Visible-light-driven
photocatalytic reductive azaarylation has
been widely used to construct the important imine-containing azaarene
derivatives. In addition to the direct use of various commercially
available cyanoazaarenes as feedstocks, the synthetic advantages include
precise regioselectivity, high efficiency, mild reaction conditions,
and good functional group tolerance. However, although many efficient
reductive azaarylation methods have been established, the example
of an enantioselective manner is still unmet, which most likely can
be ascribed to the highly reactive radical coupling as the key step
of forming stereocenters. Exploring the feasibility of enantiocontrol
thus constitutes an attractive but highly challenging task. Here,
we demonstrate that chiral hydrogen-bonding/photosensitizer catalysis
is a viable platform as it enables the realization of the first enantioselective
manifold. A variety of acyclic and cyclic enones as the reaction partners
are compatible with the dual catalyst system, leading to a wide array
of valuable enantioenriched azaarene variants with high yields and
ees. Regulating the types of chiral catalysts represents one of the
important manners to success, in which several readily accessible Cinchona alkaloid-derived bifunctional catalysts
are introduced in asymmetric photochemical reactions.
A mild and extra activator-free dehydrative alkylation of stabilized phosphonium ylides with allylic alcohols in water is developed in the presence of [Pd(allyl)Cl] 2 /dppf catalyst. A wide range of aryl, heteroaryl, alkyl and even allylic tertiary alcohols can readily react with stabilized phosphonium ylides with high regioselectivity for the efficient synthesis of functionalized skipped dienes in moderate to high yields. The role of water was investigated by means of a high-resolution mass spectrum and diffusion-ordered spectroscopy nuclear magnetic resonance, and the results revealed that water might play a crucial role in the formation of the p-allylpalladium complex via hydrogen bond. However, the present method is not suitable for water-sensitive phosphonium ylides.
The free-piston engine generator is a new power machinery, which removes the crank-shaft mechanism. This article aims to study the impact of different injection rate shapes (left triangle, rectangle, and right triangle) on the free-piston engine generator spray and combustion characteristics. An iteration numerical model was established and validated by experiment. The results reveal that there is a positive correlation between injection rate and evaporation rate, and the faster evaporation rate is more conducive to total evaporated fuel mass. The left triangle and rectangle injection modes have the better level of atomization and mixing in the early injection due to stronger turbulent kinetic energy, longer penetration, and smaller Sauter mean diameter, while the right triangle has opposite results, which impairs the mixture formation, and its higher injection pressure in the late injection duration causes more impinged fuel mass. Moreover, the temperature, pressure, peak heat release rate, and indicated thermal efficiency for the left triangle are greater than the other two injection modes. Therefore, for the free-piston engine generator, faster injection rate in the early injection stage enables to improve the fuel evaporation, atomization, and mixing processes, meanwhile enhances the level of isochoric heat release around top dead center, resulting in higher indicated thermal efficiency.
The free-piston engine generator becomes a new-type potential substitute for the conventional crankshaft combustion engine. This article presents a simulation to study the fuel spray and mixing characteristics of a diesel free-piston engine generator by comparing a corresponding crankshaft combustion engine. A full-cycle model which couples with piston dynamics, combustion, and gas exchange is developed to simulate the fuel spray, atomization, and mixing in the free-piston engine generator. The result indicates that compared with the crankshaft combustion engine, the free-piston engine generator provides a higher temperature and pressure for fuel spray and mixing during the ignition delay, but its ignition delay lasts shorter. The free-piston engine generator shows a shorter spray penetration and more fuel impingement due to its smaller combustion chamber volume during the injection process. The free-piston engine generator exhibits a lower level of air utilization and worse uniformity of fuel–air mixture in combustion chamber. In addition, the shorter ignition delay of free-piston engine generator makes the time of atomization, evaporation, and mixing of fuel shorter, and the mixing effect of free-piston engine generator is worse, resulting in less combustible mixture formed during the ignition delay. In addition, some guiding suggestions have been proposed to improve the fuel spray and fuel–air mixing characteristics of free-piston engine generator.
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