15 research outputs found
NickelâCatalyzed Interâ and Intramolecular Aryl Thioether Metathesis by Reversible Arylation
A nickelâcatalyzed aryl thioether metathesis has been developed to access highâvalue thioethers. 1,2âBis(dicyclohexylphosphino)ethane (dcype) is essential to promote this highly functionalâgroupâtolerant reaction. Furthermore, synthetically challenging macrocycles could be obtained in good yield in an unusual example of ringâclosing metathesis that does not involve alkene bonds. Inâdepth organometallic studies support a reversible Ni0/NiII pathway to product formation. Overall, this work not only provides a more sustainable alternative to previous catalytic systems based on Pd, but also presents new applications and mechanistic information that are highly relevant to the further development and application of unusual singleâbond metathesis reactions
Efficient access to unprotected primary amines by iron-catalyzed aminochlorination of alkenes
Primary amines are essential constituents of biologically active molecules and versatile intermediates in the synthesis of drugs and agrochemicals. However, their preparation from easily accessible alkenes remains challenging. Here, we report a general strategy to access primary amines from alkenes through an operationally simple iron-catalyzed aminochlorination reaction. A stable hydroxylamine derivative and benign sodium chloride act as the respective nitrogen and chlorine sources. The reaction proceeds at room temperature under air; tolerates a large scope of aliphatic and conjugated alkenes, including densely functionalized substrates; and provides excellent anti-Markovnikov regioselectivity with respect to the amino group. The reactivity of the 2-chloroalkylamine products, an understudied class of amphoteric molecules, enables facile access to linear or branched aliphatic amines, aziridines, aminonitriles, azido amines, and homoallylic amines
âArc-continent collisionâ of the Aleutian-Komandorsky arc into Kamchatka: Insight into Quaternary tectonic segmentation through Pleistocene marine terraces and morphometric analysis of fluvial drainage
Evolution of landscape in a piedmont section of Eastern Himalayan foothills along India-Bhutan border: A tectono-geomorphic perspective
Coupled Rapid Erosion and Foreland Sedimentation Control Orogenic Wedge Kinematics in the Himalayan Thrust Belt of Central Nepal
Spatial and temporal coincidence among rapid Pliocene-Holocene bedrock exhumation, development of a topographic bight, abundant monsoonal precipitation, accumulation of anomalously thick proximal foreland basin deposits, and development of an opposite-polarity salient-reentrant couple on the two most frontal major thrust faults in the Himalayan orogenic wedge of central Nepal provide a basis for a model that links these diverse phenomena and could be operating in other parts of the frontal Himalaya. Rapid bedrock erosion is documented by a concentration of young (<5 Ma) low-temperature thermochronologic ages in the Narayani River catchment basin. Where the river exits the Lesser Himalayan Zone, the Main Boundary thrust has a 15-km-amplitude reentrant. Directly south of the reentrant lies the âŒ50 km wide Chitwan wedge-top basin, which is confined by a large salient on the Main Frontal thrust. Rapid erosion and sediment flux out of the Narayani catchment basin, possibly due to anomalously intense monsoonal precipitation in this topographically depressed region of central Nepal, causes greater flexural subsidence and surface aggradation in the foreland, both of which increase initial wedge taper and render this region more susceptible to anomalous forward propagation of the thrust front. Analysis of the modern and post-early Miocene taper history of the thrust belt suggests that rapid erosion hindered forward propagation of the contemporaneous Main Boundary thrust, but simultaneously produced conditions in the foreland that eventually elevated initial taper to a critical/supercritical value promoting forelandward propagation of the Main Frontal thrust. This analysis has implications for large damaging earthquakes in the Himalaya. © 2021. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 06 March 2021This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
Overestimation of the earthquake hazard along the Himalaya: constraints in bracketing of medieval earthquakes from paleoseismic studies
Abstract The Himalaya is one of the most seismically active regions of the world. The occurrence of several large magnitude earthquakes viz. 1905 Kangra earthquake (Mw 7.8), 1934 BiharâNepal earthquake (Mw 8.2), 1950 Assam earthquake (Mw 8.4), 2005 Kashmir (Mw 7.6), and 2015 Gorkha (Mw 7.8) are the testimony to ongoing tectonic activity. In the last few decades, tremendous efforts have been made along the Himalayan arc to understand the patterns of earthquake occurrences, size, extent, and return periods. Some of the large magnitude earthquakes produced surface rupture, while some remained blind. Furthermore, due to the incompleteness of the earthquake catalogue, a very few events can be correlated with medieval earthquakes. Based on the existing paleoseismic data certainly, there exists a complexity to precisely determine the extent of surface rupture of these earthquakes and also for those events, which occurred during historic times. In this paper, we have compiled the paleo-seismological data and recalibrated the radiocarbon ages from the trenches excavated by previous workers along the entire Himalaya and compared earthquake scenario with the past. Our studies suggest that there were multiple earthquake events with overlapping surface ruptures in small patches with an average rupture length of ~300 km limiting Mw 7.8â8.0 for the Himalayan arc, rather than two or three giant earthquakes rupturing the whole front. It has been identified that the large magnitude Himalayan earthquakes, such as 1905 Kangra, 1934 BiharâNepal, and 1950 Assam, that have occurred within a time frame of 45 years. Now, if these events are dated, there is a high possibility that within the range of ±50 years, they may be considered as the remnant of one giant earthquake rupturing the entire Himalayan arc. Therefore, leading to an overestimation of seismic hazard scenario in Himalaya