Chiral complexes of aluminum containing the salcyen ligand framework, [(R,R)-salcyen]AlX (X = Me, OSiMe2 tBu), catalyze the asymmetric addition of diorgano-H-phosphonates to carbonyls: the phospho-aldol reaction. Reaction proceeds smoothly at ambient temperature in various solvents and under aerobic conditions, to afford α-hydroxyphosphonate esters (MeO)2P(O)CHR(OH), with enantiomeric excesses (ee's) <50%. Although catalyst activity is linked to the nature of X, ee's appear to be only slightly affected. Substitution within the chiral salcyen ligand framework seems to affect ee principally where there is a steric or structural change near the metal center. Although catalysis is tolerant of small quantities of water, excess water leads to attenuation of enantioselectivity through decomposition of catalyst to afford hydrated aluminas which competes through achiral phospho-aldol catalysis. Kinetic analyses reveal a second-order polynomial relationship of the form x/([A0] − x)[A0] = k 2 t + k 2 t 2, where [A0] is the initial concentration of H-phosphonate and carbonyl and x the degree of reaction. This may suggest that the metal complex must first be converted to another, more active precursor prior to catalytic turnover. Hammett analyses suggest that carbonyl binding to the metal center results in enhanced ee's, while single-crystal analyses on four aluminum complexes support the view that twisting of the ligand framework, as measured by the five-coordinate τ parameter, from a purely meridional geometry may be advantageous to stereoselectivity. Strategies for future developments are discussed in light of the results herein.
Existing wearable systems that use G-sensors to identify daily activities have been widely applied for medical, sports and military applications, while body temperature as an obvious physical characteristic that has rarely been considered in the system design and relative applications of HAR. In the context of the normalization of COVID-19, the prevention and control of the epidemic has become a top priority. Temperature monitoring plays an important role in the preliminary screening of the population for fever. Therefore, this paper proposes a wearable device embedded with inertial and temperature sensors that is used to apply human behavior recognition (HAR) to body surface temperature detection for body temperature monitoring and adjustment by evaluating recognition algorithms. The sensing system consists of an STM 32-based microcontroller, a 6-axis (accelerometer and gyroscope) sensor, and a temperature sensor to capture the original data from 10 individual participants under 4 different daily activity scenarios. Then, the collected raw data are pre-processed by signal standardization, data stacking and resampling. For HAR, several machine learning (ML) and deep learning (DL) algorithms are implemented to classify the activities. To compare the performance of different classifiers on the seven-dimensional dataset with temperature sensing signals, evaluation metrics and the algorithm running time are considered, and random forest (RF) is found to be the best-performing classifier with 88.78% recognition accuracy, which is higher than the case of the absence of temperature data (<78%). In addition, the experimental results show that participants’ body surface temperature in dynamic activities was lower compared to sitting, which can be associated with the possible missing fever population due to temperature deviations in COVID-19 prevention. According to different individual activities, epidemic prevention workers are supposed to infer the corresponding standard normal body temperature of a patient by referring to the specific values of the mean expectation and variance in the normal distribution curve provided in this paper.
The enantioselective addition of diorgano-H-phosphonates to aldehydes, the phospho-aldol reaction, is catalysed by both (R,R)-salcyen and (R,R)-salcyan complexes of aluminium at room temperature under aerobic conditions. However, whilst the former ligand system favours one absolute configuration of product a-hydroxyphosphonate ester, to the extent of <50%, the latter ligand framework favours the opposite configuration < 60%, despite both ligand systems having the same absolute backbone configuration! Structure-activity, single-crystal X-ray and computational studies shed light about these differences and provide important clues as to how to increase stereoselectivity.
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