N-heterocyclic carbenes which readily add ammonia, carbon monoxide and other small molecules,

Abstract

N-Heterocyclic carbenes (NHCs) are extremely valuable as nucleophilic organocatalysts. They are widely applied as ligands in transition-metal catalysed reactions, where they are known as particularly potent s-donors. They are commonly viewed as workhorses exhibiting reliable, but undramatic, chemical behaviour. The N / C carbene p-donation stabilises NHCs at the expense of low reactivity towards nucleophiles. In contrast to NHCs, stable (alkyl)(amino)carbenes exhibit spectacular reactivity, allowing, for example, the splitting of hydrogen and ammonia and the fixation of carbon monoxide. NHCs have been judged to be electronically not suitable for showing similar reactivity. Here, we demonstrate that a ferrocene-based NHC is able to add ammonia, methyl acrylate, tert-butyl isocyanide, and carbon monoxide-reactions typical of (alkyl)(amino)carbenes, but unprecedented for diaminocarbenes. We also show that even the simplest stable diaminocarbene, C(NiPr 2 ) 2 , adds CO. This reaction affords a b-lactam by a subsequent intramolecular process involving a C-H activation. Our results shed new light on the chemistry of diaminocarbenes and offer great potential for synthetic chemistry and catalysis.

An Isolable, Photoswitchable N‐Heterocyclic Carbene: On‐Demand Reversible Ammonia Activation

“…[10] Theaddition of gaseous NH 3 to aC 6 D 6 solution of 1o produced no noticeable change,a sd etermined by 1 HNMR spectroscopy. [10] Building on these results,w er easoned that the capture and subsequent release of NH 3 over the course of as ingle experiment should be possible by utilizing the distinct electronic states of 1.T otest this hypothesis,aC 6 D 6 solution of 1o was transferred to aq uartz low-pressure NMR tube equipped with aT eflon screw cap and cooled to À78 8 8C. [10] Building on these results,w er easoned that the capture and subsequent release of NH 3 over the course of as ingle experiment should be possible by utilizing the distinct electronic states of 1.T otest this hypothesis,aC 6 D 6 solution of 1o was transferred to aq uartz low-pressure NMR tube equipped with aT eflon screw cap and cooled to À78 8 8C.…”

“…[3] We recently developed ap hotoswitchable NHC by incorporating ap hotochromic [4] dithienylethene (DTE) [5] unit into an imidazolylidene backbone,and found that the corresponding carbene functioned as ap hotoswitchable organocatalyst for transesterification and transamidation [6] reactions as well as for ring-opening polymerizations. [10] Herein, we report the first isolable,p hotoswitchable NHC and demonstrate the remote modulation of its electronic properties in the free carbene state as well as its utility in the on-demand capture and release of ammonia. [8] All of these catalysts were effectively switched between "off" (inactive) and "on" (active) states by irradiation with UV or visible light, respectively.A lthough other DTE-functionalized NHCs have been reported, [9] the corresponding free carbenes have hitherto not been isolable and required in situ generation.…”

Metal‐Free Activation of Hydrogen, Carbon Dioxide, and Ammonia by the Open‐Shell Singlet Biradicaloid [P(μ‐NTer)]2

“…[1,15] Among the first examples of highly reactive main-group species that are able to activate stable molecules were carbenes (B,S cheme 1), featuring electron-rich, two-coordinate carbon centers. [16,17] Theh eavier Group 14 homologues,s ilylenes and germylenes,w ere also investigated;f or example,M üller et al reported on the activation of dihydrogen whereas Roesky et al investigated the addition of ammonia to silylenes. [16,17] Theh eavier Group 14 homologues,s ilylenes and germylenes,w ere also investigated;f or example,M üller et al reported on the activation of dihydrogen whereas Roesky et al investigated the addition of ammonia to silylenes.…”

Generation of 1,2-azaboretidines via reduction of ADC borane adducts

“…21b,25 A similar mechanism can be invoked to explain the formation of the 1,2-azaboretidine, since the initial reduction of 3a presumably yields a high-energy borylene (ADC = B–Dur), analogous to the aforementioned ketene (ADC = CO).…”