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Thermal Chemistry of Benzyl Isocyanate and Phenyl Isocyanate on Cu(111)Ma, Kuo-Chen 09 August 2011 (has links)
Nitrenes are reactive intermediates for many organic reactions, such
as Curtius rearrangement. The thermo- or photochemical- decomposition
of azides or isocyanates was known to generate nitrenes. We investigated
the thermal chemistry of nitrene adsorbed on Cu(111) using benzyl azide
(Bz-N=N=N), benzyl isocyanate (Bz-N=C=O) and phenyl isocyanate
(ph-N=C=O) as precursors under ultrahigh vacuum conditions using
temperature-programmed reaction/desorption (TPR/D), reflectionabsorption
infrared spectroscopy (RAIRS) and X-ray photoelectron
spectroscopy (XPS). Our study shows that despite of the isoelectronic
functionalities (-N=N=N vs. -N=C=O) these molecules undergo different
reaction pathways. For benzyl azide (Bz-N=N=N), the azido group losses
N2 ,and the phenyl group migrates from nitrogen to carbon, forming
surface bound H2C=N-Ph at 210 K. Eventually, H2 elimination and a
carbon-to-nitrogen phenyl shift give the thermally stable ph-CN final
product. XPS reveals that benzyl isocyanate (Bz-N=C=O) rearranges to
form amide intermediate on the surface, which breaks into CO2, HCN
and toluene at 410 K. RAIRs suggests that phenyl isocyanate
(ph-N=C=O) undergoes cyclodimerization, cyclotrimerization and
condensation to remove CO2 at 170 K, and phenyl group shifts from
nitrogen to carbon to produce a metal bound acyl nitrene species
(Ph-(C=O)-N---Cu) at 410 K.
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Thermal Chemistry of Benzyl Azide to Phenyl Isocyanide on Cu(111):Evidence for a Surface Imine IntermediateCheng, Cheng-Hung 03 August 2010 (has links)
Abstract
The Copper Catalyzed Azide-Alkyne Cycloaddition (CuAAC) is a paradigm of ¡§click¡¨ chemistry which has been applied in different fields. To understand the interaction between organic azides and a copper surface, we use benzyl azide (Bn¡ÐN£\¡ÐN£]¡ÝN£^) as an adsorbate on Cu(111) under ultrahigh vacuum conditions. The thermal reaction process was explored by a combination of temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoemission spectroscopy (XPS) techniques. The TPD profiles show a multilayer desorption peak at 190K, two peaks for N2 , and H2 from 270K to 390K. At 345K, peak of desorption product (m/z=103) represents phenyl cyanide (PhCN) or phenyl isocyanide (PhNC). RAIR and XP spectra demonstrate that at 190K benzyl azide on Cu(111) readily adopt the imine intermediate formalism involving N£\¡ÐN£] scission and phenyl group shift from carbon to nitrogen. The mechanism is analogous to the organic reaction of Schmidt rearrangement. To heat the surface to 250K, the CH2 group of the imine intermediate undergoes C¡ÐH bond scission to produce a surface isocyanide intermediate, (M=C=N¡ÐPh). Therefore the final desorption product is phenyl isocyanide at ~350K. Intriguingly, the thermal chemistry of benzyl azide involves both imine and isocyanide intermediacy, despite the fact that azido species usually generate nitrene or imido complexes under thermal conditions.
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