatly enhances the value of this method, the synthetic potential of which was highlighted by the synthesis of a series of high-valued commercial chemicals and pharmaceutically interesting molecules.

[Determination of Cyclohexene in workplace air by Solvent desorption gas chromatography]

Objective: To establish a solvent desorption gas chromatography method for determination of cyclohexene in workplace air. Methods: Cyclohexene in the air of workplace was collected with carbon tube and desorbed by carbon disulfide. The target toxicant was separated with the GC column and analyzed with FID detector, identified by retention time, and quantified by peak area. Results: The linear range of cyclohexene in the air of workplace was 0.77~4 050.00 μg/ml, with a correlation coefficient of 0.9999. The limit of detection was 0.23 μg/ml. The lower limit of quantification was 0.77 μg/ml. The minimum detectable concentration was 0.15 mg/m(3) under1.5 L sampling volume and 1.0 ml extraction solution volume. The within-run precision of different cyclohexene concentrations was 0.62%~1.9% and the between-run precisions was 1.5%~3.5%; The average extraction efficiency was 96.4%; Penetration capacity (100 mg of carbon tube) was 29.4 mg; The average collection efficiency was 100%; The samples could be stored for 7 days at room temperature. When placed in 4 ℃ refrigerator, the samples could be stored for 14 days. The potential coexistence of cyclohexane, hexane, benzene, toluene and ethylbenzene with cyclohexene in the air did not interfere with the results of determination. Conclusion: This method has high sensitivity, precision, accuracy and lower limit of detection and it is applicable for determination of cyclohexene in workplace air.

Effects of NOx, SO2 and RH on the SOA formation from cyclohexene photooxidation

We performed a laboratory investigation of the secondary organic aerosol (SOA) formation from cyclohexene photooxidation with different initial NOx and SO₂ concentrations at low and high relative humidity (RH). Both SOA yield and number concentration first increase drastically and then, decreased when the [VOC]₀/[NOx]₀ ratio changed from 30 to 10 and from 10 to 3. Though the presence of SO₂ could increase the SOA number concentration, the SOA yield could only increase under [VOC]₀/[NOx]₀ = 10 and high RH, and [VOC]₀/[NOx]₀ = 3 and low RH experimental conditions, while decreasing under [VOC]₀/[NOx]₀ = 10 and low RH conditions. In the presence of SO₂, the high RH and high NOx conditions were keys to efficient sulfate formation and could promote the SOA formation. The chemical composition of SOA was characterized using hybrid quadrupole-orbitrap mass spectrometer equipped with electrospray ionization (ESI-Q-Orbitrap-HRMS), and few organosulfates were identified. A visible enhancement of organosulfates and the formation of high molecular weight organic compounds were observed at high RH conditions, and this seemed to be the reason for the SOA yield increase at high RH.