The Appalachian Mountains were formed through multiple phases of Paleozoic orogenesis associated with terrane accretion. The timing, tempo, and significance of each event in New England are obscured by overprinting, the limits of geochronologic tools, and differences between lithotectonic domains. We present new monazite and xenotime geochronology, 40Ar/39Ar thermochronology, and major- and trace-element thermobarometry from major tectonic domains in southern New England and across multiple structural levels. These data show contrasting pressure-temperature-time (P-T-t) paths across tectonic domains and highlight eastward metamorphic overprinting associated with younger tectonic events. Our data and geochemical proxies suggest two major periods of crustal thickening, ca. 455–440 Ma and 400–380 Ma, and a heterogeneous record of thinning/exhumation. Ordovician (Taconic) crustal thickening postdates the interpreted accretion of the Moretown terrane by ~20 m.y. and may have been related to shallow subduction after subduction polarity reversal. Subsequent cooling and exhumation (440–430 Ma) may have been related to the end of the Taconic orogeny and opening of the Connecticut Valley basin. (Neo)Acadian tectonometamorphism is recognized in accreted terranes of New England and is absent in the Taconic block. Amphibolite- to (high-pressure) granulite-facies metamorphism, slow cooling, and protracted anatexis ca. 400–340 Ma support the existence of a long-lived orogenic plateau in southern New England. Exhumation, which began at 340–330 Ma, may have involved ductile (channel) flow. The boundary between continental Laurentia and accreted terranes has been reactivated at multiple times and is presently manifested as a 12–15 km Moho step. At the latitude of our samples, Alleghanian-age tectonism (ca. 310–285 Ma) was limited to retrograde metamorphism, and relatively minor loading and exhumation in the vicinity of the Pelham dome. Our results highlight the sensitivity of the integrative petrochronologic approach and the transition of the eastern margin of Laurentia from terrane accretion to the formation of a high-elevation plateau.
Herein we report the synthesis of Cr imido complexes in bis(alkoxide) ligand environments and their nitrene transfer reactivity with isocyanides. The reaction of Cr 2 (OR) 4 (OR = OC t Bu 2 Ph) with bulky aryl or alkyl azide results in the formation of the trigonal-planar Cr(IV) mono(imido) complexes Cr(OR) 2 (NR 1 ), whereas less bulky aryl azides form the Cr(VI) bis(imido) complexes Cr(OR) 2 (NR 1 ) 2 . Cr(IV) mono-(imido) complexes undergo facile reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR 2 ) to form the corresponding carbodiimides R 1 NCNR 2 . In contrast, no reaction of Cr(OR) 2 (NR 1 ) 2 complexes with CNR 2 is observed. The reaction of Cr(OR) 2 (NR 1 ) with excess isocyanide leads to the isolation of the Cr(II) complex Cr(OR) 2 (CNR 2 ) 4 , along with the observation of the anticipated carbodimide product. Cr(OR) 2 (CNR 2 ) 4 , which can also be obtained by treating Cr 2 (OR) 4 with 4 equiv of isocyanide, reacts with azides N 3 R 1 (R 1 = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R 1 NCNR 2 is observed from the mixtures of azides R 1 N 3 (R 1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several different aryl isocyanides CNR 2 using 2.5 mol % of Cr 2 (OR) 4 . ■ INTRODUCTIONCarbodiimides are widely used organic compounds that hydrolyze readily and thus serve as excellent dehydration agents. Large-scale applications of carbodiimides include as stabilizers and antihydrolysis agents in polymers production, as coupling agents in peptide synthesis, and in various organic transformations. 1−6 Carbodiimides are synthesized by a plethora of transition-metal-catalyzed routes, 7 including oxidative coupling of amines with isocyanides 8 and coupling of two isocyanates R′NCO with loss of 1 equiv of CO 2 , 9 as well as by transition-metal-free stoichiometric routes such as the reaction of isocyanates with phosphinimines (aza-Wittig) 10 and by tinmediated addition of silylamines to isocyanates. 11 Coupling of an [NR] (nitrene) group (originating in organoazides) with isocyanide is among the most attractive methods to synthesize carbodiimides, as (1) a large variety of azides are easily accessible using simple synthetic procedures, (2) both symmetrical and asymmetrical carbodiimides can be synthesized, and (3) the only side product in this reaction is the environmentally friendly dinitrogen. Therefore, numerous efforts have been made to make carbodiimides via coupling of isocyanides and organoazides. Most of these coupling reactions take place at low-coordinate late-transition-metal catalysts stabilized by nitrogen, carbon, or phosphorus ligands, where the MNR functionality is suggested to serve as the reactive intermediate. 12−21 The combination of the soft, electron-rich late transition metals and the hard π-donating imido functionality make the imido functionality reactive and thus susceptible to transfer. In contrast, imido compounds of earlier transition metals are significantly more stable, because the higher oxidation stat...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.